From 8ac631655dd14a375396682bbd6fb10a7415850e Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Sat, 1 Apr 2017 12:01:36 -0400
Subject: Add binary heap implementation.

* chickadee/heap.scm: New file.
* Makefile.am (SOURCES): Add it.
---
 Makefile.am        |   1 +
 chickadee/heap.scm | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 130 insertions(+)
 create mode 100644 chickadee/heap.scm

diff --git a/Makefile.am b/Makefile.am
index 0103c2b..536217b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -41,6 +41,7 @@ godir=$(libdir)/guile/$(GUILE_EFFECTIVE_VERSION)/ccache
 SOURCES = 					\
   chickadee/config.scm				\
   chickadee/utils.scm				\
+  chickadee/heap.scm				\
   chickadee/audio.scm				\
   chickadee/input/controller.scm		\
   chickadee/input/keyboard.scm			\
diff --git a/chickadee/heap.scm b/chickadee/heap.scm
new file mode 100644
index 0000000..db9bea9
--- /dev/null
+++ b/chickadee/heap.scm
@@ -0,0 +1,129 @@
+;;; Chickadee Game Toolkit
+;;; Copyright © 2017 David Thompson <davet@gnu.org>
+;;;
+;;; Chickadee is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published
+;;; by the Free Software Foundation, either version 3 of the License,
+;;; or (at your option) any later version.
+;;;
+;;; Chickadee 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
+;;; General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+(define-module (chickadee heap)
+  #:use-module (ice-9 format)
+  #:use-module (rnrs base)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-9 gnu)
+  #:use-module (srfi srfi-43)
+  #:export (make-heap
+            heap?
+            heap-empty?
+            heap-size
+            heap-min
+            heap-insert!
+            heap-remove!
+            heap-clear!))
+
+;; A binary heap.
+(define-record-type <heap>
+  (%make-heap vector size <)
+  heap?
+  (vector heap-vector set-heap-vector!)
+  (size heap-size set-heap-size!)
+  (< heap-<))
+
+(define (display-heap heap port)
+  (format port "#<heap size: ~d>" (heap-size heap)))
+
+(set-record-type-printer! <heap> display-heap)
+
+(define* (make-heap #:optional (< <))
+  "Create a new heap that uses the < procedure to determine order."
+  (%make-heap (make-vector 32 #f) 0 <))
+
+(define (heap-empty? heap)
+  "Return #t if HEAP is empty."
+  (zero? (heap-size heap)))
+
+(define (heap-capacity heap)
+  (1- (vector-length (heap-vector heap))))
+
+(define (heap-full? heap)
+  (= (heap-size heap) (heap-capacity heap)))
+
+(define (double-heap-capacity! heap)
+  (let* ((old-vec (heap-vector heap))
+         (new-vec (make-vector (* (vector-length old-vec) 2) #f)))
+    (vector-copy! new-vec 0 old-vec)
+    (set-heap-vector! heap new-vec)))
+
+(define (heap-min heap)
+  "Return the minimum element of HEAP."
+  (if (zero? (heap-size heap))
+      (error "empty heap" heap)
+      (vector-ref (heap-vector heap) 1)))
+
+(define (heap-set! heap i item)
+  (vector-set! (heap-vector heap) i item))
+
+(define (heap-ref heap i)
+  (vector-ref (heap-vector heap) i))
+
+(define (heap-insert! heap item)
+  "Add ITEM to HEAP."
+  (when (heap-full? heap)
+    (double-heap-capacity! heap))
+  (let ((hole (1+ (heap-size heap)))
+        (< (heap-< heap)))
+    (set-heap-size! heap hole)
+    (let loop ((hole hole))
+      (let* ((parent-hole (div hole 2))
+             (parent-item (heap-ref heap parent-hole)))
+        (if (and (> hole 1) (< item parent-item))
+            (begin
+              (heap-set! heap hole parent-item)
+              (loop parent-hole))
+            (heap-set! heap hole item))))))
+
+(define (heap-remove! heap)
+  "Remove the minimum element of HEAP."
+  (let ((size (1- (heap-size heap)))
+        (< (heap-< heap)))
+    (define (finish hole)
+      (heap-set! heap hole (heap-ref heap (heap-size heap)))
+      (set-heap-size! heap size)
+      *unspecified*)
+
+    (define (leaf? hole)
+      (> (* hole 2) size))
+
+    (define (smallest-child hole)
+      (let ((left-child (* hole 2))
+            (right-child (1+ (* hole 2))))
+        (if (or (= left-child size)
+                (< (heap-ref heap left-child) (heap-ref heap right-child)))
+            left-child
+            right-child)))
+
+    (heap-set! heap 1 (heap-ref heap (heap-size heap)))
+
+    (let loop ((hole 1))
+      (if (leaf? hole)
+          (finish hole)
+          (let ((child (smallest-child hole)))
+            (if (< (heap-ref heap hole) (heap-ref heap child))
+                (finish hole)
+                (begin
+                  (heap-set! heap hole (heap-ref heap child))
+                  (loop child))))))))
+
+(define (heap-clear! heap)
+  "Remove all elements from HEAP."
+  (vector-fill! (heap-vector heap) #f)
+  (set-heap-size! heap 0))
-- 
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