Knuth Morris Pratt Pattern Matching in JavaScript

"use strict";
// Knuth Morris Pratt Pattern Matching in JavaScript
//
// PURPOSE
// KMP avoids rechecking text tokens by remembering which matched pattern suffix is also a
// prefix. The prefix table tells where to resume after a mismatch.
//
// METHOD
// Build the longest proper prefix/suffix table for the pattern. During matching, fall back
// through that table without moving the text position backward. A full matched prefix gives
// the first occurrence.
//
// COMPLEXITY
// O(n+m) time and O(m) auxiliary space.
//
// CONTRACT
// Inputs are integer token sequences. Empty pattern matches at 0. Returns first matching
// index or -1. KMP does not define Unicode tokenization.
// 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 kmp_match(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));
    if ((m == 0)) {
        return 0;
    }
    let prefix = new Int32Array(alg_size(alg_add(m, 1)));
    let i = 1;
    let j = 0;
    while ((i < m)) {
        while (((j > 0) && (pattern[i] != pattern[j]))) {
            j = prefix[alg_sub(j, 1)];
        }
        if ((pattern[i] == pattern[j])) {
            j = alg_add(j, 1);
        }
        prefix[i] = j;
        i = alg_add(i, 1);
    }
    i = 0;
    j = 0;
    while ((i < n)) {
        while (((j > 0) && (text[i] != pattern[j]))) {
            j = prefix[alg_sub(j, 1)];
        }
        if ((text[i] == pattern[j])) {
            j = alg_add(j, 1);
        }
        if ((j == m)) {
            return alg_add(alg_sub(i, m), 1);
        }
        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 = kmp_match(text, pattern);
console.log(result);