A diff look names a file as the repository does: git's side prefix dropped by git's rules, a plain diff's names as written

In a git section (one with a `diff --git` line) the one-letter side prefix
git writes, `a/`/`b/` or with diff.mnemonicPrefix `c/`, `i/`, `w/`, `o/`,
is dropped from its paths only when both paths of that line carry one,
different ones: `--no-prefix` writes none, so its `a/x a/x` is a real
directory `a` and is kept. A plain `diff -u` never strips: its names are
used as written, past the timestamp. `/dev/null` stays itself.

The walker does this once (diff.Walk.side, diff.gitHeader), so the path a
look resolves, and the one a miss says, are the repository's name:
`Look: src/x.zig: no such file here`, `Look: lib/old.zig: deleted by this
diff`. The look no longer tries a stripped name beside the written one.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 64 insertions, 12 deletions

diff --git a/src/look.zig b/src/look.zig
index e22f5705..afb5b760 100644
--- a/src/look.zig
+++ b/src/look.zig
@@ -1527,9 +1527,11 @@ const DiffLook = union(enum) { none, done, at: []const u8 };
 /// line names (diff.targetAt): the new file at a `@@` line's first new
 /// line, at a context or added line's own line, and at a removed line's
 /// place, the new line now standing where it was. `---`/`+++` open their
-/// file. The path is the repository's (walking up from the diff's
-/// directory to a `.git` or `.jj`), `a/` and `b/` dropped, else the
-/// directory's own. A file that is not there opens nothing and says so.
+/// file. The path is the repository's name (diff.Walk.side: git's side
+/// prefixes dropped by git's rules, a plain diff's names as written),
+/// found from the repository root (walking up from the diff's directory to
+/// a `.git` or `.jj`), else the directory. A file that is not there opens
+/// nothing and says so, by that name.
 ///
 /// Which line: the one a click just gave (`clicked`: exec.pointerOperand
 /// expanded to it and kept its row), else, for a look written over 9P, a
@@ -1573,8 +1575,7 @@ fn diffLook(p: *Pardes, id: usize, pane: *Pane, operand: []const u8, clicked: bo
 }
 
 /// The file a diff names, from `base` (the diff's directory): tried in the
-/// repository `base` is in, then in `base`, as git writes it (`b/x`, the
-/// repository's `x`) and as written.
+/// repository `base` is in, then in `base`.
 fn diffFile(base: []const u8, path: []const u8, out: *[4096]u8) ?[]const u8 {
     if (comptime !platform_has_fs) return null;
     if (path.len == 0) return null;
@@ -1593,11 +1594,9 @@ fn diffFile(base: []const u8, path: []const u8, out: *[4096]u8) ?[]const u8 {
     } else null;
     for ([_]?[]const u8{ root, base }) |maybe| {
         const at = maybe orelse continue;
-        for ([_][]const u8{ diff.stripSide(path), path }) |name| {
-            var joined: [4096]u8 = undefined;
-            const full = std.fmt.bufPrint(&joined, "{s}/{s}", .{ at, name }) catch continue;
-            if (fs.resolveOs(full, out)) |found| if (!found.dir) return found.path;
-        }
+        var joined: [4096]u8 = undefined;
+        const full = std.fmt.bufPrint(&joined, "{s}/{s}", .{ at, path }) catch continue;
+        if (fs.resolveOs(full, out)) |found| if (!found.dir) return found.path;
     }
     return null;
 }
@@ -2175,10 +2174,10 @@ test "Look on a diff line of a file that is not here says so and opens nothing"
     var after: usize = 0;
     for (p.panes) |slot| after += @intFromBool(slot != null);
     try std.testing.expectEqual(before, after);
-    try std.testing.expectEqualStrings("Look: b/src/elsewhere.zig: no such file here", pane.msg[0..pane.msg_len]);
+    try std.testing.expectEqualStrings("Look: src/elsewhere.zig: no such file here", pane.msg[0..pane.msg_len]);
     DiffFixture.click(p, pane, 11);
     try std.testing.expect(p.look_missed);
-    try std.testing.expectEqualStrings("Look: a/old.zig: deleted by this diff", pane.msg[0..pane.msg_len]);
+    try std.testing.expectEqualStrings("Look: old.zig: deleted by this diff", pane.msg[0..pane.msg_len]);
 }
 
 test "Look on plain diff -u output: names as written, timestamps cut, next to the diff" {
@@ -2303,3 +2302,56 @@ test "Look outside a diff pane expands and looks as ever" {
     const operand = exec.pointerOperand(p, pane, pointed.sel);
     try std.testing.expect(operand.text == null or !std.mem.eql(u8, operand.text.?, "+const a = 1;"));
 }
+
+test "Look on a diff under each git prefix opens the repository's file, and a real a/ directory under --no-prefix" {
+    if (!platform_has_fs) return;
+    const Case = struct { text: []const u8, file: []const u8 };
+    const cases = [_]Case{
+        .{ .text = "diff --git a/src/x.zig b/src/x.zig\n--- a/src/x.zig\n+++ b/src/x.zig\n@@ -1,2 +1,2 @@\n-one\n+uno\n two\n", .file = "/src/x.zig" },
+        .{ .text = "diff --git c/src/x.zig w/src/x.zig\n--- c/src/x.zig\n+++ w/src/x.zig\n@@ -1,2 +1,2 @@\n-one\n+uno\n two\n", .file = "/src/x.zig" },
+        .{ .text = "diff --git i/src/x.zig w/src/x.zig\n--- i/src/x.zig\n+++ w/src/x.zig\n@@ -1,2 +1,2 @@\n-one\n+uno\n two\n", .file = "/src/x.zig" },
+        // --no-prefix: `a/x.zig` is the file x.zig in the directory a, and
+        // a `x.zig` at the top is never taken for it.
+        .{ .text = "diff --git a/x.zig a/x.zig\n--- a/x.zig\n+++ a/x.zig\n@@ -1,2 +1,2 @@\n-one\n+uno\n two\n", .file = "/a/x.zig" },
+    };
+    for (cases) |c| {
+        var fx: DiffFixture = undefined;
+        try fx.init(true, &.{ .{ "src/x.zig", "uno\ntwo\n" }, .{ "a/x.zig", "uno\ntwo\n" }, .{ "x.zig", "top\n" } }, c.text);
+        defer fx.tmp.cleanup();
+        const p = try Pardes.init(std.testing.allocator, .{ .cols = 100, .rows = 30 });
+        defer p.deinit();
+        const pane = try fx.open(p, c.text);
+        DiffFixture.click(p, pane, 6); // `two`, a context line: new line 2
+        try std.testing.expect(!p.look_missed);
+        try expectAt(p, c.file, 2);
+        DiffFixture.click(p, pane, 1); // `---`
+        try std.testing.expect(!p.look_missed);
+        try std.testing.expect(std.mem.endsWith(u8, p.panes[p.active].?.file.?.path, c.file));
+    }
+}
+
+test "Look on a diff says a missing or deleted file by its repository name, a plain diff's as written" {
+    if (!platform_has_fs) return;
+    const git_text = "diff --git c/src/gone.zig w/src/gone.zig\n--- c/src/gone.zig\n+++ w/src/gone.zig\n@@ -1 +1 @@\n-a\n+b\n" ++
+        "diff --git a/lib/old.zig b/lib/old.zig\ndeleted file mode 100644\n--- a/lib/old.zig\n+++ /dev/null\n@@ -1 +0,0 @@\n-a\n";
+    const plain_text = "--- a/x.zig\t2026-09-30 10:00:00 +0000\n+++ b/x.zig\t2026-09-30 11:00:00 +0000\n@@ -1 +1 @@\n-a\n+b\n";
+    var fx: DiffFixture = undefined;
+    try fx.init(true, &.{.{ "x.zig", "b\n" }}, git_text);
+    defer fx.tmp.cleanup();
+    const p = try Pardes.init(std.testing.allocator, .{ .cols = 100, .rows = 30 });
+    defer p.deinit();
+    var pane = try fx.open(p, git_text);
+    DiffFixture.click(p, pane, 5);
+    try std.testing.expect(p.look_missed);
+    try std.testing.expectEqualStrings("Look: src/gone.zig: no such file here", pane.msg[0..pane.msg_len]);
+    DiffFixture.click(p, pane, 11);
+    try std.testing.expectEqualStrings("Look: lib/old.zig: deleted by this diff", pane.msg[0..pane.msg_len]);
+    DiffFixture.click(p, pane, 6); // `diff --git` of the deleted file
+    try std.testing.expectEqualStrings("Look: lib/old.zig: deleted by this diff", pane.msg[0..pane.msg_len]);
+    // A plain diff's `b/x.zig` is a path as written: there is no b/ here,
+    // and the x.zig at the top is not it.
+    pane = try fx.open(p, plain_text);
+    DiffFixture.click(p, pane, 4);
+    try std.testing.expect(p.look_missed);
+    try std.testing.expectEqualStrings("Look: b/x.zig: no such file here", pane.msg[0..pane.msg_len]);
+}