Container With Most Water in C#

using System;
// Container With Most Water in C#
//
// PURPOSE
// Two pointers maximize the area between a pair of height bars.
//
// METHOD
// Evaluate width times the smaller height. Move the smaller-height endpoint inward:
// retaining it with a shorter width cannot improve the area.
//
// COMPLEXITY
// O(n) time, O(1) auxiliary space.
//
// CONTRACT
// Nonnegative heights. Areas must fit signed 32-bit. Fewer than two bars 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
// 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: 49.
// 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 max_container(ReadOnlySpan<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            for (int alg_i = 0; alg_i < a.Length; alg_i++) {
                int alg_value = a[alg_i];
                alg_require((alg_value >= 0));
            }
            int left = 0;
            int right = (n - 1);
            int best = 0;
            while ((left < right)) {
                best = Math.Max(best, ((right - left) * Math.Min(a[left], a[right])));
                if ((a[left] <= a[right])) {
                    left = (left + 1);
                } else {
                    right = (right - 1);
                }
            }
            return best;
        }
    }

    static void Main() {
        int[] a = new int[] {1, 8, 6, 2, 5, 4, 8, 3, 7};
        int result = max_container(a);
        Console.WriteLine(result);
    }
}