using System;
using System.Runtime.CompilerServices;

namespace Benchmark
{
    public readonly struct Identity
    {
        public Identity(string prefix, int index, int revision)
        {
            Prefix = prefix;
            Index = index;
            Revision = revision;
        }

        public string Prefix { get; }
        public int Index { get; }
        public int Revision { get; }

        public static bool TryParse(string value, out Identity identity)
        {
            identity = default;
            var position = value.Length;
            if (!TryGetNumberOld(ref value, null, ref position, out var revision)) return false;
            if (value[position] != '-') return false;
            if (!TryGetNumberOld(ref value, 7, ref position, out var index)) return false;
            if (!TryGetPrefixOld(ref value, ref position, out var prefix)) return false;
            identity = new Identity(prefix, index, revision);
            return true;        }

        public static bool TryParse(Span<char> value, out Identity identity)
        {
            identity = default;
            var position = value.Length;
            if (!TryGetNumber(ref value, null, ref position, out var revision)) return false;
            if (value[position] != '-') return false;
            if (!TryGetNumber(ref value, 7, ref position, out var index)) return false;
            if (!TryGetPrefix(ref value, ref position, out var prefix)) return false;
            identity = new Identity(prefix, index, revision);
            return true;
        }


        private static bool TryGetPrefixOld(ref string identity, ref int lastPosition, out string prefix)
        {
            prefix = identity[(lastPosition + 1)..];
            if (prefix.Length < 3) return false;
            for (int i = 0; i < prefix.Length; i++)
            {
                if (prefix[i] < 'A' || prefix[i] > 'Z') return false;
            }
            return true;
        }

        private static bool TryGetPrefix(ref Span<char> identity, ref int lastPosition, out string prefix)
        {
            var span = identity[(lastPosition + 1)..];
            prefix = span.ToString();
            if (span.Length < 3) return false;
            for (int i = 0; i < span.Length; i++)
            {
                if (span[i] < 'A' || span[i] > 'Z') return false;
            }

            return true;
        }

        private static bool TryGetNumberOld(ref string identity, int? exactLength, ref int currentPosition, out int revision)
        {
            revision = 0;
            var start = currentPosition;
            for (currentPosition = start - 1; currentPosition >= 0; currentPosition--)
            {
                var current = identity[currentPosition];
                if (current == '0') continue;
                if (current < '1' || current > '9') break;
                revision += (current - '0') * IntegerPow(10, start - currentPosition - 1);
            }

            if (exactLength != null && start - (currentPosition + 1) != exactLength) return false;
            return revision != default;
        }

        private static bool TryGetNumber(ref Span<char> identity, int? exactLength, ref int currentPosition, out int revision)
        {
            revision = 0;
            var start = currentPosition;
            for (currentPosition = start - 1; currentPosition >= 0; currentPosition--)
            {
                var current = identity[currentPosition];
                if (current == '0') continue;
                if (current < '1' || current > '9') break;
                revision += (current - '0') * IntegerPow(10, start - currentPosition - 1);
            }

            if (exactLength != null && start - (currentPosition + 1) != exactLength) return false;
            return revision != default;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static int IntegerPow(int x, int y)
        {
            if (y == 0) return 1;
            var result = x;
            for (var i = 1; i < y; i++)
            {
                result *= x;
            }

            return result;
        }
    }
}