From d74f4394d358440a24bb00ae2c89e0cffd7d74a5 Mon Sep 17 00:00:00 2001
From: remes2000 <nieruchalskimichal@gmail.com>
Date: Sun, 2 Aug 2020 13:40:25 +0200
Subject: [PATCH] Implement longest common substring algorithm

---
 README.md                                     |  1 +
 src/algorithms/string/index.js                |  4 +-
 .../string/longest_common_substring.js        | 40 ++++++++++++++++
 .../string/testLongestCommonSubstring.js      | 46 +++++++++++++++++++
 4 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 src/algorithms/string/longest_common_substring.js
 create mode 100644 test/algorithms/string/testLongestCommonSubstring.js

diff --git a/README.md b/README.md
index ce93985d..3d8f7129 100644
--- a/README.md
+++ b/README.md
@@ -79,6 +79,7 @@ Various Math algorithms:
 Various String algorithms:
 
 - [Levenshtein Distance](https://github.com/manrajgrover/algorithms-js/blob/master/src/algorithms/string/levenshtein_distance.js)
+- [Longest Common Substring](https://github.com/manrajgrover/algorithms-js/blob/master/src/algorithms/string/longest_common_substring.js)
 
 #### Geometry
 Various Geometry algorithms:
diff --git a/src/algorithms/string/index.js b/src/algorithms/string/index.js
index ad535890..61fd95a8 100644
--- a/src/algorithms/string/index.js
+++ b/src/algorithms/string/index.js
@@ -1,5 +1,7 @@
 const levenshteindistance = require('./levenshtein_distance');
+const longestcommonsubstring = require('./longest_common_substring');
 
 module.exports = {
-  levenshteindistance
+  levenshteindistance,
+  longestcommonsubstring
 };
diff --git a/src/algorithms/string/longest_common_substring.js b/src/algorithms/string/longest_common_substring.js
new file mode 100644
index 00000000..c360bcd5
--- /dev/null
+++ b/src/algorithms/string/longest_common_substring.js
@@ -0,0 +1,40 @@
+/**
+ * Returns Longest Common Substring of two strings
+ * @param  {String} firstWord First string
+ * @param  {String} secondWord Second string
+ * @return {String} One of the possbile longest common substring for given inputs
+ */
+function longestcommonsubstring(firstWord = '', secondWord = '') {
+  const firstWordSize = firstWord.length;
+  const secondWordSize = secondWord.length;
+  const table = [...Array(firstWordSize + 1)].map(() => Array(secondWordSize + 1));
+  let maxSubstringLength = 0;
+  let maxSubstringRow = 0;
+  let maxSubstringCol = 0;
+  for (let i = 0; i <= firstWordSize; i += 1) {
+    for (let j = 0; j <= secondWordSize; j += 1) {
+      if (i === 0 || j === 0) {
+        table[i][j] = 0;
+      } else if (firstWord.charAt(i - 1) === secondWord.charAt(j - 1)) {
+        table[i][j] = table[i - 1][j - 1] + 1;
+        if (table[i][j] > maxSubstringLength) {
+          maxSubstringLength = table[i][j];
+          maxSubstringRow = i;
+          maxSubstringCol = j;
+        }
+      } else {
+        table[i][j] = 0;
+      }
+    }
+  }
+
+  let result = '';
+  while (table[maxSubstringRow][maxSubstringCol] !== 0) {
+    result = firstWord.charAt(maxSubstringRow - 1) + result;
+    maxSubstringRow -= 1;
+    maxSubstringCol -= 1;
+  }
+  return result;
+}
+
+module.exports = longestcommonsubstring;
diff --git a/test/algorithms/string/testLongestCommonSubstring.js b/test/algorithms/string/testLongestCommonSubstring.js
new file mode 100644
index 00000000..47a0c7a9
--- /dev/null
+++ b/test/algorithms/string/testLongestCommonSubstring.js
@@ -0,0 +1,46 @@
+/* eslint-env mocha */
+const longestcommonsubstring = require('../../../src').algorithms.string.longestcommonsubstring;
+const assert = require('assert');
+
+describe('Longest Common Substring', () => {
+  it('should find abcd for xyzabcd and abcdxyz', () => {
+    const stringA = 'xyzabcd';
+    const stringB = 'abcdxyz';
+
+    const result = longestcommonsubstring(stringA, stringB);
+    assert.equal(result, 'abcd');
+  });
+
+  it('should find Geeks for GeeksforGeeks and GeeksQuiz', () => {
+    const stringA = 'GeeksForGeeks';
+    const stringB = 'GeeksQuiz';
+
+    const result = longestcommonsubstring(stringA, stringB);
+    assert.equal(result, 'Geeks');
+  });
+
+  it('should return empty string when one or both of inputs is empty', () => {
+    const resultOne = longestcommonsubstring('abcdef', '');
+    assert.equal(resultOne, '');
+    const resultTwo = longestcommonsubstring('', 'abcasds');
+    assert.equal(resultTwo, '');
+    const resultThree = longestcommonsubstring('', '');
+    assert.equal(resultThree, '');
+  });
+
+  it('should return empty string for letter and aggghhhww', () => {
+    const stringA = 'letter';
+    const stringB = 'aggghhhww';
+
+    const result = longestcommonsubstring(stringA, stringB);
+    assert.equal(result, '');
+  });
+
+  it('should return a for xysysysyysahhfghf and aaaaaa', () => {
+    const stringA = 'xysysysyysahhfghf';
+    const stringB = 'aaaaaa';
+
+    const result = longestcommonsubstring(stringA, stringB);
+    assert.equal(result, 'a');
+  });
+});