1 /**********************************************************************
2  * File:        dppoint.h
3  * Description: Simple generic dynamic programming class.
4  * Author:      Ray Smith
5  * Created:     Wed Mar 25 18:57:01 PDT 2009
6  *
7  * (C) Copyright 2009, Google Inc.
8  ** Licensed under the Apache License, Version 2.0 (the "License");
9  ** you may not use this file except in compliance with the License.
10  ** You may obtain a copy of the License at
11  ** http://www.apache.org/licenses/LICENSE-2.0
12  ** Unless required by applicable law or agreed to in writing, software
13  ** distributed under the License is distributed on an "AS IS" BASIS,
14  ** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15  ** See the License for the specific language governing permissions and
16  ** limitations under the License.
17  *
18  **********************************************************************/
19 
20 #ifndef TESSERACT_CCSTRUCT_DPPOINT_H_
21 #define TESSERACT_CCSTRUCT_DPPOINT_H_
22 
23 #include <cstdint>
24 
25 namespace tesseract {
26 
27 // A simple class to provide a dynamic programming solution to a class of
28 // 1st-order problems in which the cost is dependent only on the current
29 // step and the best cost to that step, with a possible special case
30 // of using the variance of the steps, and only the top choice is required.
31 // Useful for problems such as finding the optimal cut points in a fixed-pitch
32 // (vertical or horizontal) situation.
33 // Skeletal Example:
34 // DPPoint* array = new DPPoint[width];
35 // for (int i = 0; i < width; i++) {
36 //   array[i].AddLocalCost(cost_at_i)
37 // }
38 // DPPoint* best_end = DPPoint::Solve(..., array);
39 // while (best_end != nullptr) {
40 //   int cut_index = best_end - array;
41 //   best_end = best_end->best_prev();
42 // }
43 // delete [] array;
44 class DPPoint {
45 public:
46   // The cost function evaluates the total cost at this (excluding this's
47   // local_cost) and if it beats this's total_cost, then
48   // replace the appropriate values in this.
49   using CostFunc = int64_t (DPPoint::*)(const DPPoint *);
50 
DPPoint()51   DPPoint()
52       : local_cost_(0)
53       , total_cost_(INT32_MAX)
54       , total_steps_(1)
55       , best_prev_(nullptr)
56       , n_(0)
57       , sig_x_(0)
58       , sig_xsq_(0) {}
59 
60   // Solve the dynamic programming problem for the given array of points, with
61   // the given size and cost function.
62   // Steps backwards are limited to being between min_step and max_step
63   // inclusive.
64   // The return value is the tail of the best path.
65   static DPPoint *Solve(int min_step, int max_step, bool debug, CostFunc cost_func, int size,
66                         DPPoint *points);
67 
68   // A CostFunc that takes the variance of step into account in the cost.
69   int64_t CostWithVariance(const DPPoint *prev);
70 
71   // Accessors.
total_cost()72   int total_cost() const {
73     return total_cost_;
74   }
Pathlength()75   int Pathlength() const {
76     return total_steps_;
77   }
best_prev()78   const DPPoint *best_prev() const {
79     return best_prev_;
80   }
AddLocalCost(int new_cost)81   void AddLocalCost(int new_cost) {
82     local_cost_ += new_cost;
83   }
84 
85 private:
86   // Code common to different cost functions.
87 
88   // Update the other members if the cost is lower.
89   void UpdateIfBetter(int64_t cost, int32_t steps, const DPPoint *prev, int32_t n, int32_t sig_x,
90                       int64_t sig_xsq);
91 
92   int32_t local_cost_;       // Cost of this point on its own.
93   int32_t total_cost_;       // Sum of all costs in best path to here.
94                              // During cost calculations local_cost is excluded.
95   int32_t total_steps_;      // Number of steps in best path to here.
96   const DPPoint *best_prev_; // Pointer to prev point in best path from here.
97   // Information for computing the variance part of the cost.
98   int32_t n_;       // Number of steps in best path to here for variance.
99   int32_t sig_x_;   // Sum of step sizes for computing variance.
100   int64_t sig_xsq_; // Sum of squares of steps for computing variance.
101 };
102 
103 } // namespace tesseract.
104 
105 #endif // TESSERACT_CCSTRUCT_DPPOINT_H_
106