From c7d96461cf89df0a8df80232fa293ec909cc9397 Mon Sep 17 00:00:00 2001
From: Max Cahill <1bardesign@gmail.com>
Date: Wed, 1 Apr 2020 19:28:22 +1100
Subject: [PATCH] [added] intersect.point_in_poly and required plumbing

---
 intersect.lua | 21 +++++++++++++++++++++
 vec2.lua      | 11 +++++++++++
 2 files changed, 32 insertions(+)

diff --git a/intersect.lua b/intersect.lua
index bb8f687..c5da48a 100644
--- a/intersect.lua
+++ b/intersect.lua
@@ -397,4 +397,25 @@ function intersect.aabb_aabb_collide_continuous(
 	return false
 end
 
+--check if a point is in a polygon
+--point is the point to test
+--poly is a list of points in order
+--based on winding number, so re-intersecting areas are counted as solid rather than inverting
+function intersect.point_in_poly(point, poly)
+	local wn = 0
+	for i, a in ipairs(poly) do
+		local b = poly[i + 1] or poly[1]
+		if a.y <= point.y then
+			if b.y > point.y and vec2.winding_side(a, b, point) > 0 then
+				wn = wn + 1
+			end
+		else
+			if b.y <= point.y and vec2.winding_side(a, b, point) < 0 then
+				wn = wn - 1
+			end
+		end
+	end
+	return wn ~= 0
+end
+
 return intersect
diff --git a/vec2.lua b/vec2.lua
index 2bfa6ba..02efe4e 100644
--- a/vec2.lua
+++ b/vec2.lua
@@ -532,6 +532,17 @@ function vec2.vrej(a, b)
 	return a:copy():vreji(b)
 end
 
+--get the winding side of p, relative to the line a-b
+-- (this is based on the signed area of the triangle a-b-p)
+-- return value:
+--	>0 when p left of line
+--	=0 when p on line
+--	<0 when p right of line
+function vec2.winding_side(a, b, p)
+	return (b.x - a.x) * (p.y - a.y)
+		 - (p.x - a.x) * (b.y - a.y)
+end
+
 -----------------------------------------------------------
 -- vector extension methods for special purposes
 --   (any common vector ops worth naming)