Function stbi__jpeg_huff_decode

lite/example/cpp_example/mge/cv/stb_image.h:2242–2291

decode a jpeg huffman value from the bitstream

// decode a jpeg huffman value from the bitstream
stbi_inline static int stbi__jpeg_huff_decode(stbi__jpeg* j, stbi__huffman* h) {
    unsigned int temp;
    int c, k;

    if (j->code_bits < 16)
        stbi__grow_buffer_unsafe(j);

    // look at the top FAST_BITS and determine what symbol ID it is,
    // if the code is <= FAST_BITS
    c = (j->code_buffer >> (32 - FAST_BITS)) & ((1 << FAST_BITS) - 1);
    k = h->fast[c];
    if (k < 255) {
        int s = h->size[k];
        if (s > j->code_bits)
            return -1;
        j->code_buffer <<= s;
        j->code_bits -= s;
        return h->values[k];
    }

    // naive test is to shift the code_buffer down so k bits are
    // valid, then test against maxcode. To speed this up, we've
    // preshifted maxcode left so that it has (16-k) 0s at the
    // end; in other words, regardless of the number of bits, it
    // wants to be compared against something shifted to have 16;
    // that way we don't need to shift inside the loop.
    temp = j->code_buffer >> 16;
    for (k = FAST_BITS + 1;; ++k)
        if (temp < h->maxcode[k])
            break;
    if (k == 17) {
        // error! code not found
        j->code_bits -= 16;
        return -1;
    }

    if (k > j->code_bits)
        return -1;

    // convert the huffman code to the symbol id
    c = ((j->code_buffer >> (32 - k)) & stbi__bmask[k]) + h->delta[k];
    STBI_ASSERT(
            (((j->code_buffer) >> (32 - h->size[c])) & stbi__bmask[h->size[c]]) ==
            h->code[c]);

    // convert the id to a symbol
    j->code_bits -= k;
    j->code_buffer <<= k;
    return h->values[c];
}

// bias[n] = (-1<<n) + 1
static const int stbi__jbias[16] = {0,     -1,    -3,     -7,    -15,   -31,