In Place Array Reversal in C#

using System;
// In Place Array Reversal in C#
//
// PURPOSE
// Reversal exchanges symmetric positions until the two pointers meet. It is a building block
// for rotation and sequence manipulation.
//
// METHOD
// Swap the leftmost and rightmost unprocessed values, then move both pointers inward.
// Everything outside the pointers is already in its final reversed position.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space.
//
// CONTRACT
// Reverses n elements in place and returns 0. Empty arrays and single elements are 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
// C# / .NET 10: ReadOnlySpan<int> inputs, Span<int> outputs, primitive workspace arrays,
// Array.BinarySearch for LIS, and checked integer arithmetic. Invalid arguments and overflow
// throw exceptions.
//
// EXAMPLE
// Expected return: 0.
// Expected a after call: [3, 2, 1].
// Run: Use Program.cs in a net10.0 console project; dotnet run.
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

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

    static int reverse_array(Span<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            int left = 0;
            int right = (n - 1);
            int temp = 0;
            while ((left < right)) {
                temp = a[left];
                a[left] = a[right];
                a[right] = temp;
                left = (left + 1);
                right = (right - 1);
            }
            return 0;
        }
    }

    static void Main() {
        int[] a = new int[] {1, 2, 3};
        int result = reverse_array(a);
        Console.WriteLine(result);
        Console.WriteLine("[" + string.Join(", ", a) + "]");
        // Expected a: [3, 2, 1]
    }
}