Longest Palindromic Subsequence in JavaScript

"use strict";
// Longest Palindromic Subsequence in JavaScript
//
// PURPOSE
// A longest palindromic subsequence may skip tokens while preserving order. It differs from
// a contiguous palindromic substring.
//
// METHOD
// Scan left endpoints backward and right endpoints forward. Equal endpoints extend the inner
// solution by two; otherwise discard one endpoint. Retain one row and the old diagonal.
//
// COMPLEXITY
// O(n squared) time, O(n) auxiliary space.
//
// CONTRACT
// Integer tokens. Returns length; empty input returns 0.
// 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: 5.
// 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 palindromic_subsequence(a) {
    alg_require(a instanceof Int32Array);
    let n = a.length;
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require((a.length == n));
    let dp = new Int32Array(alg_size(alg_add(n, 1)));
    let left = alg_sub(n, 1);
    let right = 0;
    let diagonal = 0;
    let old = 0;
    while ((left >= 0)) {
        dp[left] = 1;
        diagonal = 0;
        right = alg_add(left, 1);
        while ((right < n)) {
            old = dp[right];
            if ((a[left] == a[right])) {
                dp[right] = alg_add(diagonal, 2);
            } else {
                dp[right] = Math.max(dp[right], dp[alg_sub(right, 1)]);
            }
            diagonal = old;
            right = alg_add(right, 1);
        }
        left = alg_sub(left, 1);
    }
    if ((n == 0)) {
        return 0;
    }
    return dp[alg_sub(n, 1)];
}

let a = new Int32Array([1, 2, 3, 2, 1]);
let result = palindromic_subsequence(a);
console.log(result);