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+//
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+// weiler_atherton.cxx
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+// math
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+//
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+// Created by Sam Jaffe on 5/21/19.
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+// Copyright © 2019 Sam Jaffe. All rights reserved.
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+//
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+
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+#include <vector>
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+
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+#include "expect/expect.hpp"
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+#include "game/math/common.hpp"
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+#include "game/math/shape.hpp"
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+
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+namespace math {
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+ // https://github.com/cabooom/weiler-atherton
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+ using polygon = std::vector<dim2::point>;
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+
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+ struct weiler_atherton {
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+ weiler_atherton(polygon const & s, polygon const & c);
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+
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+ static void inject_point(std::vector<dim2::point> & vec,
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+ dim2::line const & on, dim2::point const & pt);
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+ void test_intersection(dim2::line const & s, dim2::line const & c);
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+
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+ polygon const &subject, clipping;
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+ std::vector<dim2::point> p_subject, p_clipping;
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+ std::vector<dim2::point> enters, exits;
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+ };
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+
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+ weiler_atherton::weiler_atherton(polygon const & s, polygon const & c)
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+ : subject(s), clipping(c), p_subject(s), p_clipping(c) {}
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+
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+ void weiler_atherton::inject_point(std::vector<dim2::point> & vec,
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+ dim2::line const & on,
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+ dim2::point const & pt) {
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+ auto start = std::find(vec.begin(), vec.end(), on.first);
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+ auto end = std::find(vec.begin(), vec.end(), on.second);
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+ auto lnsq = [](dim2::point const & a) { return (a).dot(a); };
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+ float dist = lnsq(pt - *start);
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+ auto cmp = [&](dim2::point const & a) { return lnsq(*start - a) >= dist; };
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+ vec.insert(std::find_if(start, end, cmp), pt);
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+ }
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+
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+ void weiler_atherton::test_intersection(dim2::line const & s,
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+ dim2::line const & c) {
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+ expects_graceful(intersects(s, c), void());
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+ auto point = lines::intersection(s, c);
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+ if (orient(s, point) == anticlockwise) {
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+ enters.emplace_back(point);
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+ } else {
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+ exits.emplace_back(point);
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+ }
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+ inject_point(p_subject, s, point);
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+ inject_point(p_clipping, c, point);
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+ }
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+
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+ std::vector<polygon> intersection(polygon const & lhs, polygon const & rhs) {
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+ std::vector<polygon> rval;
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+ expects_graceful(lhs.size() >= 3 && rhs.size() >= 3, rval);
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+ // TODO: Assert that lhs and rhs are clockwise polygons with orient
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+
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+ weiler_atherton wa(lhs, rhs);
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+
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+ for (std::size_t i = 0; i < lhs.size(); ++i) {
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+ std::size_t i2 = (i + 1) % lhs.size();
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+ dim2::line left{lhs[i], lhs[i2]};
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+ for (std::size_t j = 0; j < rhs.size(); ++j) {
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+ std::size_t j2 = (j + 1) % rhs.size();
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+ wa.test_intersection(left, {rhs[j], rhs[j2]});
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+ }
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+ }
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+
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+ return rval;
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+ }
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+}
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Sam Jaffe