using System;

namespace DeukBot4.Utilities
{
    public static class Lehvenstein
    {
        /// <summary>
        /// Calculates the Levenshtein distance between two strings--the number of changes that need to be made for the first string to become the second.
        /// </summary>
        /// <param name="first">The first string, used as a source.</param>
        /// <param name="second">The second string, used as a target.</param>
        /// <returns>The number of changes that need to be made to convert the first string to the second.</returns>
        /// <remarks>
        /// From http://www.merriampark.com/ldcsharp.htm
        /// </remarks>
        public static int LevenshteinDistance(string first, string second)
        {
            if (first == null)
            {
                throw new ArgumentNullException(nameof(first));
            }

            if (second == null)
            {
                throw new ArgumentNullException(nameof(second));
            }

            var n = first.Length;
            var m = second.Length;
            var d = new int[n + 1, m + 1]; // matrix

            if (n == 0) return m;
            if (m == 0) return n;

            for (var i = 0; i <= n; d[i, 0] = i++)
            {
            }

            for (var j = 0; j <= m; d[0, j] = j++)
            {
            }

            for (var i = 1; i <= n; i++)
            {
                for (var j = 1; j <= m; j++)
                {
                    var cost = (second.Substring(j - 1, 1) == first.Substring(i - 1, 1) ? 0 : 1); // cost
                    d[i, j] = Math.Min(
                        Math.Min(
                            d[i    - 1, j] + 1,
                            d[i, j - 1]    + 1),
                        d[i - 1, j - 1] + cost);
                }
            }

            return d[n, m];
        }
    }
}