Merge pull request #537 from rahul1995/kmp-matcher

Add KMP pattern searching algorithm (Fixes #518)

Author: pomkarnath98

String/KMPPatternSearching.js

@@ -0,0 +1,55 @@
+// Implementing KMP Search Algorithm to search all the instances of pattern in
+// given text
+// Reference Book: Introduction to Algorithms, CLRS
+
+// Explanation: https://www.topcoder.com/community/competitive-programming/tutorials/introduction-to-string-searching-algorithms/
+
+const computeLPS = (pattern) => {
+  const lps = Array(pattern.length)
+  lps[0] = 0
+  for (let i = 1; i < pattern.length; i++) {
+    let matched = lps[i - 1]
+    while (matched > 0 && pattern[i] !== pattern[matched]) {
+      matched = lps[matched - 1]
+    }
+    if (pattern[i] === pattern[matched]) {
+      matched++
+    }
+    lps[i] = matched
+  }
+  return lps
+}
+
+/**
+ * Returns all indices where pattern starts in text
+ * @param {*} text a big text in which pattern string is to find
+ * @param {*} pattern the string to find
+ */
+const KMPSearch = (text, pattern) => {
+  if (!pattern || !text) {
+    return [] // no results
+  }
+
+  // lps[i] = length of proper prefix of pattern[0]...pattern[i-1]
+  //          which is also proper suffix of it
+  const lps = computeLPS(pattern)
+  const result = []
+
+  let matched = 0
+  for (let i = 0; i < text.length; i++) {
+    while (matched > 0 && text[i] !== pattern[matched]) {
+      matched = lps[matched - 1]
+    }
+    if (text[i] === pattern[matched]) {
+      matched++
+    }
+    if (matched === pattern.length) {
+      result.push(i - pattern.length + 1)
+      matched = lps[matched - 1]
+    }
+  }
+
+  return result
+}
+
+export { KMPSearch }

String/test/KMPPatternSearching.test.js

@@ -0,0 +1,27 @@
+import { KMPSearch } from '../KMPPatternSearching'
+
+describe('KMP Matcher', () => {
+  it('TC1: expects to return matching indices for pattern in text', () => {
+    const text = 'ABC ABCDAB ABCDABCDABDE'
+    const pattern = 'ABCDABD'
+    expect(KMPSearch(text, pattern)).toStrictEqual([15])
+  })
+
+  it('TC2: expects to return matching indices for pattern in text', () => {
+    const text = 'ABC ABCDABD ABCDABCDABDE'
+    const pattern = 'ABCDABD'
+    expect(KMPSearch(text, pattern)).toStrictEqual([4, 16])
+  })
+
+  it('TC3: expects to return matching indices for pattern in text', () => {
+    const text = 'AAAAA'
+    const pattern = 'AAA'
+    expect(KMPSearch(text, pattern)).toStrictEqual([0, 1, 2])
+  })
+
+  it('TC4: expects to return matching indices for pattern in text', () => {
+    const text = 'ABCD'
+    const pattern = 'BA'
+    expect(KMPSearch(text, pattern)).toStrictEqual([])
+  })
+})