#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#include "base64.h"

static const char encoding_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static char decoding_table[256];
static int decoding_table_built = 0;

static void build_decoding_table() {
    for (int i = 0; i < 64; i++) {
        decoding_table[(unsigned char) encoding_table[i]] = i;
    }
    decoding_table_built = 1;
}

size_t base64_decode(const char *in, size_t in_len, unsigned char *out) {
    if (!decoding_table_built) {
        build_decoding_table();
    }

    if (in_len % 4 != 0) {
        return 0;
    }

    size_t out_len = in_len / 4 * 3;
    if (in[in_len - 1] == '=') {
        out_len--;
    }
    if (in[in_len - 2] == '=') {
        out_len--;
    }

    for (size_t i = 0, j = 0; i < in_len;) {
        uint32_t sextet_a = in[i] == '=' ? 0 : decoding_table[(unsigned char)in[i]];
        i++;
        uint32_t sextet_b = in[i] == '=' ? 0 : decoding_table[(unsigned char)in[i]];
        i++;
        uint32_t sextet_c = in[i] == '=' ? 0 : decoding_table[(unsigned char)in[i]];
        i++;
        uint32_t sextet_d = in[i] == '=' ? 0 : decoding_table[(unsigned char)in[i]];
        i++;

        uint32_t triple = (sextet_a << 18) + (sextet_b << 12) + (sextet_c << 6) + sextet_d;

        if (j < out_len) {
            out[j++] = (triple >> 16) & 0xFF;
        }
        if (j < out_len) {
            out[j++] = (triple >> 8) & 0xFF;
        }
        if (j < out_len) {
            out[j++] = triple & 0xFF;
        }
    }

    return out_len;
}