Adler32.cpp

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/*
 * Copyright notice:
 *
 * (C) 1995-2024 Jean-loup Gailly and Mark Adler
 *
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 *
 * 1. The origin of this software must not be misrepresented; you must not
 *    claim that you wrote the original software. If you use this software
 *    in a product, an acknowledgment in the product documentation would be
 *    appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 *    misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 *
 * Jean-loup Gailly        Mark Adler
 * jloup@gzip.org          madler@alumni.caltech.edu
 */
 
#include <sourcepp/crypto/Adler32.h>
 
using namespace sourcepp;
 
constexpr uint32_t BASE = 65521u;    /* largest prime smaller than 65536 */
constexpr std::size_t NMAX = 5552u;
 
#define DO1(buffer,i)  {adler += static_cast<unsigned char>((buffer)[i]); sum2 += adler;}
#define DO2(buffer,i)  DO1(buffer,i) DO1(buffer,i+1)
#define DO4(buffer,i)  DO2(buffer,i) DO2(buffer,i+2)
#define DO8(buffer,i)  DO4(buffer,i) DO4(buffer,i+4)
#define DO16(buffer)   DO8(buffer,0) DO8(buffer,8)
 
uint32_t crypto::computeAdler32(std::span<const std::byte> buffer) {
    const std::byte* cur = buffer.data();
    uint64_t len = buffer.size();
 
    uint32_t adler = 0;
 
    /* split Adler-32 into component sums */
    uint32_t sum2 = (adler >> 16) & 0xffff;
    adler &= 0xffff;
 
    /* in case user likes doing a byte at a time, keep it fast */
    if (len == 1) {
        adler += static_cast<unsigned char>(buffer[0]);
        if (adler >= BASE)
            adler -= BASE;
        sum2 += adler;
        if (sum2 >= BASE)
            sum2 -= BASE;
        return adler | (sum2 << 16);
    }
 
    /* initial Adler-32 value (deferred check for len == 1 speed) */
    if (!cur)
        return 1L;
 
    /* in case short lengths are provided, keep it somewhat fast */
    if (len < 16) {
        while (len--) {
            adler += static_cast<unsigned char>(*cur++);
            sum2 += adler;
        }
        if (adler >= BASE)
            adler -= BASE;
        sum2 %= BASE;            /* only added so many BASE's */
        return adler | (sum2 << 16);
    }
 
    /* do length NMAX blocks -- requires just one modulo operation */
    while (len >= NMAX) {
        len -= NMAX;
        uint32_t n = NMAX / 16;    /* NMAX is divisible by 16 */
        do {
            DO16(buffer)           /* 16 sums unrolled */
            cur += 16;
        } while (--n);
        adler %= BASE;
        sum2 %= BASE;
    }
 
    /* do remaining bytes (less than NMAX, still just one modulo) */
    if (len) {                  /* avoid modulos if none remaining */
        while (len >= 16) {
            len -= 16;
            DO16(buffer)
            cur += 16;
        }
        while (len--) {
            adler += static_cast<unsigned char>(*cur++);
            sum2 += adler;
        }
        adler %= BASE;
        sum2 %= BASE;
    }
 
    /* return recombined sums */
    return adler | (sum2 << 16);
}