Iterative Segment Tree Point Assignment in JavaScript

"use strict";
// Iterative Segment Tree Point Assignment in JavaScript
//
// PURPOSE
// A point assignment changes a leaf and recomputes only its ancestors.
//
// METHOD
// Write the new leaf value, move upward by dividing the index by two, and recompute each
// parent from its children.
//
// COMPLEXITY
// O(log n) time, O(1) auxiliary space.
//
// CONTRACT
// tree is an existing iterative sum tree of length 2*n; n>=1, 0<=index<n. Mutates tree;
// 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 tree after call: [0, 17, 10, 7, 1, 9, 3, 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 segment_set(tree, index, value) {
    alg_require(tree instanceof Int32Array);
    alg_require(Number.isInteger(index) && index >= -2147483648 && index <= 2147483647);
    alg_require(Number.isInteger(value) && value >= -2147483648 && value <= 2147483647);
    let n = Math.floor(tree.length / 2);
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require((tree.length == alg_mul(2, n)));
    alg_require(((n >= 1) && (index >= 0) && (index < n)));
    index = alg_add(index, n);
    tree[index] = value;
    index = Math.floor(index / 2);
    while ((index > 0)) {
        tree[index] = alg_add(tree[alg_mul(2, index)], tree[alg_add(alg_mul(2, index), 1)]);
        index = Math.floor(index / 2);
    }
    return 0;
}

let tree = new Int32Array([0, 10, 3, 7, 1, 2, 3, 4]);
let index = 1;
let value = 9;
let result = segment_set(tree, index, value);
console.log(result);
// Expected tree: [0, 17, 10, 7, 1, 9, 3, 4]
console.log(Array.from(tree));