inside.c

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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);
}