ABCDEFGHPPP.Generalized.cs

using System;
using System.Linq;
using System.Text;

public class Program {
    public static void Main(string[] args) {
        var @base = 16;
        var pLength = 4;
        var n = (pLength - 1) * 4;
        var max = Max(pLength - 1, @base);
        var min = Min(pLength - 1, @base);
        var count = 0;
        System.Diagnostics.Stopwatch timer = System.Diagnostics.Stopwatch.StartNew();

        foreach (var ppp in Enumerable.Range(Min(pLength, @base), Max(pLength - 1, @base) * 2).Select(o => IntToString(o, @base)).Where(o => o.All(p => o.First() == p)).Select(o => StringToInt(o, @base)))
        for (int ef = min; ef <= max; ef++) {
            var p = IntToString(ppp, @base).First();
            int gh = ppp - ef;
            for (int cd = min; cd <= max; cd++) {
                int ab = ef + cd;
                if (new int[] { ef, gh, ab, cd }.SelectMany(o => IntToString(o, @base)).Distinct().Where(o => o != p).Count() == n && gh >=min && gh <= max && ab >= min && ab <= max) {
                    count++;
                }
            }
        }

        timer.Stop();
        Console.WriteLine(timer.Elapsed + " - " + count);
        Console.ReadLine();
    }

    static string digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz./";

    public static int Min(int length, int @base) {
        return StringToInt("1".PadRight(length, '0'), @base);
    }

    public static int Max(int length, int @base) {
        return StringToInt(digits[@base - 1].ToString().PadRight(length, digits[@base - 1]), @base);
    }

    public static int StringToInt(string value, int @base) {
        var result = 0;
        for (int i = 0, n = value.Length; i < n; i++) {
            result += digits.IndexOf(value[i]) * (int)Math.Pow(@base, n - 1 - i);
        }
        return result;
    }

    public static string IntToString(int value, int @base) {
        StringBuilder buffer = new StringBuilder();
        for (; value > 0; value = value / @base) buffer.Append(digits[value % @base]);
        return new String(buffer.ToString().Reverse().ToArray());
    }
}