Add F-Zero and Phantasy Star Online patches

Based on code in GCMM.

Fixes #2

header.go

@@ -61,6 +61,18 @@ func (h *header) blocks() int {
 	return int(h.CardSize) << 4 //nolint:gomnd
 }
 
+func (h *header) serialNumbers() (uint32, uint32) {
+	buf := new(bytes.Buffer)
+	buf.Grow(binary.Size(h))
+
+	_ = binary.Write(buf, binary.BigEndian, h)
+
+	serial := make([]uint32, 8) //nolint:gomnd
+	_ = binary.Read(buf, binary.BigEndian, &serial)
+
+	return serial[0] ^ serial[2] ^ serial[4] ^ serial[6], serial[1] ^ serial[3] ^ serial[5] ^ serial[7]
+}
+
 func (h *header) generateChecksums() ([]byte, []byte, error) {
 	b, err := h.MarshalBinary()
 	if err != nil {

memcard.go

@@ -93,6 +93,10 @@ func (mc *memoryCard) count() int {
 	return count
 }
 
+func (mc *memoryCard) serialNumbers() (uint32, uint32) {
+	return mc.header.serialNumbers()
+}
+
 func (mc *memoryCard) checksum() error {
 	if err := mc.header.checksum(); err != nil {
 		return err

patches.go

@@ -0,0 +1,119 @@
+package gc
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+const (
+	lengthFZero = 0x8000
+	lengthPSO   = 0x6000
+)
+
+//nolint:gomnd
+func patchFZero(r io.Reader, mc *memoryCard) (io.Reader, error) {
+	serial1, serial2 := mc.serialNumbers()
+
+	b, err := io.ReadAll(r)
+	if err != nil {
+		return nil, fmt.Errorf("unable to read save data: %w", err)
+	}
+
+	if len(b) < lengthFZero {
+		return nil, errInvalidLength
+	}
+
+	b[0x2066] = byte(serial1 >> 24)
+	b[0x2067] = byte(serial1 >> 16)
+	b[0x7580] = byte(serial2 >> 24)
+	b[0x7581] = byte(serial2 >> 16)
+	b[0x2060] = byte(serial1 >> 8)
+	b[0x2061] = byte(serial1)
+	b[0x2200] = byte(serial2 >> 8)
+	b[0x2201] = byte(serial2)
+
+	var chksum uint16 = 0xffff
+
+	for i := 2; i < 0x8000; i++ {
+		chksum ^= uint16(b[i])
+		for j := 8; j > 0; j-- {
+			if chksum&1 == 1 {
+				chksum = (chksum >> 1) ^ 0x8408
+			} else {
+				chksum >>= 1
+			}
+		}
+	}
+
+	b[0x0000] = byte(^chksum >> 8)
+	b[0x0001] = byte(^chksum)
+
+	return bytes.NewReader(b), nil
+}
+
+const (
+	offsetPSO12 = 0x00
+	offsetPSO3  = 0x10
+)
+
+func patchPSO12(r io.Reader, mc *memoryCard) (io.Reader, error) {
+	return patchPSO(r, mc, offsetPSO12)
+}
+
+func patchPSO3(r io.Reader, mc *memoryCard) (io.Reader, error) {
+	return patchPSO(r, mc, offsetPSO3)
+}
+
+//nolint:gomnd
+func patchPSO(r io.Reader, mc *memoryCard, offset int) (io.Reader, error) {
+	serial1, serial2 := mc.serialNumbers()
+
+	b, err := io.ReadAll(r)
+	if err != nil {
+		return nil, fmt.Errorf("unable to read save data: %w", err)
+	}
+
+	if len(b) < lengthPSO {
+		return nil, errInvalidLength
+	}
+
+	b[0x2158] = byte(serial1 >> 24)
+	b[0x2159] = byte(serial1 >> 16)
+	b[0x215a] = byte(serial1 >> 8)
+	b[0x215b] = byte(serial1)
+	b[0x215c] = byte(serial2 >> 24)
+	b[0x215d] = byte(serial2 >> 16)
+	b[0x215e] = byte(serial2 >> 8)
+	b[0x215f] = byte(serial2)
+
+	lut := make([]uint32, 256)
+
+	for i := 0; i < 256; i++ {
+		chksum := uint32(i)
+		for j := 8; j > 0; j-- {
+			if chksum&1 == 1 {
+				chksum = (chksum >> 1) ^ 0xedb88320
+			} else {
+				chksum >>= 1
+			}
+		}
+
+		lut[i] = chksum
+	}
+
+	var chksum uint32 = 0xdebb20e3
+
+	for i := 0x204c; i < 0x2164+offset; i++ {
+		chksum = (chksum>>8)&0xffffff ^ lut[byte(chksum)^b[i]]
+	}
+
+	chksum ^= 0xffffffff
+
+	b[0x2048] = byte(chksum >> 24)
+	b[0x2049] = byte(chksum >> 16)
+	b[0x204a] = byte(chksum >> 8)
+	b[0x204b] = byte(chksum)
+
+	return bytes.NewReader(b), nil
+}

writer.go

@@ -36,6 +36,13 @@ func (w *fileWriter) Write(p []byte) (int, error) {
 	return w.buf.Write(p) //nolint:wrapcheck
 }
 
+var gamePatches = map[string]func(io.Reader, *memoryCard) (io.Reader, error){
+	"f_zero.dat":  patchFZero,
+	"PSO_SYSTEM":  patchPSO12,
+	"PSO3_SYSTEM": patchPSO3,
+}
+
+//nolint:cyclop,funlen
 func (w *fileWriter) Close() error {
 	w.w.mu.Lock()
 	defer w.w.mu.Unlock()
@@ -67,13 +74,25 @@ func (w *fileWriter) Close() error {
 		return errNoFreeSpace
 	}
 
+	var (
+		r   io.Reader = w.buf
+		err error
+	)
+
+	patchFunc, ok := gamePatches[e.filename()]
+	if ok {
+		if r, err = patchFunc(w.buf, mc); err != nil {
+			return err
+		}
+	}
+
 	// Set e.FirstBlock to the correct location
 	e.FirstBlock = mc.blockMap[mc.activeBlockMap()].LastAllocatedBlock + 1
 
 	// Write out the blocks
 	lastBlock := e.FirstBlock + e.FileLength - reservedBlocks
 	for i := e.FirstBlock - reservedBlocks; i < lastBlock; i++ {
-		_, _ = w.buf.Read(mc.blocks[i][:])
+		_, _ = r.Read(mc.blocks[i][:])
 
 		if i+1 < lastBlock {
 			mc.blockMap[mc.activeBlockMap()].Blocks[i] = i + reservedBlocks + 1