Longest Palindromic Subsequence in C#

using System;
// Longest Palindromic Subsequence in C#
//
// PURPOSE
// A longest palindromic subsequence may skip tokens while preserving order. It differs from
// a contiguous palindromic substring.
//
// METHOD
// Scan left endpoints backward and right endpoints forward. Equal endpoints extend the inner
// solution by two; otherwise discard one endpoint. Retain one row and the old diagonal.
//
// COMPLEXITY
// O(n squared) time, O(n) auxiliary space.
//
// CONTRACT
// Integer tokens. Returns length; empty input 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: 5.
// 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 palindromic_subsequence(ReadOnlySpan<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            int[] dp = new int[alg_size((n + 1))];
            int left = (n - 1);
            int right = 0;
            int diagonal = 0;
            int old = 0;
            while ((left >= 0)) {
                dp[left] = 1;
                diagonal = 0;
                right = (left + 1);
                while ((right < n)) {
                    old = dp[right];
                    if ((a[left] == a[right])) {
                        dp[right] = (diagonal + 2);
                    } else {
                        dp[right] = Math.Max(dp[right], dp[(right - 1)]);
                    }
                    diagonal = old;
                    right = (right + 1);
                }
                left = (left - 1);
            }
            if ((n == 0)) {
                return 0;
            }
            return dp[(n - 1)];
        }
    }

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