fuzz_regression_helpers.h

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#pragma once
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
 
static unsigned char * b64_decode(const char * data, size_t * out_len) {
    // A standard lookup table for Base64 decoding. -1 represents an invalid character.
    static const int b64_decode_table[] = {
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -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};
 
    size_t in_len = strlen(data);
    if (in_len % 4 != 0)
        return NULL;  // Base64 strings must have a length that is a multiple of 4.
 
    // Calculate the output length, accounting for padding characters.
    *out_len = in_len / 4 * 3;
    if (data[in_len - 1] == '=')
        (*out_len)--;
    if (data[in_len - 2] == '=')
        (*out_len)--;
 
    unsigned char * out = (unsigned char *)malloc(*out_len);
    if (out == NULL)
        return NULL;
 
    // Process the input string in 4-character chunks.
    for (size_t i = 0, j = 0; i < in_len;) {
        int sextet_a = b64_decode_table[(unsigned char)data[i++]];
        int sextet_b = b64_decode_table[(unsigned char)data[i++]];
        int sextet_c = b64_decode_table[(unsigned char)data[i++]];
        int sextet_d = b64_decode_table[(unsigned char)data[i++]];
 
        // Validate the characters and handle padding.
        if (sextet_a == -1 || sextet_b == -1 || (data[i - 2] != '=' && sextet_c == -1) ||
            (data[i - 1] != '=' && sextet_d == -1)) {
            free(out);
            return NULL;
        }
 
        // Combine the four 6-bit sextets into a 24-bit triple.
        unsigned int triple = (sextet_a << 3 * 6) + (sextet_b << 2 * 6) + (sextet_c << 1 * 6) + (sextet_d << 0 * 6);
 
        // Extract the three 8-bit bytes from the triple.
        if (j < *out_len)
            out[j++] = (triple >> 2 * 8) & 0xFF;
        if (j < *out_len)
            out[j++] = (triple >> 1 * 8) & 0xFF;
        if (j < *out_len)
            out[j++] = (triple >> 0 * 8) & 0xFF;
    }
 
    return out;
}