Array Rotation by Reversal in Java

// Array Rotation by Reversal in Java
//
// PURPOSE
// Three reversals rotate an array right without a second array.
//
// METHOD
// Normalize k modulo length. Reverse the whole array, then reverse the first k elements and
// the remaining suffix.
//
// COMPLEXITY
// O(n) time, O(1) auxiliary space.
//
// CONTRACT
// k >= 0. Rotates right in place, returns 0. Empty input is valid.
// 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 a after call: [4, 1, 2, 3].
// 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 rotate_right(int[] a, int k) {
        java.util.Objects.requireNonNull(a, "a");
        int n = a.length;
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require((a.length == n));
        alg_require((k >= 0));
        if ((n == 0)) {
            return 0;
        }
        k = (k % n);
        int phase = 0;
        int left = 0;
        int right = 0;
        int temp = 0;
        while ((phase < 3)) {
            if ((phase == 0)) {
                left = 0;
                right = Math.subtractExact(n, 1);
            } else {
                if ((phase == 1)) {
                    left = 0;
                    right = Math.subtractExact(k, 1);
                } else {
                    left = k;
                    right = Math.subtractExact(n, 1);
                }
            }
            while ((left < right)) {
                temp = a[left];
                a[left] = a[right];
                a[right] = temp;
                left = Math.addExact(left, 1);
                right = Math.subtractExact(right, 1);
            }
            phase = Math.addExact(phase, 1);
        }
        return 0;
    }

    public static void main(String[] args) {
        int[] a = new int[] {1, 2, 3, 4};
        int k = 1;
        int result = rotate_right(a, k);
        System.out.println(result);
        // Expected a: [4, 1, 2, 3]
        System.out.println(java.util.Arrays.toString(a));
    }
}