1 files changed, 85 insertions, 0 deletions
diff --git a/src/2020/5/mod.rs b/src/2020/5/mod.rs
new file mode 100644
index 0000000..d0005f8
--- /dev/null
+++ b/src/2020/5/mod.rs
@@ -0,0 +1,85 @@
+use anyhow::Context as _;
+use std::convert::TryInto as _;
+
+pub fn part1() -> anyhow::Result<i64> {
+    let input = crate::util::read_file_str("data/5.txt")?;
+    let mut max = 0;
+    for line in input.lines() {
+        let id = seat_id(line)?;
+        if id > max {
+            max = id;
+        }
+    }
+    Ok(max)
+}
+
+pub fn part2() -> anyhow::Result<i64> {
+    let mut seats = vec![false; 1024];
+    let input = crate::util::read_file_str("data/5.txt")?;
+    for line in input.lines() {
+        let id = seat_id(line)?;
+        seats[id as usize] = true;
+    }
+    let first = seats
+        .iter()
+        .position(|b| *b)
+        .context("failed to find taken seat")?;
+    let seat = seats
+        .iter()
+        .skip(first)
+        .position(|b| !*b)
+        .context("failed to find free seat")?
+        + first;
+    if !seats[seat - 1] || seats[seat] || !seats[seat + 1] {
+        return Err(anyhow::anyhow!("invalid seat found"));
+    }
+    Ok(seat.try_into()?)
+}
+
+fn seat_id(desc: &str) -> anyhow::Result<i64> {
+    if desc.len() != 10 {
+        return Err(anyhow::anyhow!("invalid desc {}", desc));
+    }
+    let row_desc = &desc[0..7];
+    let col_desc = &desc[7..10];
+
+    let mut min_row = 0;
+    let mut max_row = 127;
+    for c in row_desc.chars() {
+        let mid = (max_row + min_row) / 2;
+        match c {
+            'F' => {
+                max_row = mid;
+            }
+            'B' => {
+                min_row = mid + 1;
+            }
+            _ => return Err(anyhow::anyhow!("invalid desc {}", desc)),
+        }
+    }
+    if min_row != max_row {
+        return Err(anyhow::anyhow!("bug"));
+    }
+    let row = min_row;
+
+    let mut min_col = 0;
+    let mut max_col = 7;
+    for c in col_desc.chars() {
+        let mid = (max_col + min_col) / 2;
+        match c {
+            'L' => {
+                max_col = mid;
+            }
+            'R' => {
+                min_col = mid + 1;
+            }
+            _ => return Err(anyhow::anyhow!("invalid desc {}", desc)),
+        }
+    }
+    if min_col != max_col {
+        return Err(anyhow::anyhow!("bug"));
+    }
+    let col = min_col;
+
+    Ok(row * 8 + col)
+}