#include <stdio.h>

static int decode_matrix[128] =
    { -1,-1,-1,-1,-1,-1,-1,-1 // A..Z = 0..25
    , -1,-1,-1,-1,-1,-1,-1,-1 // a..z = 26..51
    , -1,-1,-1,-1,-1,-1,-1,-1 // 0..9 = 52..61
    , -1,-1,-1,-1,-1,-1,-1,-1 // +,/ = 62,63
    , -1,-1,-1,-1,-1,-1,-1,-1
    , -1,-1,-1,62,-1,-1,-1,63
    , 52,53,54,55,56,57,58,59
    , 60,61,-1,-1,-1,-1,-1,-1
    , -1, 0, 1, 2, 3, 4, 5, 6
    ,  7, 8, 9,10,11,12,13,14
    , 15,16,17,18,19,20,21,22
    , 23,24,25,-1,-1,-1,-1,-1
    , -1,26,27,28,29,30,31,32
    , 33,34,35,36,37,38,39,40
    , 41,42,43,44,45,46,47,48
    , 49,50,51,-1,-1,-1,-1,-1 };

static int decode_base64_char (char c)
{
    if ((unsigned char) c < 128) {
        return decode_matrix[c];
    } else {
        return -1;
    }
}

static char* write_64 (int data, char* out, int* bit_pos)
{
    switch (*bit_pos) {
    case 6: // ======xx xxxx----
        *out &= 0xfc;
        *out |= data >> 4;
        *(out + 1) = data << 4;
        *bit_pos = 4;
        return out + 1;
    case 4: // ====xxxx xx------
        *out &= 0xf0;
        *out |= data >> 2;
        *(out + 1) = data << 6;
        *bit_pos = 2;
        return out + 1;
    case 2: // ==xxxxxx --------
        *out &= 0xc0;
        *out |= data;
        *bit_pos = 0;
        return out + 1;
    case 0: // xxxxxx-- --------
        *out = data << 2;
        *bit_pos = 6;
        return out;
    default: return out;
    }
}

static char* finish_64 (char* out, int bit_pos)
{
    switch (bit_pos) {
    case 6: // ======00
        *out &= 0xfc;
        return out + 1;
        break;
    case 4: // ====0000
        *out &= 0xf0;
        return out + 1;
        break;
    case 2: // ==000000
        *out &= 0xc0;
        return out + 1;
        break;
    case 0: // --------
    default:
        return out;
    }
}

char* decode_base64 (char* output, const char* input)
{
    int decoded;
    int bit_pos = 0;
    for (;;) {
        if (*input == '=' || *input == '\0') {
            break;
        }

        decoded = decode_base64_char(*input++);
        if (decoded == -1) {
            return NULL;
        }
        output = write_64(decoded, output, &bit_pos);
    }
    output = finish_64(output, bit_pos);
    *output = '\0';
    return output;
}

int main (int argc, char** argv)
{
    char* input;
    char* output;
    if (argc > 1) {
        input = argv[1];
    } else {
        input = "aGVsbG8sIHdvcmxkIQ==";
    }

    output = malloc(strlen(input) / 8 * 6 + 2);

    if (decode_base64(output, input)) {
        printf("%s\n", output);
    } else {
        printf("invalid base64\n");
    }
    return 0;
}