Monotonic Stack Next Greater Element in JavaScript

"use strict";
// Monotonic Stack Next Greater Element in JavaScript
//
// PURPOSE
// A monotonic stack finds each position's next strictly greater value without rescanning the
// suffix.
//
// METHOD
// Store indices with decreasing values. On a larger value, pop and resolve every smaller
// pending index. Each index is pushed and popped at most once.
//
// COMPLEXITY
// O(n) time and O(n) auxiliary space.
//
// CONTRACT
// output has n entries. Writes next greater INDEX or -1 for each position, 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: 0.
// Expected output after call: [2, 2, -1].
// 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 next_greater(a, output) {
    alg_require(a instanceof Int32Array);
    alg_require(output instanceof Int32Array);
    let n = a.length;
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require((a.length == n));
    alg_require((output.length == n));
    let stack = new Int32Array(alg_size(alg_add(n, 1)));
    let top = 0;
    let i = 0;
    let index = 0;
    while ((i < n)) {
        output[i] = alg_sub(0, 1);
        while (((top > 0) && (a[stack[alg_sub(top, 1)]] < a[i]))) {
            top = alg_sub(top, 1);
            index = stack[top];
            output[index] = i;
        }
        stack[top] = i;
        top = alg_add(top, 1);
        i = alg_add(i, 1);
    }
    return 0;
}

let a = new Int32Array([2, 1, 3]);
let output = new Int32Array([0, 0, 0]);
let result = next_greater(a, output);
console.log(result);
// Expected output: [2, 2, -1]
console.log(Array.from(output));