feat(video): track background and sprite mosaic state

Add vertical mosaic counters and background/sprite sampling regressions.

src/video/mosaic.rs

@@ -0,0 +1,190 @@
+//! Vertical mosaic phase for row capture. Size writes preserve four-bit counters.
+//! Background phase follows the displayed row. OBJ phase runs one row ahead
+//! for sprite preparation. Per-access size latching is not modeled.
+
+use crate::display::{CYCLES_PER_FRAME, CYCLES_PER_LINE, VISIBLE_LINES};
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct VerticalMosaic {
+    pub background: u8,
+    pub object: u8,
+    started: bool,
+}
+
+impl VerticalMosaic {
+    pub fn advance(&mut self, phase: u32, cycles: u32, sizes: u16) {
+        if cycles == 0 {
+            return;
+        }
+        let object_height = u32::from(sizes >> 12) + 1;
+        if !self.started {
+            // Reset has no preceding row227. Row0 uses phase0; row1 uses the next phase.
+            self.object = u8::from(object_height != 1);
+            self.started = true;
+        }
+        self.background = progress(
+            phase,
+            cycles,
+            self.background,
+            u32::from((sizes >> 4) & 15) + 1,
+        )
+        .counter;
+        self.object = progress(
+            (phase + CYCLES_PER_LINE) % CYCLES_PER_FRAME,
+            cycles,
+            self.object,
+            object_height,
+        )
+        .counter;
+    }
+}
+
+pub(super) struct Progress {
+    pub counter: u8,
+    /// Number of zero transitions since the last frame reload, if any.
+    pub resets: u32,
+    pub first_reset: bool,
+    pub frame_reloaded: bool,
+    pub crossed_line: bool,
+}
+
+/// Summarize fixed-size line transitions without iterating over lines or frames.
+/// A frame wrap discards earlier history because affine origins reload at row0.
+pub(super) fn progress(phase: u32, cycles: u32, counter: u8, height: u32) -> Progress {
+    debug_assert!(counter < 16 && (1..=16).contains(&height));
+    let end = u64::from(phase) + u64::from(cycles);
+    let wrapped = end >= u64::from(CYCLES_PER_FRAME);
+    let first = if wrapped { 0 } else { phase / CYCLES_PER_LINE };
+    let last = (end % u64::from(CYCLES_PER_FRAME)) as u32 / CYCLES_PER_LINE;
+    let initial = if wrapped { 0 } else { u32::from(counter) };
+    let first_reset = first < VISIBLE_LINES
+        && (first == VISIBLE_LINES - 1 || initial + 1 == height || initial == 15);
+    // Normal increments enter rows1..159. Entry to row160 unconditionally resets.
+    let lines = last.min(VISIBLE_LINES - 1) - first.min(VISIBLE_LINES - 1);
+    let until_zero = if initial < height {
+        height - initial
+    } else {
+        16 - initial
+    };
+    let (mut current, mut resets) = if lines < until_zero {
+        (initial + lines, 0)
+    } else {
+        let remaining = lines - until_zero;
+        (remaining % height, 1 + remaining / height)
+    };
+    if first < VISIBLE_LINES && last >= VISIBLE_LINES {
+        current = 0;
+        resets += 1;
+    }
+    Progress {
+        counter: current as u8,
+        resets,
+        first_reset,
+        frame_reloaded: wrapped,
+        crossed_line: wrapped || first != last,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn every_counter_height_and_visible_line_matches_iterated_transitions() {
+        for counter in 0..16 {
+            for height in 1..=16 {
+                for first in 0..160 {
+                    let mut expected = counter;
+                    let mut resets = 0;
+                    for last in first + 1..=160 {
+                        let next = expected + 1;
+                        expected = if last == 160 || next == height {
+                            0
+                        } else {
+                            next & 15
+                        };
+                        resets += u32::from(expected == 0);
+                        let p = progress(
+                            first * CYCLES_PER_LINE,
+                            (last - first) * CYCLES_PER_LINE,
+                            counter as u8,
+                            height,
+                        );
+                        assert_eq!(
+                            u32::from(p.counter),
+                            expected,
+                            "counter={counter} height={height} first={first} last={last}"
+                        );
+                        assert_eq!(p.resets, resets);
+                        assert_eq!(
+                            p.first_reset,
+                            first == 159 || counter + 1 == height || counter == 15
+                        );
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn random_clock_batches_and_maximum_batch_match_line_iteration() {
+        let mut seed = 0x3712_bc09u32;
+        for case in 0..500 {
+            let mut next = || {
+                seed = seed.wrapping_mul(1664525).wrapping_add(1013904223);
+                seed
+            };
+            let phase = next() % CYCLES_PER_FRAME;
+            let cycles = if case == 0 {
+                u32::MAX
+            } else {
+                next() % (7 * CYCLES_PER_FRAME)
+            };
+            let height = next() % 16 + 1;
+            let counter = if phase / CYCLES_PER_LINE >= 160 {
+                0
+            } else {
+                (next() & 15) as u8
+            };
+            let mut expected = counter;
+            let mut resets = 0;
+            for boundary in u64::from(phase / CYCLES_PER_LINE) + 1
+                ..=(u64::from(phase) + u64::from(cycles)) / u64::from(CYCLES_PER_LINE)
+            {
+                let row = boundary % 228;
+                if row == 0 {
+                    expected = 0;
+                    resets = 0;
+                } else if row <= 160 {
+                    let value = u32::from(expected) + 1;
+                    expected = if row == 160 || value == height {
+                        0
+                    } else {
+                        (value & 15) as u8
+                    };
+                    resets += u32::from(expected == 0);
+                }
+            }
+            let p = progress(phase, cycles, counter, height);
+            assert_eq!(p.counter, expected, "phase={phase} cycles={cycles}");
+            assert_eq!(p.resets, resets);
+        }
+    }
+
+    #[test]
+    fn size_shrink_preserves_counter_until_four_bit_wrap_and_zero_cycles_do_nothing() {
+        let mut mosaic = VerticalMosaic::default();
+        mosaic.advance(0, 6 * CYCLES_PER_LINE, 0x3070); // BG8, OBJ4.
+        assert_eq!((mosaic.background, mosaic.object), (6, 3)); // OBJ prepares row7.
+        mosaic.advance(6 * CYCLES_PER_LINE, 0, 0);
+        assert_eq!((mosaic.background, mosaic.object), (6, 3));
+        mosaic.advance(6 * CYCLES_PER_LINE, 9 * CYCLES_PER_LINE, 0x1010); // BG2, OBJ2.
+        assert_eq!((mosaic.background, mosaic.object), (15, 12));
+        mosaic.advance(15 * CYCLES_PER_LINE, CYCLES_PER_LINE, 0x1010);
+        assert_eq!((mosaic.background, mosaic.object), (0, 13));
+        mosaic.advance(16 * CYCLES_PER_LINE, 144 * CYCLES_PER_LINE, 0xffff);
+        assert_eq!((mosaic.background, mosaic.object), (0, 0));
+        mosaic.advance(160 * CYCLES_PER_LINE, 68 * CYCLES_PER_LINE, 0xffff);
+        assert_eq!((mosaic.background, mosaic.object), (0, 1)); // Row1 phase, row0 already prepared.
+    }
+}

tests/mosaic.rs

@@ -0,0 +1,47 @@
+use gba_rust::{
+    io::*,
+    memory::{Memory, MemoryError, OAM_START as OAM, PALETTE_START as PAL, VRAM_START as VRAM},
+    video::{rgb555_to_rgb888 as rgb, Framebuffer, HEIGHT, WIDTH},
+};
+
+#[path = "mosaic/backgrounds.rs"]
+mod backgrounds;
+#[path = "mosaic/demo.rs"]
+mod demo;
+#[path = "mosaic/objects.rs"]
+mod objects;
+
+fn memory() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    for index in 0..128 {
+        m.write16(OAM + index * 8, 0x200).unwrap();
+    }
+    for index in 0..256 {
+        let color = (index & 31) | (((index >> 3) & 31) << 5) | (((index * 7) & 31) << 10);
+        m.write16(PAL + index * 2, color as u16).unwrap();
+        m.write16(PAL + 0x200 + index * 2, color as u16).unwrap();
+    }
+    m
+}
+
+fn render(m: &Memory) -> Framebuffer {
+    let mut f = Framebuffer::default();
+    m.render_frame(&mut f).unwrap();
+    f
+}
+
+fn object(m: &mut Memory, index: u32, a: u16, b: u16, c: u16) {
+    m.write16(OAM + index * 8, a).unwrap();
+    m.write16(OAM + index * 8 + 2, b).unwrap();
+    m.write16(OAM + index * 8 + 4, c).unwrap();
+}
+
+fn fill(m: &mut Memory, address: u32, bytes: u32, value: u16) {
+    for offset in (0..bytes).step_by(2) {
+        m.write16(address + offset, value).unwrap();
+    }
+}
+
+fn at(frame: &Framebuffer, x: usize, y: usize) -> u32 {
+    frame.pixels()[y * WIDTH + x]
+}

tests/mosaic/backgrounds.rs

@@ -0,0 +1,212 @@
+use super::*;
+
+fn bitmap(mode: u16, page: u16) -> Memory {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x400 | mode | (page * 16)).unwrap();
+    m.write16(BG2PA, 256).unwrap();
+    m.write16(BG2PD, 256).unwrap();
+    let (w, h) = if mode == 5 { (160, 128) } else { (240, 160) };
+    for y in 0..h {
+        for x in 0..w {
+            let offset = (y * w + x) as u32;
+            let index = (x * 3 + y * 7) as u16 & 255;
+            if mode == 4 {
+                if x % 2 == 0 {
+                    let next = ((x + 1) * 3 + y * 7) as u16 & 255;
+                    m.write16(VRAM + page as u32 * 0xa000 + offset, index | (next << 8))
+                        .unwrap();
+                }
+            } else {
+                let color =
+                    (x as u16 & 31) | ((y as u16 & 31) << 5) | (((x + y) as u16 & 31) << 10);
+                m.write16(VRAM + page as u32 * 0xa000 + offset * 2, color)
+                    .unwrap();
+            }
+        }
+    }
+    m
+}
+
+#[test]
+fn write_only_register_merges_bytes_and_ignores_upper_word_half() {
+    let mut m = bitmap(3, 0);
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write32(MOSAIC, 0xffff_0034).unwrap(); // BG 5x4, OBJ1x1.
+    assert_eq!(m.read32(MOSAIC).unwrap(), 0);
+    m.write8(MOSAIC + 1, 0xff).unwrap(); // Must retain BG dimensions.
+    let f = render(&m);
+    assert_eq!(at(&f, 4, 3), at(&f, 0, 0));
+    assert_ne!(at(&f, 5, 4), at(&f, 0, 0));
+    m.write8(MOSAIC, 0).unwrap();
+    assert_ne!(at(&render(&m), 1, 0), at(&f, 0, 0));
+    assert_eq!(m.read16(MOSAIC + 2).unwrap(), 0);
+    assert_eq!(
+        m.read8(MOSAIC + 0x10000),
+        Err(MemoryError::Unmapped(MOSAIC + 0x10000))
+    );
+    assert_eq!(
+        m.write16(MOSAIC + 1, 0),
+        Err(MemoryError::Unaligned(MOSAIC + 1))
+    );
+    assert_eq!(m.cycles(), 0);
+}
+
+#[test]
+fn every_bg_size_and_bitmap_page_repeats_screen_grid_samples() {
+    for mode in [3, 4, 5] {
+        for page in 0..=u16::from(mode != 3) {
+            let mut m = bitmap(mode, page);
+            let base = render(&m);
+            m.write16(BG2CNT, 0x40).unwrap();
+            // Exercise every field value, including asymmetric blocks and partial LCD blocks.
+            for w in 1..=16 {
+                let h = 17 - w;
+                m.write16(MOSAIC, ((h - 1) * 16 + w - 1) as u16).unwrap();
+                let f = render(&m);
+                for y in 0..HEIGHT {
+                    for x in 0..WIDTH {
+                        assert_eq!(
+                            at(&f, x, y),
+                            at(&base, x / w * w, y / h * h),
+                            "mode={mode} page={page} size={w}x{h} at={x},{y}"
+                        );
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn mosaic_flag_is_independent_of_dimensions_and_one_by_one_is_identity() {
+    let mut m = bitmap(3, 0);
+    let base = render(&m);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    assert_eq!(render(&m).pixels(), base.pixels()); // Flag is off.
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write16(MOSAIC, 0xff00).unwrap(); // OBJ fields do not affect BG.
+    assert_eq!(render(&m).pixels(), base.pixels());
+    m.write16(MOSAIC, 0x000f).unwrap();
+    let f = render(&m);
+    assert_eq!(at(&f, 15, 1), at(&base, 0, 1));
+    assert_ne!(at(&f, 15, 1), at(&base, 0, 0));
+}
+
+#[test]
+fn text_mosaic_samples_before_scroll_and_tile_flips_in_both_depths() {
+    for eight_bit in [false, true] {
+        let mut m = memory();
+        m.write16(DISPCNT, 0x100).unwrap();
+        let control = 0x1000 | if eight_bit { 0x80 } else { 0 };
+        m.write16(BG0CNT, control).unwrap();
+        m.write16(BG0HOFS, 7).unwrap();
+        m.write16(BG0VOFS, 5).unwrap();
+        fill(&mut m, VRAM + 0x8000, 0x800, 0x0c00); // Both tile flips.
+        for y in 0..8 {
+            for x in (0..8).step_by(if eight_bit { 2 } else { 4 }) {
+                let mut value = 0;
+                for i in 0..if eight_bit { 2 } else { 4 } {
+                    let index = ((x + i + y * 3) % 15) as u16;
+                    value |= index << (i * if eight_bit { 8 } else { 4 });
+                }
+                m.write16(
+                    VRAM + (y * if eight_bit { 8 } else { 4 } + x / if eight_bit { 1 } else { 2 })
+                        as u32,
+                    value,
+                )
+                .unwrap();
+            }
+        }
+        let base = render(&m);
+        m.write16(BG0CNT, control | 0x40).unwrap();
+        m.write16(MOSAIC, 0x54).unwrap(); // 5x6 crosses tile boundaries.
+        let f = render(&m);
+        for y in 0..HEIGHT {
+            for x in 0..WIDTH {
+                assert_eq!(at(&f, x, y), at(&base, x / 5 * 5, y / 6 * 6));
+            }
+        }
+    }
+}
+
+#[test]
+fn affine_mosaic_samples_screen_positions_before_signed_transform() {
+    for mode in [1, 2, 3, 4, 5] {
+        let mut m = if mode >= 3 { bitmap(mode, 0) } else { memory() };
+        m.write16(DISPCNT, 0x400 | mode).unwrap();
+        let control = if mode <= 2 { 0x3000 } else { 0 };
+        m.write16(BG2CNT, control).unwrap();
+        if mode <= 2 {
+            for offset in (0..64).step_by(2) {
+                m.write16(VRAM + offset, (offset as u16 * 0x101).wrapping_add(0x100))
+                    .unwrap();
+            }
+        }
+        for (reg, value) in [
+            (BG2PA, 181),
+            (BG2PB, 91),
+            (BG2PC, (-181i16) as u16),
+            (BG2PD, 256),
+        ] {
+            m.write16(reg, value).unwrap();
+        }
+        m.write32(BG2X, (-769i32) as u32).unwrap();
+        m.write32(BG2Y, 31 * 256 + 127).unwrap();
+        let base = render(&m);
+        m.write16(BG2CNT, control | 0x40).unwrap();
+        m.write16(MOSAIC, 0x76).unwrap();
+        let f = render(&m);
+        for y in 0..HEIGHT {
+            for x in 0..WIDTH {
+                assert_eq!(
+                    at(&f, x, y),
+                    at(&base, x / 7 * 7, y / 8 * 8),
+                    "mode={mode} pixel={x},{y}"
+                );
+            }
+        }
+    }
+}
+
+#[test]
+fn independent_background_flags_preserve_transparency_and_priority() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x300).unwrap();
+    m.write16(BG0CNT, 0x1040).unwrap();
+    m.write16(BG1CNT, 0x1105).unwrap();
+    m.write16(PAL + 2, 31).unwrap();
+    m.write16(PAL + 4, 0x3e0).unwrap();
+    fill(&mut m, VRAM, 32, 0x1010);
+    fill(&mut m, VRAM + 0x4000, 32, 0x2222);
+    m.write16(MOSAIC, 2).unwrap();
+    let f = render(&m);
+    for x in 0..18 {
+        assert_eq!(at(&f, x, 0), rgb(if x / 3 % 2 == 0 { 0x3e0 } else { 31 }));
+    }
+    m.write16(BG0CNT, 0x1000).unwrap();
+    m.write16(BG1CNT, 0x1145).unwrap();
+    let f = render(&m);
+    for x in 0..18 {
+        assert_eq!(at(&f, x, 0), rgb(if x % 2 == 0 { 0x3e0 } else { 31 }));
+    }
+}
+
+#[test]
+fn windows_and_color_effects_use_output_coordinate_not_mosaic_sample() {
+    let mut m = bitmap(3, 0);
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write16(MOSAIC, 0x77).unwrap();
+    m.write16(DISPCNT, 0x2403).unwrap();
+    m.write16(WIN0H, 0x0306).unwrap();
+    m.write16(WIN0V, 0x0008).unwrap();
+    m.write16(WININ, 4).unwrap();
+    m.write16(WINOUT, 0x24).unwrap();
+    m.write16(BLDCNT, 0x84).unwrap();
+    m.write16(BLDY, 16).unwrap();
+    let f = render(&m);
+    for x in 0..8 {
+        assert_eq!(at(&f, x, 0), if (3..6).contains(&x) { 0 } else { 0xffffff });
+    }
+    m.write16(DISPCNT, 0xffff).unwrap();
+    assert!(render(&m).pixels().iter().all(|&p| p == 0xffffff));
+}

tests/mosaic/demo.rs

@@ -0,0 +1,100 @@
+use super::*;
+use gba_rust::{
+    input::{Button, Buttons},
+    mosaic_demo::{MosaicDemo, MOSAIC_STATE},
+    tile_demo::TileDemo,
+};
+
+#[test]
+fn cpu_demo_cycles_all_sizes_and_wraps_without_host_video_writes() {
+    let mut demo = MosaicDemo::new().unwrap();
+    let mut frame = Framebuffer::default();
+    assert_eq!(demo.machine().memory().read16(DISPCNT).unwrap(), 0);
+    assert_eq!(demo.machine().memory().read16(VRAM).unwrap(), 0);
+    for update in 1..=129 {
+        demo.frame(Buttons::default(), &mut frame).unwrap();
+        let m = demo.machine().memory();
+        assert_eq!(m.read32(MOSAIC_STATE).unwrap(), update);
+        assert_eq!(m.read16(DISPCNT).unwrap(), 0x1340);
+        assert_eq!(m.read16(BG0CNT).unwrap(), 0x1844);
+        assert_eq!(m.read16(BG1CNT).unwrap(), 0xd0c1);
+        assert_eq!(m.read16(OAM).unwrap(), 0x1048);
+        assert_eq!(m.read32(MOSAIC).unwrap(), 0);
+        assert_eq!(m.display_position().scanline, 160);
+        let size = (1 + (update / 8) % 16) as usize;
+        // Check terrain and foreground away from the sprite for every size.
+        for y in 0..48 {
+            for x in 0..WIDTH {
+                let sx = x / size * size;
+                let sy = y / size * size;
+                let cross = sx / 8 % 4 == 1 && sy / 8 % 4 == 1 && (sx % 8 == 3 || sy % 8 == 3);
+                let color = if cross {
+                    0x3ff
+                } else {
+                    [0x260, 0x3a0, 0x7d20, 0x7e80]
+                        [((sx / 32 + sy / 32) % 2) * 2 + (sx / 2 + sy / 2) % 2]
+                };
+                assert_eq!(at(&frame, x, y), rgb(color), "update={update} at={x},{y}");
+            }
+        }
+    }
+}
+
+#[test]
+fn both_bypasses_restore_original_scene_with_scroll_rotation_zoom_and_reset() {
+    let mut demo = MosaicDemo::new().unwrap();
+    let mut base = TileDemo::new().unwrap();
+    let mut f = Framebuffer::default();
+    let mut reference = Framebuffer::default();
+    for i in 0..32 {
+        let buttons = Buttons::default()
+            .with(Button::Left, i < 16)
+            .with(Button::Down, true)
+            .with(Button::Start, i == 31)
+            .with(Button::L, (8..24).contains(&i))
+            .with(Button::R, (16..28).contains(&i));
+        base.frame(buttons, &mut reference).unwrap();
+        demo.frame(buttons.with(Button::A, true).with(Button::B, true), &mut f)
+            .unwrap();
+        assert_eq!(f.pixels(), reference.pixels());
+        let m = demo.machine().memory();
+        assert_ne!(m.read16(OAM).unwrap() & 0x1000, 0);
+        assert_eq!(m.read16(OAM + 2).unwrap() & 0x1000, 0); // Z no longer flips.
+        assert_eq!(m.read16(OAM + 4).unwrap() & 0xc00, 0); // X no longer lowers priority.
+        assert_eq!(
+            m.read32(MOSAIC_STATE + 4).unwrap(),
+            base.machine().memory().read32(MOSAIC_STATE + 4).unwrap()
+        );
+    }
+    assert_eq!(demo.machine().memory().read32(MOSAIC_STATE + 4).unwrap(), 0);
+    assert_eq!(demo.machine().memory().read32(MOSAIC_STATE + 8).unwrap(), 0);
+}
+
+#[test]
+fn dma_word_write_sets_bg_and_obj_fields_while_unused_halfword_ignores_writes() {
+    use gba_rust::{
+        cpu::Cpu,
+        dma::DMA_BASE,
+        machine::{Machine, StepKind},
+    };
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1140).unwrap();
+    m.write16(BG0CNT, 0x1040).unwrap();
+    fill(&mut m, VRAM, 32, 0x1212);
+    m.write16(PAL + 4, 31).unwrap();
+    m.write16(PAL + 2, 0x3e0).unwrap();
+    object(&mut m, 0, 0x1000, 0, 0);
+    fill(&mut m, VRAM + 0x10000, 32, 0x0101);
+    m.write16(PAL + 0x202, 0x7c00).unwrap();
+    m.write32(0x0200_0000, 0xffff_0302).unwrap(); // BG3x1, OBJ4x1.
+    m.write32(DMA_BASE + 36, 0x0200_0000).unwrap();
+    m.write32(DMA_BASE + 40, MOSAIC).unwrap();
+    m.write32(DMA_BASE + 44, 0x8400_0001).unwrap();
+    let mut machine = Machine::new(Cpu::new(0x0300_0000), m);
+    assert_eq!(machine.step().unwrap(), StepKind::Dma { channel: 3 });
+    let f = render(machine.memory());
+    assert_eq!(at(&f, 3, 0), rgb(0x7c00));
+    assert_eq!(at(&f, 8, 0), rgb(31)); // BG samples x6, not x8's independent phase.
+    assert_eq!(at(&f, 9, 0), rgb(0x3e0));
+    assert_eq!(machine.memory().read32(MOSAIC).unwrap(), 0);
+}

tests/mosaic/objects.rs

@@ -0,0 +1,215 @@
+use super::*;
+
+fn scene(a: u16, b: u16) -> Memory {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    object(&mut m, 0, a | 7, b | 5 | 0x4000, 0); // 16x16 at (5,7).
+    for y in 0..16 {
+        for x in (0..16).step_by(2) {
+            let tile = y / 8 * 2 + x / 8;
+            let address = VRAM + 0x10000 + tile * 64 + y % 8 * 8 + x % 8;
+            let first = 1 + (x + y * 5) % 254;
+            let second = 1 + (x + 1 + y * 5) % 254;
+            m.write16(address, (first | second << 8) as u16).unwrap();
+        }
+    }
+    m
+}
+
+#[test]
+fn regular_obj_uses_screen_grid_clamps_partial_first_blocks_and_preserves_flips() {
+    for flips in [0, 0x1000, 0x2000, 0x3000] {
+        let mut m = scene(0x2000, flips);
+        let base = render(&m);
+        m.write16(OAM, 0x3007).unwrap(); // 8bpp plus mosaic, y7.
+        for width in 1..=16 {
+            let height = 17 - width;
+            m.write16(MOSAIC, (((height - 1) << 12) | ((width - 1) << 8)) as u16)
+                .unwrap();
+            let f = render(&m);
+            for y in 0..32 {
+                for x in 0..32 {
+                    let expected = if (5..21).contains(&x) && (7..23).contains(&y) {
+                        at(
+                            &base,
+                            (x / width * width).max(5),
+                            (y / height * height).max(7),
+                        )
+                    } else {
+                        0
+                    };
+                    assert_eq!(
+                        at(&f, x, y),
+                        expected,
+                        "flip={flips:x} size={width}x{height} at={x},{y}"
+                    );
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn affine_and_double_size_mosaic_repeat_transformed_samples() {
+    for doubled in [false, true] {
+        let mut m = scene(0x2100 | if doubled { 0x200 } else { 0 }, 0);
+        let matrix: [i16; 4] = if doubled {
+            [64, 32, -32, 64]
+        } else {
+            [128, 64, -64, 128]
+        };
+        for (i, value) in matrix.into_iter().enumerate() {
+            m.write16(OAM + 6 + i as u32 * 8, value as u16).unwrap();
+        }
+        let base = render(&m);
+        m.write16(OAM, 0x3107 | if doubled { 0x200 } else { 0 })
+            .unwrap();
+        m.write16(MOSAIC, 0x4500).unwrap(); // OBJ6x5; BG1x1.
+        let f = render(&m);
+        let size = if doubled { 32 } else { 16 };
+        for y in 0..48 {
+            for x in 0..48 {
+                let expected = if (5..5 + size).contains(&x) && (7..7 + size).contains(&y) {
+                    at(&base, (x / 6 * 6).max(5), (y / 5 * 5).max(7))
+                } else {
+                    0
+                };
+                assert_eq!(at(&f, x, y), expected, "double={doubled} at={x},{y}");
+            }
+        }
+    }
+}
+
+#[test]
+fn obj_flag_and_high_byte_are_independent_and_vertical_sampling_wraps_at_screen_top() {
+    let mut m = scene(0x2000, 0);
+    let base = render(&m);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    assert_eq!(render(&m).pixels(), base.pixels()); // Flag disabled.
+    m.write16(OAM, 0x3007).unwrap();
+    m.write8(MOSAIC + 1, 0).unwrap(); // BG15 bits do not affect OBJ.
+    assert_eq!(render(&m).pixels(), base.pixels());
+    object(&mut m, 0, 0x20fc, 0x41fc, 0); // 16x16, wraps from (-4,-4).
+    let base = render(&m);
+    m.write16(OAM, 0x30fc).unwrap();
+    m.write16(MOSAIC, 0x3400).unwrap();
+    let f = render(&m);
+    for y in 0..12 {
+        for x in 0..12 {
+            assert_eq!(at(&f, x, y), at(&base, x / 5 * 5, y / 4 * 4));
+        }
+    }
+    assert_eq!(at(&f, 12, 0), 0);
+    assert_eq!(at(&f, 0, 12), 0);
+}
+
+#[test]
+fn transparent_samples_repeat_but_non_mosaic_samples_restart_the_latch() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    m.write16(MOSAIC, 0x0300).unwrap(); // OBJ4x1.
+    m.write16(PAL + 0x202, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1010); // First sample transparent, second red.
+    object(&mut m, 0, 0x1000, 0, 0);
+    assert!(render(&m).pixels().iter().all(|&p| p == 0));
+    m.write16(VRAM + 0x10000, 0x0101).unwrap();
+    let f = render(&m);
+    for x in 0..4 {
+        assert_eq!(at(&f, x, 0), rgb(31));
+    }
+    // A lower-priority non-mosaic sprite fills transparent texels before mosaic.
+    // Its non-mosaic metadata makes those pixels latch immediately.
+    fill(&mut m, VRAM + 0x10020, 32, 0x2222);
+    m.write16(PAL + 0x204, 0x3e0).unwrap();
+    object(&mut m, 1, 0, 0, 0x401);
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), rgb(31));
+    assert_eq!(at(&f, 1, 0), rgb(0x3e0));
+}
+
+#[test]
+fn horizontal_latch_restarts_for_non_mosaic_and_higher_priority_pixels() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1040).unwrap();
+    m.write16(MOSAIC, 0x0f00).unwrap();
+    for (tile, color) in [(0, 31), (1, 0x3e0), (2, 0x7c00)] {
+        fill(
+            &mut m,
+            VRAM + 0x10000 + tile * 32,
+            32,
+            (tile as u16 + 1) * 0x1111,
+        );
+        m.write16(PAL + 0x202 + tile * 2, color).unwrap();
+    }
+    object(&mut m, 0, 0x1000, 1, 0x800);
+    object(&mut m, 1, 0x1000, 3, 0x401);
+    object(&mut m, 2, 0, 5, 2);
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), 0);
+    assert_eq!(at(&f, 1, 0), rgb(31));
+    assert_eq!(at(&f, 3, 0), rgb(0x3e0));
+    assert_eq!(at(&f, 5, 0), rgb(0x7c00));
+    assert_eq!(at(&f, 13, 0), 0); // Leaving the canvas resets the latch.
+}
+
+#[test]
+fn transparent_higher_priority_obj_changes_metadata_without_replacing_color() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x1140).unwrap();
+    m.write16(BG0CNT, 0x1001).unwrap(); // BG priority1.
+    fill(&mut m, VRAM, 32, 0x3333);
+    m.write16(PAL + 6, 0x7c00).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1111);
+    m.write16(PAL + 0x202, 31).unwrap();
+    object(&mut m, 0, 0x400, 0, 0x800); // Semi-transparent red, priority2.
+    object(&mut m, 1, 0x1000, 0, 0x401); // Transparent tile1, priority1, mosaic.
+    fill(&mut m, VRAM + 0x10040, 32, 0x2222);
+    object(&mut m, 2, 0, 0, 0x402); // Same priority as metadata; cannot replace red.
+    m.write16(MOSAIC, 0x0300).unwrap();
+    m.write16(BLDCNT, 0x100).unwrap();
+    m.write16(BLDALPHA, 0x0808).unwrap();
+    assert_eq!(at(&render(&m), 0, 0), rgb(0x3c0f)); // Alpha flag retained, priority now ties BG.
+}
+
+#[test]
+fn object_window_ignores_mosaic_and_window_edges_do_not_quantize() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x9100).unwrap();
+    m.write16(BG0CNT, 0x1000).unwrap();
+    fill(&mut m, VRAM, 32, 0x1111);
+    m.write16(PAL + 2, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x1010);
+    object(&mut m, 0, 0x1800, 0, 0);
+    m.write16(MOSAIC, 0xffff).unwrap();
+    m.write16(WINOUT, 0x0100).unwrap();
+    let f = render(&m);
+    for x in 0..8 {
+        assert_eq!(at(&f, x, 0), if x % 2 == 0 { 0 } else { rgb(31) });
+    }
+    m.write16(VRAM + 0x10004, 0x0101).unwrap(); // Next source row remains distinct.
+    assert_eq!(at(&render(&m), 0, 1), rgb(31));
+}
+
+#[test]
+fn mosaic_latch_precedes_window_masking_and_semi_transparent_blending() {
+    let mut m = memory();
+    m.write16(DISPCNT, 0x3100).unwrap();
+    m.write16(BG0CNT, 0x1000).unwrap();
+    fill(&mut m, VRAM, 32, 0x2222);
+    m.write16(PAL + 4, 0x7c00).unwrap();
+    m.write16(PAL + 0x202, 31).unwrap();
+    fill(&mut m, VRAM + 0x10000, 32, 0x0101);
+    object(&mut m, 0, 0x1400, 0, 0);
+    m.write16(MOSAIC, 0x0300).unwrap();
+    m.write16(WIN0H, 0x0001).unwrap();
+    m.write16(WIN0V, 0x0008).unwrap();
+    m.write16(WININ, 1).unwrap(); // First pixel masks OBJ but must not reset its latch.
+    m.write16(WINOUT, 0x31).unwrap();
+    m.write16(BLDCNT, 0x100).unwrap();
+    m.write16(BLDALPHA, 0x0808).unwrap();
+    let f = render(&m);
+    assert_eq!(at(&f, 0, 0), rgb(0x7c00));
+    for x in 1..4 {
+        assert_eq!(at(&f, x, 0), rgb(0x3c0f));
+    }
+}

tests/mosaic_tracking.rs

@@ -0,0 +1,351 @@
+use gba_rust::{
+    cpu::Cpu,
+    display::{
+        CYCLES_PER_FRAME as FRAME, CYCLES_PER_LINE as LINE, HBLANK_START as HB, VBLANK_START as VB,
+    },
+    dma::DMA_BASE,
+    io::*,
+    machine::Machine,
+    memory::{Memory, OAM_START as OAM, PALETTE_START as PAL, ROM_START, VRAM_START as VRAM},
+    video::{rgb555_to_rgb888 as rgb, Framebuffer, HEIGHT, WIDTH},
+};
+
+fn color(row: usize) -> u16 {
+    ((row % 64 + 1) * 0x101) as u16
+}
+
+fn scene() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    m.set_scanline_rendering(true);
+    m.write16(DISPCNT, 0x1140).unwrap();
+    m.write16(BG0CNT, 0x10c0).unwrap(); // 8bpp, mosaic, map block16.
+    for index in 0..64 {
+        m.write16(PAL + (index + 1) * 2, color(index as usize))
+            .unwrap();
+        m.write16(PAL + 0x200 + (index + 1) * 2, color(index as usize))
+            .unwrap();
+    }
+    // Eight text tiles, each with a distinct color per row. The map repeats vertically.
+    for y in 0..64 {
+        for x in (0..8).step_by(2) {
+            m.write16(VRAM + y * 8 + x, (y as u16 + 1) * 0x101).unwrap();
+        }
+    }
+    for y in 0..32 {
+        for x in 0..32 {
+            m.write16(VRAM + 0x8000 + (y * 32 + x) * 2, (y % 8) as u16)
+                .unwrap();
+        }
+    }
+    for index in 0..128 {
+        m.write16(OAM + index * 8, 0x200).unwrap();
+    }
+    m.write16(OAM, 0x3000).unwrap(); // Mosaic 8bpp sprite at (32,0), 16x16.
+    m.write16(OAM + 2, 0x4020).unwrap();
+    for y in 0..16 {
+        for x in (0..16).step_by(2) {
+            let tile = y / 8 * 2 + x / 8;
+            m.write16(
+                VRAM + 0x10000 + tile * 64 + y % 8 * 8 + x % 8,
+                (y as u16 + 1) * 0x101,
+            )
+            .unwrap();
+        }
+    }
+    m
+}
+
+fn bitmap() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    m.set_scanline_rendering(true);
+    m.write16(DISPCNT, 0x403).unwrap();
+    m.write16(BG2CNT, 0x40).unwrap();
+    m.write16(BG2PA, 256).unwrap();
+    m.write16(BG2PD, 256).unwrap();
+    for y in 0..160 {
+        for x in 0..240 {
+            m.write16(VRAM + (y * 240 + x) * 2, color(y as usize))
+                .unwrap();
+        }
+    }
+    m
+}
+
+fn present(m: &Memory) -> Framebuffer {
+    let mut f = Framebuffer::default();
+    assert!(m.present_frame(&mut f).unwrap());
+    f
+}
+
+fn finish(m: &mut Memory) -> Framebuffer {
+    let p = m.display_position();
+    m.advance_cycles(VB - u32::from(p.scanline) * LINE - u32::from(p.line_cycle));
+    present(m)
+}
+
+fn at(f: &Framebuffer, x: usize, y: usize) -> u32 {
+    f.pixels()[y * WIDTH + x]
+}
+
+#[test]
+fn all_static_heights_match_snapshots_with_independent_background_and_sprite_sizes() {
+    for height in 1..=16 {
+        let mut m = scene();
+        m.write16(MOSAIC, ((height - 1) << 4) | ((16 - height) << 12))
+            .unwrap();
+        let mut expected = Framebuffer::default();
+        m.render_frame(&mut expected).unwrap();
+        assert_eq!(
+            finish(&mut m).pixels(),
+            expected.pixels(),
+            "height={height}"
+        );
+    }
+}
+
+#[test]
+fn growing_a_block_preserves_counter_instead_of_realigning_to_screen_y() {
+    let mut m = scene();
+    m.write16(MOSAIC, 0x30).unwrap(); // BG4, OBJ1.
+    m.advance_cycles(5 * LINE + HB);
+    m.write8(MOSAIC, 0x70).unwrap(); // BG8: phase1 remains phase1.
+    let f = finish(&mut m);
+    for y in 0..24 {
+        let source = if y < 4 {
+            0
+        } else if y < 12 {
+            4
+        } else if y < 20 {
+            12
+        } else {
+            20
+        };
+        assert_eq!(at(&f, 0, y), rgb(color(source)), "row={y}");
+        if y < 16 {
+            assert_eq!(at(&f, 32, y), rgb(color(y)));
+        }
+    }
+    let mut snapshot = Framebuffer::default();
+    m.render_frame(&mut snapshot).unwrap();
+    assert_eq!(at(&snapshot, 0, 8), rgb(color(8)));
+    assert_eq!(at(&f, 0, 8), rgb(color(4)));
+    assert_eq!(present(&m).pixels(), f.pixels());
+}
+
+#[test]
+fn shrinking_below_current_phase_waits_for_four_bit_wrap() {
+    let mut m = scene();
+    m.write16(MOSAIC, 0x70).unwrap();
+    m.advance_cycles(6 * LINE);
+    m.write8(MOSAIC, 0x10).unwrap(); // Phase6 > new height2; no immediate reset.
+    m.advance_cycles(0);
+    let f = finish(&mut m);
+    for y in 0..24 {
+        let source = if y < 16 { 0 } else { y / 2 * 2 };
+        assert_eq!(at(&f, 0, y), rgb(color(source)), "row={y}");
+    }
+}
+
+#[test]
+fn affine_mosaic_advances_by_current_height_when_counter_wraps_not_by_elapsed_rows() {
+    let mut m = bitmap();
+    m.write16(MOSAIC, 0x70).unwrap();
+    m.advance_cycles(6 * LINE + HB);
+    m.write8(MOSAIC, 0x10).unwrap();
+    let f = finish(&mut m);
+    for y in 0..24 {
+        let source = if y < 16 { 0 } else { 2 + (y - 16) / 2 * 2 };
+        assert_eq!(at(&f, 0, y), rgb(color(source)), "row={y}");
+    }
+}
+
+#[test]
+fn affine_reference_writes_and_coefficients_do_not_reset_shared_mosaic_phase() {
+    let mut m = bitmap();
+    m.write16(MOSAIC, 0x70).unwrap();
+    m.advance_cycles(5 * LINE);
+    m.write32(BG2Y, 20 * 256).unwrap();
+    m.write16(BG2PD, 2 * 256).unwrap();
+    m.write8(MOSAIC, 0x10).unwrap();
+    let f = finish(&mut m);
+    for y in 0..20 {
+        let source = if y < 5 {
+            0
+        } else if y < 16 {
+            20
+        } else {
+            24 + (y - 16) / 2 * 4
+        };
+        assert_eq!(at(&f, 0, y), rgb(color(source)), "row={y}");
+    }
+}
+
+#[test]
+fn mosaic_flags_and_display_enable_do_not_gate_counters() {
+    for disabled in [false, true] {
+        let mut m = scene();
+        m.write16(MOSAIC, 0x7070).unwrap();
+        m.write16(BG0CNT, 0x1080).unwrap();
+        m.write16(OAM, 0x2000).unwrap();
+        if disabled {
+            m.write16(DISPCNT, 0).unwrap();
+        }
+        m.advance_cycles(6 * LINE);
+        m.write16(MOSAIC, 0x1010).unwrap();
+        m.write16(BG0CNT, 0x10c0).unwrap();
+        m.write16(OAM, 0x3000).unwrap();
+        m.write16(DISPCNT, 0x1140).unwrap();
+        let f = finish(&mut m);
+        for y in 6..16 {
+            assert_eq!(at(&f, 0, y), rgb(color(0)));
+            // Row6 was prepared on row5, before the attribute/enable write.
+            let source = if y == 6 && !disabled { 6 } else { 0 };
+            assert_eq!(at(&f, 32, y), rgb(color(source)));
+        }
+    }
+}
+
+#[test]
+fn forced_blank_does_not_pause_counters() {
+    let mut m = scene();
+    m.write16(MOSAIC, 0x7070).unwrap();
+    m.write16(DISPCNT, 0x11c0).unwrap();
+    m.advance_cycles(6 * LINE);
+    m.write16(MOSAIC, 0x1010).unwrap();
+    m.write16(DISPCNT, 0x1140).unwrap();
+    let f = finish(&mut m);
+    assert!(f.pixels()[..6 * WIDTH].iter().all(|&p| p == 0xffffff));
+    for y in 6..16 {
+        assert_eq!(at(&f, 0, y), rgb(color(0)));
+    }
+}
+
+#[test]
+fn sprite_counter_is_independent_and_clamps_partial_first_blocks_before_transform() {
+    for affine in [false, true] {
+        let mut m = scene();
+        m.write16(OAM, if affine { 0x3103 } else { 0x3003 })
+            .unwrap(); // y3.
+        if affine {
+            m.write16(OAM + 6, 256).unwrap();
+            m.write16(OAM + 30, 256).unwrap();
+        }
+        m.write16(MOSAIC, 0x7030).unwrap(); // BG4, OBJ8.
+        m.advance_cycles(6 * LINE);
+        m.write8(MOSAIC + 1, 0x10).unwrap(); // OBJ2 only.
+        let f = finish(&mut m);
+        for y in 0..24 {
+            assert_eq!(at(&f, 0, y), rgb(color(y / 4 * 4)));
+            let source = if !(3..19).contains(&y) {
+                y / 4 * 4
+            } else if y < 16 {
+                0
+            } else if y < 18 {
+                13
+            } else {
+                15
+            };
+            assert_eq!(at(&f, 32, y), rgb(color(source)), "affine={affine} row={y}");
+        }
+    }
+}
+
+#[test]
+fn object_window_ignores_live_mosaic_phase() {
+    let mut m = scene();
+    m.write16(DISPCNT, 0x9140).unwrap();
+    m.write16(WINOUT, 0x0100).unwrap(); // Show BG only inside OBJ window.
+    m.write16(BG0CNT, 0x1080).unwrap();
+    m.write16(OAM, 0x3800).unwrap(); // Mosaic-enabled window sprite.
+    for y in (1..16).step_by(2) {
+        for x in (0..16).step_by(2) {
+            let tile = y / 8 * 2 + x / 8;
+            m.write16(VRAM + 0x10000 + tile * 64 + y % 8 * 8 + x % 8, 0)
+                .unwrap();
+        }
+    }
+    m.write16(MOSAIC, 0x7000).unwrap();
+    m.advance_cycles(6 * LINE);
+    m.write16(MOSAIC, 0x1000).unwrap();
+    let f = finish(&mut m);
+    for y in 0..24 {
+        assert_eq!(
+            at(&f, 32, y),
+            if y < 16 && y % 2 == 0 {
+                rgb(color(y))
+            } else {
+                0
+            }
+        );
+    }
+}
+
+#[test]
+fn vertical_hold_uses_live_palette_but_sprite_vram_changes_wait_for_preparation() {
+    let mut m = scene();
+    m.write16(MOSAIC, 0x7070).unwrap();
+    m.advance_cycles(3 * LINE + HB);
+    m.write16(PAL + 2, 31).unwrap();
+    m.write16(VRAM + 0x10000, 0x0202).unwrap();
+    let f = finish(&mut m);
+    assert_eq!(at(&f, 0, 3), rgb(color(0)));
+    assert_eq!(at(&f, 0, 4), rgb(31));
+    assert_eq!(at(&f, 32, 3), rgb(color(0)));
+    assert_eq!(at(&f, 32, 4), rgb(color(0))); // Prepared before row3 HBlank.
+    assert_eq!(at(&f, 32, 5), rgb(color(1)));
+}
+
+#[test]
+fn hblank_dma_size_writes_preserve_phase_and_affect_next_line_transition() {
+    let mut m = scene();
+    m.write16(MOSAIC, 0x7070).unwrap();
+    for y in 0..160 {
+        m.write16(0x0200_0000 + y * 2, if y < 6 { 0x7070 } else { 0x3010 })
+            .unwrap();
+    }
+    m.write32(DMA_BASE, 0x0200_0000).unwrap();
+    m.write32(DMA_BASE + 4, MOSAIC).unwrap();
+    m.write32(DMA_BASE + 8, 0xa240_0001).unwrap();
+    m.write8(HALTCNT, 0).unwrap();
+    let mut machine = Machine::new(Cpu::new(ROM_START), m);
+    machine.run_until_vblank(1000).unwrap();
+    let f = present(machine.memory());
+    // Independent recurrence, driven by the table value written after each row.
+    let (mut bg, mut obj) = (0usize, 0usize);
+    for y in 0..HEIGHT {
+        assert_eq!(at(&f, 0, y), rgb(color(y - bg)), "row={y}");
+        if y < 16 {
+            assert_eq!(at(&f, 32, y), rgb(color(y - obj)));
+        }
+        let (bg_height, obj_height) = if y < 6 { (8, 8) } else { (2, 4) };
+        bg = if bg + 1 == bg_height {
+            0
+        } else {
+            (bg + 1) % 16
+        };
+        obj = if obj + 1 == obj_height {
+            0
+        } else {
+            (obj + 1) % 16
+        };
+    }
+    assert!(machine.halted());
+}
+
+#[test]
+fn vblank_resets_counters_and_capture_toggles_do_not_reset_them() {
+    let mut m = scene();
+    m.set_scanline_rendering(false);
+    m.write16(MOSAIC, 0x7070).unwrap();
+    m.advance_cycles(6 * LINE);
+    m.write16(MOSAIC, 0x1010).unwrap();
+    m.advance_cycles(VB - 6 * LINE);
+    m.write16(MOSAIC, 0x5050).unwrap(); // New size during VBlank.
+    m.advance_cycles(FRAME - VB);
+    m.set_scanline_rendering(true);
+    let f = finish(&mut m);
+    for y in 0..16 {
+        assert_eq!(at(&f, 0, y), rgb(color(y / 6 * 6)));
+        assert_eq!(at(&f, 32, y), rgb(color(y / 6 * 6)));
+    }
+}