pithsync.cpp

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/**********************************************************************
 * File:        pithsync.cpp  (Formerly pitsync2.c)
 * Description: Code to find the optimum fixed pitch segmentation of some blobs.
 * Author:      Ray Smith
 *
 * (C) Copyright 1992, Hewlett-Packard Ltd.
 ** 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 <cmath>
#include <cfloat>       // for FLT_MAX
#include <vector>       // for std::vector
#include "makerow.h"
#include "pitsync1.h"
#include "topitch.h"
#include "pithsync.h"
#include "tprintf.h"
 
/**********************************************************************
 * FPCUTPT::setup
 *
 * Constructor to make a new FPCUTPT.
 **********************************************************************/
 
void FPCUTPT::setup(                     //constructor
                    FPCUTPT *cutpts,     //predecessors
                    int16_t array_origin,  //start coord
                    STATS *projection,   //vertical occupation
                    int16_t zero_count,    //official zero
                    int16_t pitch,         //proposed pitch
                    int16_t x,             //position
                    int16_t offset         //dist to gap
                   ) {
                                 //half of pitch
  int16_t half_pitch = pitch / 2 - 1;
  uint32_t lead_flag;              //new flag
  int32_t ind;                     //current position
 
  if (half_pitch > 31)
    half_pitch = 31;
  else if (half_pitch < 0)
    half_pitch = 0;
  lead_flag = 1 << half_pitch;
 
  pred = nullptr;
  mean_sum = 0;
  sq_sum = offset * offset;
  cost = sq_sum;
  faked = false;
  terminal = false;
  fake_count = 0;
  xpos = x;
  region_index = 0;
  mid_cuts = 0;
  if (x == array_origin) {
    back_balance = 0;
    fwd_balance = 0;
    for (ind = 0; ind <= half_pitch; ind++) {
      fwd_balance >>= 1;
      if (projection->pile_count (ind) > zero_count)
        fwd_balance |= lead_flag;
    }
  }
  else {
    back_balance = cutpts[x - 1 - array_origin].back_balance << 1;
    back_balance &= lead_flag + lead_flag - 1;
    if (projection->pile_count (x) > zero_count)
      back_balance |= 1;
    fwd_balance = cutpts[x - 1 - array_origin].fwd_balance >> 1;
    if (projection->pile_count (x + half_pitch) > zero_count)
      fwd_balance |= lead_flag;
  }
}
 
 
/**********************************************************************
 * FPCUTPT::assign
 *
 * Constructor to make a new FPCUTPT.
 **********************************************************************/
 
void FPCUTPT::assign(                         //constructor
        FPCUTPT* cutpts,         //predecessors
        int16_t array_origin,      //start coord
        int16_t x,                 //position
        bool faking,            //faking this one
        bool mid_cut,           //cheap cut.
        int16_t offset,            //dist to gap
        STATS* projection,       //vertical occupation
        float projection_scale,  //scaling
        int16_t zero_count,        //official zero
        int16_t pitch,             //proposed pitch
        int16_t pitch_error        //allowed tolerance
) {
  int index;                     //test index
  int balance_index;             //for balance factor
  int16_t balance_count;           //ding factor
  int16_t r_index;                 //test cut number
  FPCUTPT *segpt;                //segment point
  int32_t dist;                    //from prev segment
  double sq_dist;                //squared distance
  double mean;                   //mean pitch
  double total;                  //total dists
  double factor;                 //cost function
                                 //half of pitch
  int16_t half_pitch = pitch / 2 - 1;
  uint32_t lead_flag;              //new flag
 
  if (half_pitch > 31)
    half_pitch = 31;
  else if (half_pitch < 0)
    half_pitch = 0;
  lead_flag = 1 << half_pitch;
 
  back_balance = cutpts[x - 1 - array_origin].back_balance << 1;
  back_balance &= lead_flag + lead_flag - 1;
  if (projection->pile_count (x) > zero_count)
    back_balance |= 1;
  fwd_balance = cutpts[x - 1 - array_origin].fwd_balance >> 1;
  if (projection->pile_count (x + half_pitch) > zero_count)
    fwd_balance |= lead_flag;
 
  xpos = x;
  cost = FLT_MAX;
  pred = nullptr;
  faked = faking;
  terminal = false;
  region_index = 0;
  fake_count = INT16_MAX;
  for (index = x - pitch - pitch_error; index <= x - pitch + pitch_error;
  index++) {
    if (index >= array_origin) {
      segpt = &cutpts[index - array_origin];
      dist = x - segpt->xpos;
      if (!segpt->terminal && segpt->fake_count < INT16_MAX) {
        balance_count = 0;
        if (textord_balance_factor > 0) {
          if (textord_fast_pitch_test) {
            lead_flag = back_balance ^ segpt->fwd_balance;
            balance_count = 0;
            while (lead_flag != 0) {
              balance_count++;
              lead_flag &= lead_flag - 1;
            }
          }
          else {
            for (balance_index = 0;
              index + balance_index < x - balance_index;
              balance_index++)
            balance_count +=
                (projection->pile_count (index + balance_index) <=
                zero_count) ^ (projection->pile_count (x -
                balance_index)
                <= zero_count);
          }
          balance_count =
            static_cast<int16_t>(balance_count * textord_balance_factor /
            projection_scale);
        }
        r_index = segpt->region_index + 1;
        total = segpt->mean_sum + dist;
        balance_count += offset;
        sq_dist =
          dist * dist + segpt->sq_sum + balance_count * balance_count;
        mean = total / r_index;
        factor = mean - pitch;
        factor *= factor;
        factor += sq_dist / (r_index) - mean * mean;
        if (factor < cost && segpt->fake_count + faked <= fake_count) {
          cost = factor;         //find least cost
          pred = segpt;          //save path
          mean_sum = total;
          sq_sum = sq_dist;
          fake_count = segpt->fake_count + faked;
          mid_cuts = segpt->mid_cuts + mid_cut;
          region_index = r_index;
        }
      }
    }
  }
}
 
 
/**********************************************************************
 * FPCUTPT::assign_cheap
 *
 * Constructor to make a new FPCUTPT on the cheap.
 **********************************************************************/
 
void FPCUTPT::assign_cheap(                         //constructor
                           FPCUTPT *cutpts,         //predecessors
                           int16_t array_origin,    //start coord
                           int16_t x,               //position
                           bool faking,             //faking this one
                           bool mid_cut,            //cheap cut.
                           int16_t offset,          //dist to gap
                           STATS *projection,       //vertical occupation
                           float projection_scale,  //scaling
                           int16_t zero_count,      //official zero
                           int16_t pitch,           //proposed pitch
                           int16_t pitch_error      //allowed tolerance
                          ) {
  int index;                     //test index
  int16_t balance_count;           //ding factor
  int16_t r_index;                 //test cut number
  FPCUTPT *segpt;                //segment point
  int32_t dist;                    //from prev segment
  double sq_dist;                //squared distance
  double mean;                   //mean pitch
  double total;                  //total dists
  double factor;                 //cost function
                                 //half of pitch
  int16_t half_pitch = pitch / 2 - 1;
  uint32_t lead_flag;              //new flag
 
  if (half_pitch > 31)
    half_pitch = 31;
  else if (half_pitch < 0)
    half_pitch = 0;
  lead_flag = 1 << half_pitch;
 
  back_balance = cutpts[x - 1 - array_origin].back_balance << 1;
  back_balance &= lead_flag + lead_flag - 1;
  if (projection->pile_count (x) > zero_count)
    back_balance |= 1;
  fwd_balance = cutpts[x - 1 - array_origin].fwd_balance >> 1;
  if (projection->pile_count (x + half_pitch) > zero_count)
    fwd_balance |= lead_flag;
 
  xpos = x;
  cost = FLT_MAX;
  pred = nullptr;
  faked = faking;
  terminal = false;
  region_index = 0;
  fake_count = INT16_MAX;
  index = x - pitch;
  if (index >= array_origin) {
    segpt = &cutpts[index - array_origin];
    dist = x - segpt->xpos;
    if (!segpt->terminal && segpt->fake_count < INT16_MAX) {
      balance_count = 0;
      if (textord_balance_factor > 0) {
        lead_flag = back_balance ^ segpt->fwd_balance;
        balance_count = 0;
        while (lead_flag != 0) {
          balance_count++;
          lead_flag &= lead_flag - 1;
        }
        balance_count = static_cast<int16_t>(balance_count * textord_balance_factor
          / projection_scale);
      }
      r_index = segpt->region_index + 1;
      total = segpt->mean_sum + dist;
      balance_count += offset;
      sq_dist =
        dist * dist + segpt->sq_sum + balance_count * balance_count;
      mean = total / r_index;
      factor = mean - pitch;
      factor *= factor;
      factor += sq_dist / (r_index) - mean * mean;
      cost = factor;             //find least cost
      pred = segpt;              //save path
      mean_sum = total;
      sq_sum = sq_dist;
      fake_count = segpt->fake_count + faked;
      mid_cuts = segpt->mid_cuts + mid_cut;
      region_index = r_index;
    }
  }
}
 
 
/**********************************************************************
 * check_pitch_sync
 *
 * Construct the lattice of possible segmentation points and choose the
 * optimal path. Return the optimal path only.
 * The return value is a measure of goodness of the sync.
 **********************************************************************/
 
double check_pitch_sync2(                          //find segmentation
                         BLOBNBOX_IT *blob_it,     //blobs to do
                         int16_t blob_count,         //no of blobs
                         int16_t pitch,              //pitch estimate
                         int16_t pitch_error,        //tolerance
                         STATS *projection,        //vertical
                         int16_t projection_left,    //edges //scale factor
                         int16_t projection_right,
                         float projection_scale,
                         int16_t &occupation_count,  //no of occupied cells
                         FPSEGPT_LIST *seg_list,   //output list
                         int16_t start,              //start of good range
                         int16_t end                 //end of good range
                        ) {
  bool faking;                  //illegal cut pt
  bool mid_cut;                 //cheap cut pt.
  int16_t x;                       //current coord
  int16_t blob_index;              //blob number
  int16_t left_edge;               //of word
  int16_t right_edge;              //of word
  int16_t array_origin;            //x coord of array
  int16_t offset;                  //dist to legal area
  int16_t zero_count;              //projection zero
  int16_t best_left_x = 0;         //for equals
  int16_t best_right_x = 0;        //right edge
  TBOX this_box;                  //bounding box
  TBOX next_box;                  //box of next blob
  FPSEGPT *segpt;                //segment point
  double best_cost;              //best path
  double mean_sum;               //computes result
  FPCUTPT *best_end;             //end of best path
  int16_t best_fake;               //best fake level
  int16_t best_count;              //no of cuts
  BLOBNBOX_IT this_it;           //copy iterator
  FPSEGPT_IT seg_it = seg_list;  //output iterator
 
  //      tprintf("Computing sync on word of %d blobs with pitch %d\n",
  //              blob_count, pitch);
  //      if (blob_count==8 && pitch==27)
  //              projection->print(stdout,true);
  zero_count = 0;
  if (pitch < 3)
    pitch = 3;                   //nothing ludicrous
  if ((pitch - 3) / 2 < pitch_error)
    pitch_error = (pitch - 3) / 2;
  this_it = *blob_it;
  this_box = box_next (&this_it);//get box
  //      left_edge=this_box.left();                                              //left of word
  //      right_edge=this_box.right();
  //      for (blob_index=1;blob_index<blob_count;blob_index++)
  //      {
  //              this_box=box_next(&this_it);
  //              if (this_box.right()>right_edge)
  //                      right_edge=this_box.right();
  //      }
  for (left_edge = projection_left; projection->pile_count (left_edge) == 0
    && left_edge < projection_right; left_edge++);
  for (right_edge = projection_right; projection->pile_count (right_edge) == 0
    && right_edge > left_edge; right_edge--);
  ASSERT_HOST (right_edge >= left_edge);
  if (pitsync_linear_version >= 4)
    return check_pitch_sync3 (projection_left, projection_right, zero_count,
      pitch, pitch_error, projection,
      projection_scale, occupation_count, seg_list,
      start, end);
  array_origin = left_edge - pitch;
  // array of points
  std::vector<FPCUTPT> cutpts(right_edge - left_edge + pitch * 2 + 1);
  for (x = array_origin; x < left_edge; x++)
                                 //free cuts
    cutpts[x - array_origin].setup(&cutpts[0], array_origin, projection,
      zero_count, pitch, x, 0);
  for (offset = 0; offset <= pitch_error; offset++, x++)
                                 //not quite free
    cutpts[x - array_origin].setup(&cutpts[0], array_origin, projection,
      zero_count, pitch, x, offset);
 
  this_it = *blob_it;
  best_cost = FLT_MAX;
  best_end = nullptr;
  this_box = box_next (&this_it);//first box
  next_box = box_next (&this_it);//second box
  blob_index = 1;
  while (x < right_edge - pitch_error) {
    if (x > this_box.right () + pitch_error && blob_index < blob_count) {
      this_box = next_box;
      next_box = box_next (&this_it);
      blob_index++;
    }
    faking = false;
    mid_cut = false;
    if (x <= this_box.left ())
      offset = 0;
    else if (x <= this_box.left () + pitch_error)
      offset = x - this_box.left ();
    else if (x >= this_box.right ())
      offset = 0;
    else if (x >= next_box.left () && blob_index < blob_count) {
      offset = x - next_box.left ();
      if (this_box.right () - x < offset)
        offset = this_box.right () - x;
    }
    else if (x >= this_box.right () - pitch_error)
      offset = this_box.right () - x;
    else if (x - this_box.left () > pitch * pitsync_joined_edge
    && this_box.right () - x > pitch * pitsync_joined_edge) {
      mid_cut = true;
      offset = 0;
    }
    else {
      faking = true;
      offset = projection->pile_count (x);
    }
    cutpts[x - array_origin].assign (&cutpts[0], array_origin, x,
      faking, mid_cut, offset, projection,
      projection_scale, zero_count, pitch,
      pitch_error);
    x++;
  }
 
  best_fake = INT16_MAX;
  best_cost = INT32_MAX;
  best_count = INT16_MAX;
  while (x < right_edge + pitch) {
    offset = x < right_edge ? right_edge - x : 0;
    cutpts[x - array_origin].assign (&cutpts[0], array_origin, x,
      false, false, offset, projection,
      projection_scale, zero_count, pitch,
      pitch_error);
    cutpts[x - array_origin].terminal = true;
    if (cutpts[x - array_origin].index () +
    cutpts[x - array_origin].fake_count <= best_count + best_fake) {
      if (cutpts[x - array_origin].fake_count < best_fake
        || (cutpts[x - array_origin].fake_count == best_fake
      && cutpts[x - array_origin].cost_function () < best_cost)) {
        best_fake = cutpts[x - array_origin].fake_count;
        best_cost = cutpts[x - array_origin].cost_function ();
        best_left_x = x;
        best_right_x = x;
        best_count = cutpts[x - array_origin].index ();
      }
      else if (cutpts[x - array_origin].fake_count == best_fake
        && x == best_right_x + 1
      && cutpts[x - array_origin].cost_function () == best_cost) {
      //exactly equal
        best_right_x = x;
      }
    }
    x++;
  }
  ASSERT_HOST (best_fake < INT16_MAX);
 
  best_end = &cutpts[(best_left_x + best_right_x) / 2 - array_origin];
  if (this_box.right () == textord_test_x
  && this_box.top () == textord_test_y) {
    for (x = left_edge - pitch; x < right_edge + pitch; x++) {
      tprintf ("x=%d, C=%g, s=%g, sq=%g, prev=%d\n",
        x, cutpts[x - array_origin].cost_function (),
        cutpts[x - array_origin].sum (),
        cutpts[x - array_origin].squares (),
        cutpts[x - array_origin].previous ()->position ());
    }
  }
  occupation_count = -1;
  do {
    for (x = best_end->position () - pitch + pitch_error;
      x < best_end->position () - pitch_error
      && projection->pile_count (x) == 0; x++);
    if (x < best_end->position () - pitch_error)
      occupation_count++;
                                 //copy it
    segpt = new FPSEGPT (best_end);
    seg_it.add_before_then_move (segpt);
    best_end = best_end->previous ();
  }
  while (best_end != nullptr);
  seg_it.move_to_last ();
  mean_sum = seg_it.data ()->sum ();
  mean_sum = mean_sum * mean_sum / best_count;
  if (seg_it.data ()->squares () - mean_sum < 0)
    tprintf ("Impossible sqsum=%g, mean=%g, total=%d\n",
      seg_it.data ()->squares (), seg_it.data ()->sum (), best_count);
  //      tprintf("blob_count=%d, pitch=%d, sync=%g, occ=%d\n",
  //              blob_count,pitch,seg_it.data()->squares()-mean_sum,
  //              occupation_count);
  return seg_it.data ()->squares () - mean_sum;
}
 
 
/**********************************************************************
 * check_pitch_sync
 *
 * Construct the lattice of possible segmentation points and choose the
 * optimal path. Return the optimal path only.
 * The return value is a measure of goodness of the sync.
 **********************************************************************/
 
double check_pitch_sync3(                          //find segmentation
                         int16_t projection_left,    //edges //to be considered 0
                         int16_t projection_right,
                         int16_t zero_count,
                         int16_t pitch,              //pitch estimate
                         int16_t pitch_error,        //tolerance
                         STATS *projection,        //vertical
                         float projection_scale,   //scale factor
                         int16_t &occupation_count,  //no of occupied cells
                         FPSEGPT_LIST *seg_list,   //output list
                         int16_t start,              //start of good range
                         int16_t end                 //end of good range
                        ) {
  bool faking;                   //illegal cut pt
  bool mid_cut;                  //cheap cut pt.
  int16_t left_edge;             //of word
  int16_t right_edge;            //of word
  int16_t x;                     //current coord
  int16_t array_origin;          //x coord of array
  int16_t offset;                //dist to legal area
  int16_t projection_offset;     //from scaled projection
  int16_t prev_zero;             //previous zero dist
  int16_t next_zero;             //next zero dist
  int16_t zero_offset;           //scan window
  int16_t best_left_x = 0;       //for equals
  int16_t best_right_x = 0;      //right edge
  FPSEGPT *segpt;                //segment point
  int minindex;                  //next input position
  int test_index;                //index to mins
  double best_cost;              //best path
  double mean_sum;               //computes result
  FPCUTPT *best_end;             //end of best path
  int16_t best_fake;             //best fake level
  int16_t best_count;            //no of cuts
  FPSEGPT_IT seg_it = seg_list;  //output iterator
 
  end = (end - start) % pitch;
  if (pitch < 3)
    pitch = 3;                   //nothing ludicrous
  if ((pitch - 3) / 2 < pitch_error)
    pitch_error = (pitch - 3) / 2;
                                 //min dist of zero
  zero_offset = static_cast<int16_t>(pitch * pitsync_joined_edge);
  for (left_edge = projection_left; projection->pile_count (left_edge) == 0
    && left_edge < projection_right; left_edge++);
  for (right_edge = projection_right; projection->pile_count (right_edge) == 0
    && right_edge > left_edge; right_edge--);
  array_origin = left_edge - pitch;
  // array of points
  std::vector<FPCUTPT> cutpts(right_edge - left_edge + pitch * 2 + 1);
  // local min results
  std::vector<bool> mins(pitch_error * 2 + 1);
  for (x = array_origin; x < left_edge; x++)
                                 //free cuts
    cutpts[x - array_origin].setup(&cutpts[0], array_origin, projection,
      zero_count, pitch, x, 0);
  prev_zero = left_edge - 1;
  for (offset = 0; offset <= pitch_error; offset++, x++)
                                 //not quite free
    cutpts[x - array_origin].setup(&cutpts[0], array_origin, projection,
      zero_count, pitch, x, offset);
 
  best_cost = FLT_MAX;
  best_end = nullptr;
  for (offset = -pitch_error, minindex = 0; offset < pitch_error;
    offset++, minindex++)
  mins[minindex] = projection->local_min (x + offset);
  next_zero = x + zero_offset + 1;
  for (offset = next_zero - 1; offset >= x; offset--) {
    if (projection->pile_count (offset) <= zero_count) {
      next_zero = offset;
      break;
    }
  }
  while (x < right_edge - pitch_error) {
    mins[minindex] = projection->local_min (x + pitch_error);
    minindex++;
    if (minindex > pitch_error * 2)
      minindex = 0;
    faking = false;
    mid_cut = false;
    offset = 0;
    if (projection->pile_count (x) <= zero_count) {
      prev_zero = x;
    }
    else {
      for (offset = 1; offset <= pitch_error; offset++)
        if (projection->pile_count (x + offset) <= zero_count
        || projection->pile_count (x - offset) <= zero_count)
          break;
    }
    if (offset > pitch_error) {
      if (x - prev_zero > zero_offset && next_zero - x > zero_offset) {
        for (offset = 0; offset <= pitch_error; offset++) {
          test_index = minindex + pitch_error + offset;
          if (test_index > pitch_error * 2)
            test_index -= pitch_error * 2 + 1;
          if (mins[test_index])
            break;
          test_index = minindex + pitch_error - offset;
          if (test_index > pitch_error * 2)
            test_index -= pitch_error * 2 + 1;
          if (mins[test_index])
            break;
        }
      }
      if (offset > pitch_error) {
        offset = projection->pile_count (x);
        faking = true;
      }
      else {
        projection_offset =
          static_cast<int16_t>(projection->pile_count (x) / projection_scale);
        if (projection_offset > offset)
          offset = projection_offset;
        mid_cut = true;
      }
    }
    if ((start == 0 && end == 0)
      || !textord_fast_pitch_test
      || (x - projection_left - start) % pitch <= end)
      cutpts[x - array_origin].assign(&cutpts[0], array_origin, x,
        faking, mid_cut, offset, projection,
        projection_scale, zero_count, pitch,
        pitch_error);
    else
      cutpts[x - array_origin].assign_cheap(&cutpts[0], array_origin, x,
        faking, mid_cut, offset,
        projection, projection_scale,
        zero_count, pitch,
        pitch_error);
    x++;
    if (next_zero < x || next_zero == x + zero_offset)
      next_zero = x + zero_offset + 1;
    if (projection->pile_count (x + zero_offset) <= zero_count)
      next_zero = x + zero_offset;
  }
 
  best_fake = INT16_MAX;
  best_cost = INT32_MAX;
  best_count = INT16_MAX;
  while (x < right_edge + pitch) {
    offset = x < right_edge ? right_edge - x : 0;
    cutpts[x - array_origin].assign(&cutpts[0], array_origin, x,
      false, false, offset, projection,
      projection_scale, zero_count, pitch,
      pitch_error);
    cutpts[x - array_origin].terminal = true;
    if (cutpts[x - array_origin].index () +
    cutpts[x - array_origin].fake_count <= best_count + best_fake) {
      if (cutpts[x - array_origin].fake_count < best_fake
        || (cutpts[x - array_origin].fake_count == best_fake
      && cutpts[x - array_origin].cost_function () < best_cost)) {
        best_fake = cutpts[x - array_origin].fake_count;
        best_cost = cutpts[x - array_origin].cost_function ();
        best_left_x = x;
        best_right_x = x;
        best_count = cutpts[x - array_origin].index ();
      }
      else if (cutpts[x - array_origin].fake_count == best_fake
        && x == best_right_x + 1
      && cutpts[x - array_origin].cost_function () == best_cost) {
      //exactly equal
        best_right_x = x;
      }
    }
    x++;
  }
  ASSERT_HOST (best_fake < INT16_MAX);
 
  best_end = &cutpts[(best_left_x + best_right_x) / 2 - array_origin];
  //      for (x=left_edge-pitch;x<right_edge+pitch;x++)
  //      {
  //              tprintf("x=%d, C=%g, s=%g, sq=%g, prev=%d\n",
  //                      x,cutpts[x-array_origin].cost_function(),
  //                      cutpts[x-array_origin].sum(),
  //                      cutpts[x-array_origin].squares(),
  //                      cutpts[x-array_origin].previous()->position());
  //      }
  occupation_count = -1;
  do {
    for (x = best_end->position () - pitch + pitch_error;
      x < best_end->position () - pitch_error
      && projection->pile_count (x) == 0; x++);
    if (x < best_end->position () - pitch_error)
      occupation_count++;
                                 //copy it
    segpt = new FPSEGPT (best_end);
    seg_it.add_before_then_move (segpt);
    best_end = best_end->previous ();
  }
  while (best_end != nullptr);
  seg_it.move_to_last ();
  mean_sum = seg_it.data ()->sum ();
  mean_sum = mean_sum * mean_sum / best_count;
  if (seg_it.data ()->squares () - mean_sum < 0)
    tprintf ("Impossible sqsum=%g, mean=%g, total=%d\n",
      seg_it.data ()->squares (), seg_it.data ()->sum (), best_count);
  return seg_it.data ()->squares () - mean_sum;
}