Denise: horizontal DIW comparator flip-flop model

src/bus/tests.rs

@@ -3447,7 +3447,42 @@ fn beam_timed_display_window_changes_clip_later_bitplane_rows() {
 }
 
 #[test]
-fn beam_timed_display_window_clips_later_bitplane_pixels_on_same_line() {
+fn hblank_tail_color_write_paints_following_row_from_left_edge() {
+    // A copper COLOR00 write in the horizontal-blank tail (hpos < 0x12)
+    // belongs to the previous output row's invisible tail; the following
+    // row must show the new colour from its first framebuffer column.
+    let mut bus = empty_bus();
+    bus.denise.diwstrt = 0x2C81;
+    bus.denise.diwstop = 0x2DC1;
+    bus.denise.bplcon0 = 0x0200;
+    bus.denise.palette.write_ocs(0, 0x0000);
+    bus.current_frame_render_base = bus.capture_render_snapshot();
+    bus.current_frame_render_events.push(BeamRegisterWrite {
+        vpos: RENDER_VISIBLE_START_VPOS + 20,
+        hpos: 0x06,
+        offset: 0x180,
+        value: 0x0F00,
+        source: BeamWriteSource::Copper,
+    });
+    bus.current_frame_render_events.push(BeamRegisterWrite {
+        vpos: RENDER_VISIBLE_START_VPOS + 20,
+        hpos: 0xDE,
+        offset: 0x180,
+        value: 0x0333,
+        source: BeamWriteSource::Copper,
+    });
+
+    let mut fb = vec![0; FB_PIXELS];
+    bitplane::render(&mut bus, &mut fb);
+
+    let row = 20 * FB_WIDTH;
+    assert_eq!(fb[row], rgb12_to_rgba8(0x0F00), "x=0");
+    assert_eq!(fb[row + 8], rgb12_to_rgba8(0x0F00), "x=8");
+    assert_eq!(fb[row + 20], rgb12_to_rgba8(0x0F00), "x=20");
+}
+
+#[test]
+fn beam_timed_diwstrt_rewrite_after_window_open_does_not_reclip_line() {
     let mut bus = empty_bus();
     bus.agnus.dmacon = DMACON_DMAEN | DMACON_BPLEN;
     bus.denise.diwstrt = 0x2C81;
@@ -3483,8 +3518,11 @@ fn beam_timed_display_window_clips_later_bitplane_pixels_on_same_line() {
     let mut fb = vec![0; FB_PIXELS];
     bitplane::render(&mut bus, &mut fb);
 
+    // The window flip-flop opened at the original HSTART before the
+    // rewrite reached the comparators; a later HSTART only re-matches an
+    // already-open window, so the line shows bitplanes continuously.
     assert_eq!(fb[68], rgb12_to_rgba8(0x0F00));
-    assert_eq!(fb[106], rgb12_to_rgba8(0x0000));
+    assert_eq!(fb[106], rgb12_to_rgba8(0x0F00));
     assert_eq!(fb[108], rgb12_to_rgba8(0x0F00));
 }
 
@@ -3566,9 +3604,14 @@ fn beam_timed_diwstrt_extends_later_bitplane_pixels_left_on_same_line() {
     let mut fb = vec![0; FB_PIXELS];
     bitplane::render(&mut bus, &mut fb);
 
-    assert_eq!(fb[94], rgb12_to_rgba8(0x0000));
-    assert_eq!(fb[96], rgb12_to_rgba8(0x0F00));
+    // The rewrite reaches the comparators before the new HSTART's match
+    // position, so the window opens there (not at the write position) and
+    // shows bitplanes up to the fetched row's end.
+    assert_eq!(fb[64], rgb12_to_rgba8(0x0000));
+    assert_eq!(fb[68], rgb12_to_rgba8(0x0F00));
+    assert_eq!(fb[94], rgb12_to_rgba8(0x0F00));
     assert_eq!(fb[132], rgb12_to_rgba8(0x0F00));
+    assert_eq!(fb[160], rgb12_to_rgba8(0x0000));
 }
 
 #[test]