makecw.c

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/*************************************************************************
 * Copyright (c) 2011 AT&T Intellectual Property 
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors: Details at https://graphviz.org
 *************************************************************************/
 
/* 
 * Force the vertices of a polygon to be in CW order.
 * 
 * Works for polygons with concavities.
 * Does not work for twisted polygons.
 *
 * ellson@graphviz.org    October 2nd, 1996
 */
 
#include "makecw.h"
#include <pathplan/pathutil.h>
#include <stddef.h>
 
void make_CW(Ppoly_t * poly)
{
    Ppoint_t *P;
    Ppoint_t tP;
    double area = 0.0;
 
    P = poly->ps;
    const size_t n = poly->pn;
    /* points or lines don't have a rotation */
    if (n > 2) {
        /* check CW or CCW by computing (twice the) area of poly */
        for (size_t i = 1; i < n - 1; i++) {
            area += area2(P[0], P[i + 1], P[i]);
        }
        /* if the area is -ve then the rotation needs to be reversed */
        /* the starting point is left unchanged */
        if (area < 0.0) {
            for (size_t i = 1, j = n - 1; i < 1 + n / 2; i++, j--) {
                tP = P[i];
                P[i] = P[j];
                P[j] = tP;
            }
        }
    }
}