hash.c

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <inttypes.h>

#define CST 11246805480246805480ULL

static uint64_t load64_le(uint8_t const* V)
{
#if !defined(__LITTLE_ENDIAN__)
#error This code only works with little endian systems
#endif
  uint64_t Ret = *((uint64_t const*)V);
  return Ret;
}

static uint64_t hash(const uint8_t* Data, const size_t Len)
{
   uint64_t Ret = 0;
   const size_t NBlocks = Len/8;
   for (size_t I = 0; I < NBlocks; ++I) {
     const uint64_t V = load64_le(&Data[I*sizeof(uint64_t)]);
     Ret = (Ret ^ V)*CST;
   }
   uint64_t LastV = 0;
   for (size_t I = 0; I < (Len-NBlocks*8); ++I) {
     LastV |= ((uint64_t)Data[NBlocks*8+I]) << (I*8);
   }
   Ret = (Ret^LastV)*CST;
   return Ret;
}

int main(int argc, char** argv)
{
  if (argc < 2) {
    fprintf(stderr, "Usage: %s string\n", argv[0]);
    return 1;
  }
  const char* Str = argv[1];
  const uint64_t H = hash((const uint8_t*) Str, strlen(Str));
  printf("%016" PRIX64 "\n", H);
  return 0;
}