Manacher Longest Palindromic Substring in JavaScript

"use strict";
// Manacher Longest Palindromic Substring in JavaScript
//
// PURPOSE
// Manacher reuses mirrored palindrome radii to find a longest contiguous palindrome in
// linear time.
//
// METHOD
// Compute odd and even center radii in separate passes. Reuse the mirror radius within the
// current rightmost palindrome, then expand only beyond its known boundary.
//
// COMPLEXITY
// O(n) time and O(n) auxiliary space.
//
// CONTRACT
// Integer tokens. Returns longest palindrome LENGTH; empty input returns zero.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// Node.js 18+: Int32Array inputs and workspaces provide compact integer storage. Automatic
// lengths and checked arithmetic reject values outside signed 32-bit. Invalid inputs or
// overflow throw RangeError.
//
// EXAMPLE
// Expected return: 4.
// Run: node example.js
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

function alg_require(valid) { if (!valid) throw new RangeError("invalid algorithm input"); }
function alg_checked(v) { if (!Number.isInteger(v) || v < -2147483648 || v > 2147483647) throw new RangeError("32-bit result overflow"); return v; }
function alg_add(a,b) { return alg_checked(a+b); }
function alg_sub(a,b) { return alg_checked(a-b); }
function alg_mul(a,b) { return alg_checked(a*b); }
function alg_size(n) { alg_require(Number.isInteger(n) && n>=0 && n<=4000000); return n; }

function manacher(a) {
    alg_require(a instanceof Int32Array);
    let n = a.length;
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require((a.length == n));
    let radius = new Int32Array(alg_size(alg_add(n, 1)));
    let parity = 0;
    let left = 0;
    let right = alg_sub(0, 1);
    let i = 0;
    let k = 0;
    let best = 0;
    while ((parity < 2)) {
        left = 0;
        right = alg_sub(0, 1);
        i = 0;
        while ((i < n)) {
            k = alg_sub(1, parity);
            if ((i <= right)) {
                k = Math.min(radius[alg_add(alg_sub(alg_add(left, right), i), parity)], alg_add(alg_sub(right, i), 1));
            }
            while (((alg_sub(alg_sub(i, k), parity) >= 0) && (alg_add(i, k) < n) && (a[alg_sub(alg_sub(i, k), parity)] == a[alg_add(i, k)]))) {
                k = alg_add(k, 1);
            }
            radius[i] = k;
            best = Math.max(best, alg_add(alg_sub(alg_mul(2, k), 1), parity));
            if ((alg_sub(alg_add(i, k), 1) > right)) {
                left = alg_add(alg_sub(alg_sub(i, k), parity), 1);
                right = alg_sub(alg_add(i, k), 1);
            }
            i = alg_add(i, 1);
        }
        parity = alg_add(parity, 1);
    }
    return best;
}

let a = new Int32Array([1, 2, 2, 1]);
let result = manacher(a);
console.log(result);