1 files changed, 83 insertions, 0 deletions
diff --git a/cart/cart.go b/cart/cart.go
new file mode 100644
index 0000000..2438c1b
--- /dev/null
+++ b/cart/cart.go
@@ -0,0 +1,83 @@
+package cart
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+)
+
+type Cartridge struct {
+	PRG       []uint8
+	CHR       []uint8
+	CHRIsRAM  bool
+	MapperID  uint8
+	Mirroring Mirroring
+	Battery   bool
+
+	Mapper Mapper
+}
+
+func Load(path string) (*Cartridge, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return Parse(data)
+}
+
+func Parse(data []byte) (*Cartridge, error) {
+	if len(data) < 16 || !bytes.Equal(data[:4], []byte("NES\x1A")) {
+		return nil, errors.New("cart: not an iNES file")
+	}
+
+	prgSize := int(data[4]) * 16 * 1024
+	chrSize := int(data[5]) * 8 * 1024
+	flags6, flags7 := data[6], data[7]
+
+	isNES2 := flags7&0x0C == 0x08
+	mapperID := flags6 >> 4
+	if isNES2 || bytes.Equal(data[12:16], []byte{0, 0, 0, 0}) {
+		mapperID |= flags7 & 0xF0
+	}
+
+	c := &Cartridge{
+		MapperID: mapperID,
+		Battery:  flags6&0x02 != 0,
+	}
+
+	switch {
+	case flags6&0x08 != 0:
+		c.Mirroring = FourScreen
+	case flags6&0x01 != 0:
+		c.Mirroring = Vertical
+	default:
+		c.Mirroring = Horizontal
+	}
+
+	offset := 16
+	if flags6&0x04 != 0 {
+		offset += 512
+	}
+	if len(data) < offset+prgSize+chrSize {
+		return nil, fmt.Errorf("cart: file truncated: need %d bytes, have %d",
+			offset+prgSize+chrSize, len(data))
+	}
+
+	c.PRG = data[offset : offset+prgSize]
+	offset += prgSize
+
+	if chrSize == 0 {
+		c.CHR = make([]uint8, 8*1024)
+		c.CHRIsRAM = true
+	} else {
+		c.CHR = data[offset : offset+chrSize]
+	}
+
+	m, err := newMapper(c)
+	if err != nil {
+		return nil, err
+	}
+	c.Mapper = m
+	return c, nil
+}