path: root/src/ppu.c

#include <stdio.h>
#include "ppu.h"
#include <string.h>

// LCDC bits
#define LCDC_BG_EN     0x01
#define LCDC_OBJ_EN    0x02
#define LCDC_OBJ_SIZE  0x04
#define LCDC_BG_MAP    0x08
#define LCDC_TILE_DATA 0x10
#define LCDC_WIN_EN    0x20
#define LCDC_WIN_MAP   0x40
#define LCDC_LCD_EN    0x80

// STAT bits
#define STAT_LYC_INT   0x40
#define STAT_M2_INT    0x20
#define STAT_M1_INT    0x10
#define STAT_M0_INT    0x08
#define STAT_LYC_FLAG  0x04

void gb_hdma_hblank(GB *gb); // from gb.c

// DMG shades (greenish palette), index 0..3
static const u8 dmg_shades[4][3] = {
    {224, 248, 208}, {136, 192, 112}, {52, 104, 86}, {8, 24, 32}
};

void ppu_reset(GB *gb) {
    PPU *p = &gb->ppu;
    p->lcdc = 0x91;
    p->stat = 0x85;
    p->bgp = 0xFC;
    p->obp0 = 0xFF;
    p->obp1 = 0xFF;
    p->ly = 0;
    p->mode = 2;
    p->dots = 0;
    p->vram_bank = 0;
    memset(p->bg_pal, 0xFF, sizeof(p->bg_pal));
    memset(p->obj_pal, 0xFF, sizeof(p->obj_pal));
}

// ---- VRAM / OAM access ----
u8 ppu_read(GB *gb, u16 addr) {
    PPU *p = &gb->ppu;
    if (addr >= 0x8000 && addr < 0xA000)
        return p->vram[p->vram_bank * 0x2000 + (addr - 0x8000)];
    if (addr >= 0xFE00 && addr < 0xFEA0)
        return p->oam[addr - 0xFE00];
    return 0xFF;
}
void ppu_write(GB *gb, u16 addr, u8 val) {
    PPU *p = &gb->ppu;
    if (addr >= 0x8000 && addr < 0xA000)
        p->vram[p->vram_bank * 0x2000 + (addr - 0x8000)] = val;
    else if (addr >= 0xFE00 && addr < 0xFEA0)
        p->oam[addr - 0xFE00] = val;
}

static inline u8 vram_at(PPU *p, int bank, u16 addr) {
    return p->vram[bank * 0x2000 + (addr - 0x8000)];
}

// ---- registers ----
u8 ppu_read_reg(GB *gb, u16 addr) {
    PPU *p = &gb->ppu;
    switch (addr) {
        case 0xFF40: return p->lcdc;
        case 0xFF41: return p->stat | 0x80;
        case 0xFF42: return p->scy;
        case 0xFF43: return p->scx;
        case 0xFF44: return p->ly;
        case 0xFF45: return p->lyc;
        case 0xFF47: return p->bgp;
        case 0xFF48: return p->obp0;
        case 0xFF49: return p->obp1;
        case 0xFF4A: return p->wy;
        case 0xFF4B: return p->wx;
        case 0xFF68: return p->bcps;
        case 0xFF69: return p->bg_pal[p->bcps & 0x3F];
        case 0xFF6A: return p->ocps;
        case 0xFF6B: return p->obj_pal[p->ocps & 0x3F];
    }
    return 0xFF;
}

static void check_lyc(GB *gb) {
    PPU *p = &gb->ppu;
    if (p->ly == p->lyc) p->stat |= STAT_LYC_FLAG;
    else p->stat &= ~STAT_LYC_FLAG;
}

void ppu_write_reg(GB *gb, u16 addr, u8 val) {
    PPU *p = &gb->ppu;
    switch (addr) {
        case 0xFF40: {
            bool was_on = p->lcdc & LCDC_LCD_EN;
            p->lcdc = val;
            if (was_on && !(val & LCDC_LCD_EN)) {
                p->ly = 0; p->dots = 0; p->mode = 0;
                p->stat &= ~0x03;
            } else if (!was_on && (val & LCDC_LCD_EN)) {
                p->mode = 2; p->dots = 0; p->ly = 0;
                check_lyc(gb);
            }
            break;
        }
        case 0xFF41: p->stat = (p->stat & 0x07) | (val & 0x78); break;
        case 0xFF42: p->scy = val; break;
        case 0xFF43: p->scx = val; break;
        case 0xFF44: break; // LY read-only
        case 0xFF45: p->lyc = val; check_lyc(gb); break;
        case 0xFF47: p->bgp = val; break;
        case 0xFF48: p->obp0 = val; break;
        case 0xFF49: p->obp1 = val; break;
        case 0xFF4A: p->wy = val; break;
        case 0xFF4B: p->wx = val; break;
        case 0xFF68: p->bcps = val & 0xBF; break;
        case 0xFF69:
            p->bg_pal[p->bcps & 0x3F] = val;
            if (p->bcps & 0x80) p->bcps = 0x80 | ((p->bcps + 1) & 0x3F);
            break;
        case 0xFF6A: p->ocps = val & 0xBF; break;
        case 0xFF6B:
            p->obj_pal[p->ocps & 0x3F] = val;
            if (p->ocps & 0x80) p->ocps = 0x80 | ((p->ocps + 1) & 0x3F);
            break;
    }
}

// ---- palette helpers ----
static void cgb_color(u8 *pal, int palnum, int cidx, u8 out[3]) {
    int i = (palnum * 4 + cidx) * 2;
    u16 c = pal[i] | (pal[i+1] << 8);
    int r = c & 0x1F, g = (c >> 5) & 0x1F, b = (c >> 10) & 0x1F;
    // scale 5-bit to 8-bit with a mild color-correction curve
    out[0] = (r * 13 + g * 2 + b) >> 1;
    out[1] = (g * 3 + b) << 1;
    out[2] = (r * 3 + g * 2 + b * 11) >> 1;
}

// ---- scanline rendering ----
static void render_scanline(GB *gb) {
    PPU *p = &gb->ppu;
    int ly = p->ly;
    if (ly >= SCREEN_H) return;

    u8 bg_prio[SCREEN_W];    // BG color index (for sprite priority)
    u8 bg_hasprio[SCREEN_W]; // CGB BG-to-OAM priority bit

    bool cgb = gb->cgb_mode;
    bool bg_master = (p->lcdc & LCDC_BG_EN) || cgb; // in CGB, bit0 = master priority

    // ----- Background & Window -----
    for (int x = 0; x < SCREEN_W; x++) {
        bg_prio[x] = 0;
        bg_hasprio[x] = 0;

        bool use_win = (p->lcdc & LCDC_WIN_EN) && (ly >= p->wy) && (x + 7 >= p->wx);
        int px, py;
        u16 map_base;
        if (use_win) {
            px = x + 7 - p->wx;
            py = p->wly;
            map_base = (p->lcdc & LCDC_WIN_MAP) ? 0x9C00 : 0x9800;
        } else {
            px = (x + p->scx) & 0xFF;
            py = (ly + p->scy) & 0xFF;
            map_base = (p->lcdc & LCDC_BG_MAP) ? 0x9C00 : 0x9800;
        }
        int tx = px / 8, ty = py / 8;
        u16 map_addr = map_base + ty * 32 + tx;
        u8 tile = vram_at(p, 0, map_addr);

        u8 attr = cgb ? vram_at(p, 1, map_addr) : 0;
        int tbank = (attr & 0x08) ? 1 : 0;
        bool xflip = attr & 0x20, yflip = attr & 0x40;
        int palnum = attr & 0x07;
        bool prio = attr & 0x80;

        int row = py % 8, col = px % 8;
        if (yflip) row = 7 - row;
        if (xflip) col = 7 - col;

        u16 tile_addr;
        if (p->lcdc & LCDC_TILE_DATA)
            tile_addr = 0x8000 + tile * 16 + row * 2;
        else
            tile_addr = 0x9000 + ((s8)tile) * 16 + row * 2;

        u8 lo = vram_at(p, tbank, tile_addr);
        u8 hi = vram_at(p, tbank, tile_addr + 1);
        int bit = 7 - col;
        int cidx = ((lo >> bit) & 1) | (((hi >> bit) & 1) << 1);

        bg_prio[x] = cidx;
        bg_hasprio[x] = prio;

        u8 rgb[3];
        if (!bg_master) {
            rgb[0] = dmg_shades[0][0]; rgb[1] = dmg_shades[0][1]; rgb[2] = dmg_shades[0][2];
        } else if (cgb) {
            cgb_color(p->bg_pal, palnum, cidx, rgb);
        } else {
            int shade = (p->bgp >> (cidx * 2)) & 3;
            memcpy(rgb, dmg_shades[shade], 3);
        }
        memcpy(p->fb[ly][x], rgb, 3);
    }
    if ((p->lcdc & LCDC_WIN_EN) && ly >= p->wy && p->wx < 167)
        p->wly++;

    // ----- Sprites -----
    if (!(p->lcdc & LCDC_OBJ_EN)) return;
    int h = (p->lcdc & LCDC_OBJ_SIZE) ? 16 : 8;

    // gather up to 10 sprites on this line
    int chosen[10], n = 0;
    for (int i = 0; i < 40 && n < 10; i++) {
        int sy = p->oam[i*4] - 16;
        if (ly >= sy && ly < sy + h) chosen[n++] = i;
    }
    // draw back-to-front: lower priority first.
    // DMG: smaller X = higher priority (drawn last). CGB: lower OAM index = higher.
    for (int a = 0; a < n; a++) {
        for (int b = a + 1; b < n; b++) {
            bool swap;
            if (cgb) swap = chosen[b] < chosen[a];
            else {
                int xa = p->oam[chosen[a]*4+1], xb = p->oam[chosen[b]*4+1];
                swap = (xb > xa) || (xb == xa && chosen[b] < chosen[a]);
            }
            if (swap) { int t = chosen[a]; chosen[a] = chosen[b]; chosen[b] = t; }
        }
    }

    for (int k = 0; k < n; k++) {
        int i = chosen[k];
        int sy = p->oam[i*4] - 16;
        int sx = p->oam[i*4+1] - 8;
        u8 tile = p->oam[i*4+2];
        u8 attr = p->oam[i*4+3];
        bool xflip = attr & 0x20, yflip = attr & 0x40;
        bool behind = attr & 0x80;
        if (h == 16) tile &= 0xFE;

        int row = ly - sy;
        if (yflip) row = h - 1 - row;
        int tbank = (cgb && (attr & 0x08)) ? 1 : 0;

        u16 tile_addr = 0x8000 + tile * 16 + row * 2;
        u8 lo = vram_at(p, tbank, tile_addr);
        u8 hi = vram_at(p, tbank, tile_addr + 1);

        for (int col = 0; col < 8; col++) {
            int x = sx + col;
            if (x < 0 || x >= SCREEN_W) continue;
            int bit = xflip ? col : 7 - col;
            int cidx = ((lo >> bit) & 1) | (((hi >> bit) & 1) << 1);
            if (cidx == 0) continue; // transparent

            // priority resolution
            if (cgb) {
                // BG master priority (LCDC.0) forces BG over OBJ when set
                if ((p->lcdc & LCDC_BG_EN)) {
                    if ((behind || bg_hasprio[x]) && bg_prio[x] != 0) continue;
                }
            } else {
                if (behind && bg_prio[x] != 0) continue;
            }

            u8 rgb[3];
            if (cgb) {
                cgb_color(p->obj_pal, attr & 0x07, cidx, rgb);
            } else {
                u8 pal = (attr & 0x10) ? p->obp1 : p->obp0;
                int shade = (pal >> (cidx * 2)) & 3;
                memcpy(rgb, dmg_shades[shade], 3);
            }
            memcpy(p->fb[ly][x], rgb, 3);
        }
    }
}

// ---- STAT interrupt line (edge-triggered) ----
static void update_stat(GB *gb, bool lyc_check) {
    PPU *p = &gb->ppu;
    static bool prev_line = false;
    if (lyc_check) check_lyc(gb);

    bool line = false;
    switch (p->mode) {
        case 0: if (p->stat & STAT_M0_INT) line = true; break;
        case 1: if (p->stat & STAT_M1_INT) line = true; break;
        case 2: if (p->stat & STAT_M2_INT) line = true; break;
    }
    if ((p->stat & STAT_LYC_INT) && (p->stat & STAT_LYC_FLAG)) line = true;

    if (line && !prev_line) gb_request_interrupt(gb, INT_LCDSTAT);
    prev_line = line;
}

void ppu_tick(GB *gb, int tcycles) {
    PPU *p = &gb->ppu;
    if (!(p->lcdc & LCDC_LCD_EN)) return;

    for (int t = 0; t < tcycles; t++) {
        p->dots++;

        switch (p->mode) {
        case 2: // OAM scan
            if (p->dots >= 80) { p->mode = 3; p->stat = (p->stat & ~3) | 3; }
            break;
        case 3: // drawing
            if (p->dots >= 80 + 172) {
                p->mode = 0;
                p->stat = (p->stat & ~3);
                render_scanline(gb);
                gb_hdma_hblank(gb);
                update_stat(gb, false);
            }
            break;
        case 0: // hblank
            if (p->dots >= 456) {
                p->dots -= 456;
                p->ly++;
                if (p->ly == 144) {
                    p->mode = 1;
                    p->stat = (p->stat & ~3) | 1;
                    p->wly = 0;
                    gb_request_interrupt(gb, INT_VBLANK);
                    p->frame_ready = true;
                    update_stat(gb, true);
                } else {
                    p->mode = 2;
                    p->stat = (p->stat & ~3) | 2;
                    update_stat(gb, true);
                }
            }
            break;
        case 1: // vblank
            if (p->dots >= 456) {
                p->dots -= 456;
                p->ly++;
                if (p->ly > 153) {
                    p->ly = 0;
                    p->mode = 2;
                    p->stat = (p->stat & ~3) | 2;
                }
                update_stat(gb, true);
            }
            break;
        }
    }
}