Created Rabin Fingerprinting module within util directory

Author: Bruce-Feldman

src/utils/hash/rolling/Rabin_Fingerprint.js

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+/**
+ * Generates fingerprints using Rabin scheme with x = 2 (for potential compiler optimizations).
+ * Guaranteed not to over or underflow if function assumptions are met.
+ */
+export default class RabinFingerprint {
+  /**
+   * @param { function() : number } [primeGenerator]
+   * @assumes Output from any function call is prime less than Number.MAX_SAFE_INTEGER / 2.
+   */
+  constructor(primeGenerator) {
+    this.prime = primeGenerator();
+  }
+
+  /**
+   * @param { array[number] } [values]
+   * @returns {number} - The hash value after digesting input.
+   * @assumes All array elements are non-negative.
+   * @note First element in array is considered to be oldest value.
+   */
+  init(values) {
+    this.val = 0;
+    this.len = values.length;
+
+    for (let i = 0; i < values.length; i += 1) {
+      this.val = (((this.val * 2) % this.prime) + (values[i] % this.prime)) % this.prime;
+    }
+
+    return this.val;
+  }
+
+  /*
+   * @param {number} [oldValue]
+   * @param {number} [newValue]
+   * @returns {number} - The hash value after removing the oldest value & inserting the newest.
+   * @assumes Instance has already been initialized.
+   * @assumes oldValue is the oldest value still processed by the hash.
+   * @assumes newValue is non-negative.
+   */
+  roll(oldValue, newValue) {
+    let oldVal = oldValue % this.prime;
+    for (let i = 1; i < this.len; i += 1) {
+      oldVal = (oldVal * 2) % this.prime;
+    }
+    this.val = (this.val + this.prime - (oldVal % this.prime)) % this.prime;
+
+    const newVal = newValue % this.prime;
+    this.val = (((this.val * 2) % this.prime) + (newVal % this.prime)) % this.prime;
+
+    return this.val;
+  }
+}

src/utils/hash/rolling/__test__/Rabin_Fingerprint.test.js

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+import RabinFingerprint from '../Rabin_Fingerprint';
+
+describe('Rabin fingerprint Hash Family', () => {
+  it('should hash deterministically', () => {
+    const primeVals = [3, 5, 19, 53, 97, 401, 7039, 193939];
+    for (let primeIdx = 0; primeIdx < primeVals.length; primeIdx += 1) {
+      const primeVal = primeVals[primeIdx];
+      const hasher = new RabinFingerprint(() => primeVal);
+
+      // Test basic values
+      expect(hasher.init([])).toEqual(0);
+      expect(hasher.init([1])).toEqual(1);
+
+      // Test overflow
+      const largeVal = Number.MAX_SAFE_INTEGER;
+      expect(hasher.init([primeVal])).toEqual(0);
+      expect(hasher.init([largeVal])).toEqual(largeVal % primeVal);
+
+      const numLargeVal = 2; // 2 ^ numLargeVal fits in javascript number
+      const largeValues = new Array(numLargeVal).fill(largeVal);
+
+      const expVal = ((largeVal % primeVal) * ((2 ** numLargeVal) - 1)) % primeVal;
+      expect(hasher.init(largeValues)).toEqual(expVal);
+
+      // Test using Fermat's little theorem
+      const fermatValues = new Array(primeVal).fill(primeVal);
+      const numFermatTests = 100;
+      for (let i = 0; i < numFermatTests; i += 1) {
+        const randomValue = Math.floor(Math.random() * largeVal);
+        fermatValues[0] = randomValue;
+        expect(hasher.init(fermatValues)).toEqual(randomValue % primeVal);
+      }
+    }
+  });
+
+  it('should roll appropriately', () => {
+    const primeVals = [3, 5, 19, 53, 97, 401, 7039, 193939];
+
+    for (let primeIdx = 0; primeIdx < primeVals.length; primeIdx += 1) {
+      const primeVal = primeVals[primeIdx];
+      const hasher = new RabinFingerprint(() => primeVal);
+
+      // Test basic values
+      const largeVal = Number.MAX_SAFE_INTEGER;
+      expect(hasher.init([0])).toEqual(0);
+      expect(hasher.roll(0, 1)).toEqual(1);
+      expect(hasher.roll(1, primeVal)).toEqual(0);
+      expect(hasher.roll(primeVal, largeVal)).toEqual(largeVal % primeVal);
+
+      const numRollTest = 100;
+      let previousValue = largeVal;
+      for (let i = 0; i < numRollTest; i += 1) {
+        const randomVal = Math.floor(Math.random() * largeVal);
+        expect(hasher.roll(previousValue, randomVal)).toEqual(randomVal % primeVal);
+        previousValue = randomVal;
+      }
+    }
+  });
+});