1 files changed, 33 insertions, 0 deletions
diff --git a/src/File.zig b/src/File.zig
index d16d4fc0..c738ee58 100644
--- a/src/File.zig
+++ b/src/File.zig
@@ -72,6 +72,9 @@ pub const State = struct {
     /// Its file was deleted on disk: said once (`deleted`) until it is
     /// written or found again.
     disk_gone: bool = false,
+    /// A +Pager pane (`pardes -`) whose text is a diff (`git diff`, `show`,
+    /// `log -p`): drawn and looked at as a diff (Pane.colorAlgo).
+    pager_diff: bool = false,
     /// Seconds of the last content change, for the filesystem's stat; zero until edited.
     mtime: u32 = 0,
     /// Non-null for a generated output buffer rather than an on-disk file.
@@ -998,6 +1001,36 @@ pub fn setContentSpan(p: *Pardes, f: *State, new: []u8, span: ?Span) void {
     f.highlights = &.{};
     f.highlight_start = 0;
     f.syntax_dirty = true;
+    if (std.mem.endsWith(u8, f.path, "/+Pager")) f.pager_diff = looksLikeDiff(new[0..@min(new.len, 64 * 1024)]);
+}
+
+/// Whether `head`, a text's first bytes, holds a diff: a `diff` header, or
+/// a unified hunk's `---`, `+++` and `@@` lines.
+pub fn looksLikeDiff(head: []const u8) bool {
+    if (std.mem.startsWith(u8, head, "diff ") or std.mem.indexOf(u8, head, "\ndiff ") != null) return true;
+    const minus = std.mem.indexOf(u8, head, "--- ") orelse return false;
+    const plus = std.mem.indexOfPos(u8, head, minus, "\n+++ ") orelse return false;
+    return std.mem.indexOfPos(u8, head, plus, "\n@@ ") != null;
+}
+
+test "a +Pager pane holding a diff is drawn and looked at as one; holding other text, as text" {
+    const p = try Pardes.init(std.testing.allocator, .{ .tty_only = true });
+    defer p.deinit();
+    const pane = try p.setTestFile("");
+    const f = &pane.file.?;
+    p.gpa.free(f.path);
+    f.path = try p.gpa.dupe(u8, "/tmp/+Pager");
+    setContent(p, f, try p.gpa.dupe(u8, "commit abc\n\ndiff --git a/x b/x\n--- a/x\n+++ b/x\n@@ -1 +1 @@\n-a\n+b\n"));
+    try std.testing.expect(pane.colorAlgo() == .diff);
+    try std.testing.expect(pardes.look.diffLines(p, pane) != null);
+    setContent(p, f, try p.gpa.dupe(u8, "commit abc\n\n    a plain log\n"));
+    try std.testing.expect(pane.colorAlgo() != .diff);
+}
+
+test "a pager's text is a diff when it holds a diff header or a unified hunk" {
+    try std.testing.expect(looksLikeDiff("commit abc\nAuthor: x\n\n    msg\n\ndiff --git a/f b/f\n"));
+    try std.testing.expect(looksLikeDiff("--- a/f\n+++ b/f\n@@ -1 +1 @@\n-a\n+b\n"));
+    try std.testing.expect(!looksLikeDiff("commit abc\nAuthor: x\n\n    plain log\n"));
 }
 
 pub fn restoreSnap(pane: *Pane, f: *State, snap: Snapshot) void {