From 2e44978ff371b70e2143807d52a7f1e7561a248f Mon Sep 17 00:00:00 2001
From: lisyarus <lisyarus@gmail.com>
Date: Thu, 15 Sep 2022 14:42:05 +0300
Subject: [PATCH] Add ear clipping simple polygon triangulation

---
 .../psemek/cg/triangulation/ear_clipping.hpp  | 74 +++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 libs/cg/include/psemek/cg/triangulation/ear_clipping.hpp

diff --git a/libs/cg/include/psemek/cg/triangulation/ear_clipping.hpp b/libs/cg/include/psemek/cg/triangulation/ear_clipping.hpp
new file mode 100644
index 00000000..afde9669
--- /dev/null
+++ b/libs/cg/include/psemek/cg/triangulation/ear_clipping.hpp
@@ -0,0 +1,74 @@
+#pragma once
+
+#include <psemek/geom/point.hpp>
+#include <psemek/geom/orientation.hpp>
+#include <psemek/geom/contains.hpp>
+
+#include <vector>
+
+namespace psemek::cg
+{
+
+	template <typename Iterator, typename OutputIterator, typename IndexType = std::size_t>
+	void ear_clipping(Iterator begin, Iterator end, OutputIterator out)
+	{
+		IndexType const count = std::distance(begin, end);
+
+		std::vector<IndexType> next(count, 0);
+
+		for (IndexType i = 0; i < count; ++i)
+		{
+			next[i] = (i + 1) % count;
+		}
+
+		IndexType j = 0;
+		for (IndexType i = 0; i + 2 < count; ++i)
+		{
+			for (;; j = (j + 1) % count)
+			{
+				if (next[j] == j) continue;
+
+				bool ear = true;
+
+				auto l = next[j];
+				auto h = next[l];
+
+				geom::simplex triangle{*(begin + j), *(begin + l), *(begin + h)};
+
+				if (geom::orientation(triangle[0], triangle[1], triangle[2]) == geom::sign_t::negative)
+					continue;
+
+				for (IndexType k = next[h]; k != j; k = next[k])
+				{
+					if (geom::contains(triangle, *(begin + k)))
+					{
+						ear = false;
+						break;
+					}
+				}
+
+				if (!ear)
+					continue;
+
+				*out++ = j;
+				*out++ = l;
+				*out++ = h;
+
+				next[l] = l;
+
+				next[j] = h;
+
+				break;
+			}
+		}
+	}
+
+	template <typename IndexType, typename Iterator>
+	std::vector<IndexType> ear_clipping(Iterator begin, Iterator end)
+	{
+		std::vector<IndexType> result;
+		ear_clipping<Iterator, std::back_insert_iterator<std::vector<IndexType>>, IndexType>(begin, end, std::back_inserter(result));
+		return result;
+	}
+
+}