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+;;; guile-2d
+;;; Copyright (C) 2013 David Thompson <dthompson2@worcester.edu>
+;;;
+;;; Guile-2d is free software: you can redistribute it and/or modify it
+;;; under the terms of the GNU Lesser General Public License as
+;;; published by the Free Software Foundation, either version 3 of the
+;;; License, or (at your option) any later version.
+;;;
+;;; Guile-2d is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; Lesser General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU Lesser General Public
+;;; License along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Rects are axis-aligned bounding boxes that can be used for
+;; performing simple collision detection.
+;;
+;;; Code:
+
+(define-module (2d rect)
+  #:use-module (srfi srfi-9)
+  #:export (<rect>
+            make-rect
+            rect?
+            rect-x
+            rect-y
+            rect-x2
+            rect-y2
+            rect-width
+            rect-height
+            rect-move
+            rect-inflate
+            rect-union
+            rect-contains?
+            rect-collides?))
+
+(define-record-type <rect>
+  (make-rect x y width height)
+  rect?
+  (x rect-x)
+  (y rect-y)
+  (width rect-width)
+  (height rect-height))
+
+(define (rect-x2 rect)
+  (+ (rect-x rect) (rect-width rect)))
+
+(define (rect-y2 rect)
+  (+ (rect-y rect) (rect-height rect)))
+
+(define (rect-move rect x y)
+  "Moves a rect by the given offset."
+  (make-rect (+ (rect-x rect) x)
+             (+ (rect-y rect) y)
+             (rect-width rect)
+             (rect-height rect)))
+
+(define (rect-inflate rect width height)
+  "Grows the rect by the given amount. The rect stays centered around
+its current center."
+  (make-rect (+ (rect-x rect) (/ width 2))
+             (+ (rect-y rect) (/ height 2))
+             (+ (rect-width rect) width)
+             (+ (rect-height rect) height)))
+
+(define (rect-union rect1 rect2)
+  "Returns a rect that covers the area of rect1 and rect2."
+  (let ((x1 (min (rect-x  rect1) (rect-x  rect2)))
+        (x2 (max (rect-x2 rect1) (rect-x2 rect2)))
+        (y1 (min (rect-y  rect1) (rect-y  rect2)))
+        (y2 (max (rect-y2 rect1) (rect-y2 rect2))))
+    (make-rect x1 y1 (- x2 x1) (- y2 y1))))
+
+(define (rect-contains? rect1 rect2)
+  "Tests if rect2 is completely within rect1."
+  (and (>= (rect-x  rect2) (rect-x  rect1))
+       (<= (rect-x  rect2) (rect-x2 rect1))
+       (>= (rect-x2 rect2) (rect-x  rect1))
+       (<= (rect-x2 rect2) (rect-x2 rect1))
+       (>= (rect-y  rect2) (rect-y  rect1))
+       (<= (rect-y  rect2) (rect-y2 rect1))
+       (>= (rect-y2 rect2) (rect-y  rect1))
+       (<= (rect-y2 rect2) (rect-y2 rect1))))
+
+(define (rect-collides? rect1 rect2)
+  "Tests if rect2 overlaps rect1."
+  (or (and (>= (rect-x  rect2) (rect-x  rect1))
+           (<= (rect-x  rect2) (rect-x2 rect1)))
+      (and (>= (rect-x2 rect2) (rect-x  rect1))
+           (<= (rect-x2 rect2) (rect-x2 rect1)))
+      (and (>= (rect-y  rect2) (rect-y  rect1))
+           (<= (rect-y  rect2) (rect-y2 rect1)))
+      (and (>= (rect-y2 rect2) (rect-y  rect1))
+           (<= (rect-y2 rect2) (rect-y2 rect1)))))