Rabin Karp Pattern Matching in Java

// Rabin Karp Pattern Matching in Java
//
// 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
// Java 17+: primitive int arrays, Arrays.binarySearch for LIS, and Math exact arithmetic.
// Invalid input throws IllegalArgumentException; arithmetic overflow throws
// ArithmeticException.
//
// EXAMPLE
// Expected return: 2.
// Run: javac AlgorithmExample.java && java AlgorithmExample
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

class AlgorithmExample {
    static void alg_require(boolean valid) { if (!valid) throw new IllegalArgumentException("invalid algorithm input"); }
    static int alg_size(int n) { alg_require(n>=0 && n<=4000000); return n; }

    static int rabin_karp(int[] text, int[] pattern) {
        java.util.Objects.requireNonNull(text, "text");
        java.util.Objects.requireNonNull(pattern, "pattern");
        int n = text.length;
        int 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 (int alg_i = 0; alg_i < text.length; alg_i++) {
            int alg_value = text[alg_i];
            alg_require(((alg_value >= 0) && (alg_value <= 255)));
        }
        for (int alg_i = 0; alg_i < pattern.length; alg_i++) {
            int alg_value = pattern[alg_i];
            alg_require(((alg_value >= 0) && (alg_value <= 255)));
        }
        if ((m == 0)) {
            return 0;
        }
        if ((m > n)) {
            return Math.negateExact(1);
        }
        int power = 1;
        int target_hash = 0;
        int window_hash = 0;
        int i = 0;
        int j = 0;
        while ((i < m)) {
            target_hash = (Math.addExact(Math.multiplyExact(target_hash, 256), pattern[i]) % 10007);
            window_hash = (Math.addExact(Math.multiplyExact(window_hash, 256), text[i]) % 10007);
            if ((i < Math.subtractExact(m, 1))) {
                power = (Math.multiplyExact(power, 256) % 10007);
            }
            i = Math.addExact(i, 1);
        }
        i = 0;
        while ((i <= Math.subtractExact(n, m))) {
            if ((target_hash == window_hash)) {
                j = 0;
                while (((j < m) && (text[Math.addExact(i, j)] == pattern[j]))) {
                    j = Math.addExact(j, 1);
                }
                if ((j == m)) {
                    return i;
                }
            }
            if ((i < Math.subtractExact(n, m))) {
                window_hash = (Math.addExact(Math.subtractExact(window_hash, (Math.multiplyExact(text[i], power) % 10007)), 10007) % 10007);
                window_hash = (Math.addExact(Math.multiplyExact(window_hash, 256), text[Math.addExact(i, m)]) % 10007);
            }
            i = Math.addExact(i, 1);
        }
        return Math.negateExact(1);
    }

    public static void main(String[] args) {
        int[] text = new int[] {1, 2, 1, 2, 3};
        int[] pattern = new int[] {1, 2, 3};
        int result = rabin_karp(text, pattern);
        System.out.println(result);
    }
}