fix(core): index moved hunk lines that start with ++ or --

.changeset/quiet-diffs-moved.md

@@ -0,0 +1,5 @@
+---
+"hunkdiff": patch
+---
+
+Keep moved-line classification aligned when a diff hunk adds or removes a line whose content begins with `++` or `--`.

src/core/changeset/fromPatch.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, test } from "bun:test";
+import { changesetFromPatch } from "./fromPatch";
+
+/**
+ * A hunk body line is always exactly one sign char plus its content, so a content line
+ * that happens to start with `++` or `--` (e.g. `++i;`, `--flag`) reads `+++…`/`---…`
+ * in the patch. File headers never appear inside a hunk, so content lines decide their
+ * own index in the per-side line arrays.
+ */
+const ADDITION_PLUS_PREFIX_PATCH = [
+  "diff --git a/add_plus.txt b/add_plus.txt",
+  "index 0000000..1111111 100644",
+  "--- a/add_plus.txt",
+  "+++ b/add_plus.txt",
+  "@@ -1,3 +1,4 @@",
+  " one",
+  "\u001b[1;35m-two\u001b[m",
+  "\u001b[32m+++ plus\u001b[m",
+  "\u001b[36m+three\u001b[m",
+  " four",
+  "",
+].join("\n");
+
+const DELETION_MINUS_PREFIX_PATCH = [
+  "diff --git a/del_minus.txt b/del_minus.txt",
+  "index 0000000..2222222 100644",
+  "--- a/del_minus.txt",
+  "+++ b/del_minus.txt",
+  "@@ -1,3 +1,3 @@",
+  " one",
+  "\u001b[1;35m--- flag\u001b[m",
+  "\u001b[36m+two\u001b[m",
+  " three",
+  "",
+].join("\n");
+
+describe("collectLineMoveKinds", () => {
+  test("indexes an added content line that starts with ++', not its following lines' move kind", () => {
+    const changeset = changesetFromPatch(ADDITION_PLUS_PREFIX_PATCH, "t", "probe", null);
+    const file = changeset.files.find((entry) => entry.path === "add_plus.txt")!;
+
+    expect(file.metadata.additionLines).toEqual(["one\n", "++ plus\n", "three\n", "four\n"]);
+    expect(file.lineMoveKinds?.additionLines).toEqual([undefined, undefined, "moved", undefined]);
+  });
+
+  test("indexes a deleted content line that starts with --', keeping its move kind", () => {
+    const changeset = changesetFromPatch(DELETION_MINUS_PREFIX_PATCH, "t", "probe", null);
+    const file = changeset.files.find((entry) => entry.path === "del_minus.txt")!;
+
+    expect(file.metadata.deletionLines).toEqual(["one\n", "-- flag\n", "three\n"]);
+    expect(file.lineMoveKinds?.deletionLines).toEqual([undefined, "moved", undefined]);
+  });
+});

src/core/changeset/fromPatch.ts

@@ -96,13 +96,15 @@ function collectLineMoveKinds(patchText: string): DiffLineMoveKinds[] {
       continue;
     }
 
-    if (plainLine.startsWith("+") && !plainLine.startsWith("+++")) {
+    // Every hunk-body `+`/`-` line is content — a line whose content starts with `++`
+    // reads `+++…` just like a file header, but headers never appear inside a hunk.
+    if (plainLine.startsWith("+")) {
       activeMoveKinds.additionLines[additionLineIndex] = movedLineKindFromAnsi(rawLine, "addition");
       additionLineIndex += 1;
       continue;
     }
 
-    if (plainLine.startsWith("-") && !plainLine.startsWith("---")) {
+    if (plainLine.startsWith("-")) {
       activeMoveKinds.deletionLines[deletionLineIndex] = movedLineKindFromAnsi(rawLine, "deletion");
       deletionLineIndex += 1;
       continue;