Modular Exponentiation in JavaScript

"use strict";
// Modular Exponentiation in JavaScript
//
// PURPOSE
// Modular exponentiation computes a power while reducing every intermediate product modulo
// m. It is widely used in number theory, though this teaching version is not constant time.
//
// METHOD
// Reduce the base, then process exponent bits by squaring. Multiply the result on odd bits
// and reduce after each product.
//
// COMPLEXITY
// O(log exponent) modular multiplications, O(1) auxiliary space.
//
// CONTRACT
// base and exponent are nonnegative, modulus > 0. Products of two residues must fit the
// integer type; this is not a cryptographic implementation.
// 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: 24.
// 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 modular_power(base, exponent, modulus) {
    alg_require(Number.isInteger(base) && base >= -2147483648 && base <= 2147483647);
    alg_require(Number.isInteger(exponent) && exponent >= -2147483648 && exponent <= 2147483647);
    alg_require(Number.isInteger(modulus) && modulus >= -2147483648 && modulus <= 2147483647);
    alg_require(((base >= 0) && (exponent >= 0) && (modulus > 0) && (modulus <= 46340)));
    let result = (1 % modulus);
    base = (base % modulus);
    while ((exponent > 0)) {
        if (((exponent % 2) == 1)) {
            result = (alg_mul(result, base) % modulus);
        }
        base = (alg_mul(base, base) % modulus);
        exponent = Math.floor(exponent / 2);
    }
    return result;
}

let base = 2;
let exponent = 10;
let modulus = 1000;
let result = modular_power(base, exponent, modulus);
console.log(result);