diff --git a/include/qbsp/brush.hh b/include/qbsp/brush.hh
index 020dcb40..eaf700a1 100644
--- a/include/qbsp/brush.hh
+++ b/include/qbsp/brush.hh
@@ -49,5 +49,6 @@ bool PlaneEqual(const qbsp_plane_t *p1, const qbsp_plane_t *p2);
 bool PlaneInvEqual(const qbsp_plane_t *p1, const qbsp_plane_t *p2);
 
 vec_t BrushVolume (const brush_t *brush);
+int BrushMostlyOnSide (const brush_t *brush, const plane_t *plane);
 
 #endif
diff --git a/qbsp/brush.cc b/qbsp/brush.cc
index 64590d23..df10fb23 100644
--- a/qbsp/brush.cc
+++ b/qbsp/brush.cc
@@ -1243,3 +1243,38 @@ vec_t BrushVolume (const brush_t *brush)
     return volume;
 }
 
+/*
+==================
+BrushMostlyOnSide
+
+from q3map
+==================
+*/
+int BrushMostlyOnSide (const brush_t *brush, const plane_t *plane)
+{
+    vec_t max;
+    int side;
+    
+    max = 0;
+    side = SIDE_FRONT;
+    
+    for (const face_t *face = brush->faces; face; face = face->next) {
+        const winding_t *w = &face->w;
+        if (!w->numpoints)
+            continue;
+        
+        for (int j=0 ; j<w->numpoints ; j++) {
+            const vec_t d = DotProduct (w->points[j], plane->normal) - plane->dist;
+            if (d > max) {
+                max = d;
+                side = SIDE_FRONT;
+            }
+            if (-d > max) {
+                max = -d;
+                side = SIDE_BACK;
+            }
+        }
+    }
+    
+    return side;
+}
diff --git a/qbsp/test_qbsp.cc b/qbsp/test_qbsp.cc
index adfadf4d..7302a993 100644
--- a/qbsp/test_qbsp.cc
+++ b/qbsp/test_qbsp.cc
@@ -121,7 +121,8 @@ TEST(qbsp, duplicatePlanes) {
     FreeMem(brush, BRUSH, 1);
 }
 
-TEST(qbsp, brushVolume) {
+static brush_t *load128x128x32Brush()
+{
     /* 128x128x32 rectangular brush */
     const char *map = R"(
     {
@@ -138,12 +139,40 @@ TEST(qbsp, brushVolume) {
     )";
     
     mapentity_t worldspawn = LoadMap(map);
-    ASSERT_EQ(1, worldspawn.nummapbrushes);
+    Q_assert(1 == worldspawn.nummapbrushes);
     
     brush_t *brush = LoadBrush(&worldspawn.mapbrush(0), vec3_origin, 0);
-    ASSERT_NE(nullptr, brush);
+    Q_assert(nullptr != brush);
+    
+    return brush;
+}
+
+TEST(qbsp, BrushVolume) {
+    brush_t *brush = load128x128x32Brush();
     
     EXPECT_FLOAT_EQ((128*128*32), BrushVolume(brush));
+}
+
+TEST(qbsp, BrushMostlyOnSide1) {
+    brush_t *brush = load128x128x32Brush();
+    
+    plane_t plane1;
+    VectorSet(plane1.normal, -1, 0, 0);
+    plane1.dist = -100;
+    
+    EXPECT_EQ(SIDE_FRONT, BrushMostlyOnSide(brush, &plane1));
+    
+    FreeMem(brush, BRUSH, 1);
+}
+
+TEST(qbsp, BrushMostlyOnSide2) {
+    brush_t *brush = load128x128x32Brush();
+    
+    plane_t plane2;
+    VectorSet(plane2.normal, 1, 0, 0);
+    plane2.dist = 100;
+    
+    EXPECT_EQ(SIDE_BACK, BrushMostlyOnSide(brush, &plane2));
     
     FreeMem(brush, BRUSH, 1);
 }