Iterative Binary Tree Inorder in JavaScript

"use strict";
// Iterative Binary Tree Inorder in JavaScript
//
// PURPOSE
// Inorder traversal visits each binary-tree node once without recursion.
//
// METHOD
// Keep a stack of nodes and visit states. State zero descends left, state one descends
// right, and state two exits. Emit the node at the traversal-specific state.
//
// COMPLEXITY
// O(n) time and O(n) allocated stack space; live stack usage is O(tree height).
//
// CONTRACT
// Parallel left_child and right_child arrays contain -1 or node IDs. root=-1 means empty.
// The structure must be a valid tree, not cyclic or shared. output has n entries; writes
// node IDs and returns visited count.
// 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: 3.
// Expected output after call: [1, 0, 2].
// 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 tree_inorder(left_child, right_child, root, output) {
    alg_require(left_child instanceof Int32Array);
    alg_require(right_child instanceof Int32Array);
    alg_require(Number.isInteger(root) && root >= -2147483648 && root <= 2147483647);
    alg_require(output instanceof Int32Array);
    let n = left_child.length;
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require((left_child.length == n));
    alg_require((right_child.length == n));
    alg_require((output.length == n));
    alg_require(((root >= alg_sub(0, 1)) && (root < n)));
    for (let alg_i = 0; alg_i < left_child.length; alg_i++) {
        let alg_value = left_child[alg_i];
        alg_require(((alg_value >= alg_sub(0, 1)) && (alg_value < n)));
    }
    for (let alg_i = 0; alg_i < right_child.length; alg_i++) {
        let alg_value = right_child[alg_i];
        alg_require(((alg_value >= alg_sub(0, 1)) && (alg_value < n)));
    }
    let stack = new Int32Array(alg_size(alg_add(n, 1)));
    let state = new Int32Array(alg_size(alg_add(n, 1)));
    let top = 0;
    let u = 0;
    let phase = 0;
    let count = 0;
    if ((root == alg_sub(0, 1))) {
        return 0;
    }
    stack[0] = root;
    top = 1;
    while ((top > 0)) {
        u = stack[alg_sub(top, 1)];
        phase = state[alg_sub(top, 1)];
        if ((phase == 1)) {
            output[count] = u;
            count = alg_add(count, 1);
        }
        if ((phase == 0)) {
            state[alg_sub(top, 1)] = 1;
            if ((left_child[u] != alg_sub(0, 1))) {
                stack[top] = left_child[u];
                state[top] = 0;
                top = alg_add(top, 1);
            }
        } else {
            if ((phase == 1)) {
                state[alg_sub(top, 1)] = 2;
                if ((right_child[u] != alg_sub(0, 1))) {
                    stack[top] = right_child[u];
                    state[top] = 0;
                    top = alg_add(top, 1);
                }
            } else {
                top = alg_sub(top, 1);
            }
        }
    }
    return count;
}

let left_child = new Int32Array([1, -1, -1]);
let right_child = new Int32Array([2, -1, -1]);
let root = 0;
let output = new Int32Array([0, 0, 0]);
let result = tree_inorder(left_child, right_child, root, output);
console.log(result);
// Expected output: [1, 0, 2]
console.log(Array.from(output));