Add Rabin.

Author: trekhleb

src/algorithms/string/rabin-karp/__test__/rabinKarp.test.js

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+import { rabinKarp, hashWord, reHashWord } from '../rabinKarp';
+
+describe('rabinKarp', () => {
+  it('should correctly calculates hash and re-hash', () => {
+    expect(hashWord('a')).toBe(97);
+    expect(hashWord('b')).toBe(98);
+    expect(hashWord('abc')).toBe(941094);
+    expect(hashWord('bcd')).toBe(950601);
+    expect(reHashWord(hashWord('abc'), 'abc', 'bcd')).toBe(950601);
+    expect(reHashWord(hashWord('abc'), 'abc', 'bcd')).toBe(hashWord('bcd'));
+  });
+
+  it('should find substring in a string', () => {
+    expect(rabinKarp('abcbcglx', 'abca')).toBe(-1);
+    expect(rabinKarp('abcbcglx', 'bcgl')).toBe(3);
+    expect(rabinKarp('abcxabcdabxabcdabcdabcy', 'abcdabcy')).toBe(15);
+    expect(rabinKarp('abcxabcdabxabcdabcdabcy', 'abcdabca')).toBe(-1);
+    expect(rabinKarp('abcxabcdabxaabcdabcabcdabcdabcy', 'abcdabca')).toBe(12);
+    expect(rabinKarp('abcxabcdabxaabaabaaaabcdabcdabcy', 'aabaabaaa')).toBe(11);
+  });
+});

src/algorithms/string/rabin-karp/rabinKarp.js

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+/**
+ * A prime number used to create
+ * the hash representation of a word
+ *
+ * Bigger the prime number,
+ * bigger the hash value
+ */
+const PRIME = 97;
+
+/**
+ * Function that creates hash representation of the word.
+ *
+ * @param {string} word
+ * @return {number}
+ */
+export function hashWord(word) {
+  let hash = 0;
+
+  for (let charIndex = 0; charIndex < word.length; charIndex += 1) {
+    hash += word[charIndex].charCodeAt(0) * (PRIME ** charIndex);
+  }
+
+  return hash;
+}
+
+/**
+ * Function that creates hash representation of the word
+ * based on previous word (shifted by one character left) hash value.
+ *
+ * Recalculates the hash representation of a word so that it isn't
+ * necessary to traverse the whole word again
+ *
+ * @param {number} prevHash
+ * @param {string} prevWord
+ * @param {string} newWord
+ * @return {number}
+ */
+export function reHashWord(prevHash, prevWord, newWord) {
+  const newWordLastIndex = newWord.length - 1;
+  let newHash = prevHash - prevWord[0].charCodeAt(0);
+  newHash /= PRIME;
+  newHash += newWord[newWordLastIndex].charCodeAt(0) * (PRIME ** newWordLastIndex);
+
+  return newHash;
+}
+
+/**
+ * @param {string} text
+ * @param {string} word
+ * @return {number}
+ */
+export function rabinKarp(text, word) {
+  // Calculate word hash that we will use for comparison with other substring hashes.
+  const wordHash = hashWord(word);
+
+  let prevSegment = null;
+  let currentSegmentHash = null;
+
+  // Go through all substring of the text that may match
+  for (let charIndex = 0; charIndex <= text.length - word.length; charIndex += 1) {
+    const currentSegment = text.substring(charIndex, charIndex + word.length);
+
+    // Calculate the hash of current substring.
+    if (currentSegmentHash === null) {
+      currentSegmentHash = hashWord(currentSegment);
+    } else {
+      currentSegmentHash = reHashWord(currentSegmentHash, prevSegment, currentSegment);
+    }
+
+    prevSegment = currentSegment;
+
+    // Compare the hash of current substring and seeking string.
+    if (wordHash === currentSegmentHash) {
+      // In case if hashes match let's check substring char by char.
+      let numberOfMatches = 0;
+
+      for (let deepCharIndex = 0; deepCharIndex < word.length; deepCharIndex += 1) {
+        if (word[deepCharIndex] === text[charIndex + deepCharIndex]) {
+          numberOfMatches += 1;
+        }
+      }
+
+      if (numberOfMatches === word.length) {
+        return charIndex;
+      }
+    }
+  }
+
+  return -1;
+}