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zopflib/src/zopfli/blocksplitter.c  view on Meta::CPAN

/*
Copyright 2011 Google Inc. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Author: lode.vandevenne@gmail.com (Lode Vandevenne)
Author: jyrki.alakuijala@gmail.com (Jyrki Alakuijala)
*/

#include "blocksplitter.h"

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#include "deflate.h"
#include "squeeze.h"
#include "tree.h"
#include "util.h"

/*
The "f" for the FindMinimum function below.
i: the current parameter of f(i)
context: for your implementation
*/
typedef double FindMinimumFun(size_t i, void* context);

/*
Finds minimum of function f(i) where is is of type size_t, f(i) is of type
double, i is in range start-end (excluding end).
Outputs the minimum value in *smallest and returns the index of this value.
*/
static size_t FindMinimum(FindMinimumFun f, void* context,
                          size_t start, size_t end, double* smallest) {
  if (end - start < 1024) {
    double best = ZOPFLI_LARGE_FLOAT;
    size_t result = start;
    size_t i;
    for (i = start; i < end; i++) {
      double v = f(i, context);
      if (v < best) {
        best = v;
        result = i;
      }
    }
    *smallest = best;
    return result;
  } else {
    /* Try to find minimum faster by recursively checking multiple points. */
#define NUM 9  /* Good value: 9. */
    size_t i;
    size_t p[NUM];
    double vp[NUM];
    size_t besti;
    double best;
    double lastbest = ZOPFLI_LARGE_FLOAT;
    size_t pos = start;

    for (;;) {
      if (end - start <= NUM) break;

      for (i = 0; i < NUM; i++) {
        p[i] = start + (i + 1) * ((end - start) / (NUM + 1));
        vp[i] = f(p[i], context);
      }
      besti = 0;
      best = vp[0];
      for (i = 1; i < NUM; i++) {
        if (vp[i] < best) {
          best = vp[i];
          besti = i;
        }
      }
      if (best > lastbest) break;

      start = besti == 0 ? start : p[besti - 1];
      end = besti == NUM - 1 ? end : p[besti + 1];

      pos = p[besti];
      lastbest = best;
    }
    *smallest = lastbest;
    return pos;
#undef NUM
  }
}

/*
Returns estimated cost of a block in bits.  It includes the size to encode the
tree and the size to encode all literal, length and distance symbols and their