Create SentinelSearch.php (#124)

* Create SentinelSearch.php

Added Sentinel Search algorithm in Searches folder

* Update SentinelSearch.php

* Update SearchesTest.php

Added tests for SentinelSearch.php

* Update DIRECTORY.md

Added link for sentinel search

* Update SearchesTest.php

Testcase corrected

* Updated SentinelSearch.php

* Updated SearchesTest.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

* Update Searches/SentinelSearch.php

---------

Co-authored-by: Brandon Johnson <[email protected]>

Author: Abhishek-Pashte

DIRECTORY.md

@@ -66,9 +66,11 @@
   * [Jumpsearch](./Searches/JumpSearch.php)
   * [Linearsearch](./Searches/LinearSearch.php)
   * [Lowerbound](./Searches/LowerBound.php)
+  * [SentinelSearch](./Searches/SentinelSearch.php)
   * [Ternarysearch](./Searches/TernarySearch.php)
   * [Twopointers](./Searches/TwoPointers.php)
   * [Upperbound](./Searches/UpperBound.php)
+  
 
 ## Sorting
   * [Arraykeyssort](./Sorting/ArrayKeysSort.php)

Searches/SentinelSearch.php

@@ -0,0 +1,42 @@
+<?php
+
+/* SentinelSearch
+    Input : -
+    parameter 1: Array
+    parameter 2: Target element
+
+    Output : -
+    Returns index of element if found, else -1
+*/
+function SentinelSearch($list, $target)
+{
+    //Length of array
+    $len = sizeof($list);
+
+    //Store last element of array
+    $lastElement = $list[$len - 1];
+
+    //Put target at the last position of array known as 'Sentinel'
+    if ($lastElement == $target) {
+        return ($len - 1);
+    }
+    //Put target at last index of array
+    $list[$len - 1] = $target;
+
+    //Initialize variable to traverse through array
+    $i = 0;
+
+    //Traverse through array to search target
+    while ($list[$i] != $target) {
+        $i++;
+    }
+    //Put last element at it's position
+    $list[$len - 1] = $lastElement;
+
+    //If i in less than length, It means element is present in array
+    if ($i < ($len - 1)) {
+        return $i;
+    } else {
+        return -1;
+    }
+}

tests/Searches/SearchesTest.php

@@ -12,6 +12,7 @@
 require_once __DIR__ . '/../../Searches/ExponentialSearch.php';
 require_once __DIR__ . '/../../Searches/TernarySearch.php';
 require_once __DIR__ . '/../../Searches/InterpolationSearch.php';
+require_once __DIR__ . '/../../Searches/SentinelSearch.php';
 require_once __DIR__ . '/../../Searches/TwoPointers.php';
 
 
@@ -204,6 +205,23 @@ public function testInterpolationSearch()
         $this->assertEquals(12, $result);
     }
 
+    public function testSentinelSearch()
+    {
+        $list = [1,3,5,2,4,13,18,23,25,30];
+        $target = 1;
+        $result = SentinelSearch($list, $target);
+        $this->assertEquals(0, $result);
+        $target = 2;
+        $result = SentinelSearch($list, $target);
+        $this->assertEquals(3, $result);
+        $target = 1000;
+        $result = SentinelSearch($list, $target);
+        $this->assertEquals(-1, $result);
+        $target = -2;
+        $result = SentinelSearch($list, $target);
+        $this->assertEquals(-1, $result);
+    }
+
     public function testTwoPointers()
     {
         $list = [1, 2, 4, 7, 8, 10, 11, 12, 15];