Rabin Karp Search Algorithm (TheAlgorithms#1545)

aladin002dz · appgurueu · github-actions · web-flow · commit d74f242ac4f8 · 2023-10-30T11:00:31.000+05:30
* Search: Rabin-Karp algorithm

* Prettier Style

* Search: Rabin-Karp adding reference

* Search: Rabin-Karp styling and remove unecessary logging

* Search: Rabin-Karp review notes

* Simplify return

* Updated Documentation in README.md

---------

Co-authored-by: Lars Müller &lt;34514239+appgurueu@users.noreply.github.com&gt;
Co-authored-by: github-actions &lt;${GITHUB_ACTOR}@users.noreply.github.com&gt;
diff --git a/DIRECTORY.md b/DIRECTORY.md
@@ -311,6 +311,7 @@
   * [LinearSearch](Search/LinearSearch.js)
   * [Minesweeper](Search/Minesweeper.js)
   * [QuickSelectSearch](Search/QuickSelectSearch.js)
+  * [RabinKarp](Search/RabinKarp.js)
   * [SlidingWindow](Search/SlidingWindow.js)
   * [StringSearch](Search/StringSearch.js)
   * [TernarySearch](Search/TernarySearch.js)
diff --git a/Search/RabinKarp.js b/Search/RabinKarp.js
@@ -0,0 +1,64 @@
+/*
+ * Implements the Rabin-Karp algorithm for pattern searching.
+ *
+ * The Rabin-Karp algorithm is a string searching algorithm that uses hashing to find patterns in strings.
+ * It is faster than naive string matching algorithms because it avoids comparing every character in the text.
+ *
+ * This implementation uses a rolling hash function to efficiently compute the hash values of substrings.
+ * It also uses a modulo operator to reduce the size of the hash values, which helps to prevent hash collisions.
+ *
+ * The algorithm returns an array of indices where the pattern is found in the text. If the pattern is not
+ * found, the algorithm returns an empty array.
+ *
+ * [Reference](https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm)
+ */
+
+const BASE = 256 // The number of characters in the alphabet
+const MOD = 997 // A prime number used for the hash function
+
+function rabinKarpSearch(text, pattern) {
+  const patternLength = pattern.length
+  const textLength = text.length
+  const hashPattern = hash(pattern, patternLength)
+  const hashText = []
+  const indices = []
+
+  // Calculate the hash of the first substring in the text
+  hashText[0] = hash(text, patternLength)
+
+  // Precompute BASE^(patternLength-1) % MOD
+  const basePow = Math.pow(BASE, patternLength - 1) % MOD
+
+  for (let i = 1; i <= textLength - patternLength + 1; i++) {
+    // Update the rolling hash by removing the first character
+    // and adding the next character in the text
+    hashText[i] =
+      (BASE * (hashText[i - 1] - text.charCodeAt(i - 1) * basePow) +
+        text.charCodeAt(i + patternLength - 1)) %
+      MOD
+
+    // In case of hash collision, check character by character
+    if (hashText[i] < 0) {
+      hashText[i] += MOD
+    }
+
+    // Check if the hashes match and perform a character-wise comparison
+    if (hashText[i] === hashPattern) {
+      if (text.substring(i, i + patternLength) === pattern) {
+        indices.push(i) // Store the index where the pattern is found
+      }
+    }
+  }
+
+  return indices
+}
+
+function hash(str, length) {
+  let hashValue = 0
+  for (let i = 0; i < length; i++) {
+    hashValue = (hashValue * BASE + str.charCodeAt(i)) % MOD
+  }
+  return hashValue
+}
+
+export { rabinKarpSearch }
diff --git a/Search/test/RabinKarp.test.js b/Search/test/RabinKarp.test.js
@@ -0,0 +1,30 @@
+import { rabinKarpSearch } from '../RabinKarp'
+
+describe('Rabin-Karp Search', function () {
+  it('should find the pattern in the text', function () {
+    const text = 'ABABDABACDABABCABAB'
+    const pattern = 'DAB'
+    const expected = [4, 9]
+
+    const result = rabinKarpSearch(text, pattern)
+    expect(result).to.deep.equal(expected)
+  })
+
+  it('should handle multiple occurrences of the pattern', function () {
+    const text = 'ABABABABABAB'
+    const pattern = 'ABAB'
+    const expected = [2, 4, 6, 8]
+
+    const result = rabinKarpSearch(text, pattern)
+    expect(result).to.deep.equal(expected)
+  })
+
+  it('should handle pattern not found', function () {
+    const text = 'ABCD'
+    const pattern = 'XYZ'
+    const expected = []
+
+    const result = rabinKarpSearch(text, pattern)
+    expect(result).to.deep.equal(expected)
+  })
+})
