Stabilize wavediff output for buffered file2-only changes

src/diff.rs

@@ -370,12 +370,22 @@ fn compare_signal_channels<W: Write>(
     }
 
     // Anything still in the buffer was in file2 but never matched by file1.
-    for ((time, handle), value) in &buffered2 {
+    // Sort rows before printing so diff output does not depend on HashMap
+    // iteration order.
+    let owned: Vec<((u64, usize), OwnedSignalValue)> = buffered2.drain().collect();
+    if !owned.is_empty() {
         has_differences = true;
+    }
+    let mut file2_only_rows: Vec<(u64, String, usize)> = Vec::new();
+    for (idx, ((time, handle), _)) in owned.iter().enumerate() {
         for name in format_handle_names(*handle, &hier2.signal_map, &hier2.names) {
-            writeln!(writer, "{} {} {} (only in file2)", time, name, value)?;
+            file2_only_rows.push((*time, name, idx));
         }
     }
+    file2_only_rows.sort_by(|a, b| a.0.cmp(&b.0).then_with(|| a.1.cmp(&b.1)));
+    for (time, name, idx) in file2_only_rows {
+        writeln!(writer, "{} {} {} (only in file2)", time, name, owned[idx].1)?;
+    }
 
     // Drain any remaining file2 batches we never read from the channel.
     if !source2_ended {

tests/data/buffered_file2_base.vcd

@@ -0,0 +1,14 @@
+$timescale 1 ns $end
+$scope module t $end
+$var wire 1 ! a $end
+$var wire 1 " b $end
+$var wire 1 # c $end
+$upscope $end
+$enddefinitions $end
+$dumpvars
+0!
+0"
+0#
+$end
+#20
+1!

tests/data/buffered_file2_extra.vcd

@@ -0,0 +1,18 @@
+$timescale 1 ns $end
+$scope module t $end
+$var wire 1 ! a $end
+$var wire 1 " b $end
+$var wire 1 # c $end
+$upscope $end
+$enddefinitions $end
+$dumpvars
+0!
+0"
+0#
+$end
+#10
+1!
+1"
+1#
+#20
+1!

tests/data/expected/buffered_file2_base.cat

@@ -0,0 +1,4 @@
+0 t.a 0
+0 t.b 0
+0 t.c 0
+20 t.a 1

tests/data/expected/buffered_file2_base.names

@@ -0,0 +1,3 @@
+t.a
+t.b
+t.c

tests/data/expected/buffered_file2_base.vcd.attrs

@@ -0,0 +1,3 @@
+t.a  wire  1
+t.b  wire  1
+t.c  wire  1

tests/data/expected/buffered_file2_extra.cat

@@ -0,0 +1,7 @@
+0 t.a 0
+0 t.b 0
+0 t.c 0
+10 t.a 1
+10 t.b 1
+10 t.c 1
+20 t.a 1

tests/data/expected/buffered_file2_extra.names

@@ -0,0 +1,3 @@
+t.a
+t.b
+t.c

tests/data/expected/buffered_file2_extra.vcd.attrs

@@ -0,0 +1,3 @@
+t.a  wire  1
+t.b  wire  1
+t.c  wire  1

tests/diff_test.rs

@@ -137,6 +137,22 @@ fn test_edge_time_diff() {
     assert_eq!(output, expected, "Expected exact diff output");
 }
 
+#[test]
+fn test_buffered_file2_only_diffs_are_sorted() {
+    let (has_diff, output) = run_wave_diff_test(
+        "tests/data/buffered_file2_base.vcd",
+        "tests/data/buffered_file2_extra.vcd",
+    );
+    assert!(has_diff, "file2-only intermediate changes should differ");
+
+    let expected = "\
+10 t.a 1 (only in file2)
+10 t.b 1 (only in file2)
+10 t.c 1 (only in file2)
+";
+    assert_eq!(output, expected, "Expected deterministic file2-only diff output");
+}
+
 #[test]
 fn test_new_sig_diff() {
     let (has_name_diff, msg) = check_signal_names(