Manacher Longest Palindromic Substring in C#

using System;
// Manacher Longest Palindromic Substring in C#
//
// PURPOSE
// Manacher reuses mirrored palindrome radii to find a longest contiguous palindrome in
// linear time.
//
// METHOD
// Compute odd and even center radii in separate passes. Reuse the mirror radius within the
// current rightmost palindrome, then expand only beyond its known boundary.
//
// COMPLEXITY
// O(n) time and O(n) auxiliary space.
//
// CONTRACT
// Integer tokens. Returns longest palindrome LENGTH; empty input returns zero.
// 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: 4.
// 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 manacher(ReadOnlySpan<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            int[] radius = new int[alg_size((n + 1))];
            int parity = 0;
            int left = 0;
            int right = (-1);
            int i = 0;
            int k = 0;
            int best = 0;
            while ((parity < 2)) {
                left = 0;
                right = (-1);
                i = 0;
                while ((i < n)) {
                    k = (1 - parity);
                    if ((i <= right)) {
                        k = Math.Min(radius[(((left + right) - i) + parity)], ((right - i) + 1));
                    }
                    while (((((i - k) - parity) >= 0) && ((i + k) < n) && (a[((i - k) - parity)] == a[(i + k)]))) {
                        k = (k + 1);
                    }
                    radius[i] = k;
                    best = Math.Max(best, (((2 * k) - 1) + parity));
                    if ((((i + k) - 1) > right)) {
                        left = (((i - k) - parity) + 1);
                        right = ((i + k) - 1);
                    }
                    i = (i + 1);
                }
                parity = (parity + 1);
            }
            return best;
        }
    }

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