Rabin Karp Pattern Matching in JavaScript

"use strict";
// Rabin Karp Pattern Matching in JavaScript
//
// PURPOSE
// Rabin-Karp uses a rolling hash to filter candidate matches, then compares tokens to
// guarantee correctness despite hash collisions.
//
// METHOD
// Hash the pattern and first window modulo a small prime. At each position verify matching
// hashes, then remove the old token and append the new one.
//
// COMPLEXITY
// Expected O(n+m) time on suitable data; O(nm) worst case due to verification. O(1)
// auxiliary space.
//
// CONTRACT
// Tokens are integers in [0,255]. Empty pattern matches at zero. Returns first index or -1;
// not a cryptographic hash.
// 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: 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 rabin_karp(text, pattern) {
    alg_require(text instanceof Int32Array);
    alg_require(pattern instanceof Int32Array);
    let n = text.length;
    let m = pattern.length;
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require(((m >= 0) && (m <= 1000000)));
    alg_require((pattern.length == m));
    alg_require((text.length == n));
    for (let alg_i = 0; alg_i < text.length; alg_i++) {
        let alg_value = text[alg_i];
        alg_require(((alg_value >= 0) && (alg_value <= 255)));
    }
    for (let alg_i = 0; alg_i < pattern.length; alg_i++) {
        let alg_value = pattern[alg_i];
        alg_require(((alg_value >= 0) && (alg_value <= 255)));
    }
    if ((m == 0)) {
        return 0;
    }
    if ((m > n)) {
        return alg_sub(0, 1);
    }
    let power = 1;
    let target_hash = 0;
    let window_hash = 0;
    let i = 0;
    let j = 0;
    while ((i < m)) {
        target_hash = (alg_add(alg_mul(target_hash, 256), pattern[i]) % 10007);
        window_hash = (alg_add(alg_mul(window_hash, 256), text[i]) % 10007);
        if ((i < alg_sub(m, 1))) {
            power = (alg_mul(power, 256) % 10007);
        }
        i = alg_add(i, 1);
    }
    i = 0;
    while ((i <= alg_sub(n, m))) {
        if ((target_hash == window_hash)) {
            j = 0;
            while (((j < m) && (text[alg_add(i, j)] == pattern[j]))) {
                j = alg_add(j, 1);
            }
            if ((j == m)) {
                return i;
            }
        }
        if ((i < alg_sub(n, m))) {
            window_hash = (alg_add(alg_sub(window_hash, (alg_mul(text[i], power) % 10007)), 10007) % 10007);
            window_hash = (alg_add(alg_mul(window_hash, 256), text[alg_add(i, m)]) % 10007);
        }
        i = alg_add(i, 1);
    }
    return alg_sub(0, 1);
}

let text = new Int32Array([1, 2, 1, 2, 3]);
let pattern = new Int32Array([1, 2, 3]);
let result = rabin_karp(text, pattern);
console.log(result);