Add sprite 0 hit logic

src/cart.c

@@ -58,7 +58,7 @@ int nes_cart_load_mem(void* data, int size, nes* sys) {
             mem->ppu.bank[page] = mem->ppu.pal_bank;
         }
 
-        nes_PPU_set_mirroring(
+        nes_ppu_set_mirroring(
                 &mem->ppu, hdr->flags_6 & ines_Flag_Vert_Mirror
             );
 

src/nes.c

@@ -51,13 +51,14 @@ static int nes_hsync(nes* sys) {
 
     sys->ppu.scanline++;
     if (nes_ppu_frame_end_line == sys->ppu.scanline) {
-        sys->ppu.scanline = 0;
+        sys->ppu.scanline = nes_ppu_prerender_line;
     }
 
     switch (sys->ppu.scanline) {
     case nes_ppu_prerender_line:
-        // TODO: Reset PPU Status
-        // TODO: Get Sprite 0 Hit
+        sys->core.memory.ppu.status &= ~(   ppu_Status_VBlank |
+                                            ppu_Status_Hit);
+        nes_ppu_find_hit_line(&sys->ppu, &sys->core.memory.ppu);
         break;
 
     case nes_ppu_postrender_line:
@@ -77,8 +78,18 @@ int nes_loop(nes* sys) {
     int dot = 0;
 
     while (0 == status) {
-        // TODO: Partial scanline if sprite 0 hit
         int dots_remaining = nes_ppu_scanline_dots - dot;
+        if (    sys->ppu.hit_line == sys->ppu.scanline &&
+                (sys->core.memory.ppu.mask & (ppu_Mask_Sprite | ppu_Mask_Back)) &&
+                !(sys->core.memory.ppu.status & ppu_Status_Hit)) {
+            int hit_dot = ((oam_sprite*)sys->core.memory.ppu.oam)->x;
+            if (dot >= hit_dot) {
+//                printf("Line %d sprite 0 hit @ %d\n", sys->ppu.hit_line, hit_dot);
+                sys->core.memory.ppu.status |= ppu_Status_Hit;
+            } else {
+                dots_remaining = hit_dot - dot;
+            }
+        }
         int cpu_cycles = (dots_remaining + 2) / 3;
         dot += 3 * f6502_step(&sys->core, cpu_cycles);
 

src/ppu.c

@@ -10,7 +10,7 @@ static const uint8_t mirror_schemes[][4] = {
 };
 
 
-void nes_PPU_set_mirroring(nes_PPU_Memory* mem,
+void nes_ppu_set_mirroring(nes_PPU_Memory* mem,
                            nes_Nametable_Mirroring mirror) {
     for (int i = 0; i < 4; ++i) {
         mem->bank[nes_PPU_Nametable_Bank_Index + i] =
@@ -18,3 +18,35 @@ void nes_PPU_set_mirroring(nes_PPU_Memory* mem,
     }
 }
 
+void nes_ppu_find_hit_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
+    int sprite_bank = !!(mem->ctrl & ppu_Control_Sprite_Bank);
+    oam_sprite sprite = *(oam_sprite*)mem->oam;
+    int sprite_height = (mem->ctrl & ppu_Control_Sprite_Size) ?
+                        16 : 8;
+    int stride = 1;
+    int y_off = 0;
+    if (sprite.attr & oam_Attr_Flip_Y) {
+        stride = -1;
+        y_off = sprite_height - 1;
+        sprite_bank = (sprite.index & 1);
+        sprite.index &= 0xFEU;
+    }
+
+    sprite_bank = (sprite_bank << 2) | (sprite.index >> 6);
+
+    const int data_off = ((sprite.index & 0x3FU) << 4) +
+                         ((y_off & 8) << 1) + (y_off & 7);
+    uint8_t* data = mem->bank[sprite_bank] + data_off;
+
+    ppu->hit_line = -1;
+
+    if (sprite.y + 1 <= nes_ppu_render_h && sprite.y > 0) {
+        for (int line = 0; line < sprite_height; ++line) {
+            if (data[0] | data[8]) {
+                ppu->hit_line = sprite.y + 1 + line + nes_ppu_visible_line;
+                break;
+            }
+            data += stride;
+        }
+    }
+}

src/ppu.h

@@ -4,6 +4,9 @@
 #include <stdint.h>
 
 
+#define nes_ppu_render_w    (256U)
+#define nes_ppu_render_h    (240U)
+
 #define nes_ppu_scanline_dots   (341U)
 #define nes_ppu_prerender_line  (0U)
 #define nes_ppu_visible_line    (1U)
@@ -11,6 +14,20 @@
 #define nes_ppu_vblank_line     (242U)
 #define nes_ppu_frame_end_line  (262U)
 
+typedef struct {
+    uint8_t y;
+    uint8_t index;
+    uint8_t attr;
+    uint8_t x;
+} oam_sprite;
+
+typedef enum {
+    oam_Attr_Pal_Mask   = 0b00000011,
+    oam_Attr_Background = 0b00100000,
+    oam_Attr_Flip_X     = 0b01000000,
+    oam_Attr_Flip_Y     = 0b10000000,
+} oam_Attribute;
+
 
 typedef enum {
     ppu_reg_ctrl = 0,
@@ -35,6 +52,17 @@ typedef enum {
     ppu_Control_VBlank          = 0b10000000,
 } nes_ppu_Control;
 
+typedef enum {
+    ppu_Mask_Greyscale      = 0b00000001,
+    ppu_Mask_Left_Back      = 0b00000010,
+    ppu_Mask_Left_Sprite    = 0b00000100,
+    ppu_Mask_Back           = 0b00001000,
+    ppu_Mask_Sprite         = 0b00010000,
+    ppu_Mask_More_Red       = 0b00100000,
+    ppu_Mask_More_Green     = 0b01000000,
+    ppu_Mask_More_Blue      = 0b10000000,
+} nes_ppu_Mask;
+
 typedef enum {
     ppu_Status_Open_Bus_Mask    = 0b00011111,
     ppu_Status_Overflow         = 0b00100000,
@@ -96,13 +124,15 @@ typedef enum {
     nes_Mirror_Four,
 } nes_Nametable_Mirroring;
 
-void nes_PPU_set_mirroring(nes_PPU_Memory* mem,
+void nes_ppu_set_mirroring(nes_PPU_Memory* mem,
                            nes_Nametable_Mirroring mirror);
 
-
 typedef struct {
     int scanline;
+    int hit_line;
 } nes_PPU;
 
+void nes_ppu_find_hit_line(nes_PPU*, nes_PPU_Memory*);
+
 
 #endif // NESE_PPU_H_