Reduce memory footprint for inline transformations (#20296)

Closes https://github.com/zed-industries/zed/issues/18062

This pull request prevents the `scores` matrix for the streaming diff
from growing quadratically.

Previously, we would store rows and columns respectively for all
characters in the old and new text. However, every time we receive a
chunk, we will always advance the position in the matrix to the very
latest character in the new text. This means we can avoid storing scores
for the new characters that were already reported.

Randomized tests still pass and I also made sure that the diffs we
produce are identical.

Release Notes:

- Improved memory footprint for inline transformations
([#18062](https://github.com/zed-industries/zed/issues/18062))

crates/assistant/src/streaming_diff.rs

@@ -28,13 +28,36 @@ impl Matrix {
         self.cols = cols;
     }
 
+    fn swap_columns(&mut self, col1: usize, col2: usize) {
+        if col1 == col2 {
+            return;
+        }
+
+        if col1 >= self.cols {
+            panic!("column out of bounds");
+        }
+
+        if col2 >= self.cols {
+            panic!("column out of bounds");
+        }
+
+        unsafe {
+            let ptr = self.cells.as_mut_ptr();
+            std::ptr::swap_nonoverlapping(
+                ptr.add(col1 * self.rows),
+                ptr.add(col2 * self.rows),
+                self.rows,
+            );
+        }
+    }
+
     fn get(&self, row: usize, col: usize) -> f64 {
         if row >= self.rows {
             panic!("row out of bounds")
         }
 
         if col >= self.cols {
-            panic!("col out of bounds")
+            panic!("column out of bounds")
         }
         self.cells[col * self.rows + row]
     }
@@ -45,7 +68,7 @@ impl Matrix {
         }
 
         if col >= self.cols {
-            panic!("col out of bounds")
+            panic!("column out of bounds")
         }
 
         self.cells[col * self.rows + row] = value;
@@ -106,26 +129,32 @@ impl StreamingDiff {
 
     pub fn push_new(&mut self, text: &str) -> Vec<CharOperation> {
         self.new.extend(text.chars());
-        self.scores.resize(self.old.len() + 1, self.new.len() + 1);
+        self.scores.swap_columns(0, self.scores.cols - 1);
+        self.scores
+            .resize(self.old.len() + 1, self.new.len() - self.new_text_ix + 1);
+        self.equal_runs.retain(|(_i, j), _| *j == self.new_text_ix);
 
         for j in self.new_text_ix + 1..=self.new.len() {
-            self.scores.set(0, j, j as f64 * Self::INSERTION_SCORE);
+            let relative_j = j - self.new_text_ix;
+
+            self.scores
+                .set(0, relative_j, j as f64 * Self::INSERTION_SCORE);
             for i in 1..=self.old.len() {
-                let insertion_score = self.scores.get(i, j - 1) + Self::INSERTION_SCORE;
-                let deletion_score = self.scores.get(i - 1, j) + Self::DELETION_SCORE;
+                let insertion_score = self.scores.get(i, relative_j - 1) + Self::INSERTION_SCORE;
+                let deletion_score = self.scores.get(i - 1, relative_j) + Self::DELETION_SCORE;
                 let equality_score = if self.old[i - 1] == self.new[j - 1] {
                     let mut equal_run = self.equal_runs.get(&(i - 1, j - 1)).copied().unwrap_or(0);
                     equal_run += 1;
                     self.equal_runs.insert((i, j), equal_run);
 
                     let exponent = cmp::min(equal_run as i32 / 4, Self::MAX_EQUALITY_EXPONENT);
-                    self.scores.get(i - 1, j - 1) + Self::EQUALITY_BASE.powi(exponent)
+                    self.scores.get(i - 1, relative_j - 1) + Self::EQUALITY_BASE.powi(exponent)
                 } else {
                     f64::NEG_INFINITY
                 };
 
                 let score = insertion_score.max(deletion_score).max(equality_score);
-                self.scores.set(i, j, score);
+                self.scores.set(i, relative_j, score);
             }
         }
 
@@ -133,7 +162,7 @@ impl StreamingDiff {
         let mut next_old_text_ix = self.old_text_ix;
         let next_new_text_ix = self.new.len();
         for i in self.old_text_ix..=self.old.len() {
-            let score = self.scores.get(i, next_new_text_ix);
+            let score = self.scores.get(i, next_new_text_ix - self.new_text_ix);
             if score > max_score {
                 max_score = score;
                 next_old_text_ix = i;
@@ -174,7 +203,9 @@ impl StreamingDiff {
 
             let (prev_i, prev_j) = [insertion_score, deletion_score, equality_score]
                 .iter()
-                .max_by_key(|cell| cell.map(|(i, j)| OrderedFloat(self.scores.get(i, j))))
+                .max_by_key(|cell| {
+                    cell.map(|(i, j)| OrderedFloat(self.scores.get(i, j - self.new_text_ix)))
+                })
                 .unwrap()
                 .unwrap();