diff --git a/src/data-structures/hash-table/HashTable.js b/src/data-structures/hash-table/HashTable.js index b8b523ea..c10a0a95 100644 --- a/src/data-structures/hash-table/HashTable.js +++ b/src/data-structures/hash-table/HashTable.js @@ -6,6 +6,27 @@ import LinkedList from '../linked-list/LinkedList'; // are being handled. const defaultHashTableSize = 32; +// This is the table used for the Pearson hashing. +// Which contains shuffled number from 0 to 255. +const table = [ + 84, 124, 126, 180, 160, 221, 178, 97, 71, 13, 102, 167, 209, 153, 232, 237, + 243, 24, 205, 228, 182, 83, 111, 245, 86, 99, 59, 26, 172, 225, 9, 94, + 37, 179, 138, 250, 154, 142, 132, 36, 134, 233, 188, 100, 29, 57, 161, 17, + 41, 136, 198, 61, 241, 46, 201, 137, 214, 10, 45, 109, 145, 204, 15, 119, + 21, 16, 252, 170, 47, 12, 95, 246, 249, 116, 7, 64, 149, 219, 171, 120, + 212, 216, 80, 74, 186, 192, 210, 190, 85, 78, 196, 181, 150, 110, 176, 127, + 140, 22, 98, 82, 18, 53, 230, 240, 139, 62, 70, 151, 157, 159, 213, 207, + 56, 81, 147, 168, 49, 229, 189, 235, 152, 130, 222, 215, 141, 6, 175, 2, + 244, 28, 224, 96, 91, 27, 129, 227, 199, 251, 69, 242, 223, 158, 8, 23, + 19, 31, 236, 25, 68, 254, 112, 247, 208, 148, 38, 114, 40, 55, 88, 231, + 32, 93, 163, 253, 90, 30, 89, 146, 113, 200, 177, 101, 135, 169, 128, 48, + 174, 66, 4, 155, 14, 166, 60, 193, 162, 0, 58, 79, 92, 239, 72, 203, + 123, 115, 67, 217, 106, 52, 173, 20, 35, 50, 5, 44, 11, 143, 206, 73, + 42, 34, 75, 131, 184, 220, 125, 156, 202, 238, 164, 103, 77, 248, 195, 183, + 211, 65, 226, 105, 43, 191, 3, 54, 234, 187, 194, 117, 121, 107, 1, 218, + 185, 76, 133, 33, 39, 255, 197, 63, 51, 144, 122, 87, 104, 118, 165, 108, +]; + export default class HashTable { /** * @param {number} hashTableSize @@ -25,20 +46,33 @@ export default class HashTable { * @return {number} */ hash(key) { - // For simplicity reasons we will just use character codes sum of all characters of the key - // to calculate the hash. + // Using Pearson hashing algorithm to generate the hash number. // - // But you may also use more sophisticated approaches like polynomial string hash to reduce the + // But you may also use other approaches like polynomial string hash to reduce the // number of collisions: // // hash = charCodeAt(0) * PRIME^(n-1) + charCodeAt(1) * PRIME^(n-2) + ... + charCodeAt(n-1) // // where charCodeAt(i) is the i-th character code of the key, n is the length of the key and // PRIME is just any prime number like 31. - const hash = Array.from(key).reduce( - (hashAccumulator, keySymbol) => (hashAccumulator + keySymbol.charCodeAt(0)), - 0, - ); + let hash = 0; + + // Produce a 32-bits hash number. + for (let i = 0; i < 4; i += 1) { + // charCodeAt may return number from 0 to 65536, + // should remap to range 0 to 255. + let hashPerRound = table[(key.charCodeAt(0) + i) % 256]; + for (let j = 1; j < key.length; j += 1) { + hashPerRound = table[hashPerRound ^ (key.charCodeAt(j) % 256)]; + } + + // Left shift by 8-bits and OR the per-rounded hash. + hash = (hash << 8) | hashPerRound; + } + + // Bit mask to clear the left most bit, + // so the result will be a positive number. + hash &= 0x7FFFFFFF; // Reduce hash number so it would fit hash table size. return hash % this.buckets.length; diff --git a/src/data-structures/hash-table/__test__/HashTable.test.js b/src/data-structures/hash-table/__test__/HashTable.test.js index 86bbf3ad..054ad09f 100644 --- a/src/data-structures/hash-table/__test__/HashTable.test.js +++ b/src/data-structures/hash-table/__test__/HashTable.test.js @@ -12,9 +12,9 @@ describe('HashTable', () => { it('should generate proper hash for specified keys', () => { const hashTable = new HashTable(); - expect(hashTable.hash('a')).toBe(1); - expect(hashTable.hash('b')).toBe(2); - expect(hashTable.hash('abc')).toBe(6); + expect(hashTable.hash('a')).toBe(18); + expect(hashTable.hash('b')).toBe(21); + expect(hashTable.hash('abc')).toBe(21); }); it('should set, read and delete data with collisions', () => { @@ -22,7 +22,7 @@ describe('HashTable', () => { expect(hashTable.hash('a')).toBe(1); expect(hashTable.hash('b')).toBe(2); - expect(hashTable.hash('c')).toBe(0); + expect(hashTable.hash('c')).toBe(2); expect(hashTable.hash('d')).toBe(1); hashTable.set('a', 'sky-old'); @@ -37,9 +37,9 @@ describe('HashTable', () => { const stringifier = (value) => `${value.key}:${value.value}`; - expect(hashTable.buckets[0].toString(stringifier)).toBe('c:earth'); + expect(hashTable.buckets[0].toString(stringifier)).toBe(''); expect(hashTable.buckets[1].toString(stringifier)).toBe('a:sky,d:ocean'); - expect(hashTable.buckets[2].toString(stringifier)).toBe('b:sea'); + expect(hashTable.buckets[2].toString(stringifier)).toBe('b:sea,c:earth'); expect(hashTable.get('a')).toBe('sky'); expect(hashTable.get('d')).toBe('ocean'); @@ -94,7 +94,7 @@ describe('HashTable', () => { hashTable.set('b', 'beta'); hashTable.set('c', 'gamma'); - expect(hashTable.getValues()).toEqual(['gamma', 'alpha', 'beta']); + expect(hashTable.getValues()).toEqual(['alpha', 'beta', 'gamma']); }); it('should get all the values from empty hash table', () => { @@ -105,13 +105,12 @@ describe('HashTable', () => { it('should get all the values in case of hash collision', () => { const hashTable = new HashTable(3); - // Keys `ab` and `ba` in current implementation should result in one hash (one bucket). // We need to make sure that several items from one bucket will be serialized. hashTable.set('ab', 'one'); hashTable.set('ba', 'two'); hashTable.set('ac', 'three'); - expect(hashTable.getValues()).toEqual(['one', 'two', 'three']); + expect(hashTable.getValues()).toEqual(['three', 'one', 'two']); }); });