Takes a byte[] array with binary data, and outputs that to the .p8 pico 8 cartridge format

P8Writer.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

public class P8Writer {
    public const int RomSize = 0x4300;
    
    private byte[] data;
    private StringBuilder str;

    public P8Writer(byte[] data) {
        this.data = data;
    }

    public string Write() {
        str = new StringBuilder();
        str.Append(@"pico-8 cartridge // http://www.pico-8.com
version 16
__lua__
-- data cartridge

");

        if (data.Length > 0) {
            var gfx = new byte[0x2000];
            Buffer.BlockCopy(data, 0, gfx, 0, Math.Min(gfx.Length, data.Length));
            WriteGfx(gfx);
        }
        if (data.Length > 0x3000) {
            var gff = new byte[0x100];
            Buffer.BlockCopy(data, 0x3000, gff, 0, Math.Min(gff.Length, data.Length - 0x3000));
            WriteGff(gff);
        }
        if (data.Length > 0x2000) {
            var map = new byte[0x1000];
            Buffer.BlockCopy(data, 0x2000, map, 0, Math.Min(map.Length, data.Length - 0x2000));
            WriteMap(map);
        }
        if (data.Length > 0x3200) {
            var sfx = new byte[0x1100];
            Buffer.BlockCopy(data, 0x3200, sfx, 0, Math.Min(sfx.Length, data.Length - 0x3200));
            WriteSfx(sfx);
        }
        if (data.Length > 0x3100) {
            var music = new byte[0x100];
            Buffer.BlockCopy(data, 0x3100, music, 0, Math.Min(music.Length, data.Length - 0x3100));
            WriteMusic(music);
        }

        if (data.Length > 0x4300) {
            throw new Exception("Data won't fit!");
        }

        return str.ToString();
    }

    private void WriteGfx(byte[] data) {
        str.AppendLine("__gfx__");

        for (int i = 0, o = 0; i < 128; i++) {
            for (int j = 0; j < 64; j++, o++) {
                str.Append(Reverse(data[o].ToString("x2")));
            }
            str.AppendLine();
        }
    }
    
    private string Reverse(string s) {
        var str = new StringBuilder();
        for (int i = s.Length - 1; i >= 0; i--) {
            str.Append(s[i]);
        }
        return str.ToString();
    }

    private void WriteGff(byte[] data) {
        str.AppendLine("__gff__");

        for (int i = 0, o = 0; i < 2; i++) {
            for (int j = 0; j < 128; j++, o++) {
                str.Append(data[o].ToString("x2"));
            }
            str.AppendLine();
        }
    }

    private void WriteMap(byte[] data) {
        str.AppendLine("__map__");

        for (int i = 0, o = 0; i < 32; i++) {
            for (int j = 0; j < 128; j++, o++) {
                str.Append(data[o].ToString("x2"));
            }
            str.AppendLine();
        }
    }

    private void WriteSfx(byte[] data) {
        str.AppendLine("__sfx__");

        for (int i = 0, o = 0; i < 64; i++) {
            str.Append(data[o + 64].ToString("x2")); // editor mode
            str.Append(data[o + 65].ToString("x2")); // speed
            str.Append(data[o + 66].ToString("x2")); // loop start
            str.Append(data[o + 67].ToString("x2")); // loop end

            for (int j = 0; j < 32; j++, o += 2) {
                // first byte
                var pitch = data[o] & 0x3F;             // 00111111
                var waveform1 = (data[o] & 0x40) >> 6;  // 01000000
                var waveform2 = (data[o] & 0x80) >> 7;  // 10000000

                // second byte
                var waveform3 = data[o + 1] & 0x1;      // 00000001
                var volume = (data[o + 1] & 0xE) >> 1;  // 00001110
                var effect = (data[o + 1] & 0x70) >> 4; // 01110000
                var custom = (data[o + 1] & 0x80) >> 7; // 10000000

                // 0x?? - pitch
                str.Append(pitch.ToString("x2"));

                // 0x? - waveform
                var waveform = waveform1 | (waveform2 << 1) | (waveform3 << 2) | (custom << 3);
                str.Append(waveform.ToString("x1"));

                // 0x? - volume
                str.Append(volume.ToString("x1"));

                // 0x? - effect
                str.Append(effect.ToString("x1"));
            }

            str.AppendLine();
            o += 4;
        }
    }

    private void WriteMusic(byte[] data) {
        str.AppendLine("__music__");

        for (int i = 0, o = 0; i < 64; i++, o += 4) {
            byte flags = 0;

            if ((data[o] & 128) != 0) {
                data[o] &= 0x7F;
                flags |= 1;
            }
            if ((data[o + 1] & 128) != 0) {
                data[o + 1] &= 0x7F;
                flags |= 2;
            }
            if ((data[o + 2] & 128) != 0) {
                data[o + 2] &= 0x7F;
                flags |= 4;
            }
            if ((data[o + 3] & 128) != 0) {
                data[o + 3] &= 0x7F;
                flags |= 8;
            }

            str.AppendFormat("{0} {1}{2}{3}{4}",
                flags.ToString("x2"), data[o].ToString("x2"), data[o + 1].ToString("x2"), data[o + 2].ToString("x2"), data[o + 3].ToString("x2"));
            str.AppendLine();
        }
    }
}