Kadane Maximum Subarray in C#

using System;
// Kadane Maximum Subarray in C#
//
// PURPOSE
// Kadane algorithm finds the largest sum of a contiguous nonempty subarray. At each
// position, a best ending subarray either extends the previous one or starts here.
//
// METHOD
// Track the best sum ending at the current position and the best seen anywhere. If extending
// is worse than restarting, start at the current value.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space.
//
// CONTRACT
// n >= 1. Returns the maximum sum, including the largest element when all values are
// negative. Sums must fit the integer type.
// 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: 6.
// 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 maximum_subarray(ReadOnlySpan<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            alg_require((n >= 1));
            int ending = a[0];
            int best = a[0];
            int i = 1;
            while ((i < n)) {
                ending = Math.Max(a[i], (ending + a[i]));
                best = Math.Max(best, ending);
                i = (i + 1);
            }
            return best;
        }
    }

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