decode

    pub fn decode(decoder: *const Base64Decoder, dest: []u8, source: []const u8) Error!void {
        if (decoder.pad_char != null and source.len % 4 != 0) return error.InvalidPadding;
        var dest_idx: usize = 0;
        var fast_src_idx: usize = 0;
        var acc: u12 = 0;
        var acc_len: u4 = 0;
        var leftover_idx: ?usize = null;
        while (fast_src_idx + 16 < source.len and dest_idx + 15 < dest.len) : ({
            fast_src_idx += 16;
            dest_idx += 12;
        }) {
            var bits: u128 = 0;
            inline for (0..4) |i| {
                var new_bits: u128 = decoder.fast_char_to_index[0][source[fast_src_idx + i * 4]];
                new_bits |= decoder.fast_char_to_index[1][source[fast_src_idx + 1 + i * 4]];
                new_bits |= decoder.fast_char_to_index[2][source[fast_src_idx + 2 + i * 4]];
                new_bits |= decoder.fast_char_to_index[3][source[fast_src_idx + 3 + i * 4]];
                if ((new_bits & invalid_char_tst) != 0) return error.InvalidCharacter;
                bits |= (new_bits << (24 * i));
            }
            std.mem.writeInt(u128, dest[dest_idx..][0..16], bits, .little);
        }
        while (fast_src_idx + 4 < source.len and dest_idx + 3 < dest.len) : ({
            fast_src_idx += 4;
            dest_idx += 3;
        }) {
            var bits = decoder.fast_char_to_index[0][source[fast_src_idx]];
            bits |= decoder.fast_char_to_index[1][source[fast_src_idx + 1]];
            bits |= decoder.fast_char_to_index[2][source[fast_src_idx + 2]];
            bits |= decoder.fast_char_to_index[3][source[fast_src_idx + 3]];
            if ((bits & invalid_char_tst) != 0) return error.InvalidCharacter;
            std.mem.writeInt(u32, dest[dest_idx..][0..4], bits, .little);
        }
        const remaining = source[fast_src_idx..];
        for (remaining, fast_src_idx..) |c, src_idx| {
            const d = decoder.char_to_index[c];
            if (d == invalid_char) {
                if (decoder.pad_char == null or c != decoder.pad_char.?) return error.InvalidCharacter;
                leftover_idx = src_idx;
                break;
            }
            acc = (acc << 6) + d;
            acc_len += 6;
            if (acc_len >= 8) {
                acc_len -= 8;
                dest[dest_idx] = @as(u8, @truncate(acc >> acc_len));
                dest_idx += 1;
            }
        }
        if (acc_len > 4 or (acc & (@as(u12, 1) << acc_len) - 1) != 0) {
            return error.InvalidPadding;
        }
        if (leftover_idx == null) return;
        const leftover = source[leftover_idx.?..];
        if (decoder.pad_char) |pad_char| {
            const padding_len = acc_len / 2;
            var padding_chars: usize = 0;
            for (leftover) |c| {
                if (c != pad_char) {
                    return if (c == Base64Decoder.invalid_char) error.InvalidCharacter else error.InvalidPadding;
                }
                padding_chars += 1;
            }
            if (padding_chars != padding_len) return error.InvalidPadding;
        }
    }