From 3de244c50ca6fbad75c21985f564e48eb041f94f Mon Sep 17 00:00:00 2001
From: Michael Reeves <mike.reeves@gmail.com>
Date: Wed, 15 Jul 2015 00:20:46 -0700
Subject: [PATCH] Merge Sort example

---
 MergeSort.php | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 MergeSort.php

diff --git a/MergeSort.php b/MergeSort.php
new file mode 100644
index 0000000..ea98d60
--- /dev/null
+++ b/MergeSort.php
@@ -0,0 +1,84 @@
+<?php
+// Simple little merge sort recursive function based on:
+// Coursera's Stanford Algorithms: Design and Analysis, Part 1 Course
+// ref: https://www.coursera.org/course/algo
+
+function mergeSort($a) {
+
+  $len = count($a);
+  if ($len == 1) {
+    return $a;
+  }
+  $chunk_size = ceil($len/2);
+  $tmp = array_chunk($a, $chunk_size); // first chunk will be larger
+  $b1 = $tmp[0];
+  $b2 = $tmp[1];
+
+  if (count($b1) > 1) {
+    $b1 = mergeSort($b1);  
+  } 
+  if (count($b2) > 1) {
+    $b2 = mergeSort($b2);
+  }
+
+  $i = 0;
+  $j = 0;
+  $b1_max = count($b1);
+  $b2_max = count($b2);
+  $output = array();
+  while ($i <= $b1_max || $j <= $b2_max){
+    if (!isset($b1[$i]) && !isset($b2[$j])) {
+      return $output;
+    } elseif (!isset($b1[$i])) {
+      $output[] = $b2[$j];
+      $j++;
+    } elseif (!isset($b2[$j])) {
+      $output[] = $b1[$i];
+      $i++;
+    } elseif ($b1[$i] > $b2[$j]) {
+      $output[] = $b2[$j];
+      $j++;
+    } elseif ($b1[$i] < $b2[$j]) {
+      $output[] = $b1[$i];
+      $i++;
+    } elseif ($b1[$i] == $b2[$j]) {
+      $output[] = $b1[$i];
+      $output[] = $b2[$j];
+      $i++;
+      $j++;
+    } else {
+      die("unexpected case!");
+    }
+  }
+}
+
+// Test Cases
+$a1 = array(2,3,7,1,9,6,4,0);
+echo "********** array with even count:\n";
+echo "input:\n".var_export($a1, true)."\n";
+echo "output:\n".var_export(mergeSort($a1), true)."\n\n";
+
+$a2 = array(6,3,-5,1,0,20,-100,5);
+echo "********** array with negative numbers:\n";
+echo "input:\n".var_export($a2, true)."\n";
+echo "output:\n".var_export(mergeSort($a2), true)."\n\n";
+
+$a3 = array(6,3,2,1,4,20,7);
+echo "********** odd array length:\n";
+echo "input:\n".var_export($a3, true)."\n";
+echo "output:\n".var_export(mergeSort($a3), true)."\n\n";
+
+$a4 = array(4,3,2,1,4,20,2);
+echo "********** duplicate numbers array:\n";
+echo "input:\n".var_export($a4, true)."\n";
+echo "output:\n".var_export(mergeSort($a4), true)."\n\n";
+
+$a5 = array(4);
+echo "********** 1 number array:\n";
+echo "input:\n".var_export($a5, true)."\n";
+echo "output:\n".var_export(mergeSort($a5), true)."\n\n";
+
+$a6 = array(1,2,3,4,5,6,7,8,9,10);
+echo "********** already sorted array:\n";
+echo "input:\n".var_export($a6, true)."\n";
+echo "output:\n".var_export(mergeSort($a6), true)."\n\n";