Check if input string is a palindrome, if character deletions are allowed

palindrome.js

/**
Copyright 2020 Cardin Lee

MIT License

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*/

/**
Palindrome

Task: Determine if a given string is a palindrome, if a certain number of character deletions are allowed.

Procedure:
1. Put 1 pointer to the start of the string, 1 pointer to the end of the string
2. A counter has the value 2, e.g. the number of characters that can be deleted. When counter reaches -1, terminate with failure.
3. Generate a permutation of steps that we can advance the pointer while within the counter limits.
4. Iterate through the permutation and the string.
 */

/** @enum {number} */
const DIR = {
    ADVANCE: 1,
    RETREAT: -1
};

class Config {
    /**
     * 
     * @param {boolean} longestPalindrome
     * If true, it favors nearest longest palindrome.
     * If false, it favors nearest palindrome.
     * Whether it's really the longest palindrome, I doubt so.
     * 
     * @param {number} delAllowed 
     */
    constructor(longestPalindrome = true, delAllowed = 2) {
        this.longestPalindrome = longestPalindrome;
        this.delAllowed = delAllowed;
    }
}

class Ptr {
    /**
     * 
     * @param {string} str 
     */
    constructor(str) {
        this.l = -1;
        this.r = str.length;
    }

    hasEnded() {
        // R and L is adjacent (diff 1)
        // R and L is identical (diff 0)
        return this.r - this.l <= 1;
    }

    /**
     * 
     * @param {string} str 
     */
    compareChars(str) {
        return str[this.l] == str[this.r];
    }

    /**
     * 
     * @param {number} offsetL 
     * @param {number} offsetR 
     * @param {number} [dir]
     */
    adjust(offsetL, offsetR, dir = DIR.ADVANCE) {
        // note that r is opposite dir
        this.l += offsetL * dir;
        this.r += offsetR * -dir;
    }
}

/**
 * Runtime: O (v log v), where v = val
 * 
 * @param {number} val 
 * @param {boolean} longestPalindrome
 * @returns All (ordered) permutations of number-pairs that sum to `val`
 */
function permutePairs(val, longestPalindrome) {
    let permute = [];
    for (let i = 0; i <= val; ++i) {
        for (let j = 0; (j + i) <= val; ++j) {
            permute.push([i, j]);
        }
    }
    if (longestPalindrome) {
        permute.sort((a, b) => (a[0] + a[1]) - (b[0] + b[1]));
    }
    return permute;
}
// console.log(permutePairs(4, true).join(" | ")); // debug permutePairs()

/**
 * Runtime: O (d log d), where d = delRemain
 * 
 * ptr will be updated to the last-known correct palindrome position
 * 
 * @param {string} str 
 * @param {Ptr} ptr 
 * @param {Iterable<number[]>} delPairs
 */
function validate(str, ptr, delPairs) {
    for (let [i, j] of delPairs) {
        ptr.adjust(i, j);

        if (ptr.compareChars(str)) {
            return i + j;
        } else {
            ptr.adjust(i, j, DIR.RETREAT);
        }
    }
    return -1;
}

/**
 * Runtime: O(n), where n is the length of palindromeArr
 * @param {string[]} palindromeArr 
 * @param {Ptr} ptr
 */
function createPalindrome(palindromeArr, ptr) {
    let i = (ptr.l == ptr.r) ? palindromeArr.length - 2 : palindromeArr.length - 1;
    for (; i >= 0; --i) {
        palindromeArr.push(palindromeArr[i]);
    }
    return palindromeArr.join("");
}

/**
 * Runtime: O(nd log d), where n is input.length, d is cfg.delAllowed
 * aka O(n) if d is constant
 * 
 * @param {string} input 
 * @param {Config} cfg
 */
function run(input, cfg) {
    const ptr = new Ptr(input);
    let delRemain = cfg.delAllowed;
    let palindromeArr = [];
    // O(d log d)
    let delPermutePairs = permutePairs(cfg.delAllowed, cfg.longestPalindrome);
    
    // O(n) * O(d log d)
    while (!ptr.hasEnded()) {
        ptr.adjust(1, 1);
        // O(d log d)
        const result = validate(input, ptr, delPermutePairs);
        if (result == -1) {
            break;
        } else {
            palindromeArr.push(input[ptr.l]);
            delRemain -= result;
            // O(d log d)
            delPermutePairs = delPermutePairs.filter(x => x[0] + x[1] <= delRemain);
        }
    }
    if (ptr.hasEnded()) {
        // O(n)
        const palindrome = createPalindrome(palindromeArr, ptr);
        console.log(input, "is palindrome", palindrome);
        return palindrome;
    } else {
        console.log(input, "is not palindrome");
        return null;
    }
}

module.exports.checkPalindrome = run;

let cfg = new Config(false, 2);
console.assert(run("abcba", cfg) == "abcba");
console.assert(run("abczba", cfg) == "abcba");
console.assert(run("abczzba", cfg) == "abcba");
console.assert(run("abczzzba", cfg) == "abzzzba");
console.assert(run("abwcfba", cfg) == "abwba");
console.assert(run("awbcbfa", cfg) == "abcba");
console.assert(!run("awefwefwwbcbfa", cfg));

cfg = new Config(true, 2);
console.assert(run("abcba", cfg) == "abcba");
console.assert(run("abczba", cfg) == "abcba");
console.assert(run("abczzba", cfg) == "abzzba");
console.assert(run("abczzzba", cfg) == "abzzzba");
console.assert(run("abwcfba", cfg) == "abwba");
console.assert(run("awbcbfa", cfg) == "abcba");
console.assert(!run("awefwefwwbcbfa", cfg));