Sonic The Hedgehog (Game Gear) romhack: level select, modify level name, set number of lives

splash.py

#!/usr/bin/env python3

import sys
import pathlib

space = 0xEB
# fmt: off
alphabet_0_8 = [
    *range(0x34, 0x38),  # ABCD
    *range(0x44, 0x48),  # EFGH
    *range(0x40, 0x44),  # IJKL
    0x50, 0x51, 0x52,  # MNO
    0x60, 0x61, 0x62,  # POR
    0x70,  # S
    0x80, 0x81,  # TU
    0x54, # V
    *range(0x3C, 0x40) # WXYZ
]
alphabet_0_8_others = {
    "©": 0xCF,
    " ": space,
        }
# fmt: on
alphabet_9_17_base = 0x1E  # A
alphabet_9_17 = [
    alphabet_9_17_base + row + col for row in range(0, 193, 16) for col in range(2)
]
alphabet_9_17_others = {
    '"': 0xE9,
    "©": 0xAB,
    "-": 0xE8,
    " ": space,
}
message_base_offset = 0x118B  # Green Hill
message_next_len = 15
lives_offset = 0x137E


def main():
    if len(sys.argv) != 4:
        print(f"usage: {sys.argv[0]} <string 0-12 chars> <level 0-18> <lives 0-99>")
        return 1
    message = sys.argv[1]
    level = int(sys.argv[2])
    lives = int(sys.argv[3])
    assert level <= 18 and level >= 0
    assert lives < 100 and lives >= 0
    rom = bytearray(pathlib.Path("./sonic1.gg").read_bytes())

    base = message_base_offset + message_next_len * (level // 3)
    written = patch_level_name(rom, base, message, tile_alphabet(level))
    patch_level_selection(rom, level)
    rom[lives_offset] = lives

    pathlib.Path(f"./sonic-lvl{level}-{lives}lives-{written}.gg").write_bytes(rom)


def patch_level_name(rom, base, message, chars):
    i = 0
    written = ""
    for offset in range(12):
        if i == len(message):
            rom[base + offset] = space
        while i < len(message):
            c = message[i].upper()
            i += 1
            if c in chars:
                written += c
                rom[base + offset] = chars[c]
                break
            print(f"Ignored character {c} not in alphabet")
    if i < len(message):
        print(f"{len(message)-i} characters ignored: {message[i:]}")
    return written


def tile_alphabet(level):
    alphabet = {}
    source = alphabet_0_8
    if level > 8:
        source = alphabet_9_17
        alphabet.update(alphabet_9_17_others)
    else:
        alphabet.update(alphabet_0_8_others)
    for i in range(26):
        alphabet[chr(i + ord("A"))] = source[i]
    return alphabet


def patch_level_selection(rom, level):
    assert level >= 0 and level < 256
    patch = {
        # fmt: off
        0x138C: [
            0xCD, 0xE0, 0x7F,  # CALL $7FE0
            0x00,  # NOP
        ],
        0x7FE0: [
            0x3E, level,  # LD A, $level
            0x32, 0x38, 0xD2,   # LD ($D238), A
            0xAF,         # XOR A
            0xC9,         # RET
            ],
        # fmt: on
    }
    for k, v in patch.items():
        rom[k : k + len(v)] = v


if __name__ == "__main__":
    main()