133 lines
2.3 KiB
C
133 lines
2.3 KiB
C
/*
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* p2d_attrib.c - Determine various polygon attributes
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*
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* Written 2012, 2015 by Werner Almesberger
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* Copyright 2012, 2015 Werner Almesberger
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*/
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#include <stdbool.h>
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#include <stdio.h>
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#include <math.h>
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#include <assert.h>
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#include "poly2d.h"
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/*
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* Angle in counter-clockwise direction to turn at point B when coming from A
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* in order to face towards C.
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*/
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static double angle_3(const struct v2d *a, const struct v2d *b,
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const struct v2d *c)
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{
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double ax, ay, bx, by;
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double cs, aa, bb;
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double cross, angle;
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ax = b->x - a->x;
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ay = b->y - a->y;
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bx = c->x - b->x;
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by = c->y - b->y;
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cross = ax * by - ay * bx;
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if (!cross)
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return 0;
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aa = hypot(ax, ay);
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bb = hypot(bx, by);
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cs = (ax * bx + ay * by) / aa / bb;
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if (cs <= -1)
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angle = 180;
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else if (cs >= 1)
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angle = 0;
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else
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angle = acos(cs) / M_PI * 180.0;
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return cross >= 0 ? angle : -angle;
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}
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/*
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* If we predominantly turn to the right, then the path must be clockwise.
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*/
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bool p2d_is_cw(const struct p2d *p)
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{
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const struct v2d *v;
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double a = 0;
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assert(p2d_vertices(p) >= 3);
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assert(p2d_is_closed(p));
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assert(p2d_no_intersect(p));
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v = p->v;
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do {
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a += angle_3(v, v->next, v->next->next);
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v = v->next;
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}
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while (v != p->v);
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assert(!isnan(a)); /* require -std=c99 or -std=gnu99 */
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return a < 0;
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}
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bool p2d_is_closed(const struct p2d *p)
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{
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return p->v == p->last || p->last->next;
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}
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/*
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* Known bug: if the polygon intersects on a vertex, the intersection may
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* go unnoticed.
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*/
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bool p2d_no_intersect(const struct p2d *p)
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{
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const struct v2d *v, *u;
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v = p->v;
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while (v) {
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u = v->next;
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if (!u || u == p->v)
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return 1;
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u = u->next;
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if (!u || u == p->v)
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return 1;
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while (u && u->next && u->next != v) {
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if (v2d_intersect(v, v->next, u, u->next,
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NULL, NULL) > 0)
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return 0;
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u = u->next;
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if (u == p->v)
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break;
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}
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v = v->next;
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if (v == p->v)
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break;
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}
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return 1;
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}
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unsigned p2d_vertices(const struct p2d *p)
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{
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const struct v2d *v;
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unsigned n = 0;
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v = p->v;
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while (v) {
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n++;
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v = v->next;
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if (v == p->v)
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break;
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}
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return n;
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}
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