@technobuddha/library/longestCommonSubstring/longestCommonSubstring.ts

A large library of useful functions

import create2DArray    from '../create2DArray';

type Options = {
    /** compare the two strings in case insensitive mode */
    caseInsensitive?: boolean;
};

/**
 * Implementation of Longest Common Substring problem.
 * https://en.wikipedia.org/wiki/Longest_common_substring_problem
 *
 * Returns the longest possible substring that is substring of both of given strings.
 *
 * @param array1 First string.
 * @param array2 Second string.
 * @returns A string that is common to both strings such that there is no
 * common substring with size greater than the length of the string.
 */
export function longestCommonSubstring(string1: string, string2: string, { caseInsensitive = false }: Options = {}): string {
    const ci1 = caseInsensitive ? string1.toLowerCase() : string1;
    const ci2 = caseInsensitive ? string2.toLowerCase() : string2;

    const comparsions = create2DArray(ci1.length + 1, ci2.length + 1, 0);
    let maxSubStrLength = 0;
    let lastMaxSubStrIndex = -1;

    for(let i = 0; i < ci1.length; ++i) {
        const c1 = ci1.charAt(i);

        for(let j = 0; j < ci2.length; ++j) {
            const c2 = ci2.charAt(j);

            if(c1 === c2) {
                if(i > 0 && j > 0)
                    comparsions[i][j] = comparsions[i - 1][j - 1] + 1;
                else
                    comparsions[i][j] = 1;
            } else { comparsions[i][j] = 0; }

            if(comparsions[i][j] > maxSubStrLength) {
                maxSubStrLength = comparsions[i][j];
                lastMaxSubStrIndex = i;
            }
        }
    }

    return string1.slice(lastMaxSubStrIndex - maxSubStrLength + 1, lastMaxSubStrIndex + 1);
}

export default longestCommonSubstring;