hashFn.h

/* Copyright (c) 2015 Brian R. Bondy. Distributed under the MPL2 license.
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef HASHFN_H_
#define HASHFN_H_

#include "./base.h"

typedef uint64_t uint64Array[30];
static int precomputedArraySize = sizeof(uint64Array) / sizeof(uint64_t);

inline uint64_t customPow(uint64Array *precomputedPowers, bool usePrecomputed,
    uint64_t base, int exp) {
  if (usePrecomputed && exp < precomputedArraySize) {
    return (*precomputedPowers)[exp];
  }

  // TOOD: Optimization possible here when passed in toSize which is bigger
  // than precomputedArraySize, we can start from the value of the last
  // precomputed value.
  uint64_t result = 1;
  while (exp) {
    if (exp & 1)
      result *= base;
    exp >>= 1;
    base *= base;
  }
  return result;
}


// Functor for a hashing function
// Implements a Rabin fingerprint hash function
class HashFn {
 public:
  // Initialize a HashFn with the prime p which is used as the base of the Rabin
  // fingerprint algorithm
  explicit HashFn(int p, bool precompute = true) {
    this->p = p;
    this->precompute = precompute;
    if (precompute) {
      uint64_t result = 1;
      for (int i = 0; i < precomputedArraySize; i++) {
        precomputedPowers[i] = result;
        result *= p;
      }
    }
  }

  virtual uint64_t operator()(const char *input, int len,
      unsigned char lastCharCode, uint64_t lastHash);

  virtual uint64_t operator()(const char *input, int len);

 private:
  int p;
  bool precompute;
  uint64Array precomputedPowers;
};

#endif  // HASHFN_H_