Two Pointer Pair Sum in C#

using System;
// Two Pointer Pair Sum in C#
//
// 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
// 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: 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 pair_sum(ReadOnlySpan<int> a, int target) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            int left = 0;
            int right = (n - 1);
            int total = 0;
            while ((left < right)) {
                total = (a[left] + a[right]);
                if ((total == target)) {
                    return 1;
                }
                if ((total < target)) {
                    left = (left + 1);
                } else {
                    right = (right - 1);
                }
            }
            return 0;
        }
    }

    static void Main() {
        int[] a = new int[] {1, 2, 4, 7};
        int target = 9;
        int result = pair_sum(a, target);
        Console.WriteLine(result);
    }
}