Separate CHANCE and WEIGHT. CHANCE is a probability, WEIGHT is a raw number used as a generation weight (i.e. WEIGHT is what the old CHANCE used to be).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@7501 c06c8d41-db1a-0410-9941-cceddc491573

26 files changed, 581 insertions, 297 deletions

diff --git a/crawl-ref/docs/level_design.txt b/crawl-ref/docs/level_design.txt
index 95cce6b6c4..f8c9932d1d 100644
--- a/crawl-ref/docs/level_design.txt
+++ b/crawl-ref/docs/level_design.txt
@@ -298,12 +298,62 @@ DEPTH:    For random vaults, branch entry vaults, and minivaults, this
           have no DEPTH: constraint. Note that maps without a DEPTH: constraint
           cannot be selected as random vaults or minivaults.
 
-CHANCE:   (number with 10 being default)
+CHANCE:   <priority>:<roll> or <roll>
+
+          CHANCE allows you to control the probability that your map
+          is used on any given level with an absolute roll.
+
+          There are two ways to specify the CHANCE roll:
+
+          CHANCE: 500
+          or
+          CHANCE: 5%
+
+          If specified as a raw number, the chance of selecting the
+          vault is <number> in 10000. If specified as a percentage,
+          the chance of selecting the vault is <perc> * 100 in 10000.
+          Note that CHANCE accepts only integers, no fractions or
+          decimals.
+
+          For any map with alternatives, a CHANCE influences how
+          likely the map is to be picked instead of the alternatives.
+          If a map has a CHANCE, Crawl will roll a random number in
+          the range 1-10000, and select the map if the CHANCE is >=
+          the rolled random number.
+
+          If there are multiple alternative maps with CHANCE, they
+          will be tested in an unspecified order; the first map that
+          makes the CHANCE roll will be used. If you'd like to specify
+          an order of testing CHANCEs, specify a CHANCE with a
+          priority:
+
+             CHANCE: 10 : 20%
+
+          This specifies a CHANCE of 20%, with a priority of 10, which
+          means this vault will be checked before any other vault with
+          a lower priority (the default priority is 0).
+
+          If no map with a CHANCE is picked, Crawl will select a map
+          based on WEIGHT, ignoring vaults with a CHANCE set.
+
+          Note that the Lua equivalent for CHANCE is a two-argument
+          function:
+
+          : chance(<priority>, <number>)
+
+          These lines are all equivalent:
+          CHANCE: 5%
+          CHANCE: 500
+          CHANCE: 0 : 5%
+          CHANCE: 0 : 500
+          : chance(0, 500)
+
+WEIGHT:   (number with 10 being default)
           For entry vaults and any other vaults randomly picked from among
           a set, this type of line affects the likelihood of the given vault
-          being picked in a given game. The default CHANCE: is 10. The
+          being picked in a given game. The default WEIGHT: is 10. The
           likelihood of a vault getting picked is:
-          [vault's CHANCE: / sum of all CHANCE:s of vaults of that type]
+          [vault's WEIGHT: / sum of all WEIGHT:s of vaults of that type]
 
 PLACE:    Used to specify certain special levels. Existing special levels
           include most branch ends.
@@ -1042,7 +1092,7 @@ G.    Hints for level makers
   The same goes for traps. This is much less so if there are several places
   for the chamber (or trap) and there's even a chance it doesn't exist.
 
-  You can also use CHANCE to create modified versions of the same map. In
+  You can also use WEIGHT to create modified versions of the same map. In
   order to do this, make several maps and endow each with a chance such
   that the sum of chances add up to 10.
 
@@ -1068,13 +1118,13 @@ G.    Hints for level makers
   necromancy) is even better.
 
 * Testing your maps.
-  This is easy for entry vaults. Temporarily introducing a CHANCE: 50000
+  This is easy for entry vaults. Temporarily introducing a WEIGHT: 50000
   will make your entry appear almost always. For other vaults, you can
-  for the moment declare them as entry vaults with a huge CHANCE: as
+  for the moment declare them as entry vaults with a huge WEIGHT: as
   above (and preferably in wizard mode). For more intricate things like
   new branch ends, you have to resort to wizard mode and use the &~ command
   to go directly to the place where the map is used, say Snake:5. You may want
-  to use a high CHANCE: again, if the map has alternatives (like Snake:5, or
+  to use a high WEIGHT: again, if the map has alternatives (like Snake:5, or
   Coc:7). Minivaults can also be tested by adding a PLACE: to the definition,
   which makes it very likely that the minivault will appear in the chosen
   level.
diff --git a/crawl-ref/source/dat/altar.des b/crawl-ref/source/dat/altar.des
index 76b7b8981e..d02c6001f6 100644
--- a/crawl-ref/source/dat/altar.des
+++ b/crawl-ref/source/dat/altar.des
@@ -17,7 +17,7 @@
 NAME:   minitemple
 TAGS:   allow_dup
 # More common than the others.
-CHANCE: 20
+WEIGHT: 20
 MAP
 cccccccccc
 cBcBcBcBcc
@@ -113,7 +113,7 @@ ENDMAP
 
 NAME:   lemuel_altar_in_water
 DEPTH:  D:2-18, Swamp, Shoal
-CHANCE: 9
+WEIGHT: 9
 : local brnd = crawl.random2(13)
 : if brnd > 10 then
 TAGS:   no_monster_gen no_rotate
@@ -138,7 +138,7 @@ ENDMAP
 NAME:   lemuel_altar_in_water2
 TAGS:   no_pool_fixup no_monster_gen
 DEPTH:  D:2-18, Lair, Snake, Swamp, Shoal
-CHANCE: 1
+WEIGHT: 1
 MAP
    www
   wwwww
@@ -176,7 +176,7 @@ MAP
 ENDMAP
 
 NAME:   lemuel_sealed_altar_a
-CHANCE: 3
+WEIGHT: 3
 DEPTH:  D:2-18, Vault, Elf, Crypt
 MAP
 xxxxxxxxxxxxx
@@ -194,7 +194,7 @@ xxxxxxxxxxxxx
 ENDMAP
 
 NAME: lemuel_sealed_altar_b
-CHANCE: 7
+WEIGHT: 7
 DEPTH: D:2-18, Vault, Elf, Crypt
 MAP
 xxxxxxxxxxx
diff --git a/crawl-ref/source/dat/bazaar.des b/crawl-ref/source/dat/bazaar.des
index 6ea205b1bc..500160406e 100644
--- a/crawl-ref/source/dat/bazaar.des
+++ b/crawl-ref/source/dat/bazaar.des
@@ -209,7 +209,7 @@ ENDMAP
 NAME:   bazaar_general_marketplace
 TAGS:   bazaar allow_dup no_rotate
 ORIENT: encompass
-CHANCE: 5
+WEIGHT: 5
 KFEAT:  A = any shop
 : bazaar_message(_G)
 MAP
@@ -236,7 +236,7 @@ ENDMAP
 NAME:    bazaar_outfitter
 TAGS:    bazaar allow_dup
 ORIENT:  encompass
-CHANCE:  5
+WEIGHT:  5
 KFEAT:   A = any shop
 SHUFFLE: ABC, de
 KFEAT:   A = weapon shop / armour shop
@@ -262,7 +262,7 @@ ENDMAP
 # 4 random shops
 NAME:    bazaar_oval
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  encompass
 SUBST:   A = TVBG
 KFEAT:   B = any shop
@@ -282,7 +282,7 @@ ENDMAP
 # 2 shops
 NAME:    bazaar_mystics
 TAGS:    bazaar allow_dup
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  encompass
 SUBST:   w : wlx
 SHUFFLE: AB, def
@@ -452,7 +452,7 @@ ENDMAP
 # circle bazaar
 # 5 shops
 NAME:    bazaar_circle_1
-CHANCE:  2
+WEIGHT:  2
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 SHUFFLE: ABCD, EFGH
@@ -492,7 +492,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 ENDMAP
 
 NAME:    bazaar_circle_2
-CHANCE:  2
+WEIGHT:  2
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 SHUFFLE: EFGH
@@ -532,7 +532,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 ENDMAP
 
 NAME:    bazaar_circle_3
-CHANCE:  2
+WEIGHT:  2
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 SHUFFLE: EFGH
@@ -572,7 +572,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 ENDMAP
 
 NAME:    bazaar_circle_4
-CHANCE:  1
+WEIGHT:  1
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 SHUFFLE: ABCDEF
@@ -611,7 +611,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 ENDMAP
 
 NAME:    bazaar_circle_5
-CHANCE:  2
+WEIGHT:  2
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 KFEAT:   C = any shop
@@ -649,7 +649,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 ENDMAP
 
 NAME:    bazaar_circle_6
-CHANCE:  1
+WEIGHT:  1
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
 KFEAT:   A = any shop / antique armour shop
@@ -796,7 +796,7 @@ ENDMAP
 # ~3 shops
 NAME:    bazaar_cross1
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SHUFFLE: ABCD
 SUBST:   A = <, B = >
@@ -827,7 +827,7 @@ ENDMAP
 
 NAME:    bazaar_cross2
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  3
+WEIGHT:  3
 ORIENT:  encompass
 SHUFFLE: ABCD
 SUBST:   A = T E, B = >
@@ -909,7 +909,7 @@ ENDMAP
 # 5 shops
 NAME:    bazaar_ninerooms1
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SUBST:   k : . x
 NSUBST:  A = 2=T:30 V / *:C
@@ -946,7 +946,7 @@ ENDMAP
 
 NAME:    bazaar_ninerooms2
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SUBST:   k : . x
 NSUBST:  A = 2=T:30 V / *:C
@@ -983,7 +983,7 @@ ENDMAP
 
 NAME:    bazaar_ninerooms3
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SUBST:   z = .:50 +
 SUBST:   k : . x
@@ -1021,7 +1021,7 @@ ENDMAP
 
 NAME:    bazaar_ninerooms4
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SUBST:   z = .:50 +
 SUBST:   k : . x
@@ -1059,7 +1059,7 @@ ENDMAP
 
 NAME:    bazaar_ninerooms5
 TAGS:    bazaar allow_dup no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  encompass
 SUBST:   k : . x
 NSUBST:  A = 1:< / 1:> / 1:T C / 3=T:30 V / *:C
@@ -1100,7 +1100,7 @@ ENDMAP
 NAME:    bazaar_triangles
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
-CHANCE:  1
+WEIGHT:  1
 SHUFFLE: ACD
 SUBST:   A = <
 KFEAT:   B = any shop / antique armour shop / jewellery shop
@@ -1122,7 +1122,7 @@ ENDMAP
 NAME:    bazaar_hexagon
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
-CHANCE:  4
+WEIGHT:  4
 SHUFFLE: AC, BD
 KFEAT:   A = armour shop / weapon shop / wand shop
 KFEAT:   B = general shop / food shop
@@ -1152,7 +1152,7 @@ ENDMAP
 NAME:    bazaar_triangle_bulge
 TAGS:    bazaar allow_dup no_rotate
 ORIENT:  encompass
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: ABC
 SUBST:   A = <, B = >
 KFEAT:   C = any shop
diff --git a/crawl-ref/source/dat/clua/loadmaps.lua b/crawl-ref/source/dat/clua/loadmaps.lua
index 0e12ed4313..2f6b617635 100644
--- a/crawl-ref/source/dat/clua/loadmaps.lua
+++ b/crawl-ref/source/dat/clua/loadmaps.lua
@@ -7,10 +7,13 @@
 ------------------------------------------------------------------------------
 
 local des_files = {
-   "altar.des", "bazaar.des", "entry.des", "elf.des", "float.des", "hells.des",
-   "hive.des", "lab.des", "lair.des", "large.des", "layout.des", "mini.des", 
-   "orc.des", "pan.des", "sewer.des", "temple.des", "vaults.des", "crypt.des",
-   "zot.des"
+  -- The dummy vaults that define global vault generation odds.
+  "dummy.des", 
+  
+  "altar.des", "bazaar.des", "entry.des", "elf.des", "float.des",
+  "hells.des", "hive.des", "lab.des", "lair.des", "large.des", "layout.des",
+  "mini.des", "orc.des", "pan.des", "sewer.des", "temple.des", "vaults.des",
+  "crypt.des", "zot.des"
 }
 
 for _, file in ipairs(des_files) do
diff --git a/crawl-ref/source/dat/elf.des b/crawl-ref/source/dat/elf.des
index 19b962335b..ee2b093303 100644
--- a/crawl-ref/source/dat/elf.des
+++ b/crawl-ref/source/dat/elf.des
@@ -37,7 +37,7 @@
 NAME:   elf_arrival_dummy
 TAGS:   dummy
 PLACE:  Elf:1
-CHANCE: 50
+WEIGHT: 50
 ORIENT: float
 MAP
 {
@@ -378,7 +378,7 @@ PLACE:   Elf:7
 TAGS:    no_pool_fixup no_rotate
 LFLAGS:  no_tele_control
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 MONS:    deep elf high priest, deep elf demonologist
 MONS:    deep elf annihilator, deep elf sorcerer
 MONS:    deep elf death mage, deep elf blademaster
@@ -432,7 +432,7 @@ NAME:    elf_hall_gauntlet_narrow
 PLACE:   Elf:7
 TAGS:    no_pool_fixup no_rotate
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 MONS:    deep elf high priest, deep elf demonologist
 MONS:    deep elf annihilator, deep elf sorcerer
 MONS:    deep elf death mage, deep elf master archer
diff --git a/crawl-ref/source/dat/entry.des b/crawl-ref/source/dat/entry.des
index 932f09a06e..aac7791fcc 100644
--- a/crawl-ref/source/dat/entry.des
+++ b/crawl-ref/source/dat/entry.des
@@ -114,7 +114,7 @@ ENDMAP
 # good luck! (Shiori)
 NAME:    shiawase
 TAGS:    entry no_monster_gen no_rotate no_vmirror
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  float
 ITEM:    bread ration, meat ration, apple, orange, banana, choko, pear
 SUBST:   . = .:100 $ % d
@@ -142,7 +142,7 @@ ENDMAP
 # Angband town! (Shiori)
 NAME:    angband_town
 ORIENT:  float
-CHANCE:  1
+WEIGHT:  1
 TAGS:    entry no_rotate
 NSUBST:  { = { / > / .
 NSUBST:  = = 3== / *=x
@@ -295,7 +295,7 @@ ENDMAP
 NAME:    zelgadis_glass_entry_002
 TAGS:    entry
 ORIENT:  float
-CHANCE:  1
+WEIGHT:  1
 SHUFFLE: {[(<
 SUBST:   < = .
 NSUBST:  X = 4:+ / *:x
@@ -502,7 +502,7 @@ NAME:    lemuel_entry_005_a
 TAGS:    entry no_monster_gen
 ORIENT:  float
 SHUFFLE: {}
-CHANCE:  2
+WEIGHT:  2
 MAP
 xxxxxxxxxxxxxxxxxxx
 xxxxx{.......}xxxxx
@@ -527,7 +527,7 @@ ENDMAP
 NAME:    lemuel_entry_005_b
 TAGS:    entry no_monster_gen
 ORIENT:  float
-CHANCE:  7
+WEIGHT:  7
 SHUFFLE: {}, XYZ
 SUBST:   X=W, Y=w, Z=w
 MAP
@@ -558,7 +558,7 @@ TAGS:    entry no_monster_gen
 ORIENT:  float
 SHUFFLE: {}, ABC
 SUBST:   A:=, B:x, C:x
-CHANCE:  1
+WEIGHT:  1
 MAP  
 xxxxxxxxxxxxxxxxxxx  
 xxxWA{.......}xxxxx  
@@ -674,7 +674,7 @@ TAGS:    entry no_monster_gen
 ORIENT:  float
 SHUFFLE: {[(
 SUBST:   ? : xxcvG.
-CHANCE:  4
+WEIGHT:  4
 MAP
 xxxxxxxxxxx
 x{...(...[x
@@ -700,7 +700,7 @@ SUBST:   ? : xxcvG.
 ITEM:    meat ration / bread ration / beef jerky / spear / potion of water / \
          apple / club / hammer / knife
 SHUFFLE: {[(
-CHANCE:  3
+WEIGHT:  3
 MAP
 xxxxxxxxxxxx
 x{...(...[xx
@@ -726,7 +726,7 @@ SUBST:   ? : xxcvG.
 ITEM:    meat ration / bread ration / beef jerky / spear / potion of water / \
          apple / club / hammer / knife
 SHUFFLE: {[(
-CHANCE:  3
+WEIGHT:  3
 MAP
 xxxxxxxxxxxx
 x{...(...[xx
@@ -750,7 +750,7 @@ ENDMAP
 
 NAME:    lemuel_entry_010_a
 TAGS:    entry no_monster_gen
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  float
 SHUFFLE: {(
 SUBST:   b : bcvxxx
@@ -770,7 +770,7 @@ ENDMAP
 
 NAME:    lemuel_entry_010_b
 TAGS:    entry no_monster_gen
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  float
 SHUFFLE: {(
 SUBST:   b : bcvxxx
@@ -817,7 +817,7 @@ ENDMAP
 NAME:    david_entry_001_a
 TAGS:    entry no_rotate
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: {[(
 MAP
    xxxxx  x@x  xxxxx
@@ -843,7 +843,7 @@ ENDMAP
 NAME:    david_entry_001_b
 TAGS:    entry no_rotate no_monster_gen
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: {[, }>
 MAP
          xxxxx
@@ -1112,7 +1112,7 @@ ENDMAP
 #
 NAME:   david_entry_011_a
 TAGS:   entry no_monster_gen no_rotate
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 NSUBST: . = 1:d / *:.
 ITEM:   stone / dart
@@ -1144,7 +1144,7 @@ ENDMAP
 
 NAME:   david_entry_011_b
 TAGS:   entry no_monster_gen no_rotate
-CHANCE: 1
+WEIGHT: 1
 ORIENT: float
 NSUBST: . = 1:d / *:.
 ITEM:   stone / dart
@@ -1176,7 +1176,7 @@ ENDMAP
 
 NAME:    david_entry_011_c
 TAGS:    entry no_monster_gen no_rotate
-CHANCE:  6
+WEIGHT:  6
 ORIENT:  float
 SHUFFLE: ab, cd, ef, gh, ij, kl, mn
 SUBST:   a=., c=., e=., g=., i=., k=., m=.
@@ -1303,7 +1303,7 @@ ENDMAP
 # (but commented out). All water is deep.
 # NAME: water_fire_template
 # TAGS: entry no_monster_gen no_pool_fixup no_rotate
-# CHANCE: 1
+# WEIGHT: 1
 # ORIENT: float
 # MONS: plant
 # MAP
@@ -1333,7 +1333,7 @@ ENDMAP
 
 NAME: david_entry_015_a_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 MONS: plant
 ORIENT: float
 MAP
@@ -1363,7 +1363,7 @@ ENDMAP
 
 NAME: david_entry_015_b_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1395,7 +1395,7 @@ ENDMAP
 
 NAME: david_entry_015_c_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1427,7 +1427,7 @@ ENDMAP
 
 NAME: david_entry_015_d_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1459,7 +1459,7 @@ ENDMAP
 
 NAME: david_entry_015_e_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1491,7 +1491,7 @@ ENDMAP
 
 NAME: david_entry_015_f_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1523,7 +1523,7 @@ ENDMAP
 
 NAME: david_entry_015_g_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1555,7 +1555,7 @@ ENDMAP
 
 NAME: david_entry_015_h_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1587,7 +1587,7 @@ ENDMAP
 
 NAME: david_entry_015_i_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1619,7 +1619,7 @@ ENDMAP
 
 NAME: david_entry_015_j_water_fire
 TAGS: entry no_monster_gen no_pool_fixup no_rotate
-CHANCE: 1
+WEIGHT: 1
 NSUBST: . = 1:d / *:.
 ITEM: stone/dart
 MONS: plant
@@ -1678,7 +1678,7 @@ ENDMAP
 #
 NAME:    david_entry_017_a
 TAGS:    entry no_monster_gen no_rotate
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  float
 SHUFFLE: {[(
 MAP
@@ -1700,7 +1700,7 @@ ENDMAP
 NAME:    david_entry_017_b
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  8
+WEIGHT:  8
 SHUFFLE: cv
 MAP
 x@xxxxxxxxxxxxxxxxxxxx@x
@@ -2128,7 +2128,7 @@ NAME:    david_entry_033_a
 TAGS:    entry
 ORIENT:  float
 SHUFFLE: TV
-CHANCE:  5
+WEIGHT:  5
 MAP
   xx..@..xx
  xx...x...xx
@@ -2148,7 +2148,7 @@ ENDMAP
 NAME:   david_entry_033_b
 TAGS:   entry
 ORIENT: float
-CHANCE: 5
+WEIGHT: 5
 MAP
   xx..@..xx
  xx...x...xx
@@ -2211,7 +2211,7 @@ ENDMAP
 NAME:    david_entry_036_a
 TAGS:    entry
 ORIENT:  float
-CHANCE:  2
+WEIGHT:  2
 SHUFFLE: {[(
 MAP
 @.......xxxxx..@
@@ -2229,7 +2229,7 @@ ENDMAP
 NAME:    david_entry_036_b
 TAGS:    entry
 ORIENT:  float
-CHANCE:  2
+WEIGHT:  2
 SHUFFLE: {[(
 MAP
 @.......xxxxx..@
@@ -2248,7 +2248,7 @@ NAME:    david_entry_036_c
 TAGS:    entry
 ORIENT:  float
 SHUFFLE: {[(
-CHANCE:  2
+WEIGHT:  2
 MAP
 @.......xxxxx..@
 .[......xxxxx...
@@ -2266,7 +2266,7 @@ NAME:    david_entry_036_d
 TAGS:    entry
 ORIENT:  float
 SHUFFLE: {[(
-CHANCE:  2
+WEIGHT:  2
 MAP
 @.......xxxxx..@
 .[......xxxxx...
@@ -2284,7 +2284,7 @@ NAME:    david_entry_036_e
 TAGS:    entry
 ORIENT:  float
 SHUFFLE: {[(
-CHANCE:  2
+WEIGHT:  2
 MAP
 @.......xxxxx..@
 .[......xxxxx...
@@ -2321,7 +2321,7 @@ ENDMAP
 NAME:   david_entry_038_a
 TAGS:   entry
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 MAP
 xxxxxxxxxxxxx
 x%.x%.=..=.[x
@@ -2337,7 +2337,7 @@ ENDMAP
 NAME:   david_entry_038_b
 TAGS:   entry
 ORIENT: float
-CHANCE: 2
+WEIGHT: 2
 MAP
 xxxxxxxxxxxxx
 x.%x%.x..=.[x
@@ -2353,7 +2353,7 @@ ENDMAP
 NAME:   david_entry_038_c
 TAGS:   entry
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 MAP
 xxxxxxxxxxxxx
 x.%x%%x..=.[x
@@ -2369,7 +2369,7 @@ ENDMAP
 NAME:   david_entry_038_d
 TAGS:   entry
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 MAP
 xxxxxxxxxxxxx
 x%.=..=.%x.[x
@@ -2385,7 +2385,7 @@ ENDMAP
 NAME:   david_entry_038_e
 TAGS:   entry
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 MAP
 xxxxxxxxxxxxx
 x..=..=..x%[x
@@ -2401,7 +2401,7 @@ ENDMAP
 NAME:   david_entry_038_f
 TAGS:   entry
 ORIENT: float
-CHANCE: 2
+WEIGHT: 2
 MAP
 xxxxxxxxxxxxx
 x..=..x%.=%[x
@@ -2417,7 +2417,7 @@ ENDMAP
 NAME:   david_entry_038_g
 TAGS:   entry
 ORIENT: float
-CHANCE: 2
+WEIGHT: 2
 MAP
 xxxxxxxxxxxxx
 x%.x..x..x%[x
@@ -2564,7 +2564,7 @@ ENDMAP
 NAME:    david_entry_045_a
 TAGS:    entry
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: {[(
 MAP
 xxxxxx@.xxx
@@ -2582,7 +2582,7 @@ ENDMAP
 NAME:    david_entry_045_b
 TAGS:    entry
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: {[(
 MAP
 xxxxxx@.xxx
@@ -2644,7 +2644,7 @@ ENDMAP
 NAME:    david_entry_048_a
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: cvba
 NSUBST:  . = 1:d / *:.
 ITEM:    stone / dart
@@ -2663,7 +2663,7 @@ ENDMAP
 NAME:    david_entry_048_b
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: cvba
 NSUBST:  . = 1:d / *:.
 ITEM:    stone / dart
@@ -2830,7 +2830,7 @@ ENDMAP
 NAME:    david_entry_055_a
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: lw
 MAP
       xxxxxxxxxxx
@@ -2855,7 +2855,7 @@ ENDMAP
 NAME:    david_entry_055_b
 TAGS:    entry no_monster_gen
 ORIENT:  float
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: {(, lw
 MAP
       xxxxx@xxxxx
@@ -2883,7 +2883,7 @@ ENDMAP
 NAME:    david_entry_056_a
 TAGS:    entry no_rotate
 ORIENT:  float
-CHANCE:  3
+WEIGHT:  3
 ITEM:    nothing, nothing, nothing
 SHUFFLE: cccbvxxx, def}
 MAP
@@ -2911,7 +2911,7 @@ ENDMAP
 NAME:    david_entry_056_b
 TAGS:    entry no_rotate
 ORIENT:  float
-CHANCE:  7
+WEIGHT:  7
 SHUFFLE: cccbvxxx
 MAP
    ....................   
@@ -2941,7 +2941,7 @@ ORIENT:  float
 SHUFFLE: {[AB
 MONS:    gnoll
 SUBST:   A=., B=.
-CHANCE:  3
+WEIGHT:  3
 MAP
 @....B........@
 ..............[
@@ -2964,7 +2964,7 @@ ORIENT:  float
 SHUFFLE: {[AB
 SUBST:   A=., B=.
 MONS:    gnoll
-CHANCE:  3
+WEIGHT:  3
 MAP
 @...B.........@
 ..............[
@@ -2987,7 +2987,7 @@ ORIENT:  float
 SHUFFLE: {[AB
 SUBST:   A=., B=.
 MONS:    gnoll
-CHANCE:  2
+WEIGHT:  2
 MAP
 @.....B.......@
 ..............[
@@ -3010,7 +3010,7 @@ ORIENT:  float
 SHUFFLE: {[AB
 SUBST:   A=., B=.
 MONS:    gnoll
-CHANCE:  2
+WEIGHT:  2
 MAP
 @......A......@
 ..............[
@@ -3100,7 +3100,7 @@ TAGS:    entry no_monster_gen
 ORIENT:  northwest
 SHUFFLE: Cc, {Y
 SUBST:   ?=l. , Y=.
-CHANCE:  6
+WEIGHT:  6
 MAP
 xxxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxxx
@@ -3125,7 +3125,7 @@ ENDMAP
 NAME:    david_entry_061_b_altar
 TAGS:    entry no_monster_gen
 ORIENT:  northwest
-CHANCE:  4
+WEIGHT:  4
 SHUFFLE: Cc, {Y
 SUBST:   Y=.
 MAP
@@ -3176,7 +3176,7 @@ ENDMAP
 #
 NAME:   david_entry_063_tiny
 TAGS:   entry
-CHANCE: 10
+WEIGHT: 10
 ORIENT: float
 MAP
 {
@@ -3187,7 +3187,7 @@ ENDMAP
 #
 NAME:   david_entry_064_a_tiny
 TAGS:   entry
-CHANCE: 2
+WEIGHT: 2
 ORIENT: float
 MAP
 xxx
@@ -3197,7 +3197,7 @@ ENDMAP
 
 NAME:   david_entry_064_b_tiny
 TAGS:   entry
-CHANCE: 4
+WEIGHT: 4
 ORIENT: float
 MAP
 @....
@@ -3208,7 +3208,7 @@ ENDMAP
 
 NAME:   david_entry_064_c_tiny
 TAGS:   entry
-CHANCE: 4
+WEIGHT: 4
 ORIENT: float
 MAP
 @....
@@ -3223,7 +3223,7 @@ ENDMAP
 #
 NAME:   david_entry_065_a_tiny
 TAGS:   entry
-CHANCE: 1
+WEIGHT: 1
 ORIENT: float
 MAP
 xxxxx
@@ -3235,7 +3235,7 @@ ENDMAP
 
 NAME:   david_entry_065_b_tiny
 TAGS:   entry
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 MAP
 @......
@@ -3248,7 +3248,7 @@ ENDMAP
 
 NAME:   david_entry_065_c_tiny
 TAGS:   entry
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 MAP
 @...xx
@@ -3261,7 +3261,7 @@ ENDMAP
 
 NAME:   david_entry_065_d_tiny
 TAGS:   entry
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 MAP
 @xxxxxx
@@ -3277,7 +3277,7 @@ ENDMAP
 #
 NAME:   david_entry_066_a_tiny
 TAGS:   entry
-CHANCE: 4
+WEIGHT: 4
 ORIENT: float
 MAP
 xxxxx
@@ -3287,7 +3287,7 @@ ENDMAP
 
 NAME:   david_entry_066_b_tiny
 TAGS:   entry
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 SUBST:  v : vcbxxx
 MAP
@@ -3300,7 +3300,7 @@ ENDMAP
 
 NAME:   david_entry_066_d_tiny
 TAGS:   entry
-CHANCE: 3
+WEIGHT: 3
 ORIENT: float
 MAP
 xxxx
@@ -3373,7 +3373,7 @@ ENDMAP
 NAME:    david_entry_069_a
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  3
+WEIGHT:  3
 SHUFFLE: bxxxx
 MAP
  .......    .......
@@ -3392,7 +3392,7 @@ ENDMAP
 NAME:    david_entry_069_b
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  3
+WEIGHT:  3
 SHUFFLE: bxxxx
 MAP
  .......    .......
@@ -3411,7 +3411,7 @@ ENDMAP
 NAME: david_entry_069_c
 TAGS: entry no_monster_gen no_rotate
 ORIENT: float
-CHANCE: 3
+WEIGHT: 3
 SHUFFLE: bxxxx
 MAP
  .......    .......
@@ -3430,7 +3430,7 @@ ENDMAP
 NAME:    david_entry_069_d
 TAGS:    entry no_monster_gen no_rotate
 ORIENT:  float
-CHANCE:  1
+WEIGHT:  1
 SHUFFLE: bxxxx
 MAP
  .......    .......
@@ -3568,7 +3568,7 @@ ENDMAP
 NAME:    erik_entry_001_a
 TAGS:    entry
 ORIENT:  float
-CHANCE:  1
+WEIGHT:  1
 SHUFFLE: {[(
 SUBST:   X : x:20 G:4 l:1
 SUBST:   c : cxxx
@@ -3596,7 +3596,7 @@ NAME:    erik_entry_001_b
 TAGS:    entry
 ORIENT:  float
 MONS:    gnoll
-CHANCE:  3
+WEIGHT:  3
 SHUFFLE: {[(
 SUBST:   c : cxxx
 MAP
@@ -3623,7 +3623,7 @@ NAME:    erik_entry_001_c
 TAGS:    entry
 ORIENT:  float
 MONS:    centaur, orc
-CHANCE:  3
+WEIGHT:  3
 SHUFFLE: {[(
 SUBST:   c : cxxx
 MAP
@@ -3648,7 +3648,7 @@ ENDMAP
 
 NAME:    erik_entry_001_d
 TAGS:    entry
-CHANCE:  3
+WEIGHT:  3
 ORIENT:  float
 MONS:    orc
 SHUFFLE: {[(
@@ -3956,7 +3956,7 @@ ENDMAP
 NAME: matt_entry_007_a
 TAGS: entry no_monster_gen
 ORIENT:  float
-CHANCE:  1
+WEIGHT:  1
 SHUFFLE: {[(<
 MAP
 xxx@xxx@xxx
@@ -3975,7 +3975,7 @@ ENDMAP
 NAME: matt_entry_007_b
 TAGS: entry no_monster_gen
 ORIENT:  float
-CHANCE:  3
+WEIGHT:  3
 SHUFFLE: {[(<
 MAP
 xxx@xxx@xxx
@@ -3994,7 +3994,7 @@ ENDMAP
 NAME: matt_entry_007_c
 TAGS: entry no_monster_gen
 ORIENT:  float
-CHANCE:  6
+WEIGHT:  6
 SHUFFLE: {[(<
 MAP
 xxx@xxx@xxx
@@ -4313,7 +4313,7 @@ ENDMAP
 # Mini maze
 NAME:    onia_ninara_004_a_mini_maze
 TAGS:    entry
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  float
 ITEM:    apple / sausage / nothing / stone / potion of water
 SHUFFLE: ({[, Aa, Bb, Cc, Ee, Ff
@@ -4338,7 +4338,7 @@ ENDMAP
 
 NAME:    onia_ninara_004_b_mini_maze
 TAGS:    entry
-CHANCE:  5
+WEIGHT:  5
 ORIENT:  float
 ITEM:    apple / sausage / nothing / stone / potion of water / w:30 nothing
 SHUFFLE: ({[ , AB
@@ -4447,7 +4447,7 @@ TAGS:     entry no_monster_gen no_rotate
 ORIENT:   northwest
 SUBST:    V = V T:2
 SUBST:    ' : x.
-CHANCE:   5
+WEIGHT:   5
 MAP
 xxxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxxx
@@ -4479,7 +4479,7 @@ ENDMAP
 NAME:     entry_eino_001_b
 TAGS:     entry no_monster_gen no_rotate
 ORIENT:   northwest
-CHANCE:   5
+WEIGHT:   5
 MAP
 xxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxx
@@ -4512,7 +4512,7 @@ TAGS:     entry no_monster_gen no_rotate
 ORIENT:   northwest
 NSUBST:   A = 1:= / x
 SUBST:    % = % .:5
-CHANCE:   2
+WEIGHT:   2
 MAP
 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
@@ -4538,7 +4538,7 @@ TAGS:     entry no_rotate
 ORIENT:   northeast
 SHUFFLE:  bvc
 SUBST:    ' : x.
-CHANCE:   4
+WEIGHT:   4
 MAP
 xxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxx
@@ -4567,7 +4567,7 @@ NAME:     entry_eino_002_c
 TAGS:     entry no_monster_gen no_rotate
 ORIENT:   northeast
 SUBST:    ' : x. , " : x. , ? = x.
-CHANCE:   4
+WEIGHT:   4
 MAP
 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
diff --git a/crawl-ref/source/dat/float.des b/crawl-ref/source/dat/float.des
index 11b9d752ef..ad49cbb487 100644
--- a/crawl-ref/source/dat/float.des
+++ b/crawl-ref/source/dat/float.des
@@ -27,7 +27,7 @@ DEPTH: D:1-11
 # by depth.
 TAGS: dummy
 ORIENT: float
-CHANCE: 100
+WEIGHT: 100
 MAP
 x
 ENDMAP
@@ -41,7 +41,7 @@ DEPTH: 1-, !D
 # by depth.
 TAGS: dummy
 ORIENT: float
-CHANCE: 60
+WEIGHT: 60
 MAP
 x
 ENDMAP
@@ -51,7 +51,7 @@ ENDMAP
 #
 NAME:   erik_1
 ORIENT: float
-CHANCE: 5
+WEIGHT: 5
 TAGS:   no_monster_gen
 DEPTH:  D:7-11, Lair
 #
@@ -431,7 +431,7 @@ ENDMAP
 #
 NAME:    oklob_3
 DEPTH:   D:18-27, Lair:2-10, Elf:2-7
-CHANCE:  2
+WEIGHT:  2
 ORIENT:  float
 SHUFFLE: ab" / AB'
 SUBST:   a=[, b=(, A=., B=., '=., "=>
diff --git a/crawl-ref/source/dat/hive.des b/crawl-ref/source/dat/hive.des
index 745403bfe9..db65754621 100644
--- a/crawl-ref/source/dat/hive.des
+++ b/crawl-ref/source/dat/hive.des
@@ -11,7 +11,7 @@
 NAME: hive_0
 TAGS: hive_entry
 ORIENT: float
-CHANCE: 60
+WEIGHT: 60
 MAP
 O
 ENDMAP
@@ -260,7 +260,7 @@ ENDMAP
 NAME:   Hive_Balancer
 DEPTH:  Hive:*
 TAGS:   dummy
-CHANCE: 200
+WEIGHT: 200
 MAP
 x
 ENDMAP
@@ -397,7 +397,7 @@ ENDMAP
 NAME:   lemuel_wasp_nest
 DEPTH:  Hive:2-3
 MONS:   yellow wasp / weight:2 red wasp
-CHANCE: 1
+WEIGHT: 1
 SUBST:  1=1 .:15
 SUBST:  ?=a.
 MAP
@@ -420,7 +420,7 @@ ENDMAP
 
 NAME:   lemuel_hive_grid_a
 DEPTH:  Hive:*
-CHANCE: 4
+WEIGHT: 4
 MAP
 a.a.a.a.a.a.a.a.a
 aa.a.a.a.a.a.a.aa
@@ -437,7 +437,7 @@ ENDMAP
 
 NAME:   lemuel_hive_grid_b
 DEPTH:  Hive:*
-CHANCE: 6
+WEIGHT: 6
 MAP
 a.a.a.a.aa
 aa.a.a.a.a
@@ -450,7 +450,7 @@ ENDMAP
 
 NAME:   lemuel_hive_grid_c
 DEPTH:  Hive:*
-CHANCE: 10
+WEIGHT: 10
 MAP
      aa.aa
     aa.a.aa
diff --git a/crawl-ref/source/dat/lab.des b/crawl-ref/source/dat/lab.des
index 3aed292afa..9e40008913 100644
--- a/crawl-ref/source/dat/lab.des
+++ b/crawl-ref/source/dat/lab.des
@@ -42,7 +42,7 @@ ENDMAP
 # Dummy balancer
 NAME: labyrinth_0
 TAGS: minotaur dummy
-CHANCE: 20
+WEIGHT: 20
 MAP
 x
 ENDMAP
@@ -164,7 +164,7 @@ NSUBST:  D = 1:. / *:D
 KFEAT:   d = axe trap / dart trap / needle trap / blade trap
 KFEAT:   D = teleport trap
 SUBST:   c : vvc
-CHANCE:  2
+WEIGHT:  2
 MAP
 ..............
 .cccccccccccc.
@@ -194,7 +194,7 @@ SUBST:   Y=*, Z=*
 KFEAT:   X = <
 KMONS:   X = patrolling minotaur
 KFEAT:   S = granite_statue
-CHANCE:  2
+WEIGHT:  2
 MAP
 .............
 .vvvvvvvvvvv.
@@ -226,7 +226,7 @@ ENDMAP
 # Labyrinth dummy decorator
 NAME: lab_dummy
 TAGS: lab dummy
-CHANCE: 90
+WEIGHT: 90
 MAP
 x
 ENDMAP
@@ -292,7 +292,7 @@ ENDMAP
 
 NAME:   labyrinth_glass_2
 TAGS:   lab allow_dup
-CHANCE: 1
+WEIGHT: 1
 MAP
 ......
 .nnnn.
@@ -324,7 +324,7 @@ TAGS:    lab allow_dup
 KFEAT:   Y = teleport trap
 KITEM:   Y = any good_item
 SHUFFLE: cxv
-CHANCE:  1
+WEIGHT:  1
 MAP
 .....
 .x=x.
diff --git a/crawl-ref/source/dat/lair.des b/crawl-ref/source/dat/lair.des
index 5d103a529e..7588ea89f5 100644
--- a/crawl-ref/source/dat/lair.des
+++ b/crawl-ref/source/dat/lair.des
@@ -9,7 +9,7 @@
 # Dummy Lair entry
 NAME:   lair_0_dummy
 TAGS:   lair_entry dummy
-CHANCE: 30
+WEIGHT: 30
 ORIENT: float
 MAP
 O
@@ -220,7 +220,7 @@ NAME:    RatsNest_Lair
 ORIENT:  northeast
 TAGS:    uniq_rats_nest
 DEPTH:   Lair
-CHANCE:  5
+WEIGHT:  5
 SHUFFLE: 23, 34
 MONS:    rat / green rat / nothing, grey rat / orange rat / nothing
 MONS:    green rat / orange rat / nothing, orange rat / rat
@@ -295,7 +295,7 @@ ENDMAP
 #
 NAME:   strawberry_fields_big
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 DEPTH:  Lair
 SUBST:  .= d .:150 1:8 2:4
 MONS:   plant, butterfly
@@ -323,7 +323,7 @@ ENDMAP
 
 NAME:   strawberry_fields_big_rounded
 ORIENT: float
-CHANCE: 1
+WEIGHT: 1
 DEPTH:  Lair
 SUBST:  .= d .:140 1:8 2:4
 MONS:   plant, butterfly
@@ -351,7 +351,7 @@ ENDMAP
 
 NAME:   strawberry_fields_round
 ORIENT: float
-CHANCE: 4
+WEIGHT: 4
 TAGS:   allow_dup
 DEPTH:  Lair
 SUBST:  .= d .:50 1:8 2:4
@@ -372,7 +372,7 @@ ENDMAP
 
 NAME:   strawberry_fields_patches_spacepadded
 ORIENT: float
-CHANCE: 4
+WEIGHT: 4
 DEPTH:  Lair
 SUBST:  .= d .:50 1:8 2:4 
 MONS:   plant, butterfly
@@ -408,7 +408,7 @@ SUBST:  . = .:90 a:10 2:1
 KITEM:  a : apple / apricot / pear / orange / banana / strawberry q:1 w:5 /\
             strawberry q:2 w:5 / lemon
 KFEAT:  a = .
-CHANCE: 8
+WEIGHT: 8
 MAP
  ..... 
 .......
@@ -429,7 +429,7 @@ SUBST:  _ = .:90 a:4
 KITEM:  a : apple / apricot / pear / orange / banana / strawberry q:1 w:5 /\
             strawberry q:2 w:5 / lemon
 KFEAT:  a = .
-CHANCE: 2
+WEIGHT: 2
 MAP
    ________   
   _........_  
@@ -650,7 +650,7 @@ ENDMAP
 #
 NAME:   Shoal_1
 PLACE:  Shoal:5
-CHANCE: 0
+WEIGHT: 0
 MONS:   cyclops, yaktaur, yaktaur captain
 MAP
 wwwwwwwwcccccccccccccwwwwwww
@@ -689,7 +689,7 @@ ENDMAP
 #
 NAME:   Shoal_2
 PLACE:  Shoal:5
-CHANCE: 0
+WEIGHT: 0
 MONS:   cyclops,stone giant
 SUBST:  1 = 1.
 SUBST:  W = W:2 w
@@ -727,7 +727,7 @@ ENDMAP
 #
 NAME:   Shoal_3
 PLACE:  Shoal:5
-CHANCE: 0
+WEIGHT: 0
 KMONS:  x : ice statue / orange crystal statue / silver statue
 KFEAT:  x : O
 MAP
@@ -741,7 +741,7 @@ ENDMAP
 #
 NAME:   Shoal_4
 PLACE:  Shoal:5
-CHANCE: 0
+WEIGHT: 0
 KFEAT:  O = w
 ### KITEM: O = liquid rune of Zot
 MAP
diff --git a/crawl-ref/source/dat/large.des b/crawl-ref/source/dat/large.des
index 0992a9c7f0..bea59e0766 100644
--- a/crawl-ref/source/dat/large.des
+++ b/crawl-ref/source/dat/large.des
@@ -16,7 +16,7 @@ default-depth: D:12-26
 NAME:    RatsNest_Dungeon
 ORIENT:  northeast
 TAGS:    uniq_rats_nest
-CHANCE:  5
+WEIGHT:  5
 DEPTH:   D:5-9
 SHUFFLE: 23, 34
 MONS:    rat / nothing, grey rat / nothing
@@ -1255,7 +1255,7 @@ NAME:    erik_rubicon_a
 DEPTH:   D:10-16
 ORIENT:  south
 TAGS:    no_pool_fixup no_rotate uniq_rubicon
-CHANCE:  6
+WEIGHT:  6
 ITEM:    ring of levitation / potion of levitation / nothing
 ITEM:    potion of levitation / nothing, nothing, nothing, nothing
 SHUFFLE: {[(, )gh
@@ -1290,7 +1290,7 @@ NAME:    erik_rubicon_b
 DEPTH:   D:10-16
 ORIENT:  south
 TAGS:    no_rotate uniq_rubicon
-CHANCE:  4
+WEIGHT:  4
 ITEM:    ring of levitation / potion of levitation / nothing
 ITEM:    potion of levitation, nothing, nothing, nothing, nothing
 SHUFFLE: {[(, fgW, )hi
diff --git a/crawl-ref/source/dat/mini.des b/crawl-ref/source/dat/mini.des
index e03d932937..54a2d49c52 100644
--- a/crawl-ref/source/dat/mini.des
+++ b/crawl-ref/source/dat/mini.des
@@ -37,7 +37,7 @@ default-depth: D:8-27
 NAME: mini_dummy
 DEPTH: 1-50, !D:8-27
 TAGS: dummy
-CHANCE: 90
+WEIGHT: 90
 MAP
 .
 ENDMAP
@@ -48,7 +48,7 @@ ENDMAP
 # still see you
 NAME:  glass_columns_a
 DEPTH: D, Elf, Crypt
-CHANCE: 5
+WEIGHT: 5
 MAP
 .....
 .m.m.
@@ -59,7 +59,7 @@ ENDMAP
 
 NAME:  glass_columns_b
 DEPTH: D, Elf, Crypt
-CHANCE: 3
+WEIGHT: 3
 MAP
 .......
 .m.m.m.
@@ -72,7 +72,7 @@ ENDMAP
 
 NAME:  glass_columns_c
 DEPTH: D, Elf, Crypt
-CHANCE: 2
+WEIGHT: 2
 MAP
 .........
 .m.m.m.m.
@@ -103,7 +103,7 @@ ENDMAP
 # An item encased in glass.  If the player wants it, it can be dug out.
 NAME:   item_on_display_a
 DEPTH:  D, Elf, Crypt, Vault
-CHANCE: 9
+WEIGHT: 9
 SUBST:  % = %%%*
 TAGS:   no_monster_gen no_item_gen
 MAP
@@ -118,7 +118,7 @@ ENDMAP
 # A good item encased in transparent stone.
 NAME:   item_on_display_b
 DEPTH:  D, Elf, Crypt, Vault
-CHANCE: 1
+WEIGHT: 1
 SUBST:  * = ***|
 TAGS:   no_monster_gen no_item_gen
 MAP
@@ -406,7 +406,7 @@ TAGS:    allow_dup
 NSUBST:  Z = 4=. / z
 KFEAT:   z = zot trap
 SUBST:   *: *$
-CHANCE:  3
+WEIGHT:  3
 MAP
 xxxxxxxxxx
 xxxxxxx**x
@@ -639,7 +639,7 @@ ENDMAP
 # Greed's colour
 
 NAME:    david_greed
-CHANCE:  4
+WEIGHT:  4
 DEPTH:   D:10-26, Vault, Elf
 ITEM:    nothing / any good_item
 MONS:    silver statue / orange crystal statue / ice statue
@@ -926,7 +926,7 @@ ENDMAP
 
 NAME:    minivault_16
 TAGS:    allow_dup
-CHANCE:  40
+WEIGHT:  40
 # as this replaces the former statue only minivaults
 SHUFFLE: G111
 MONS:    orange crystal statue / silver statue / ice statue
@@ -1522,7 +1522,7 @@ ENDMAP
 NAME:   small_statue_alley_a
 TAGS:   uniq_statue_alley
 DEPTH:  D:10-20
-CHANCE: 7
+WEIGHT: 7
 MAP
 xxxcccccx
 ...G...G@
@@ -1535,7 +1535,7 @@ ENDMAP
 NAME:   small_statue_alley_b
 TAGS:   uniq_statue_alley
 DEPTH:  D:3-10
-CHANCE: 2
+WEIGHT: 2
 MAP
 xxxcccccx
 ...G...G@
@@ -1548,7 +1548,7 @@ ENDMAP
 NAME:   small_statue_alley_c
 TAGS:   uniq_statue_alley
 DEPTH:  Orc
-CHANCE: 1
+WEIGHT: 1
 MAP
 xxxcccccx
 ...I...I@
diff --git a/crawl-ref/source/dat/orc.des b/crawl-ref/source/dat/orc.des
index 69a90229dd..14cacecff2 100644
--- a/crawl-ref/source/dat/orc.des
+++ b/crawl-ref/source/dat/orc.des
@@ -11,7 +11,7 @@
 
 NAME:   orc_0_dummy
 TAGS:   orc_entry
-CHANCE: 50
+WEIGHT: 50
 ORIENT: float
 MAP
 O
@@ -438,7 +438,7 @@ default-depth: Orc:*
 #
 NAME: mines_dummy
 TAGS: dummy
-CHANCE: 60
+WEIGHT: 60
 MAP
 x
 ENDMAP
@@ -486,7 +486,7 @@ ENDMAP
 NAME:   mines1_david
 TAGS:   allow_dup
 KFEAT:  C = altar_beogh
-CHANCE: 20
+WEIGHT: 20
 MAP
 ...........
 .xxxx.xxxx.
@@ -508,7 +508,7 @@ TAGS:   allow_dup
 KFEAT:  C = altar_beogh
 MONS:   orc / orc warrior w:1
 SUBST:  . = . 1:2
-CHANCE: 20
+WEIGHT: 20
 MAP
 .......
 .......
diff --git a/crawl-ref/source/dat/sewer.des b/crawl-ref/source/dat/sewer.des
index 03f7f4913e..1cc3422ca1 100644
--- a/crawl-ref/source/dat/sewer.des
+++ b/crawl-ref/source/dat/sewer.des
@@ -95,7 +95,7 @@ ENDMAP
 # The easiest one, so most probable (5 out of 9 sewers).
 NAME:     sewer_kobolds
 TAGS:     sewer no_item_gen no_monster_gen no_pool_fixup
-CHANCE:   40
+WEIGHT:   40
 ORIENT:   encompass
 KFEAT:    < = exit_portal_vault
 COLOUR:   < = darkgrey
diff --git a/crawl-ref/source/dat/temple.des b/crawl-ref/source/dat/temple.des
index b24ec5af41..e220f4c1e4 100644
--- a/crawl-ref/source/dat/temple.des
+++ b/crawl-ref/source/dat/temple.des
@@ -12,7 +12,7 @@
 #
 NAME:   temple_hall_a
 TAGS:   temple_entry
-CHANCE: 9
+WEIGHT: 9
 ORIENT: float
 SUBST:  ? : c:20 x v b G:5
 MAP
@@ -35,7 +35,7 @@ ENDMAP
 
 NAME:   temple_hall_b
 TAGS:   temple_entry no_pool_fixup no_monster_gen
-CHANCE: 1
+WEIGHT: 1
 ORIENT: float
 SUBST:  ? : v b G:5
 MAP
@@ -104,7 +104,7 @@ ENDMAP
 # Where's the entry to the temple?
 #
 NAME:   temple_secret_doors_a
-CHANCE: 1
+WEIGHT: 1
 TAGS:   temple_entry
 ORIENT: float
 SUBST:  c:cxxx
@@ -120,7 +120,7 @@ cccccccccc
 ENDMAP
 
 NAME:   temple_secret_doors_b
-CHANCE: 1
+WEIGHT: 1
 TAGS:   temple_entry
 ORIENT: float
 SUBST:  ? : ?:40 T:30 U, ? = T:30 U
@@ -137,7 +137,7 @@ ccccccccccc
 ENDMAP
 
 NAME:   temple_secret_doors_c
-CHANCE: 1
+WEIGHT: 1
 TAGS:   temple_entry
 ORIENT: float
 SUBST:  ? : ?:40 T:30 U, ? = T:30 U
@@ -156,7 +156,7 @@ ccccc@ccc
 ENDMAP
 
 NAME:   temple_secret_doors_d
-CHANCE: 1
+WEIGHT: 1
 TAGS:   temple_entry 
 ORIENT: float
 SUBST:  ? : ?:40 T:30 U, ? = T:30 U
@@ -175,7 +175,7 @@ ccccc@ccccc
 ENDMAP
 
 NAME:   temple_secret_doors_e
-CHANCE: 6
+WEIGHT: 6
 TAGS:   temple_entry no_rotate
 ORIENT: float
 SUBST:  ? : ?:40 T:30 U, ? = T:30 U
@@ -200,7 +200,7 @@ ENDMAP
 #
 NAME:    temple_pool_a
 TAGS:    temple_entry no_monster_gen no_pool_fixup no_rotate
-CHANCE:  3
+WEIGHT:  3
 ORIENT:  float
 SHUFFLE: XY
 SUBST:   Y=w, X=.
@@ -220,7 +220,7 @@ ENDMAP
 
 NAME:    temple_pool_b
 TAGS:    temple_entry no_monster_gen no_rotate
-CHANCE:  7
+WEIGHT:  7
 ORIENT:  float
 SHUFFLE: ABCDEFGHIJKL
 SUBST:   A=W, B=W, C=W, D=w, E=w, F=w, G=w, H=w, I=w, J=w, K=w, L=w
diff --git a/crawl-ref/source/dat/vaults.des b/crawl-ref/source/dat/vaults.des
index f50f8dd7b0..cdf365c456 100644
--- a/crawl-ref/source/dat/vaults.des
+++ b/crawl-ref/source/dat/vaults.des
@@ -14,7 +14,7 @@
 
 NAME:   vaults_0_dummy
 TAGS:   vault_entry
-CHANCE: 60
+WEIGHT: 60
 ORIENT: float
 MAP
 O
diff --git a/crawl-ref/source/dat/zot.des b/crawl-ref/source/dat/zot.des
index f21af392e3..d8de119f9e 100644
--- a/crawl-ref/source/dat/zot.des
+++ b/crawl-ref/source/dat/zot.des
@@ -10,7 +10,7 @@
 #
 NAME:   lemuel_baited_zot_trap
 DEPTH:  Zot:*
-CHANCE: 20
+WEIGHT: 20
 TAGS:   allow_dup
 KFEAT:  * = Zot trap
 KITEM:  * = any good_item
diff --git a/crawl-ref/source/debug.cc b/crawl-ref/source/debug.cc
index 9f1ab4cf1f..f25a886c18 100644
--- a/crawl-ref/source/debug.cc
+++ b/crawl-ref/source/debug.cc
@@ -4698,6 +4698,7 @@ static bool mg_do_build_level(int niters)
 static std::vector<level_id> mg_dungeon_places()
 {
     std::vector<level_id> places;
+
     for (int br = BRANCH_MAIN_DUNGEON; br < NUM_BRANCHES; ++br)
     {
         if (branches[br].depth == -1)
@@ -4790,6 +4791,8 @@ static void _mapgen_report_available_random_vaults(FILE *outf)
          i != places.end(); ++i)
     {
         fprintf(outf, "\n%s -------------\n", i->describe().c_str());
+        mesclr();
+        mprf("Examining random maps at %s", i->describe().c_str());
         mg_report_random_maps(outf, *i);
         fprintf(outf, "---------------------------------\n");
     }
diff --git a/crawl-ref/source/dungeon.cc b/crawl-ref/source/dungeon.cc
index a004341c0c..9f4160da0b 100644
--- a/crawl-ref/source/dungeon.cc
+++ b/crawl-ref/source/dungeon.cc
@@ -241,8 +241,7 @@ std::vector<vault_placement> Level_Vaults;
 std::string dgn_Build_Method;
 std::string dgn_Layout_Type;
 
-static int vault_chance = 9;
-static int minivault_chance = 3;
+static int can_create_vault = true;
 static bool dgn_level_vetoed = false;
 static bool use_random_maps  = true;
 static bool dgn_check_connectivity = false;
@@ -795,7 +794,7 @@ static bool _ensure_vault_placed(bool vault_success)
     if (!vault_success)
         dgn_level_vetoed = true;
     else
-        vault_chance = 0;
+        can_create_vault = false;
     return (vault_success);
 }
 
@@ -862,8 +861,7 @@ static void _reset_level()
     level_clear_vault_memory();
     dgn_colour_grid.reset(NULL);
 
-    vault_chance     = 9;
-    minivault_chance = 3;
+    can_create_vault = true;
     use_random_maps  = true;
     dgn_check_connectivity = false;
     dgn_zones        = 0;
@@ -2418,7 +2416,7 @@ static void _place_minivaults(const std::string &tag, int lo, int hi, bool force
     }
 
     std::set<int> used;
-    if (use_random_maps && minivault_chance && one_chance_in(minivault_chance))
+    if (use_random_maps)
     {
         const int vault = random_map_in_depth(level_id::current(), true);
         if (vault != -1)
@@ -2461,9 +2459,7 @@ static builder_rc_type _builder_normal(int level_number, char level_type,
     int vault = _dgn_random_map_for_place(false);
 
     // Can't have vaults on you.where_are_you != BRANCH_MAIN_DUNGEON levels.
-    if (vault == -1
-        && use_random_maps
-        && one_chance_in(vault_chance))
+    if (vault == -1 && use_random_maps && can_create_vault)
     {
         vault = random_map_in_depth(level_id::current());
 
@@ -2518,7 +2514,6 @@ static builder_rc_type _builder_normal(int level_number, char level_type,
     if (level_number > 2 && level_number < 23 && one_chance_in(3))
     {
         _plan_main(level_number, 0);
-        minivault_chance = 3;
         return BUILD_SKIP;
     }
 
@@ -2534,7 +2529,6 @@ static builder_rc_type _builder_normal(int level_number, char level_type,
 
         // Sometimes _roguey_level() generates a special room.
         _roguey_level(level_number, sr, just_roguey);
-        minivault_chance = 4;
 
         if (just_roguey)
             return BUILD_SKIP;
@@ -2841,8 +2835,7 @@ static void _place_extra_vaults()
 {
     if (!player_in_branch(BRANCH_MAIN_DUNGEON)
         && use_random_maps
-        && vault_chance
-        && one_chance_in(vault_chance))
+        && can_create_vault)
     {
         int vault = random_map_in_depth(level_id::current());
 
@@ -2851,7 +2844,7 @@ static void _place_extra_vaults()
             vault = -1;
 
         if (vault != -1 && _build_secondary_vault(you.your_level, vault, -1))
-            vault_chance = 0;
+            can_create_vault = false;
     }
 }
 
diff --git a/crawl-ref/source/luadgn.cc b/crawl-ref/source/luadgn.cc
index ca773b84d8..2d688d887c 100644
--- a/crawl-ref/source/luadgn.cc
+++ b/crawl-ref/source/luadgn.cc
@@ -534,12 +534,30 @@ static int dgn_change_branch_flags(lua_State *ls)
     return (1);
 }
 
+static int dgn_chance(lua_State *ls)
+{
+    MAP(ls, 1, map);
+    if (!lua_isnil(ls, 2) && !lua_isnil(ls, 3))
+    {
+        const int chance_priority = luaL_checkint(ls, 2);
+        const int chance = luaL_checkint(ls, 3);
+        if (chance < 0 || chance > CHANCE_ROLL)
+            luaL_argerror(ls, 2,
+                          make_stringf("Chance must be in the range [0,%d]",
+                                       CHANCE_ROLL).c_str());
+
+        map->chance_priority = chance_priority;
+        map->chance = chance;
+    }
+    PLUARET(number, map->chance);
+}
+
 static int dgn_weight(lua_State *ls)
 {
     MAP(ls, 1, map);
     if (!lua_isnil(ls, 2))
-        map->chance = luaL_checkint(ls, 2);
-    PLUARET(number, map->chance);
+        map->weight = luaL_checkint(ls, 2);
+    PLUARET(number, map->weight);
 }
 
 static int dgn_orient(lua_State *ls)
@@ -2107,7 +2125,8 @@ static const struct luaL_reg dgn_lib[] =
     { "tags_remove", dgn_tags_remove },
     { "lflags", dgn_lflags },
     { "bflags", dgn_bflags },
-    { "chance", dgn_weight },
+    { "chance", dgn_chance },
+    { "weight", dgn_weight },
     { "welcome", dgn_welcome },
     { "weight", dgn_weight },
     { "orient", dgn_orient },
diff --git a/crawl-ref/source/mapdef.cc b/crawl-ref/source/mapdef.cc
index 3eaea9b11f..fb0b1b6a3a 100644
--- a/crawl-ref/source/mapdef.cc
+++ b/crawl-ref/source/mapdef.cc
@@ -1164,7 +1164,7 @@ dlua_set_map::~dlua_set_map()
 //
 
 map_def::map_def()
-    : name(), tags(), place(), depths(), orient(), chance(),
+    : name(), tags(), place(), depths(), orient(), chance(), weight(),
       welcome_messages(), map(), mons(), items(), keyspecs(),
       prelude("dlprelude"), main("dlmain"), validate("dlvalidate"),
       veto("dlveto"), rock_colour(BLACK), floor_colour(BLACK),
@@ -1199,8 +1199,26 @@ void map_def::reinit()
 
     rock_colour = floor_colour = BLACK;
 
-    // Base chance; this is not a percentage.
-    chance = 10;
+    // Chance of using this level. Nonzero chance should be used
+    // sparingly. When selecting vaults for a place, first those
+    // vaults with chance > 0 are considered, in the order they were
+    // loaded (which is arbitrary). If random2(100) < chance, the
+    // vault is picked, and all other vaults are ignored for that
+    // random selection. weight is ignored if the vault is chosen
+    // based on its chance.
+    chance = 0;
+
+    // If multiple alternative vaults have a chance, the order in which
+    // they're tested is based on chance_priority: higher priority vaults
+    // are checked first. Vaults with the same priority are tested in
+    // unspecified order.
+    chance_priority = 0;
+
+    // Weight for this map. When selecting a map, if no map with a
+    // nonzero chance is picked, one of the other eligible vaults is
+    // picked with a probability of weight / (sum of weights of all
+    // eligible vaults).
+    weight = 10;
 
     // Clearing the map also zaps map transforms.
     map.clear();
@@ -1296,7 +1314,9 @@ void map_def::write_index(writer& outf) const
     marshallString4(outf, place_loaded_from.filename);
     marshallLong(outf, place_loaded_from.lineno);
     marshallShort(outf, orient);
+    marshallLong(outf, chance_priority);
     marshallLong(outf, chance);
+    marshallLong(outf, weight);
     marshallLong(outf, cache_offset);
     marshallString4(outf, tags);
     place.save(outf);
@@ -1310,7 +1330,9 @@ void map_def::read_index(reader& inf)
     unmarshallString4(inf, place_loaded_from.filename);
     place_loaded_from.lineno   = unmarshallLong(inf);
     orient       = static_cast<map_section_type>( unmarshallShort(inf) );
+    chance_priority = unmarshallLong(inf);
     chance       = unmarshallLong(inf);
+    weight       = unmarshallLong(inf);
     cache_offset = unmarshallLong(inf);
     unmarshallString4(inf, tags);
     place.load(inf);
@@ -1757,6 +1779,11 @@ bool map_def::has_tag_suffix(const std::string &suffix) const
         && tags.find(suffix + " ") != std::string::npos;
 }
 
+std::vector<std::string> map_def::get_tags() const
+{
+    return split_string(" ", tags);
+}
+
 const keyed_mapspec *map_def::mapspec_for_key(int key) const
 {
     keyed_specs::const_iterator i = keyspecs.find(key);
diff --git a/crawl-ref/source/mapdef.h b/crawl-ref/source/mapdef.h
index 83098d2b5a..ca9da9c113 100644
--- a/crawl-ref/source/mapdef.h
+++ b/crawl-ref/source/mapdef.h
@@ -618,7 +618,10 @@ public:
 
     depth_ranges     depths;
     map_section_type orient;
+
+    int              chance_priority;
     int              chance;
+    int              weight;
 
     std::vector<std::string> welcome_messages;
 
@@ -711,6 +714,7 @@ public:
     bool has_tag(const std::string &tag) const;
     bool has_tag_prefix(const std::string &tag) const;
     bool has_tag_suffix(const std::string &suffix) const;
+    std::vector<std::string> get_tags() const;
 
     std::vector<std::string> get_shuffle_strings() const;
     std::vector<std::string> get_subst_strings() const;
@@ -753,6 +757,8 @@ private:
             const std::string &s, bool fixed));
 };
 
+const int CHANCE_ROLL = 10000;
+
 std::string escape_string(std::string in, const std::string &toesc,
                           const std::string &escapewith);
 
diff --git a/crawl-ref/source/maps.cc b/crawl-ref/source/maps.cc
index cc896d2f22..f2c5827afb 100644
--- a/crawl-ref/source/maps.cc
+++ b/crawl-ref/source/maps.cc
@@ -22,6 +22,7 @@
 #include "dungeon.h"
 #include "enum.h"
 #include "files.h"
+#include "message.h"
 #include "monplace.h"
 #include "mapdef.h"
 #include "misc.h"
@@ -44,7 +45,8 @@ static bool resolve_map(map_def &def, const map_def &original);
 //////////////////////////////////////////////////////////////////////////
 // New style vault definitions
 
-static std::vector<map_def> vdefs;
+typedef std::vector<map_def> map_vector;
+static map_vector vdefs;
 
 /* ******************** BEGIN PUBLIC FUNCTIONS ******************* */
 
@@ -356,111 +358,249 @@ std::vector<std::string> find_map_matches(const std::string &name)
     return (matches);
 }
 
-// Returns a map for which PLACE: matches the given place.
-int random_map_for_place(const level_id &place, bool want_minivault)
-{
-    if (!place.is_valid())
-        return (-1);
+struct map_selector {
+private:
+    enum select_type
+    {
+        PLACE,
+        DEPTH,
+        TAG
+    };
 
-    int mapindex = -1;
-    int rollsize = 0;
+public:
+    bool accept(const map_def &md) const;
+    void announce(int vault) const;
 
-    for (unsigned i = 0, size = vdefs.size(); i < size; ++i)
+    bool valid() const
     {
-        // We also accept tagged levels here.
-        if (vdefs[i].place == place
-            && vdefs[i].is_minivault() == want_minivault
-            && !vdefs[i].has_tag("layout")
-            && map_matches_layout_type(vdefs[i])
-            && vault_unforbidden(vdefs[i]))
-        {
-            rollsize += vdefs[i].chance;
+        return (sel == TAG || place.is_valid());
+    }
 
-            if (rollsize && x_chance_in_y(vdefs[i].chance, rollsize))
-                mapindex = i;
-        }
+    static map_selector by_place(const level_id &place, bool mini)
+    {
+        return map_selector(map_selector::PLACE, place, "", mini, false);
     }
 
-    if (mapindex != -1 && vdefs[mapindex].has_tag("dummy"))
-        mapindex = -1;
+    static map_selector by_depth(const level_id &place, bool mini)
+    {
+        return map_selector(map_selector::DEPTH, place, "", mini, true);
+    }
 
+    static map_selector by_tag(const std::string &tag, bool mini,
+                               bool check_depth)
+    {
+        return map_selector(map_selector::TAG, level_id::current(), tag,
+                            mini, check_depth);
+    }
+
+private:
+    map_selector(select_type _typ, const level_id &_pl,
+                 const std::string &_tag,
+                 bool _mini, bool _check_depth)
+        : ignore_chance(false), preserve_dummy(false),
+          sel(_typ), place(_pl), tag(_tag),
+          mini(_mini), check_depth(_check_depth),
+          check_layout(sel == DEPTH && place == level_id::current())
+    {
+    }
+
+public:
+    bool ignore_chance;
+    bool preserve_dummy;
+    const select_type sel;
+    const level_id place;
+    const std::string tag;
+    const bool mini;
+    const bool check_depth;
+    const bool check_layout;
+};
+
+bool map_selector::accept(const map_def &mapdef) const
+{
+    switch (sel)
+    {
+    case PLACE:
+        return (mapdef.place == place
+                && mapdef.is_minivault() == mini
+                && !mapdef.has_tag("layout")
+                && map_matches_layout_type(mapdef)
+                && vault_unforbidden(mapdef));
+    case DEPTH:
+        return (mapdef.is_minivault() == mini
+                && !mapdef.place.is_valid()
+                && mapdef.is_usable_in(place)
+                // Some tagged levels cannot be selected by depth. This is
+                // the only thing preventing Pandemonium demon vaults from
+                // showing up in the main dungeon.
+                && !mapdef.has_tag_suffix("entry")
+                && !mapdef.has_tag("pan")
+                && !mapdef.has_tag("unrand")
+                && !mapdef.has_tag("bazaar")
+                && !mapdef.has_tag("layout")
+                && (!check_layout || map_matches_layout_type(mapdef))
+                && vault_unforbidden(mapdef));
+    case TAG:
+        return (mapdef.has_tag(tag) && mapdef.is_minivault() == mini
+                && (!check_depth || !mapdef.has_depth()
+                    || mapdef.is_usable_in(place))
+                && map_matches_layout_type(mapdef)
+                && vault_unforbidden(mapdef));
+    default:
+        return (false);
+    }
+}
+
+void map_selector::announce(int vault) const
+{
 #ifdef DEBUG_DIAGNOSTICS
-    if (mapindex != -1)
-        mprf(MSGCH_DIAGNOSTICS, "Found map %s for %s",
-             vdefs[mapindex].name.c_str(), place.describe().c_str());
+    if (vault != -1)
+    {
+        const char *format =
+            sel == PLACE? "[PLACE] Found map %s for %s" :
+            sel == DEPTH? "[DEPTH] Found random map %s for %s" :
+                          "[TAG] Found map %s tagged '%s'";
+
+        mprf(MSGCH_DIAGNOSTICS, format,
+             vdefs[vault].name.c_str(),
+             sel == TAG? tag.c_str() : place.describe().c_str());
+    }
 #endif
+}
 
-    return (mapindex);
+static bool _compare_vault_chance_priority(int a, int b)
+{
+    return (vdefs[b].chance_priority < vdefs[a].chance_priority);
 }
 
-int random_map_in_depth(const level_id &place, bool want_minivault)
+static std::string _vault_chance_tag(const map_def &map)
 {
+    if (map.has_tag_prefix("chance_"))
+    {
+        const std::vector<std::string> tags = map.get_tags();
+        for (int i = 0, size = tags.size(); i < size; ++i)
+        {
+            if (tags[i].find("chance_") == 0)
+                return (tags[i]);
+        }
+    }
+    return ("");
+}
+
+static int _random_map_by_selector(const map_selector &sel)
+{
+    if (!sel.valid())
+        return (-1);
+
     int mapindex = -1;
     int rollsize = 0;
 
-    const bool check_layout = (place == level_id::current());
+    typedef std::vector<unsigned> vault_indices;
+
+    // First build a list of vaults that could be used:
+    vault_indices eligible;
+
+    // Vaults that are eligible and have >0 chance.
+    vault_indices chance;
+
+    typedef std::set<std::string> tag_set;
+    tag_set chance_tags;
 
     for (unsigned i = 0, size = vdefs.size(); i < size; ++i)
-        if (vdefs[i].is_minivault() == want_minivault
-            && !vdefs[i].place.is_valid()
-            && vdefs[i].is_usable_in(place)
-            // Some tagged levels cannot be selected by depth. This is
-            // the only thing preventing Pandemonium demon vaults from
-            // showing up in the main dungeon.
-            && !vdefs[i].has_tag_suffix("entry")
-            && !vdefs[i].has_tag("pan")
-            && !vdefs[i].has_tag("unrand")
-            && !vdefs[i].has_tag("bazaar")
-            && !vdefs[i].has_tag("layout")
-            && (!check_layout || map_matches_layout_type(vdefs[i]))
-            && vault_unforbidden(vdefs[i]))
+        if (sel.accept(vdefs[i]))
         {
-            rollsize += vdefs[i].chance;
-
-            if (rollsize && x_chance_in_y(vdefs[i].chance, rollsize))
-                mapindex = i;
+            if (!sel.ignore_chance && vdefs[i].chance > 0)
+            {
+                // There may be several alternatives for a portal
+                // vault that want to be governed by one common
+                // CHANCE. In this case each vault will use a
+                // CHANCE, and a common chance_xxx tag. Pick the
+                // first such vault for the chance roll. Note that
+                // at this point we ignore chance_priority.
+                const std::string tag = _vault_chance_tag(vdefs[i]);
+                if (chance_tags.find(tag) == chance_tags.end())
+                {
+                    if (!tag.empty())
+                        chance_tags.insert(tag);
+                    chance.push_back(i);
+                }
+            }
+            else
+            {
+                eligible.push_back(i);
+            }
         }
 
-    if (mapindex != -1 && vdefs[mapindex].has_tag("dummy"))
-        mapindex = -1;
+    // Sort chances by priority, high priority first.
+    std::sort(chance.begin(), chance.end(), _compare_vault_chance_priority);
 
-    return (mapindex);
-}
+    // Check CHANCEs.
+    for (vault_indices::const_iterator i = chance.begin();
+         i != chance.end(); ++i)
+    {
+        const map_def &map(vdefs[*i]);
+        if (random2(CHANCE_ROLL) < map.chance)
+        {
+            const std::string chance_tag = _vault_chance_tag(map);
+            // If this map has a chance_ tag, convert the search into
+            // a lookup for that tag.
+            if (!chance_tag.empty())
+            {
+                map_selector msel = map_selector::by_tag(sel.tag, sel.mini,
+                                                         sel.check_depth);
+                msel.ignore_chance = true;
+                return _random_map_by_selector(msel);
+            }
 
-int random_map_for_tag(const std::string &tag,
-                       bool want_minivault,
-                       bool check_depth)
-{
-    int mapindex = -1;
-    int rollsize = 0;
+            mapindex = *i;
+            break;
+        }
+    }
 
-    for (unsigned i = 0, size = vdefs.size(); i < size; ++i)
+    if (mapindex == -1)
     {
-        if (vdefs[i].has_tag(tag) && vdefs[i].is_minivault() == want_minivault
-            && (!check_depth || !vdefs[i].has_depth()
-                || vdefs[i].is_usable_in(level_id::current()))
-            && map_matches_layout_type(vdefs[i])
-            && vault_unforbidden(vdefs[i]))
+        for (vault_indices::const_iterator i = eligible.begin();
+             i != eligible.end(); ++i)
         {
-            rollsize += vdefs[i].chance;
+            const map_def &map(vdefs[*i]);
+            rollsize += map.weight;
 
-            if (rollsize && x_chance_in_y(vdefs[i].chance, rollsize))
-                mapindex = i;
+            if (rollsize && x_chance_in_y(map.weight, rollsize))
+                mapindex = *i;
         }
     }
 
-    if (mapindex != -1 && vdefs[mapindex].has_tag("dummy"))
+    if (!sel.preserve_dummy && mapindex != -1
+        && vdefs[mapindex].has_tag("dummy"))
+    {
         mapindex = -1;
+    }
 
-#ifdef DEBUG_DIAGNOSTICS
-    if (mapindex != -1)
-        mprf(MSGCH_DIAGNOSTICS, "Found map %s tagged '%s'",
-                vdefs[mapindex].name.c_str(), tag.c_str());
-#endif
+    sel.announce(mapindex);
 
     return (mapindex);
 }
 
+// Returns a map for which PLACE: matches the given place.
+int random_map_for_place(const level_id &place, bool want_minivault)
+{
+    return _random_map_by_selector(
+        map_selector::by_place(place, want_minivault));
+}
+
+int random_map_in_depth(const level_id &place, bool want_minivault)
+{
+    return _random_map_by_selector(
+        map_selector::by_depth(place, want_minivault));
+}
+
+int random_map_for_tag(const std::string &tag,
+                       bool want_minivault,
+                       bool check_depth)
+{
+    return _random_map_by_selector(
+        map_selector::by_tag(tag, want_minivault, check_depth));
+}
+
 const map_def *map_by_index(int index)
 {
     if (index < 0 || index >= (int) vdefs.size())
@@ -745,6 +885,14 @@ void run_map_preludes()
 typedef std::pair<std::string, int> weighted_map_name;
 typedef std::vector<weighted_map_name> weighted_map_names;
 
+static int _mg_random_vault_here(const level_id &place, bool mini)
+{
+    no_messages mx;
+    map_selector sel = map_selector::by_depth(place, mini);
+    sel.preserve_dummy = true;
+    return _random_map_by_selector(sel);
+}
+
 static weighted_map_names mg_find_random_vaults(
     const level_id &place, bool wantmini)
 {
@@ -753,22 +901,23 @@ static weighted_map_names mg_find_random_vaults(
     if (!place.is_valid())
         return (wms);
 
-    for (unsigned i = 0, size = vdefs.size(); i < size; ++i)
+    typedef std::map<std::string, int> map_count_t;
+
+    map_count_t map_counts;
+
+    for (int i = 0; i < 10000; ++i)
     {
-        if (vdefs[i].is_minivault() == wantmini
-            && !vdefs[i].place.is_valid()
-            && vdefs[i].is_usable_in(place)
-            // Some tagged levels cannot be selected by depth. This is
-            // the only thing preventing Pandemonium demon vaults from
-            // showing up in the main dungeon.
-            && !vdefs[i].has_tag_suffix("entry")
-            && !vdefs[i].has_tag("pan")
-            && !vdefs[i].has_tag("unrand")
-            && !vdefs[i].has_tag("bazaar"))
-        {
-            wms.push_back(
-                weighted_map_name( vdefs[i].name, vdefs[i].chance ) );
-        }
+        const int v = _mg_random_vault_here(place, wantmini);
+        if (v == -1)
+            map_counts["(none)"]++;
+        else
+            map_counts[vdefs[v].name]++;
+    }
+
+    for (map_count_t::const_iterator i = map_counts.begin();
+         i != map_counts.end(); ++i)
+    {
+        wms.push_back(*i);
     }
 
     return (wms);
@@ -794,7 +943,7 @@ static void mg_report_random_vaults(
     for (int i = 0, size = wms.size(); i < size; ++i)
     {
         std::string curr =
-            make_stringf("%s (%.2f%%)",
+            make_stringf("%s (%.4f%%)",
                          wms[i].first.c_str(),
                          100.0 * wms[i].second / weightsum);
         if (i < size - 1)
diff --git a/crawl-ref/source/maps.h b/crawl-ref/source/maps.h
index f2f35a275e..20dc1709c1 100644
--- a/crawl-ref/source/maps.h
+++ b/crawl-ref/source/maps.h
@@ -71,7 +71,7 @@ extern depth_ranges    lc_default_depths;
 extern dlua_chunk      lc_global_prelude;
 extern bool            lc_run_global_prelude;
 
-const int              MAP_CACHE_VERSION = 1007;
+const int              MAP_CACHE_VERSION = 1008;
 
 
 #ifdef DEBUG_DIAGNOSTICS
diff --git a/crawl-ref/source/util/levcomp.lpp b/crawl-ref/source/util/levcomp.lpp
index 590cd59122..41d3b60b0f 100644
--- a/crawl-ref/source/util/levcomp.lpp
+++ b/crawl-ref/source/util/levcomp.lpp
@@ -236,7 +236,7 @@ ORIENT:             { CBEGIN(ARGUMENT); return ORIENT; }
 PLACE:              { CBEGIN(ARGUMENT); return PLACE; }
 WELCOME:            { CBEGIN(ARGUMENT); return WELCOME; }
 CHANCE:             return CHANCE;
-WEIGHT:             return CHANCE;
+WEIGHT:             return WEIGHT;
 FLAGS:              { CBEGIN(KEYWORDS); return TAGS; }
 TAGS:               { CBEGIN(KEYWORDS); return TAGS; }
 LFLAGS:             { CBEGIN(ARGUMENT); return LFLAGS; }
@@ -258,7 +258,11 @@ KMASK:              { CBEGIN(ARGUMENT); return KMASK; }
 
 ,                   return COMMA;
 
-[0-9]+              {
+:                   return COLON;
+
+%                   return PERC;
+
+[+-]?[0-9]+         {
                         clean();
                         yylval.i = atoi(yytext);
                         return INTEGER;
diff --git a/crawl-ref/source/util/levcomp.ypp b/crawl-ref/source/util/levcomp.ypp
index f2bb2b17b6..a678937ea9 100644
--- a/crawl-ref/source/util/levcomp.ypp
+++ b/crawl-ref/source/util/levcomp.ypp
@@ -53,11 +53,11 @@ level_range set_range(const char *s, int start, int end)
 %expect 2
 
 %token <i>      DEFAULT_DEPTH SHUFFLE SUBST TAGS KFEAT KITEM KMONS KMASK
-%token <i>      NAME DEPTH ORIENT PLACE CHANCE MONS ITEM MARKER COLOUR
+%token <i>      NAME DEPTH ORIENT PLACE CHANCE WEIGHT MONS ITEM MARKER COLOUR
 %token <i>      PRELUDE MAIN VALIDATE VETO NSUBST WELCOME LFLAGS BFLAGS
 %token <i>      FLOORCOL ROCKCOL
 
-%token <i>      COMMA INTEGER CHARACTER
+%token <i>      COMMA COLON PERC INTEGER CHARACTER
 
 %token <text>   STRING MAP_LINE MONSTER_NAME ITEM_INFO
 %token <text>   LUA_LINE
@@ -152,6 +152,7 @@ name            : NAME STRING
 metaline        : place
                 | depth
                 | chance
+                | weight
                 | orientation
                 | welcome
                 | mons
@@ -468,11 +469,40 @@ depth           : DEPTH {}
                 }
                 ;
 
-chance          : CHANCE INTEGER
+chance          : CHANCE INTEGER COLON INTEGER PERC
                 {
                     lc_map.main.add(
                         yylineno,
-                        make_stringf("chance(\"%d\")", $2));
+                        make_stringf("chance(%d, %d)", $2, $4 * 100));
+                }
+                |
+                CHANCE INTEGER COLON INTEGER
+                {
+                    lc_map.main.add(
+                        yylineno,
+                        make_stringf("chance(%d, %d)", $2, $4));
+                }
+                |
+                CHANCE INTEGER PERC
+                {
+                    lc_map.main.add(
+                        yylineno,
+                        make_stringf("chance(0, %d)", $2 * 100));
+                }
+                |
+                CHANCE INTEGER
+                {
+                    lc_map.main.add(
+                        yylineno,
+                        make_stringf("chance(0, %d)", $2));
+                }
+                ;
+
+weight          : WEIGHT INTEGER
+                {
+                    lc_map.main.add(
+                        yylineno,
+                        make_stringf("weight(%d)", $2));
                 }
                 ;