A draft ImHex pattern for the VGM chiptune file format

vgm.hexpat

#pragma author applecuckoo
#pragma description VGM chiptune files (WIP)
#pragma endian little
#pragma MIME audio/x-vgm

import type.magic;
import type.byte;
import std.core;
import std.string;
import std.ptr;
import std.fxpt;
import std.io;
import std.math;

struct PackedBCD<auto Digits> {
    type::Nibbles bytes[Digits];
} [[/*sealed,*/ format("format_packed_bcd")]];

fn format_packed_bcd(auto bcd) {
    str result;

    for (u32 i = 3, i < sizeof(bcd.bytes), i -= 1) {
        u8 highbyte = bcd.bytes[i].high;
        
        if (highbyte >= 10)
            return "Invalid";

        result += std::format("{}", highbyte);
        
        u8 lowbyte = bcd.bytes[i].low;
        
        if (lowbyte >= 10)
            return "Invalid";

        result += std::format("{}", lowbyte);
    }
    
    std::fxpt::fixed result_fmt;
    result_fmt = std::string::parse_int(result, 10);
    return result_fmt;
};

bitfield SN76489Flags {
    frequency : 1;
    negateOutput : 1;
    GameGearStereo : 1;
    clockDiv : 1;
    XNORNoiseMode : 1;
    padding : 3;
};

bitfield AY8910Flags {
    legacyOutput : 1;
    singleOutput : 1;
    discreteOutput : 1;
    RAWOutput : 1;
    YMclockDivLow : 1;
    padding : 3;
};

bitfield OKIM6258Flags {
    clockDiv : 2;
    ADPCMsel : 1;
    outputBitDepth : 1;
    padding : 4;
};

bitfield K054539Flags {
    reverseStereo : 1;
    disableReverb : 1;
    KeyOnUpdate : 1;
    padding : 5;
};

enum AY8190Type : u8 {
    AY8910,
    AY8912,
    AY8913,
    AY8914,
    YM2149 = 0x10,
    YM3439,
    YMZ284,
    YMZ294,
};

enum C140Type : u8 {
    C140_NamcoSystem2,
    C140_NamcoSystem21,
    ASIC219_NamcoNA,
};

struct Gd3 {
    type::Magic<"Gd3 "> ident;
    u8 version[4];
    u32 Gd3Length;
    std::string::NullString16 trackNameEng;
    std::string::NullString16 trackNameOriginal;
    std::string::NullString16 gameNameEng;
    std::string::NullString16 gameNameOriginal;
    std::string::NullString16 sysNameEng;
    std::string::NullString16 sysNameOriginal;
    std::string::NullString16 trackAuthorEnglish;
    std::string::NullString16 trackAuthorOriginal;
    std::string::NullString16 gameReleaseDate;
    std::string::NullString16 VGMConverter;
    std::string::NullString16 Notes;
};

struct Header {
    type::Magic<"Vgm "> ident;
    u32 eofOffset;
    PackedBCD<4> version; // TODO: decode this 'properly'? this is 4-bit BCD
    u32 SN76489_clk; // TODO: bit 31 and the dual chip bit indicates if a T6W28 is in play
    u32 YM2413_clk;
    Gd3 *Gd3Offset : u32 [[pointer_base("std::ptr::relative_to_parent")]];
    u32 sampleCount;
    u32 loopOffset;
    u32 loopSamples;
    // VGM 1.01 onwards
    u32 rate;
    // VGM 1.10 onwards
    u16 SN76489_feedback;
    u8 SN76489_shift_width;
    // VGM 1.51 onwards
    SN76489Flags snflags;
    // VGM 1.10 onwards
    u32 YM2612_clk;
    u32 YM2151_clk;
    // VGM 1.50 onwards
    u32 VGMOffset;
    // VGM 1.51 onwards
    u32 SegaPCM_clk;
    u32 SegaPCM_reg;
    u32 RF5C68_clk;
    u32 YM2203_clk;
    u32 YM2608_clk;
    u32 YM2610_clk; // TODO: bit 31 indicates if this is a YM2610B or not
    u32 YM3812_clk;
    u32 YM3526_clk;
    u32 Y8950_clk;
    u32 YMF262_clk;
    u32 YMF278B_clk;
    u32 YMF271_clk;
    u32 YMZ280B_clk;
    u32 RF5C164_clk;
    u32 PWM_clk;
    u32 AY8910_clk;
    AY8190Type AY8910_type;
    AY8910Flags AY8910_flags;
    u8 YM2203_flags;
    u8 YM2608_flags;
    // VGM 1.60 onwards
    u8 volumeMod;
    padding[1];
    u8 loopBase;
    // VGM 1.51 onwards
    u8 loopMod;
    // VGM 1.61 onwards
    u32 DMG_clk;
    u32 APU_clk; // TODO: bit 31 enables the FDS sound addon
    u32 MultiPCM_clk;
    u32 uPD7759_clk;
    u32 OKIM6258_clk;
    OKIM6258Flags OKIM6258_flags;
    K054539Flags K054539_flags;
    C140Type C140_type;
    padding[1];
    u32 OKIM6295_clk;
    u32 K051649_clk;
    u32 K054539_clk;
    u32 HuC6280_clk;
    u32 C140_clk;
    u32 K053260_clk;
    u32 Pokey_clk;
    u32 QSound_clk;
    // VGM 1.71 onwards
    u32 SCSP_clk;
    // VGM 1.70 onwards
    u32 extraHeaderOffset;
    // VGM 1.71 onwards
    u32 WonderSwan_clk;
    u32 VSU_clk;
    u32 SAA1099_clk;
    u32 ES5503_clk;
    u32 ES5005_clk;
    u8 ES5503_ch;
    u8 ES5505_ch; // TODO: bit 31 indicates if this chip is an ES5505 or a ES5506
    u8 C352_clockDiv;
    u32 X1_010_clk;
    u32 C352_clk;
    u32 GA20_clk;
    // VGM 1.72 onwards
    u32 Mikey_clk;
};

struct VGM {
    Header header;
};

VGM vgm @ 0x00;