init

    pub fn init(alphabet_chars: [64]u8, pad_char: ?u8) Base64Decoder {
        var result = Base64Decoder{
            .char_to_index = @splat(invalid_char),
            .fast_char_to_index = @splat(@splat(invalid_char_tst)),
            .pad_char = pad_char,
        };

        var char_in_alphabet: [256]bool = @splat(false);
        for (alphabet_chars, 0..) |c, i| {
            assert(!char_in_alphabet[c]);
            assert(pad_char == null or c != pad_char.?);

            const ci = @as(u32, @intCast(i));
            result.fast_char_to_index[0][c] = ci << 2;
            result.fast_char_to_index[1][c] = (ci >> 4) | ((ci & 0x0f) << 12);
            result.fast_char_to_index[2][c] = ((ci & 0x3) << 22) | ((ci & 0x3c) << 6);
            result.fast_char_to_index[3][c] = ci << 16;

            result.char_to_index[c] = @as(u8, @intCast(i));
            char_in_alphabet[c] = true;
        }
        return result;
    }