Extract buildLineDiff into a shared ui/src/lib/line-diff helper

Lifts the LCS-based line diff out of DocumentDiffModal so that the routine
revision history diff modal can reuse the same algorithm without duplication.
Behavior is unchanged.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/DocumentDiffModal.tsx

@@ -3,6 +3,7 @@ import { useQuery } from "@tanstack/react-query";
 import type { DocumentRevision } from "@paperclipai/shared";
 import { issuesApi } from "../api/issues";
 import { queryKeys } from "../lib/queryKeys";
+import { buildLineDiff, type DiffRow } from "../lib/line-diff";
 import { relativeTime } from "../lib/utils";
 import {
   Dialog,
@@ -27,96 +28,6 @@ function getRevisionLabel(revision: DocumentRevision) {
   return `rev ${revision.revisionNumber} — ${relativeTime(revision.createdAt)} • ${actor}`;
 }
 
-type DiffRow = {
-  kind: "context" | "removed" | "added";
-  oldLineNumber: number | null;
-  newLineNumber: number | null;
-  text: string;
-};
-
-function buildLineDiff(oldText: string, newText: string): DiffRow[] {
-  const oldLines = oldText.split("\n");
-  const newLines = newText.split("\n");
-  const oldCount = oldLines.length;
-  const newCount = newLines.length;
-  const dp = Array.from({ length: oldCount + 1 }, () => Array<number>(newCount + 1).fill(0));
-
-  for (let i = oldCount - 1; i >= 0; i -= 1) {
-    for (let j = newCount - 1; j >= 0; j -= 1) {
-      dp[i][j] = oldLines[i] === newLines[j]
-        ? dp[i + 1][j + 1] + 1
-        : Math.max(dp[i + 1][j], dp[i][j + 1]);
-    }
-  }
-
-  const rows: DiffRow[] = [];
-  let i = 0;
-  let j = 0;
-  let oldLineNumber = 1;
-  let newLineNumber = 1;
-
-  while (i < oldCount && j < newCount) {
-    if (oldLines[i] === newLines[j]) {
-      rows.push({
-        kind: "context",
-        oldLineNumber,
-        newLineNumber,
-        text: oldLines[i],
-      });
-      i += 1;
-      j += 1;
-      oldLineNumber += 1;
-      newLineNumber += 1;
-      continue;
-    }
-
-    if (dp[i + 1][j] >= dp[i][j + 1]) {
-      rows.push({
-        kind: "removed",
-        oldLineNumber,
-        newLineNumber: null,
-        text: oldLines[i],
-      });
-      i += 1;
-      oldLineNumber += 1;
-      continue;
-    }
-
-    rows.push({
-      kind: "added",
-      oldLineNumber: null,
-      newLineNumber,
-      text: newLines[j],
-    });
-    j += 1;
-    newLineNumber += 1;
-  }
-
-  while (i < oldCount) {
-    rows.push({
-      kind: "removed",
-      oldLineNumber,
-      newLineNumber: null,
-      text: oldLines[i],
-    });
-    i += 1;
-    oldLineNumber += 1;
-  }
-
-  while (j < newCount) {
-    rows.push({
-      kind: "added",
-      oldLineNumber: null,
-      newLineNumber,
-      text: newLines[j],
-    });
-    j += 1;
-    newLineNumber += 1;
-  }
-
-  return rows;
-}
-
 export function DocumentDiffModal({
   issueId,
   documentKey,

ui/src/lib/line-diff.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import { buildLineDiff } from "./line-diff";
+
+describe("buildLineDiff", () => {
+  it("emits context rows when both sides are identical", () => {
+    const rows = buildLineDiff("a\nb\nc", "a\nb\nc");
+    expect(rows).toHaveLength(3);
+    expect(rows.every((row) => row.kind === "context")).toBe(true);
+  });
+
+  it("marks added and removed lines", () => {
+    const rows = buildLineDiff("a\nb\nc", "a\nB\nc");
+    const kinds = rows.map((row) => row.kind);
+    expect(kinds).toContain("removed");
+    expect(kinds).toContain("added");
+    const removed = rows.find((row) => row.kind === "removed");
+    const added = rows.find((row) => row.kind === "added");
+    expect(removed?.text).toBe("b");
+    expect(added?.text).toBe("B");
+  });
+
+  it("handles empty old text as full insertion", () => {
+    const rows = buildLineDiff("", "x\ny");
+    expect(rows.filter((row) => row.kind === "added")).toHaveLength(2);
+  });
+});

ui/src/lib/line-diff.ts

@@ -0,0 +1,91 @@
+export type DiffRowKind = "context" | "removed" | "added";
+
+export type DiffRow = {
+  kind: DiffRowKind;
+  oldLineNumber: number | null;
+  newLineNumber: number | null;
+  text: string;
+};
+
+export function buildLineDiff(oldText: string, newText: string): DiffRow[] {
+  const oldLines = oldText.split("\n");
+  const newLines = newText.split("\n");
+  const oldCount = oldLines.length;
+  const newCount = newLines.length;
+  const dp = Array.from({ length: oldCount + 1 }, () => Array<number>(newCount + 1).fill(0));
+
+  for (let i = oldCount - 1; i >= 0; i -= 1) {
+    for (let j = newCount - 1; j >= 0; j -= 1) {
+      dp[i][j] = oldLines[i] === newLines[j]
+        ? dp[i + 1][j + 1] + 1
+        : Math.max(dp[i + 1][j], dp[i][j + 1]);
+    }
+  }
+
+  const rows: DiffRow[] = [];
+  let i = 0;
+  let j = 0;
+  let oldLineNumber = 1;
+  let newLineNumber = 1;
+
+  while (i < oldCount && j < newCount) {
+    if (oldLines[i] === newLines[j]) {
+      rows.push({
+        kind: "context",
+        oldLineNumber,
+        newLineNumber,
+        text: oldLines[i],
+      });
+      i += 1;
+      j += 1;
+      oldLineNumber += 1;
+      newLineNumber += 1;
+      continue;
+    }
+
+    if (dp[i + 1][j] >= dp[i][j + 1]) {
+      rows.push({
+        kind: "removed",
+        oldLineNumber,
+        newLineNumber: null,
+        text: oldLines[i],
+      });
+      i += 1;
+      oldLineNumber += 1;
+      continue;
+    }
+
+    rows.push({
+      kind: "added",
+      oldLineNumber: null,
+      newLineNumber,
+      text: newLines[j],
+    });
+    j += 1;
+    newLineNumber += 1;
+  }
+
+  while (i < oldCount) {
+    rows.push({
+      kind: "removed",
+      oldLineNumber,
+      newLineNumber: null,
+      text: oldLines[i],
+    });
+    i += 1;
+    oldLineNumber += 1;
+  }
+
+  while (j < newCount) {
+    rows.push({
+      kind: "added",
+      oldLineNumber: null,
+      newLineNumber,
+      text: newLines[j],
+    });
+    j += 1;
+    newLineNumber += 1;
+  }
+
+  return rows;
+}