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feat: Implementing Rabin-Karp algorithm for substring search #1757

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Rudrajiii wants to merge 2 commits into TheAlgorithms:master from Rudrajiii:master
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88 changes: 88 additions & 0 deletions String/RabinKarp.js
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/**
* Rabin-Karp algorithm for finding all occurrences of a pattern in a text.
* It uses hashing to efficiently find substrings.
* For More https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm
*
* Time Complexity:
* - Average: O(n + m)
* - Worst: O(n * m) due to potential hash collisions.
*
* Space Complexity: O(1) - only uses a fixed number of variables.
*
* @param {string} haystack - The text to search within.
* @param {string} needle - The substring to search for.
* @returns {number[]} - An array containing the starting indices of matches.
* @throws {RangeError} - If either input is an empty string.
*
* @example:
* const text = "abracadabra";
* const pattern = "abra";
* const result = rabinKarp(text, pattern);
* console.log(result); // Output: [0, 7]
*
* Explanation:
* The pattern "abra" occurs at index 0 and index 7 in the text "abracadabra".
*
* Here's how the algorithm works:
* 1. It computes the hash of the pattern and the hash of the first window of the text.
* 2. If the hashes match, it performs a character-by-character comparison to confirm a match.
* 3. The algorithm then slides over the text, recalculating the hash for each new window.
* 4. It continues until it has checked all possible windows in the text.
*/
export function rabinKarp(haystack, needle) {
if (!haystack || !needle) {
throw new RangeError('Input text and pattern cannot be empty.')
}

const base = 256 // Character set size (extended ASCII)
const prime = 101 // Prime number for hash computation
const needleLength = needle.length
const haystackLength = haystack.length
const matches = []

let needleHash = 0 // Hash value for the pattern
let haystackHash = 0 // Hash value for the current window in the text
let hashMultiplier = 1

// Calculate the multiplier used for the leading character in the hash
for (let i = 0; i < needleLength - 1; i++) {
hashMultiplier = (hashMultiplier * base) % prime
}

// Calculate the initial hash values for the pattern and the first window of the text
for (let i = 0; i < needleLength; i++) {
needleHash = (base * needleHash + needle.charCodeAt(i)) % prime
haystackHash = (base * haystackHash + haystack.charCodeAt(i)) % prime
}

// Slide over the text to check for matches
for (let i = 0; i <= haystackLength - needleLength; i++) {
// If the hash values match, check for character by character equality
if (needleHash === haystackHash) {
let j
for (j = 0; j < needleLength; j++) {
if (haystack[i + j] !== needle[j]) {
break
}
}
if (j === needleLength) {
matches.push(i) // Pattern found, add index to results
}
}

// Calculate the hash value for the next window
if (i < haystackLength - needleLength) {
haystackHash =
(base * (haystackHash - haystack.charCodeAt(i) * hashMultiplier) +
haystack.charCodeAt(i + needleLength)) %
prime

// Ensure the hash value is non-negative
if (haystackHash < 0) {
haystackHash += prime
}
}
}

return matches
}
36 changes: 36 additions & 0 deletions String/test/RabinKarp.test.js
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import { rabinKarp } from '../RabinKarp'

describe('Rabin-Karp Algorithm', () => {
it('should find occurrences of the pattern in the text', () => {
const haystack = 'abracadabra'
const needle = 'abra'
const result = rabinKarp(haystack, needle)
expect(result).toEqual([0, 7])
})

it('should return an empty array if the pattern is absent', () => {
const haystack = 'hello world'
const needle = 'test'
const result = rabinKarp(haystack, needle)
expect(result).toEqual([])
})

it('should throw RangeError for empty input', () => {
expect(() => rabinKarp('', 'pattern')).toThrow(RangeError)
expect(() => rabinKarp('text', '')).toThrow(RangeError)
})

it('should return empty for patterns longer than text', () => {
const haystack = 'short'
const needle = 'longerpattern'
const result = rabinKarp(haystack, needle)
expect(result).toEqual([])
})

it('should handle single-character patterns', () => {
const haystack = 'hello'
const needle = 'e'
const result = rabinKarp(haystack, needle)
expect(result).toEqual([1])
})
})

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