Boyer Moore Majority Vote in C#

using System;
// Boyer Moore Majority Vote in C#
//
// PURPOSE
// Majority vote cancels pairs of different values. If a value occurs more than half the
// time, it survives as the final candidate.
//
// METHOD
// Choose a new candidate whenever the balance is zero. Increment for matching values and
// decrement for different ones. Make a second pass to verify the candidate, since a majority
// need not exist.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space.
//
// CONTRACT
// Returns the INDEX of a verified majority value, or -1 if none exists. Returning an index
// avoids colliding with any valid integer value.
// 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: 0.
// 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 majority_index(ReadOnlySpan<int> a) {
        checked {
            int n = a.Length;
            alg_require(((n >= 0) && (n <= 1000000)));
            alg_require((a.Length == n));
            int candidate = 0;
            int balance = 0;
            int i = 0;
            int count = 0;
            while ((i < n)) {
                if ((balance == 0)) {
                    candidate = i;
                }
                if ((a[i] == a[candidate])) {
                    balance = (balance + 1);
                } else {
                    balance = (balance - 1);
                }
                i = (i + 1);
            }
            i = 0;
            while ((i < n)) {
                if ((a[i] == a[candidate])) {
                    count = (count + 1);
                }
                i = (i + 1);
            }
            if ((count > (n / 2))) {
                return candidate;
            }
            return (-1);
        }
    }

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