parseIntWithSign

fn parseIntWithSign(
    comptime Result: type,
    comptime Character: type,
    buf: []const Character,
    base: u8,
    comptime sign: enum { pos, neg },
) ParseIntError!Result {
    if (buf.len == 0) return error.InvalidCharacter;

    var buf_base = base;
    var buf_start = buf;
    if (base == 0) {
        // Treat is as a decimal number by default.
        buf_base = 10;
        // Detect the base by looking at buf prefix.
        if (buf.len > 2 and buf[0] == '0') {
            if (math.cast(u8, buf[1])) |c| switch (std.ascii.toLower(c)) {
                'b' => {
                    buf_base = 2;
                    buf_start = buf[2..];
                },
                'o' => {
                    buf_base = 8;
                    buf_start = buf[2..];
                },
                'x' => {
                    buf_base = 16;
                    buf_start = buf[2..];
                },
                else => {},
            };
        }
    }

    const add = switch (sign) {
        .pos => math.add,
        .neg => math.sub,
    };

    // accumulate into Accumulate which is always 8 bits or larger.  this prevents
    // `buf_base` from overflowing Result.
    const info = @typeInfo(Result);
    const Accumulate = @Int(info.int.signedness, @max(8, info.int.bits));
    var accumulate: Accumulate = 0;

    if (buf_start[0] == '_' or buf_start[buf_start.len - 1] == '_') return error.InvalidCharacter;

    for (buf_start) |c| {
        if (c == '_') continue;
        const digit = try charToDigit(math.cast(u8, c) orelse return error.InvalidCharacter, buf_base);
        if (accumulate != 0) {
            accumulate = try math.mul(Accumulate, accumulate, math.cast(Accumulate, buf_base) orelse return error.Overflow);
        } else if (sign == .neg) {
            // The first digit of a negative number.
            // Consider parsing "-4" as an i3.
            // This should work, but positive 4 overflows i3, so we can't cast the digit to T and subtract.
            accumulate = math.cast(Accumulate, -@as(i8, @intCast(digit))) orelse return error.Overflow;
            continue;
        }
        accumulate = try add(Accumulate, accumulate, math.cast(Accumulate, digit) orelse return error.Overflow);
    }

    return if (Result == Accumulate)
        accumulate
    else
        math.cast(Result, accumulate) orelse return error.Overflow;
}