dppoint.cpp

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/**********************************************************************
 * File:        dppoint.cpp
 * Description: Simple generic dynamic programming class.
 * Author:      Ray Smith
 * Created:     Wed Mar 25 19:08:01 PDT 2009
 *
 * (C) Copyright 2009, Google Inc.
 ** 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.
 *
 **********************************************************************/

#include "dppoint.h"
#include "errcode.h"
#include "tprintf.h"

namespace tesseract {

// Solve the dynamic programming problem for the given array of points, with
// the given size and cost function.
// Steps backwards are limited to being between min_step and max_step
// inclusive.
// The return value is the tail of the best path.
DPPoint* DPPoint::Solve(int min_step, int max_step, bool debug,
                        CostFunc cost_func, int size, DPPoint* points) {
  if (size <= 0 || max_step < min_step || min_step >= size)
    return nullptr;  // Degenerate, but not necessarily an error.
  ASSERT_HOST(min_step > 0);  // Infinite loop possible if this is not true.
  if (debug)
    tprintf("min = %d, max=%d\n",
            min_step, max_step);
  // Evaluate the total cost at each point.
  for (int i = 0; i < size; ++i) {
    for (int offset = min_step; offset <= max_step; ++offset) {
      DPPoint* prev = offset <= i ? points + i - offset : nullptr;
      int64_t new_cost = (points[i].*cost_func)(prev);
      if (points[i].best_prev_ != nullptr && offset > min_step * 2 &&
          new_cost > points[i].total_cost_)
        break;  // Find only the first minimum if going over twice the min.
    }
    points[i].total_cost_ += points[i].local_cost_;
    if (debug) {
      tprintf("At point %d, local cost=%d, total_cost=%d, steps=%d\n",
              i, points[i].local_cost_, points[i].total_cost_,
              points[i].total_steps_);
    }
  }
  // Now find the end of the best path and return it.
  int best_cost = points[size - 1].total_cost_;
  int best_end = size - 1;
  for (int end = best_end - 1; end >= size - min_step; --end) {
    int cost = points[end].total_cost_;
    if (cost < best_cost) {
      best_cost = cost;
      best_end = end;
    }
  }
  return points + best_end;
}

// A CostFunc that takes the variance of step into account in the cost.
int64_t DPPoint::CostWithVariance(const DPPoint* prev) {
  if (prev == nullptr || prev == this) {
    UpdateIfBetter(0, 1, nullptr, 0, 0, 0);
    return 0;
  }

  int delta = this - prev;
  int32_t n = prev->n_ + 1;
  int32_t sig_x = prev->sig_x_ + delta;
  int64_t sig_xsq = prev->sig_xsq_ + delta * delta;
  int64_t cost = (sig_xsq - sig_x * sig_x / n) / n;
  cost += prev->total_cost_;
  UpdateIfBetter(cost, prev->total_steps_ + 1, prev, n, sig_x, sig_xsq);
  return cost;
}

// Update the other members if the cost is lower.
void DPPoint::UpdateIfBetter(int64_t cost, int32_t steps, const DPPoint* prev,
                             int32_t n, int32_t sig_x, int64_t sig_xsq) {
  if (cost < total_cost_) {
    total_cost_ = cost;
    total_steps_ = steps;
    best_prev_ = prev;
    n_ = n;
    sig_x_ = sig_x;
    sig_xsq_ = sig_xsq;
  }
}

}  // namespace tesseract.