using System.Collections.Generic;

namespace PKHeX.Core
{
    /// <summary>
    /// Analyzes a <see cref="TeamLock"/> to determine if the provided parameters are a valid end product.
    /// </summary>
    /// <remarks>
    /// When generating a Trainer Team for <see cref="GameVersion.CXD"/>, the game must generate PID values that match the programmed team specifications.
    /// The game is 'locked' into a PID generating loop until a valid PID is created, after which the routine is unlocked and proceeds to generate the next member.
    /// These locks cause the <see cref="PKM.PID"/> of the current <see cref="PKM"/> to be rerolled until the requisite lock is satisfied.
    /// This locking is the same as Method H/J/K and Gen3 Cute Charm by extension, with the additional restriction of not allowing shinies.
    /// <see cref="PKM.Nature"/> locks require a certain <see cref="Nature"/>, which is derived from the <see cref="PKM.PID"/>.
    /// <see cref="PKM.Gender"/> locks require a certain gender value, which is derived from the <see cref="PKM.PID"/> and <see cref="PersonalInfo.Gender"/> ratio.
    /// <see cref="PKM.IsShiny"/> locks require the member to not be shiny. This is enforced for non-shadow members for both games, and for shadow members in <see cref="GameVersion.XD"/>.
    /// </remarks>
    public sealed class TeamLockResult
    {
        /// <summary>
        /// Seed prior to generating the team per <see cref="Specifications"/>.
        /// </summary>
        public readonly uint OriginSeed;

        /// <summary>
        /// Indicates if the input parameters can be generated by the <see cref="Specifications"/> provided.
        /// </summary>
        public readonly bool Valid;

        /// <summary>
        /// NPC Team data containing a list of <see cref="NPCLock"/> members.
        /// </summary>
        internal readonly TeamLock Specifications;

        /// <summary>
        /// Frame the <see cref="OriginSeed"/> is from, based on frames reversed from the input seed provided.
        /// </summary>
        private int OriginFrame;

        /// <summary>
        /// Required CPU Trainer Shiny Value
        /// </summary>
        /// <remarks>
        /// If this value is >= 0, the CPU Trainer Shiny Value must be equal to this value as a <see cref="SeedFrame"/> skipped over a matching interrupt frame.
        /// If this value is <see cref="NOT_FORCED"/>, the CPU Trainer Shiny Value can be anything (except matching any of the <see cref="SeedFrame"/> result members.
        /// </remarks>
        private int RCSV = NOT_FORCED;

        /// <summary>
        /// Player Trainer Shiny Value
        /// </summary>
        /// <remarks>Only used by <see cref="GameVersion.XD"/> encounters, which disallow shiny shadow members for both player &amp; CPU TSVs.</remarks>
        private readonly int TSV;

        private readonly Stack<NPCLock> Locks;
        private readonly FrameCache Cache;
        // only save a list of valid nodes we've visited to save memory while we recursively search for a full team.
        private readonly Stack<SeedFrame> Team;

        private const int NOT_FORCED = -1;

        internal TeamLockResult(TeamLock teamSpec, uint originSeed, int tsv)
        {
            Locks = new Stack<NPCLock>((Specifications = teamSpec).Locks);
            Team = new Stack<SeedFrame>();
            Cache = new FrameCache(RNG.XDRNG.Reverse(originSeed, 2), RNG.XDRNG.Prev);
            TSV = tsv;

            Valid = FindLockSeed();
            if (Valid)
                OriginSeed = Cache.GetSeed(OriginFrame);
        }

        /// <summary>
        /// Depth-first search traversal which finds a possible origin for the <see cref="Specifications"/>.
        /// </summary>
        /// <param name="frame">Frame at which the search starts/continues at.</param>
        /// <param name="prior">Prior <see cref="NPCLock"/> data. If this is the last lock in the CPU Team, this is null.</param>
        /// <returns>True if the <see cref="Specifications"/> are valid.</returns>
        private bool FindLockSeed(int frame = 0, NPCLock? prior = null)
        {
            if (Locks.Count == 0) // full team reverse-generated
                return VerifyNPC(frame);

            var current = Locks.Pop();
            var locks = GetPossibleLocks(frame, current, prior);
            foreach (var l in locks)
            {
                Team.Push(l); // possible match
                if (FindLockSeed(l.FrameID, current))
                    return true; // all locks are satisfied
                Team.Pop(); // no match, remove
            }

            Locks.Push(current); // return the lock, lock is impossible
            return false;
        }

        /// <summary>
        /// Generates a list of frames the <see cref="current"/> lock data can be generated at.
        /// </summary>
        /// <param name="ctr">Starting frame for the traversal.</param>
        /// <param name="current">Current lock criteria to satisfy. Used to find valid <see cref="SeedFrame"/> results to yield.</param>
        /// <param name="prior">Prior lock criteria. Used for determining when the traversal stops.</param>
        /// <returns>List of possible locks for the provided input.</returns>
        private IEnumerable<SeedFrame> GetPossibleLocks(int ctr, NPCLock current, NPCLock? prior)
        {
            if (prior?.Shadow != false)
                return GetSingleLock(ctr, current);

            return GetAllLocks(ctr, current, prior);
        }

        /// <summary>
        /// Returns a single <see cref="SeedFrame"/> as the <see cref="current"/> lock must match precisely.
        /// </summary>
        /// <param name="ctr">Starting frame for the traversal.</param>
        /// <param name="current">Current lock criteria to satisfy. Used to find valid <see cref="SeedFrame"/> results to yield.</param>
        /// <returns></returns>
        private IEnumerable<SeedFrame> GetSingleLock(int ctr, NPCLock current)
        {
            uint pid = Cache[ctr + 1] << 16 | Cache[ctr];
            if (current.MatchesLock(pid))
                yield return new SeedFrame(pid, ctr + (current.Seen ? 5 : 7));
        }

        /// <summary>
        /// Generates <see cref="SeedFrame"/> nodes until the traversal is ended by an interrupt frame that matches the <see cref="prior"/>.
        /// </summary>
        /// <param name="ctr">Starting frame for the traversal.</param>
        /// <param name="current">Current lock criteria to satisfy. Used to find valid <see cref="SeedFrame"/> results to yield.</param>
        /// <param name="prior">Prior lock criteria. Used for determining when the traversal stops.</param>
        /// <returns>List of possible locks for the provided input.</returns>
        /// <remarks>
        /// An "interrupt" signals the end of the traversal.
        /// Any <see cref="SeedFrame"/> afterwards (when generated forward from the CPU Trainer) will use the interrupt rather than the previous <see cref="SeedFrame"/> that was found for the <see cref="prior"/> lock.
        /// </remarks>
        private IEnumerable<SeedFrame> GetAllLocks(int ctr, NPCLock current, NPCLock prior)
        {
            // Since the prior(next) lock is generated 7+2*n frames after, the worst case break is 7 frames after the PID.
            // Continue reversing until a sequential generation case is found.
            int start = ctr;

            // Track if we ever require the CPU Trainer Shiny Value to be a value for a shiny skip.
            // We need to un-set this flag if future frames don't pan out.
            bool forcedOT = false;

            while (true)
            {
                int p7 = ctr - 7;
                if (p7 > start)
                {
                    // check for interrupting cpu team cases
                    var upper = Cache[p7 + 1];
                    var lower = Cache[p7];
                    uint cid = upper << 16 | lower;
                    var sv = (upper ^ lower) >> 3;
                    if (sv == TSV) // XD shiny checks all opponent PKM, even non-shadow.
                    {
                        // This interrupt is ignored! The result is shiny.
                    }
                    else if (prior.MatchesLock(cid)) // lock matched cpu mon
                    {
                        if (RCSV != NOT_FORCED) // CPU shiny value is required for a previous lock
                        {
                            if (sv != RCSV)
                            {
                                if (forcedOT) // current call to this method had forced the OT; clear the forced OT before breaking.
                                    RCSV = NOT_FORCED;
                                yield break; // Since we can't skip this interrupt, we're done.
                            }
                        }
                        else // No CPU shiny value forced yet. Lets try to skip this lock by requiring the eventual OT to get this shiny.
                        {
                            RCSV = (int)sv;
                            forcedOT = true;
                            // don't break
                        }
                    }
                }
                uint pid = Cache[ctr + 1] << 16 | Cache[ctr];
                if (current.MatchesLock(pid))
                    yield return new SeedFrame(pid, ctr + (current.Seen ? 5 : 7));

                ctr += 2;
            }
        }

        /// <summary>
        /// Checks to see if the generated <see cref="Team"/> is compatible with the CPU Trainer Data.
        /// </summary>
        /// <param name="ctr">Ending frame of the traversal.</param>
        /// <returns>True if the <see cref="Specifications"/> are valid.</returns>
        private bool VerifyNPC(int ctr)
        {
            var TID = Cache[ctr + 1];
            var SID = Cache[ctr];
            var CPUSV = (TID ^ SID) >> 3;
            if (RCSV != NOT_FORCED && RCSV != CPUSV)
                return false; // required CPU Trainer's shiny value did not match the required value.

            int pos = Team.Count - 1; // stack can't do a for loop :(
            foreach (var member in Team)
            {
                var pid = member.PID;
                var psv = ((pid & 0xFFFF) ^ (pid >> 16)) >> 3;

                // check for shiny for Trainer -- XD only
                // if (psv == TSV) // XD shiny checks all opponent PKM, even non-shadow.
                //    return false; // no shiny shadow mons
                // we already checked this when re-generating the team

                // check for shiny for CPU
                if (psv == CPUSV)
                {
                    if (!Specifications.Locks[pos].Shadow)
                        return false; // no shiny CPU mons
                    if (TSV != NOT_FORCED) // XD
                        return false; // no shiny shadow mons
                }
                pos--;
            }

            OriginFrame = ctr + 2;
            return true;
        }
    }
}