adler32.h

#include <stdint.h>

// contributed by Dmitry Artamonov
// this is *deterministic*
class Adler32 {
    uint32 sum1, sum2;
public:
    static const uint32_t Base = 65521;
    uint32_t hashvalue;
    int len;

    Adler32(int window) : sum1(1), sum2(0), hashvalue(0), len (window) {}

    void eat(uint8_t inchar) {
        sum1 = (sum1 + inchar) % Base;
        sum2 = (sum2 + sum1) % Base;

        hashvalue = (sum2 << 16) | sum1;
    }

    void reset() {
      sum1 = 1;
      sum2 = 0;
      hashvalue = 0;
    }

    void update(uint8_t outchar, uint8_t inchar) {
        int sum2 = (hashvalue >> 16) & 0xffff;
        int sum1 = hashvalue & 0xffff;

        sum1 += inchar - outchar;
        if (sum1 >= Base)
        {
            sum1 -= Base;
        }
        else if (sum1 < 0)
        {
            sum1 += Base;
        }

        sum2 = ((int)(sum2 - len * outchar + sum1 - 1) % (int)Base);
        if (sum2 < 0)
        {
            sum2 += Base;
        }
        hashvalue = (sum2 << 16) | sum1;
    }
};