BitReader

const BitReader = struct {
    bytes: []const u8,
    index: usize = 0,
    bits: u8 = 0,
    count: u4 = 0,

    fn initBits(comptime T: type, out: anytype, num: u16) Bits(T) {
        const UT = @Int(.unsigned, @bitSizeOf(T));
        return .{
            @bitCast(@as(UT, @intCast(out))),
            num,
        };
    }

    fn readBitsNoEof(self: *@This(), comptime T: type, num: u16) !T {
        const b, const c = try self.readBitsTuple(T, num);
        if (c < num) return error.EndOfStream;
        return b;
    }

    fn readBits(self: *@This(), comptime T: type, num: u16, out_bits: *u16) !T {
        const b, const c = try self.readBitsTuple(T, num);
        out_bits.* = c;
        return b;
    }

    fn readBitsTuple(self: *@This(), comptime T: type, num: u16) !Bits(T) {
        const UT = @Int(.unsigned, @bitSizeOf(T));
        const U = if (@bitSizeOf(T) < 8) u8 else UT;

        if (num <= self.count) return initBits(T, self.removeBits(@intCast(num)), num);

        var out_count: u16 = self.count;
        var out: U = self.removeBits(self.count);

        const full_bytes_left = (num - out_count) / 8;

        for (0..full_bytes_left) |_| {
            const byte = takeByte(self) catch |err| switch (err) {
                error.EndOfStream => return initBits(T, out, out_count),
            };

            const pos = @as(U, byte) << @intCast(out_count);
            out |= pos;
            out_count += 8;
        }

        const bits_left = num - out_count;
        const keep = 8 - bits_left;

        if (bits_left == 0) return initBits(T, out, out_count);

        const final_byte = takeByte(self) catch |err| switch (err) {
            error.EndOfStream => return initBits(T, out, out_count),
        };

        const pos = @as(U, final_byte & low_bit_mask[bits_left]) << @intCast(out_count);
        out |= pos;
        self.bits = final_byte >> @intCast(bits_left);

        self.count = @intCast(keep);
        return initBits(T, out, num);
    }

    fn takeByte(br: *BitReader) error{EndOfStream}!u8 {
        if (br.bytes.len - br.index == 0) return error.EndOfStream;
        const result = br.bytes[br.index];
        br.index += 1;
        return result;
    }

    fn removeBits(self: *@This(), num: u4) u8 {
        if (num == 8) {
            self.count = 0;
            return self.bits;
        }

        const keep = self.count - num;
        const bits = self.bits & low_bit_mask[num];
        self.bits >>= @intCast(num);
        self.count = keep;
        return bits;
    }

    fn alignToByte(self: *@This()) void {
        self.bits = 0;
        self.count = 0;
    }
}