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/*****************************************************************************
*
* MODULE: Vector library
*
* AUTHOR(S): Original author CERL, probably Dave Gerdes.
* Update to GRASS 5.7 Radim Blazek.
*
* PURPOSE: Lower level functions for reading/writing/manipulating vectors.
*
* COPYRIGHT: (C) 2001 by the GRASS Development Team
*
* This program is free software under the GNU General Public
* License (>=v2). Read the file COPYING that comes with GRASS
* for details.
*
*****************************************************************************/
#include <grass/vector.h>
double dig_x_intersect(double beg_x, double end_x, double beg_y, double end_y,
double Y)
{
double b;
/* assumes beg_y != end_y */
/* sort for numerical stability */
if (end_x < beg_x || (end_x == beg_x && end_y < beg_y)) {
b = end_x;
end_x = beg_x;
beg_x = b;
b = end_y;
end_y = beg_y;
beg_y = b;
}
/* solve simple linear equation to get X = a + b * Y
* with
* b = (end_x - beg_x) / (end_y - beg_y)
* a = beg_x - b * beg_y
*
* simplify a + b * Y:
* a + b * Y = beg_x - b * beg_y + b * Y
* a + b * Y = beg_x + b * (Y - beg_y) */
b = (end_x - beg_x) / (end_y - beg_y);
return beg_x + b * (Y - beg_y);
}
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