1 /*
2 ****************************************************************************
3 *
4 * MODULE: Vector library
5 *
6 * AUTHOR(S): Original author CERL, probably Dave Gerdes.
7 * Update to GRASS 5.7 Radim Blazek.
8 *
9 * PURPOSE: Lower level functions for reading/writing/manipulating vectors.
10 *
11 * COPYRIGHT: (C) 2001 by the GRASS Development Team
12 *
13 * This program is free software under the GNU General Public
14 * License (>=v2). Read the file COPYING that comes with GRASS
15 * for details.
16 *
17 *****************************************************************************/
18 #include <grass/vector.h>
19
20 double
dig_x_intersect(double beg_x,double end_x,double beg_y,double end_y,double Y)21 dig_x_intersect(double beg_x,
22 double end_x, double beg_y, double end_y, double Y)
23 {
24 double b;
25
26 /* assumes beg_y != end_y */
27
28 /* sort for numerical stability */
29 if (end_x < beg_x || (end_x == beg_x && end_y < beg_y)) {
30 b = end_x;
31 end_x = beg_x;
32 beg_x = b;
33
34 b = end_y;
35 end_y = beg_y;
36 beg_y = b;
37 }
38
39 /* solve simple linear equation to get X = a + b * Y
40 * with
41 * b = (end_x - beg_x) / (end_y - beg_y)
42 * a = beg_x - b * beg_y
43 *
44 * simplify a + b * Y:
45 * a + b * Y = beg_x - b * beg_y + b * Y
46 * a + b * Y = beg_x + b * (Y - beg_y) */
47
48 b = (end_x - beg_x) / (end_y - beg_y);
49
50 return beg_x + b * (Y - beg_y);
51 }
52