also add a rows_diff method

3 files changed, 37 insertions, 3 deletions

diff --git a/src/grid.rs b/src/grid.rs
index 075e604..fa6b4b9 100644
--- a/src/grid.rs
+++ b/src/grid.rs
@@ -177,7 +177,7 @@ impl Grid {
         for (idx, (row, prev_row)) in self.rows().zip(prev.rows()).enumerate()
         {
             let (mut new_contents, new_attrs, new_col) =
-                row.contents_diff(prev_row, prev_attrs);
+                row.contents_diff(prev_row, 0, self.size.cols, prev_attrs);
             if !new_contents.is_empty() {
                 contents.extend(format!("\x1b[{};1H", idx + 1).as_bytes());
                 final_row = idx.try_into().unwrap();
diff --git a/src/row.rs b/src/row.rs
index fb31594..7b2ab99 100644
--- a/src/row.rs
+++ b/src/row.rs
@@ -126,14 +126,21 @@ impl Row {
     pub fn contents_diff(
         &self,
         prev: &Self,
+        start: u16,
+        width: u16,
         attrs: crate::attrs::Attrs,
     ) -> (Vec<u8>, crate::attrs::Attrs, u16) {
         let mut skip = 0;
         let mut contents = vec![];
         let mut prev_attrs = attrs;
         let mut final_col = 0;
-        for (idx, (cell, prev_cell)) in
-            self.cells().zip(prev.cells()).enumerate()
+
+        for (idx, (cell, prev_cell)) in self
+            .cells()
+            .zip(prev.cells())
+            .skip(start as usize)
+            .take(width as usize)
+            .enumerate()
         {
             if cell == prev_cell {
                 skip += 1;
diff --git a/src/screen.rs b/src/screen.rs
index 6aa80ba..873eeb3 100644
--- a/src/screen.rs
+++ b/src/screen.rs
@@ -211,6 +211,33 @@ impl Screen {
         grid_contents
     }
 
+    /// Returns a sequence of terminal byte streams sufficient to turn the
+    /// subset of each row from `prev` (as described by `start` and `width`)
+    /// into the corresponding row subset in `self`.
+    ///
+    /// You must handle the initial row positioning yourself - each row diff
+    /// expects to start out positioned at the start of that row. Internal
+    /// terminal modes (such as application keypad mode or alternate screen
+    /// mode) will not be included here.
+    pub fn rows_diff<'a>(
+        &'a self,
+        prev: &'a Self,
+        start: u16,
+        width: u16,
+    ) -> impl Iterator<Item = Vec<u8>> + 'a {
+        self.grid().rows().zip(prev.grid().rows()).map(
+            move |(row, prev_row)| {
+                let (contents, ..) = row.contents_diff(
+                    prev_row,
+                    start,
+                    width,
+                    crate::attrs::Attrs::default(),
+                );
+                contents
+            },
+        )
+    }
+
     /// Returns the `Cell` object at the given location in the terminal, if it
     /// exists.
     pub fn cell(&self, row: u16, col: u16) -> Option<&crate::cell::Cell> {