Type only brainfuck interpreter.

brainfuck.d.ts

type StringToArray<str extends string> = str extends `${infer First}${infer Rest}`
    ? [ First, ...StringToArray<Rest> ]
    : []

type NewArray<length extends number, arr extends any[] = []> = arr["length"] extends length
    ? arr
    : NewArray<length, [ ...arr, any ]>;

type New2DArray<length extends number, arr extends any[][] = []> = arr["length"] extends length
    ? arr
    : New2DArray<length, [ ...arr, [] ]>;

type ModifyCell<cells extends any[][], pos extends any[], addOne extends boolean> = addOne extends true
    ? { [K in keyof cells]: K extends `${pos["length"]}` ? cells[K]["length"] extends 255 ? [] : [ ...cells[K], any ] : cells[K] }
    : { [K in keyof cells]: K extends `${pos["length"]}` ? cells[K] extends [ infer First, ...infer Rest ] ? Rest : NewArray<255> : cells[K] }

type PopArrayFirst<array extends any[]> = array extends [ infer First, ...infer Rest ]
    ? Rest
    : never;

type SetArrayItem<array extends any[], index extends number, item extends any> = {[K in keyof array]: K extends `${index}` ? item : array[K]};

type ConvertCellsToDecimal<cells extends any[][]> = {[K in keyof cells]: cells[K]["length"]}

type AllChars = {"\x00": 0,"\x01": 1,"\x02": 2,"\x03": 3,"\x04": 4,"\x05": 5,"\x06": 6,"\x07": 7,"\x08": 8,"\x09": 9,"\x0a": 10,"\x0b": 11,"\x0c": 12,"\x0d": 13,"\x0e": 14,"\x0f": 15,"\x10": 16,"\x11": 17,"\x12": 18,"\x13": 19,"\x14": 20,"\x15": 21,"\x16": 22,"\x17": 23,"\x18": 24,"\x19": 25,"\x1a": 26,"\x1b": 27,"\x1c": 28,"\x1d": 29,"\x1e": 30,"\x1f": 31,"\x20": 32,"\x21": 33,"\x22": 34,"\x23": 35,"\x24": 36,"\x25": 37,"\x26": 38,"\x27": 39,"\x28": 40,"\x29": 41,"\x2a": 42,"\x2b": 43,"\x2c": 44,"\x2d": 45,"\x2e": 46,"\x2f": 47,"\x30": 48,"\x31": 49,"\x32": 50,"\x33": 51,"\x34": 52,"\x35": 53,"\x36": 54,"\x37": 55,"\x38": 56,"\x39": 57,"\x3a": 58,"\x3b": 59,"\x3c": 60,"\x3d": 61,"\x3e": 62,"\x3f": 63,"\x40": 64,"\x41": 65,"\x42": 66,"\x43": 67,"\x44": 68,"\x45": 69,"\x46": 70,"\x47": 71,"\x48": 72,"\x49": 73,"\x4a": 74,"\x4b": 75,"\x4c": 76,"\x4d": 77,"\x4e": 78,"\x4f": 79,"\x50": 80,"\x51": 81,"\x52": 82,"\x53": 83,"\x54": 84,"\x55": 85,"\x56": 86,"\x57": 87,"\x58": 88,"\x59": 89,"\x5a": 90,"\x5b": 91,"\x5c": 92,"\x5d": 93,"\x5e": 94,"\x5f": 95,"\x60": 96,"\x61": 97,"\x62": 98,"\x63": 99,"\x64": 100,"\x65": 101,"\x66": 102,"\x67": 103,"\x68": 104,"\x69": 105,"\x6a": 106,"\x6b": 107,"\x6c": 108,"\x6d": 109,"\x6e": 110,"\x6f": 111,"\x70": 112,"\x71": 113,"\x72": 114,"\x73": 115,"\x74": 116,"\x75": 117,"\x76": 118,"\x77": 119,"\x78": 120,"\x79": 121,"\x7a": 122,"\x7b": 123,"\x7c": 124,"\x7d": 125,"\x7e": 126,"\x7f": 127,"\x80": 128,"\x81": 129,"\x82": 130,"\x83": 131,"\x84": 132,"\x85": 133,"\x86": 134,"\x87": 135,"\x88": 136,"\x89": 137,"\x8a": 138,"\x8b": 139,"\x8c": 140,"\x8d": 141,"\x8e": 142,"\x8f": 143,"\x90": 144,"\x91": 145,"\x92": 146,"\x93": 147,"\x94": 148,"\x95": 149,"\x96": 150,"\x97": 151,"\x98": 152,"\x99": 153,"\x9a": 154,"\x9b": 155,"\x9c": 156,"\x9d": 157,"\x9e": 158,"\x9f": 159,"\xa0": 160,"\xa1": 161,"\xa2": 162,"\xa3": 163,"\xa4": 164,"\xa5": 165,"\xa6": 166,"\xa7": 167,"\xa8": 168,"\xa9": 169,"\xaa": 170,"\xab": 171,"\xac": 172,"\xad": 173,"\xae": 174,"\xaf": 175,"\xb0": 176,"\xb1": 177,"\xb2": 178,"\xb3": 179,"\xb4": 180,"\xb5": 181,"\xb6": 182,"\xb7": 183,"\xb8": 184,"\xb9": 185,"\xba": 186,"\xbb": 187,"\xbc": 188,"\xbd": 189,"\xbe": 190,"\xbf": 191,"\xc0": 192,"\xc1": 193,"\xc2": 194,"\xc3": 195,"\xc4": 196,"\xc5": 197,"\xc6": 198,"\xc7": 199,"\xc8": 200,"\xc9": 201,"\xca": 202,"\xcb": 203,"\xcc": 204,"\xcd": 205,"\xce": 206,"\xcf": 207,"\xd0": 208,"\xd1": 209,"\xd2": 210,"\xd3": 211,"\xd4": 212,"\xd5": 213,"\xd6": 214,"\xd7": 215,"\xd8": 216,"\xd9": 217,"\xda": 218,"\xdb": 219,"\xdc": 220,"\xdd": 221,"\xde": 222,"\xdf": 223,"\xe0": 224,"\xe1": 225,"\xe2": 226,"\xe3": 227,"\xe4": 228,"\xe5": 229,"\xe6": 230,"\xe7": 231,"\xe8": 232,"\xe9": 233,"\xea": 234,"\xeb": 235,"\xec": 236,"\xed": 237,"\xee": 238,"\xef": 239,"\xf0": 240,"\xf1": 241,"\xf2": 242,"\xf3": 243,"\xf4": 244,"\xf5": 245,"\xf6": 246,"\xf7": 247,"\xf8": 248,"\xf9": 249,"\xfa": 250,"\xfb": 251,"\xfc": 252,"\xfd": 253,"\xfe": 254,"\xff": 255}

type FromCharCode<N extends number> = ["\x00","\x01","\x02","\x03","\x04","\x05","\x06","\x07","\x08","\x09","\x0a","\x0b","\x0c","\x0d","\x0e","\x0f","\x10","\x11","\x12","\x13","\x14","\x15","\x16","\x17","\x18","\x19","\x1a","\x1b","\x1c","\x1d","\x1e","\x1f","\x20","\x21","\x22","\x23","\x24","\x25","\x26","\x27","\x28","\x29","\x2a","\x2b","\x2c","\x2d","\x2e","\x2f","\x30","\x31","\x32","\x33","\x34","\x35","\x36","\x37","\x38","\x39","\x3a","\x3b","\x3c","\x3d","\x3e","\x3f","\x40","\x41","\x42","\x43","\x44","\x45","\x46","\x47","\x48","\x49","\x4a","\x4b","\x4c","\x4d","\x4e","\x4f","\x50","\x51","\x52","\x53","\x54","\x55","\x56","\x57","\x58","\x59","\x5a","\x5b","\x5c","\x5d","\x5e","\x5f","\x60","\x61","\x62","\x63","\x64","\x65","\x66","\x67","\x68","\x69","\x6a","\x6b","\x6c","\x6d","\x6e","\x6f","\x70","\x71","\x72","\x73","\x74","\x75","\x76","\x77","\x78","\x79","\x7a","\x7b","\x7c","\x7d","\x7e","\x7f","\x80","\x81","\x82","\x83","\x84","\x85","\x86","\x87","\x88","\x89","\x8a","\x8b","\x8c","\x8d","\x8e","\x8f","\x90","\x91","\x92","\x93","\x94","\x95","\x96","\x97","\x98","\x99","\x9a","\x9b","\x9c","\x9d","\x9e","\x9f","\xa0","\xa1","\xa2","\xa3","\xa4","\xa5","\xa6","\xa7","\xa8","\xa9","\xaa","\xab","\xac","\xad","\xae","\xaf","\xb0","\xb1","\xb2","\xb3","\xb4","\xb5","\xb6","\xb7","\xb8","\xb9","\xba","\xbb","\xbc","\xbd","\xbe","\xbf","\xc0","\xc1","\xc2","\xc3","\xc4","\xc5","\xc6","\xc7","\xc8","\xc9","\xca","\xcb","\xcc","\xcd","\xce","\xcf","\xd0","\xd1","\xd2","\xd3","\xd4","\xd5","\xd6","\xd7","\xd8","\xd9","\xda","\xdb","\xdc","\xdd","\xde","\xdf","\xe0","\xe1","\xe2","\xe3","\xe4","\xe5","\xe6","\xe7","\xe8","\xe9","\xea","\xeb","\xec","\xed","\xee","\xef","\xf0","\xf1","\xf2","\xf3","\xf4","\xf5","\xf6","\xf7","\xf8","\xf9","\xfa","\xfb","\xfc","\xfd","\xfe","\xff"][N];
type GetCharCode<N extends string> = N extends keyof AllChars
    ? AllChars[N] extends number
        ? AllChars[N]
        : never
    : never;

type GetNextSquareClose<source extends string[], pos extends any[] = [], opened extends any[] = []> = pos["length"] extends source["length"]
    ? unknown
    : source[pos["length"]] extends "]"
        ? opened["length"] extends 0
            ? pos
            : GetNextSquareClose<source, [ ...pos, any ], PopArrayFirst<opened>>
        : source[pos["length"]] extends "["
            ? GetNextSquareClose<source, [ ...pos, any ], [ ...opened, any ]>
            : GetNextSquareClose<source, [ ...pos, any ], opened>

type GetLastSquareOpen<source extends string[], pos extends any[] = [], closed extends any[] = []> = source[pos["length"]] extends "["
        ? closed["length"] extends 0
            ? pos
            : pos["length"] extends 0 ? unknown : GetLastSquareOpen<source, PopArrayFirst<pos>, PopArrayFirst<closed>>
        : source[pos["length"]] extends "]"
            ? pos["length"] extends 0 ? unknown : GetLastSquareOpen<source, PopArrayFirst<pos>, [ ...closed, any ]>
            : pos["length"] extends 0 ? unknown : GetLastSquareOpen<source, PopArrayFirst<pos>, closed>

type Ensure<T extends any, U extends any> = T extends U ? T : never;

type BrainfInterpreter<source extends string[], tapeLength extends number = 20, pos extends any[] = [], cells extends any[][] = New2DArray<tapeLength>, pointer extends any[] = [], input extends string[] = [], inputPointer extends any[] = [], output extends string = ""> = source[pos["length"]] extends string
    ? source[pos["length"]] extends "+"
        ? BrainfInterpreter<source, tapeLength, [ ...pos, any ], ModifyCell<cells, pointer, true>, pointer, input, inputPointer, output>
        : source[pos["length"]] extends "-"
            ? BrainfInterpreter<source, tapeLength, [ ...pos, any ], ModifyCell<cells, pointer, false>, pointer, input, inputPointer, output>
            : source[pos["length"]] extends ">"
                ? [ ...pointer, any ]["length"] extends tapeLength
                    ? "BrainFuck error: tried to move pointer off tape. (too big)"
                    : BrainfInterpreter<source, tapeLength, [ ...pos, any ], cells, [ ...pointer, [] ], input,inputPointer, output>
                : source[pos["length"]] extends "<"
                    ? pointer["length"] extends 0
                        ? "BrainFuck error: tried to move pointer off tape. (too low)"
                        : BrainfInterpreter<source, tapeLength, [ ...pos, any ], cells, PopArrayFirst<pointer>, input, inputPointer, output>
                    : source[pos["length"]] extends "."
                        ? BrainfInterpreter<source, tapeLength, [ ...pos, any ], cells, pointer, input, inputPointer, `${output}${FromCharCode<cells[pointer["length"]]["length"]>}`>
                        : source[pos["length"]] extends ","
                            ? inputPointer["length"] extends input["length"]
                                ? { reason: "no more input", pos: pos["length"], pointer: pointer["length"], output: output, cells: ConvertCellsToDecimal<cells> }
                                : BrainfInterpreter<source, tapeLength, [ ...pos, any ], SetArrayItem<cells, pointer["length"], NewArray<GetCharCode<input[inputPointer["length"]]>>>, pointer, input, [ ...inputPointer, any ], output>
                            : source[pos["length"]] extends "["
                                ? cells[pointer["length"]]["length"] extends 0
                                    ? GetNextSquareClose<source, [ ...pos, any ]> extends any[]
                                        ? BrainfInterpreter<source, tapeLength, [ ...Ensure<GetNextSquareClose<source, [ ...pos, any ]>, any[]>, any ], cells, pointer, input, inputPointer, output>
                                        : `BrainFuck error: no closing bracket found for index: ${pos["length"]}`
                                    : BrainfInterpreter<source, tapeLength, [ ...pos, any ], cells, pointer, input, inputPointer, output>
                                : source[pos["length"]] extends "]"
                                    ? pos["length"] extends 0
                                        ? `BrainFuck error: no opening bracket found for index: ${pos["length"]}`
                                        : GetLastSquareOpen<source, PopArrayFirst<pos>> extends any[]
                                            ? BrainfInterpreter<source, tapeLength, Ensure<GetLastSquareOpen<source, PopArrayFirst<pos>>, any[]>, cells, pointer, input, inputPointer, output >
                                            : `BrainFuck error: no opening bracket found for index: ${pos["length"]}, ${PopArrayFirst<pos>["length"]}`
                                    : BrainfInterpreter<source, tapeLength, [ ...pos, any ], cells, pointer, input, inputPointer, output>
    : { reason: "over", pos: pos["length"], pointer: pointer["length"], output: output, cells: ConvertCellsToDecimal<cells> }

export type RunBrainfuck<source extends string, tapeLength extends number = 20, input extends string = "", lastCells extends any[][] = New2DArray<tapeLength>> =
    BrainfInterpreter<StringToArray<source>, tapeLength, [], lastCells, [], StringToArray<input>>

test.d.ts

import { RunBrainfuck } from "./brainfuck";

// --------- PROGRAM ---------
type Program = RunBrainfuck<"+[>,----- -----]<-[+ +++++ +++++.<-]", 20, "abcdefg\x0a">;
//   ^?