From bca6a5566207abce005767bd329feee2f5319e5c Mon Sep 17 00:00:00 2001
From: Eric Wasylishen <ewasylishen@gmail.com>
Date: Sat, 22 Jul 2017 15:08:51 -0600
Subject: [PATCH] qbsp winding: add WindingArea

---
 include/qbsp/winding.hh |  2 ++
 qbsp/test_qbsp.cc       | 16 ++++++++++++++++
 qbsp/winding.cc         | 23 +++++++++++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/include/qbsp/winding.hh b/include/qbsp/winding.hh
index a8a29f16..db91a9eb 100644
--- a/include/qbsp/winding.hh
+++ b/include/qbsp/winding.hh
@@ -47,4 +47,6 @@ void MidpointWinding(const winding_t *w, vec3_t v);
 void CalcSides(const winding_t *in, const qbsp_plane_t *split, int *sides,
                vec_t *dists, int counts[3]);
 
+vec_t WindingArea(const winding_t * w);
+
 #endif
diff --git a/qbsp/test_qbsp.cc b/qbsp/test_qbsp.cc
index 730107f2..1bb15e51 100644
--- a/qbsp/test_qbsp.cc
+++ b/qbsp/test_qbsp.cc
@@ -120,3 +120,19 @@ TEST(qbsp, duplicatePlanes) {
     EXPECT_EQ(6, Brush_NumFaces(brush));
     FreeMem(brush, BRUSH, 1);
 }
+
+TEST(mathlib, WindingArea) {
+    winding_t w;
+    w.numpoints = 5;
+    
+    // poor test.. but at least checks that the colinear point is treated correctly
+    VectorSet(w.points[0], 0,0,0);
+    VectorSet(w.points[1], 0,32,0); // colinear
+    VectorSet(w.points[2], 0,64,0);
+    VectorSet(w.points[3], 64,64,0);
+    VectorSet(w.points[4], 64,0,0);
+
+    
+    EXPECT_EQ(64.0f * 64.0f, WindingArea(&w));
+}
+
diff --git a/qbsp/winding.cc b/qbsp/winding.cc
index 7a2f91ac..f2724719 100644
--- a/qbsp/winding.cc
+++ b/qbsp/winding.cc
@@ -371,3 +371,26 @@ MidpointWinding(const winding_t *w, vec3_t v)
     if (w->numpoints)
         VectorScale(v, 1.0 / w->numpoints, v);
 }
+
+/*
+==================
+WindingArea
+==================
+*/
+vec_t
+WindingArea(const winding_t * w)
+{
+    int i;
+    vec3_t d1, d2, cross;
+    vec_t total;
+    
+    total = 0;
+    for (i = 2; i < w->numpoints; i++) {
+        VectorSubtract(w->points[i - 1], w->points[0], d1);
+        VectorSubtract(w->points[i], w->points[0], d2);
+        CrossProduct(d1, d2, cross);
+        vec_t triarea = 0.5 * VectorLength(cross);
+        total += triarea;
+    }
+    return total;
+}