feat(compare): implement Document Compare dialog with local-diff and git-HEAD-diff modes

Amit Haridas

src/utils/line-diff.js

@@ -0,0 +1,58 @@
+/**
+ * Minimal LCS-based line diff for the Document Compare dialog.
+ * Textbook dynamic-programming formulation — the app has no diff library and
+ * document-sized inputs keep the (n+1) x (m+1) table affordable.
+ */
+
+function splitLines(text) {
+  if (typeof text !== 'string' || text === '') return [];
+  const lines = text.split(/\r?\n/);
+  // Editor convention: "a\n" is one line, not a line plus an empty one — drop the
+  // trailing split artifact so purely trailing-newline differences stay invisible.
+  if (lines.length > 1 && lines[lines.length - 1] === '') lines.pop();
+  return lines;
+}
+
+/**
+ * Compare two texts line by line.
+ * @param {string} oldText - Original text.
+ * @param {string} newText - Revised text.
+ * @returns {Array<{type: 'added'|'removed'|'unchanged', text: string}>} Edit script in
+ *   reading order; within a change block, removals are emitted before additions.
+ */
+function computeLineDiff(oldText, newText) {
+  const a = splitLines(oldText);
+  const b = splitLines(newText);
+  const n = a.length;
+  const m = b.length;
+
+  // lcs[i][j] = length of the longest common subsequence of a[i..] and b[j..]
+  const lcs = Array.from({ length: n + 1 }, () => new Array(m + 1).fill(0));
+  for (let i = n - 1; i >= 0; i--) {
+    for (let j = m - 1; j >= 0; j--) {
+      lcs[i][j] = a[i] === b[j] ? lcs[i + 1][j + 1] + 1 : Math.max(lcs[i + 1][j], lcs[i][j + 1]);
+    }
+  }
+
+  const result = [];
+  let i = 0;
+  let j = 0;
+  while (i < n && j < m) {
+    if (a[i] === b[j]) {
+      result.push({ type: 'unchanged', text: a[i] });
+      i++;
+      j++;
+    } else if (lcs[i + 1][j] >= lcs[i][j + 1]) {
+      result.push({ type: 'removed', text: a[i] });
+      i++;
+    } else {
+      result.push({ type: 'added', text: b[j] });
+      j++;
+    }
+  }
+  while (i < n) result.push({ type: 'removed', text: a[i++] });
+  while (j < m) result.push({ type: 'added', text: b[j++] });
+  return result;
+}
+
+module.exports = { computeLineDiff };