Add SimplePolynomialHash function.

Author: trekhleb

src/algorithms/cryptography/polynomial-hash/PolynomialHash.js

@@ -23,7 +23,6 @@ export default class PolynomialHash {
     const charCodes = Array.from(word).map(char => this.charToNumber(char));
 
     let hash = 0;
-
     for (let charIndex = 0; charIndex < charCodes.length; charIndex += 1) {
       hash *= this.base;
       hash %= this.modulus;

src/algorithms/cryptography/polynomial-hash/SimplePolynomialHash.js

@@ -0,0 +1,63 @@
+const DEFAULT_BASE = 17;
+
+export default class SimplePolynomialHash {
+  /**
+   * @param {number} [base] - Base number that is used to create the polynomial.
+   */
+  constructor(base = DEFAULT_BASE) {
+    this.base = base;
+  }
+
+  /**
+   * Function that creates hash representation of the word.
+   *
+   * Time complexity: O(word.length).
+   *
+   * @assumption: This version of the function  doesn't use modulo operator.
+   * Thus it may produce number overflows by generating numbers that are
+   * bigger than Number.MAX_SAFE_INTEGER. This function is mentioned here
+   * for simplicity and LEARNING reasons.
+   *
+   * @param {string} word - String that needs to be hashed.
+   * @return {number}
+   */
+  hash(word) {
+    let hash = 0;
+    for (let charIndex = 0; charIndex < word.length; charIndex += 1) {
+      hash += word.charCodeAt(charIndex) * (this.base ** 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.
+   *
+   * Time complexity: O(1).
+   *
+   * @assumption: This function doesn't use modulo operator and thus is not safe since
+   * it may deal with numbers that are bigger than Number.MAX_SAFE_INTEGER. This
+   * function is mentioned here for simplicity and LEARNING reasons.
+   *
+   * @param {number} prevHash
+   * @param {string} prevWord
+   * @param {string} newWord
+   * @return {number}
+   */
+  roll(prevHash, prevWord, newWord) {
+    let hash = prevHash;
+
+    const prevValue = prevWord.charCodeAt(0);
+    const newValue = newWord.charCodeAt(newWord.length - 1);
+
+    hash -= prevValue;
+    hash /= this.base;
+    hash += newValue * (this.base ** (newWord.length - 1));
+
+    return hash;
+  }
+}

src/algorithms/cryptography/polynomial-hash/__test__/SimplePolynomialHash.test.js

@@ -0,0 +1,52 @@
+import SimplePolynomialHash from '../SimplePolynomialHash';
+
+describe('PolynomialHash', () => {
+  it('should calculate new hash based on previous one', () => {
+    const bases = [3, 5];
+    const frameSizes = [5, 10];
+
+    const text = 'Lorem Ipsum is simply dummy text of the printing and '
+      + 'typesetting industry. Lorem Ipsum has been the industry\'s standard '
+      + 'galley of type and \u{ffff} scrambled it to make a type specimen book. It '
+      + 'electronic 耀 typesetting, remaining essentially unchanged. It was '
+      + 'popularised in the 1960s with the release of Letraset sheets '
+      + 'publishing software like Aldus 耀 PageMaker including versions of Lorem.';
+
+    // Check hashing for different prime base.
+    bases.forEach((base) => {
+      const polynomialHash = new SimplePolynomialHash(base);
+
+      // Check hashing for different word lengths.
+      frameSizes.forEach((frameSize) => {
+        let previousWord = text.substr(0, frameSize);
+        let previousHash = polynomialHash.hash(previousWord);
+
+        // Shift frame through the whole text.
+        for (let frameShift = 1; frameShift < (text.length - frameSize); frameShift += 1) {
+          const currentWord = text.substr(frameShift, frameSize);
+          const currentHash = polynomialHash.hash(currentWord);
+          const currentRollingHash = polynomialHash.roll(previousHash, previousWord, currentWord);
+
+          // Check that rolling hash is the same as directly calculated hash.
+          expect(currentRollingHash).toBe(currentHash);
+
+          previousWord = currentWord;
+          previousHash = currentHash;
+        }
+      });
+    });
+  });
+
+  it('should generate numeric hashed', () => {
+    const polynomialHash = new SimplePolynomialHash();
+
+    expect(polynomialHash.hash('Test')).toBe(604944);
+    expect(polynomialHash.hash('a')).toBe(97);
+    expect(polynomialHash.hash('b')).toBe(98);
+    expect(polynomialHash.hash('c')).toBe(99);
+    expect(polynomialHash.hash('d')).toBe(100);
+    expect(polynomialHash.hash('e')).toBe(101);
+    expect(polynomialHash.hash('ab')).toBe(1763);
+    expect(polynomialHash.hash('abc')).toBe(30374);
+  });
+});