Function CompressArray

src/crypto/equihash.cpp:120–157

}

void CompressArray(const unsigned char* in, size_t in_len,
                   unsigned char* out, size_t out_len,
                   size_t bit_len, size_t byte_pad)
{
    assert(bit_len >= 8);
    assert(8*sizeof(uint32_t) >= 7+bit_len);

    size_t in_width { (bit_len+7)/8 + byte_pad };
    assert(out_len == bit_len*in_len/(8*in_width));

    uint32_t bit_len_mask { ((uint32_t)1 << bit_len) - 1 };

    // The acc_bits least-significant bits of acc_value represent a bit sequence
    // in big-endian order.
    size_t acc_bits = 0;
    uint32_t acc_value = 0;

    size_t j = 0;
    for (size_t i = 0; i < out_len; i++) {
        // When we have fewer than 8 bits left in the accumulator, read the next
        // input element.
        if (acc_bits < 8) {
            acc_value = acc_value << bit_len;
            for (size_t x = byte_pad; x < in_width; x++) {
                acc_value = acc_value | (
                    (
                        // Apply bit_len_mask across byte boundaries
                        in[j+x] & ((bit_len_mask >> (8*(in_width-x-1))) & 0xFF)
                    ) << (8*(in_width-x-1))); // Big-endian
            }
            j += in_width;
            acc_bits += bit_len;
        }

        acc_bits -= 8;
        out[i] = (acc_value >> acc_bits) & 0xFF;
    }
}

// Big-endian so that lexicographic array comparison is equivalent to integer
// comparison