fcs_base64.h

// This Base64 implementation is adapted from:
//
// http://stackoverflow.com/a/6782480/7709

#pragma once

#include <stdint.h>
#include "rinutils/rinutils.h"

// We modified the encoding table to have '_' instead of '/',
// which interferes with UNIX paths.

static const char encoding_table[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
    'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',
    'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l',
    'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0',
    '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '_'};
static unsigned char decoding_table[256];
static const size_t mod_table[] = {0, 2, 1};

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

static void base64_encode(const unsigned char *data, const size_t input_length,
    // Must have adequate space, we got rid of malloc()s to avoid memory
    // fragmentation.
    char *encoded_data, size_t *const output_length)
{
    *output_length = 4 * ((input_length + 2) / 3);

    for (size_t i = 0, j = 0; i < input_length;)
    {
        const uint32_t octet_a = i < input_length ? data[i++] : 0;
        const uint32_t octet_b = i < input_length ? data[i++] : 0;
        const uint32_t octet_c = i < input_length ? data[i++] : 0;

        const uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;

        encoded_data[j++] = encoding_table[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 0 * 6) & 0x3F];
    }

    for (size_t i = 0; i < mod_table[input_length % 3]; i++)
        encoded_data[*output_length - 1 - i] = '=';

    encoded_data[*output_length] = '\0';
}

static int base64_decode(const char *data, const size_t input_length,
    unsigned char *decoded_data, size_t *const output_length)
{
    if (input_length % 4 != 0)
        return -1;

    *output_length = input_length / 4 * 3;
    if (data[input_length - 1] == '=')
        (*output_length)--;
    if (data[input_length - 2] == '=')
        (*output_length)--;

    for (size_t i = 0, j = 0; i < input_length;)
    {
#define DECODE()                                                               \
    (data[i] == '=' ? 0 & i++                                                  \
                    : decoding_table[(size_t)(unsigned char)(data[i++])])

        const uint32_t sextet_a = DECODE();
        const uint32_t sextet_b = DECODE();
        const uint32_t sextet_c = DECODE();
        const uint32_t sextet_d = DECODE();
#undef DECODE

        const uint32_t triple = (sextet_a << 3 * 6) + (sextet_b << 2 * 6) +
                                (sextet_c << 1 * 6) + (sextet_d << 0 * 6);

        if (j < *output_length)
            decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
        if (j < *output_length)
            decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
        if (j < *output_length)
            decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
    }

    return 0;
}