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position.c

/*
 * Copyright (C) 1984-2000  Mark Nudelman
 *
 * You may distribute under the terms of either the GNU General Public
 * License or the Less License, as specified in the README file.
 *
 * For more information about less, or for information on how to 
 * contact the author, see the README file.
 */


/*
 * Routines dealing with the "position" table.
 * This is a table which tells the position (in the input file) of the
 * first char on each currently displayed line.
 *
 * {{ The position table is scrolled by moving all the entries.
 *    Would be better to have a circular table 
 *    and just change a couple of pointers. }}
 */

#include "less.h"
#include "position.h"

static POSITION *table = NULL;      /* The position table */
static int table_size;

extern int sc_width, sc_height;

/*
 * Return the starting file position of a line displayed on the screen.
 * The line may be specified as a line number relative to the top
 * of the screen, but is usually one of these special cases:
 *    the top (first) line on the screen
 *    the second line on the screen
 *    the bottom line on the screen
 *    the line after the bottom line on the screen
 */
      public POSITION
position(where)
      int where;
{
      switch (where)
      {
      case BOTTOM:
            where = sc_height - 2;
            break;
      case BOTTOM_PLUS_ONE:
            where = sc_height - 1;
            break;
      case MIDDLE:
            where = (sc_height - 1) / 2;
      }
      return (table[where]);
}

/*
 * Add a new file position to the bottom of the position table.
 */
      public void
add_forw_pos(pos)
      POSITION pos;
{
      register int i;

      /*
       * Scroll the position table up.
       */
      for (i = 1;  i < sc_height;  i++)
            table[i-1] = table[i];
      table[sc_height - 1] = pos;
}

/*
 * Add a new file position to the top of the position table.
 */
      public void
add_back_pos(pos)
      POSITION pos;
{
      register int i;

      /*
       * Scroll the position table down.
       */
      for (i = sc_height - 1;  i > 0;  i--)
            table[i] = table[i-1];
      table[0] = pos;
}

/*
 * Initialize the position table, done whenever we clear the screen.
 */
      public void
pos_clear()
{
      register int i;

      for (i = 0;  i < sc_height;  i++)
            table[i] = NULL_POSITION;
}

/*
 * Allocate or reallocate the position table.
 */
      public void
pos_init()
{
      struct scrpos scrpos;

      if (sc_height <= table_size)
            return;
      /*
       * If we already have a table, remember the first line in it
       * before we free it, so we can copy that line to the new table.
       */
      if (table != NULL)
      {
            get_scrpos(&scrpos);
            free((char*)table);
      } else
            scrpos.pos = NULL_POSITION;
      table = (POSITION *) ecalloc(sc_height, sizeof(POSITION));
      table_size = sc_height;
      pos_clear();
      if (scrpos.pos != NULL_POSITION)
            table[scrpos.ln-1] = scrpos.pos;
}

/*
 * See if the byte at a specified position is currently on the screen.
 * Check the position table to see if the position falls within its range.
 * Return the position table entry if found, -1 if not.
 */
      public int
onscreen(pos)
      POSITION pos;
{
      register int i;

      if (pos < table[0])
            return (-1);
      for (i = 1;  i < sc_height;  i++)
            if (pos < table[i])
                  return (i-1);
      return (-1);
}

/*
 * See if the entire screen is empty.
 */
      public int
empty_screen()
{
      return (empty_lines(0, sc_height-1));
}

      public int
empty_lines(s, e)
      int s;
      int e;
{
      register int i;

      for (i = s;  i <= e;  i++)
            if (table[i] != NULL_POSITION)
                  return (0);
      return (1);
}

/*
 * Get the current screen position.
 * The screen position consists of both a file position and
 * a screen line number where the file position is placed on the screen.
 * Normally the screen line number is 0, but if we are positioned
 * such that the top few lines are empty, we may have to set
 * the screen line to a number > 0.
 */
      public void
get_scrpos(scrpos)
      struct scrpos *scrpos;
{
      register int i;

      /*
       * Find the first line on the screen which has something on it,
       * and return the screen line number and the file position.
       */
      for (i = 0; i < sc_height;  i++)
            if (table[i] != NULL_POSITION)
            {
                  scrpos->ln = i+1;
                  scrpos->pos = table[i];
                  return;
            }
      /*
       * The screen is empty.
       */
      scrpos->pos = NULL_POSITION;
}

/*
 * Adjust a screen line number to be a simple positive integer
 * in the range { 0 .. sc_height-2 }.
 * (The bottom line, sc_height-1, is reserved for prompts, etc.)
 * The given "sline" may be in the range { 1 .. sc_height-1 }
 * to refer to lines relative to the top of the screen (starting from 1),
 * or it may be in { -1 .. -(sc_height-1) } to refer to lines
 * relative to the bottom of the screen.
 */
      public int
adjsline(sline)
      int sline;
{
      /*
       * Negative screen line number means
       * relative to the bottom of the screen.
       */
      if (sline < 0)
            sline += sc_height;
      /*
       * Can't be less than 1 or greater than sc_height-1.
       */
      if (sline <= 0)
            sline = 1;
      if (sline >= sc_height)
            sline = sc_height - 1;
      /*
       * Return zero-based line number, not one-based.
       */
      return (sline-1);
}

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