From 5e8399d16c6848e38b89cf3bf049bc61520e5887 Mon Sep 17 00:00:00 2001
From: danij <danij@dengine.net>
Date: Mon, 22 Apr 2013 05:36:16 +0100
Subject: [PATCH] Partition: Added intersection; intercept; isParallelTo

Derived from their libdeng1 counterparts.
---
 doomsday/client/include/partition.h | 53 ++++++++++++++++++++++++++++-
 1 file changed, 52 insertions(+), 1 deletion(-)

diff --git a/doomsday/client/include/partition.h b/doomsday/client/include/partition.h
index 4b3519354b..e94014585a 100644
--- a/doomsday/client/include/partition.h
+++ b/doomsday/client/include/partition.h
@@ -21,6 +21,7 @@
 #ifndef DENG_MATH_PARTITION
 #define DENG_MATH_PARTITION
 
+#include <de/math.h>
 #include <de/Vector>
 
 namespace de {
@@ -63,9 +64,59 @@ class Partition
      * @param x  X coordinate for the point.
      * @param y  Y coordinate for the point.
      */
-    ddouble pointOnSide(ddouble x, ddouble y) const {
+    inline ddouble pointOnSide(ddouble x, ddouble y) const {
         return pointOnSide(Vector2d(x, y));
     }
+
+    /**
+     * Returns @c true iff "this" line @a other are parallel. In special
+     * case where either of the two lines having a zero-length direction
+     * then @c true is always returned.
+     */
+    bool isParallelTo(Partition const &other, ddouble epsilon = .99999999) const
+    {
+        ddouble len = direction.length();
+        if(len == 0) return true;
+
+        ddouble otherLen = other.direction.length();
+        if(otherLen == 0) return true;
+
+        ddouble dot = direction.dot(other.direction) / len / otherLen;
+
+        // If it's close enough, we'll consider them parallel.
+        epsilon = de::abs(epsilon);
+        return dot > epsilon || dot < -epsilon;
+    }
+
+    /**
+     * Determines how far along the line (relative to the origin) where the
+     * @a other line and "this" intersect.
+     *
+     * @return Intersection point expressed as a scale factor relative to the
+     * partition origin. In the special case of the two lines being parallel
+     * @c 0 is returned.
+     *
+     * @see intercept()
+     */
+    double intersection(Partition const &other) const
+    {
+        ddouble divsor = direction.x * other.direction.y -
+                         direction.y * other.direction.x;
+
+        // Special case: parallel?
+        if(divsor == 0) return 0;
+
+        Vector2d delta = origin - other.origin;
+        return (delta.y * other.direction.x - delta.x * other.direction.y) / divsor;
+    }
+
+    /**
+     * Determine the intercept point where "this" line and @a other intersect
+     * and return the point at which the two intercept.
+     */
+    inline Vector2d intercept(Partition const &other) const {
+        return origin + direction * intersection(other);
+    }
 };
 
 } // namespace de