Monotonic Stack Next Greater Element in Java

// Monotonic Stack Next Greater Element in Java
//
// PURPOSE
// A monotonic stack finds each position's next strictly greater value without rescanning the
// suffix.
//
// METHOD
// Store indices with decreasing values. On a larger value, pop and resolve every smaller
// pending index. Each index is pushed and popped at most once.
//
// COMPLEXITY
// O(n) time and O(n) auxiliary space.
//
// CONTRACT
// output has n entries. Writes next greater INDEX or -1 for each position, 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
// Java 17+: primitive int arrays, Arrays.binarySearch for LIS, and Math exact arithmetic.
// Invalid input throws IllegalArgumentException; arithmetic overflow throws
// ArithmeticException.
//
// EXAMPLE
// Expected return: 0.
// Expected output after call: [2, 2, -1].
// 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 next_greater(int[] a, int[] output) {
        java.util.Objects.requireNonNull(a, "a");
        java.util.Objects.requireNonNull(output, "output");
        int n = a.length;
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require((a.length == n));
        alg_require((output.length == n));
        int[] stack = new int[alg_size(Math.addExact(n, 1))];
        int top = 0;
        int i = 0;
        int index = 0;
        while ((i < n)) {
            output[i] = Math.negateExact(1);
            while (((top > 0) && (a[stack[Math.subtractExact(top, 1)]] < a[i]))) {
                top = Math.subtractExact(top, 1);
                index = stack[top];
                output[index] = i;
            }
            stack[top] = i;
            top = Math.addExact(top, 1);
            i = Math.addExact(i, 1);
        }
        return 0;
    }

    public static void main(String[] args) {
        int[] a = new int[] {2, 1, 3};
        int[] output = new int[] {0, 0, 0};
        int result = next_greater(a, output);
        System.out.println(result);
        // Expected output: [2, 2, -1]
        System.out.println(java.util.Arrays.toString(output));
    }
}