Two Pointer Pair Sum in Java

// Two Pointer Pair Sum in Java
//
// PURPOSE
// Two pointers find whether two distinct positions in a sorted array sum to a target. Sorted
// order tells which pointer can improve the sum.
//
// METHOD
// Compare the sum at the two ends. Increase the left pointer when the sum is too small, or
// decrease the right pointer when too large. Each move safely eliminates one position.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space.
//
// CONTRACT
// Input must be sorted ascending. Returns 1 if a pair of distinct indices exists, otherwise
// 0. Pair sums must fit.
// 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: 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 pair_sum(int[] a, int target) {
        java.util.Objects.requireNonNull(a, "a");
        int n = a.length;
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require((a.length == n));
        int left = 0;
        int right = Math.subtractExact(n, 1);
        int total = 0;
        while ((left < right)) {
            total = Math.addExact(a[left], a[right]);
            if ((total == target)) {
                return 1;
            }
            if ((total < target)) {
                left = Math.addExact(left, 1);
            } else {
                right = Math.subtractExact(right, 1);
            }
        }
        return 0;
    }

    public static void main(String[] args) {
        int[] a = new int[] {1, 2, 4, 7};
        int target = 9;
        int result = pair_sum(a, target);
        System.out.println(result);
    }
}