#nullable disable

using System;
using System.IO;

// This is a port of Dmitry Shkarin's PPMd Variant I Revision 1.
// Ported by Michael Bone (mjbone03@yahoo.com.au).

namespace SharpCompress.Compressors.PPMd.I1;

/// <summary>
/// The model.
/// </summary>
internal partial class Model
{
    public const uint SIGNATURE = 0x84acaf8fU;
    public const char VARIANT = 'I';
    public const int MAXIMUM_ORDER = 16; // maximum allowed model order

    private const byte UPPER_FREQUENCY = 5;
    private const byte INTERVAL_BIT_COUNT = 7;
    private const byte PERIOD_BIT_COUNT = 7;
    private const byte TOTAL_BIT_COUNT = INTERVAL_BIT_COUNT + PERIOD_BIT_COUNT;
    private const uint INTERVAL = 1 << INTERVAL_BIT_COUNT;
    private const uint BINARY_SCALE = 1 << TOTAL_BIT_COUNT;
    private const uint MAXIMUM_FREQUENCY = 124;
    private const uint ORDER_BOUND = 9;

    private readonly See2Context[,] _see2Contexts;
    private readonly See2Context _emptySee2Context;
    private PpmContext _maximumContext;
    private readonly ushort[,] _binarySummary = new ushort[25, 64]; // binary SEE-contexts
    private readonly byte[] _numberStatisticsToBinarySummaryIndex = new byte[256];
    private readonly byte[] _probabilities = new byte[260];
    private readonly byte[] _characterMask = new byte[256];
    private byte _escapeCount;
    private int _modelOrder;
    private int _orderFall;
    private int _initialEscape;
    private int _initialRunLength;
    private int _runLength;
    private byte _previousSuccess;
    private byte _numberMasked;
    private ModelRestorationMethod _method;
    private PpmState _foundState; // found next state transition

    private Allocator _allocator;
    private Coder _coder;
    private PpmContext _minimumContext;
    private byte _numberStatistics;
    private readonly PpmState[] _decodeStates = new PpmState[256];

    private static readonly ushort[] INITIAL_BINARY_ESCAPES =
    {
        0x3CDD,
        0x1F3F,
        0x59BF,
        0x48F3,
        0x64A1,
        0x5ABC,
        0x6632,
        0x6051,
    };

    private static ReadOnlySpan<byte> EXPONENTIAL_ESCAPES =>
        new byte[] { 25, 14, 9, 7, 5, 5, 4, 4, 4, 3, 3, 3, 2, 2, 2, 2 };

    #region Public Methods

    public Model()
    {
        // Construct the conversion table for number statistics.  Initially it will contain the following values.
        //
        // 0 2 4 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
        // 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

        _numberStatisticsToBinarySummaryIndex[0] = 2 * 0;
        _numberStatisticsToBinarySummaryIndex[1] = 2 * 1;
        for (var index = 2; index < 11; index++)
        {
            _numberStatisticsToBinarySummaryIndex[index] = 2 * 2;
        }
        for (var index = 11; index < 256; index++)
        {
            _numberStatisticsToBinarySummaryIndex[index] = 2 * 3;
        }

        // Construct the probability table.  Initially it will contain the following values (depending on the value of
        // the upper frequency).
        //
        // 00 01 02 03 04 05 06 06 07 07 07 08 08 08 08 09 09 09 09 09 10 10 10 10 10 10 11 11 11 11 11 11
        // 11 12 12 12 12 12 12 12 12 13 13 13 13 13 13 13 13 13 14 14 14 14 14 14 14 14 14 14 15 15 15 15
        // 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 17 17 17 17
        // 18 18 18 18 18 18 18 18 18 18 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20
        // 20 20 20 20 20 20 20 20 20 20 20 20 20 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 22 22
        // 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23
        // 23 23 23 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 25 25 25 25 25 25 25 25 25
        // 25 25 25 25 25 25 25 25 25 25 25 25 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26
        // 26 26 27 27

        uint count = 1;
        uint step = 1;
        uint probability = UPPER_FREQUENCY;

        for (var index = 0; index < UPPER_FREQUENCY; index++)
        {
            _probabilities[index] = (byte)index;
        }

        for (int index = UPPER_FREQUENCY; index < 260; index++)
        {
            _probabilities[index] = (byte)probability;
            count--;
            if (count == 0)
            {
                step++;
                count = step;
                probability++;
            }
        }

        // Create the context array.

        _see2Contexts = new See2Context[24, 32];
        for (var index1 = 0; index1 < 24; index1++)
        {
            for (var index2 = 0; index2 < 32; index2++)
            {
                _see2Contexts[index1, index2] = new See2Context();
            }
        }

        // Set the signature (identifying the algorithm).

        _emptySee2Context = new See2Context();
        _emptySee2Context._summary = (ushort)(SIGNATURE & 0x0000ffff);
        _emptySee2Context._shift = (byte)((SIGNATURE >> 16) & 0x000000ff);
        _emptySee2Context._count = (byte)(SIGNATURE >> 24);
    }

    /// <summary>
    /// Encode (ie. compress) a given source stream, writing the encoded result to the target stream.
    /// </summary>
    public void Encode(Stream target, Stream source, PpmdProperties properties)
    {
        if (target is null)
        {
            throw new ArgumentNullException(nameof(target));
        }

        if (source is null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        EncodeStart(properties);
        EncodeBlock(target, source, true);
    }

    internal Coder EncodeStart(PpmdProperties properties)
    {
        _allocator = properties._allocator;
        _coder = new Coder();
        _coder.RangeEncoderInitialize();
        StartModel(properties.ModelOrder, properties.RestorationMethod);
        return _coder;
    }

    internal void EncodeBlock(Stream target, Stream source, bool final)
    {
        while (true)
        {
            _minimumContext = _maximumContext;
            _numberStatistics = _minimumContext.NumberStatistics;

            var c = source.ReadByte();
            if (c < 0 && !final)
            {
                return;
            }

            if (_numberStatistics != 0)
            {
                EncodeSymbol1(c, _minimumContext);
                _coder.RangeEncodeSymbol();
            }
            else
            {
                EncodeBinarySymbol(c, _minimumContext);
                _coder.RangeShiftEncodeSymbol(TOTAL_BIT_COUNT);
            }

            while (_foundState == PpmState.ZERO)
            {
                _coder.RangeEncoderNormalize(target);
                do
                {
                    _orderFall++;
                    _minimumContext = _minimumContext.Suffix;
                    if (_minimumContext == PpmContext.ZERO)
                    {
                        goto StopEncoding;
                    }
                } while (_minimumContext.NumberStatistics == _numberMasked);
                EncodeSymbol2(c, _minimumContext);
                _coder.RangeEncodeSymbol();
            }

            if (_orderFall == 0 && (Pointer)_foundState.Successor >= _allocator._baseUnit)
            {
                _maximumContext = _foundState.Successor;
            }
            else
            {
                UpdateModel(_minimumContext);
                if (_escapeCount == 0)
                {
                    ClearMask();
                }
            }

            _coder.RangeEncoderNormalize(target);
        }

        StopEncoding:
        _coder.RangeEncoderFlush(target);
    }

    /// <summary>
    /// Dencode (ie. decompress) a given source stream, writing the decoded result to the target stream.
    /// </summary>
    public void Decode(Stream target, Stream source, PpmdProperties properties)
    {
        if (target is null)
        {
            throw new ArgumentNullException(nameof(target));
        }

        if (source is null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        DecodeStart(source, properties);
        var buffer = new byte[65536];
        int read;
        while ((read = DecodeBlock(source, buffer, 0, buffer.Length)) != 0)
        {
            target.Write(buffer, 0, read);
        }
    }

    internal Coder DecodeStart(Stream source, PpmdProperties properties)
    {
        _allocator = properties._allocator;
        _coder = new Coder();
        _coder.RangeDecoderInitialize(source);
        StartModel(properties.ModelOrder, properties.RestorationMethod);
        _minimumContext = _maximumContext;
        _numberStatistics = _minimumContext.NumberStatistics;
        return _coder;
    }

    internal int DecodeBlock(Stream source, byte[] buffer, int offset, int count)
    {
        if (_minimumContext == PpmContext.ZERO)
        {
            return 0;
        }

        var total = 0;
        while (total < count)
        {
            if (_numberStatistics != 0)
            {
                DecodeSymbol1(_minimumContext);
            }
            else
            {
                DecodeBinarySymbol(_minimumContext);
            }

            _coder.RangeRemoveSubrange();

            while (_foundState == PpmState.ZERO)
            {
                _coder.RangeDecoderNormalize(source);
                do
                {
                    _orderFall++;
                    _minimumContext = _minimumContext.Suffix;
                    if (_minimumContext == PpmContext.ZERO)
                    {
                        goto StopDecoding;
                    }
                } while (_minimumContext.NumberStatistics == _numberMasked);
                DecodeSymbol2(_minimumContext);
                _coder.RangeRemoveSubrange();
            }

            buffer[offset] = _foundState.Symbol;
            offset++;
            total++;

            if (_orderFall == 0 && (Pointer)_foundState.Successor >= _allocator._baseUnit)
            {
                _maximumContext = _foundState.Successor;
            }
            else
            {
                UpdateModel(_minimumContext);
                if (_escapeCount == 0)
                {
                    ClearMask();
                }
            }

            _minimumContext = _maximumContext;
            _numberStatistics = _minimumContext.NumberStatistics;
            _coder.RangeDecoderNormalize(source);
        }

        StopDecoding:
        return total;
    }

    #endregion

    #region Private Methods

    /// <summary>
    /// Initialise the model (unless the model order is set to 1 in which case the model should be cleared so that
    /// the statistics are carried over, allowing "solid" mode compression).
    /// </summary>
    private void StartModel(int modelOrder, ModelRestorationMethod modelRestorationMethod)
    {
        Array.Clear(_characterMask, 0, _characterMask.Length);
        _escapeCount = 1;

        // Compress in "solid" mode if the model order value is set to 1 (this will examine the current PPM context
        // structures to determine the value of orderFall).

        if (modelOrder < 2)
        {
            _orderFall = _modelOrder;
            for (
                var context = _maximumContext;
                context.Suffix != PpmContext.ZERO;
                context = context.Suffix
            )
            {
                _orderFall--;
            }
            return;
        }

        _modelOrder = modelOrder;
        _orderFall = modelOrder;
        _method = modelRestorationMethod;
        _allocator.Initialize();
        _initialRunLength = -((modelOrder < 12) ? modelOrder : 12) - 1;
        _runLength = _initialRunLength;

        // Allocate the context structure.

        _maximumContext = _allocator.AllocateContext();
        _maximumContext.Suffix = PpmContext.ZERO;
        _maximumContext.NumberStatistics = 255;
        _maximumContext.SummaryFrequency = (ushort)(_maximumContext.NumberStatistics + 2);
        _maximumContext.Statistics = _allocator.AllocateUnits(256 / 2);

        // allocates enough space for 256 PPM states (each is 6 bytes)

        _previousSuccess = 0;
        for (var index = 0; index < 256; index++)
        {
            var state = _maximumContext.Statistics[index];
            state.Symbol = (byte)index;
            state.Frequency = 1;
            state.Successor = PpmContext.ZERO;
        }

        uint probability = 0;
        for (var index1 = 0; probability < 25; probability++)
        {
            while (_probabilities[index1] == probability)
            {
                index1++;
            }
            for (var index2 = 0; index2 < 8; index2++)
            {
                _binarySummary[probability, index2] = (ushort)(
                    BINARY_SCALE - (INITIAL_BINARY_ESCAPES[index2] / (index1 + 1))
                );
            }
            for (var index2 = 8; index2 < 64; index2 += 8)
            {
                for (var index3 = 0; index3 < 8; index3++)
                {
                    _binarySummary[probability, index2 + index3] = _binarySummary[
                        probability,
                        index3
                    ];
                }
            }
        }

        probability = 0;
        for (uint index1 = 0; probability < 24; probability++)
        {
            while (_probabilities[index1 + 3] == probability + 3)
            {
                index1++;
            }
            for (var index2 = 0; index2 < 32; index2++)
            {
                _see2Contexts[probability, index2].Initialize((2 * index1) + 5);
            }
        }
    }

    private void UpdateModel(PpmContext minimumContext)
    {
        var state = PpmState.ZERO;
        PpmContext successor;
        var currentContext = _maximumContext;
        uint numberStatistics;
        uint ns1;
        uint cf;
        uint sf;
        uint s0;
        uint foundStateFrequency = _foundState.Frequency;
        var foundStateSymbol = _foundState.Symbol;
        byte symbol;
        byte flag;

        var foundStateSuccessor = _foundState.Successor;
        var context = minimumContext.Suffix;

        if ((foundStateFrequency < MAXIMUM_FREQUENCY / 4) && (context != PpmContext.ZERO))
        {
            if (context.NumberStatistics != 0)
            {
                state = context.Statistics;
                if (state.Symbol != foundStateSymbol)
                {
                    do
                    {
                        symbol = state[1].Symbol;
                        state++;
                    } while (symbol != foundStateSymbol);
                    if (state[0].Frequency >= state[-1].Frequency)
                    {
                        Swap(state[0], state[-1]);
                        state--;
                    }
                }
                cf = (uint)((state.Frequency < MAXIMUM_FREQUENCY - 9) ? 2 : 0);
                state.Frequency += (byte)cf;
                context.SummaryFrequency += (byte)cf;
            }
            else
            {
                state = context.FirstState;
                state.Frequency += (byte)((state.Frequency < 32) ? 1 : 0);
            }
        }

        if (_orderFall == 0 && foundStateSuccessor != PpmContext.ZERO)
        {
            _foundState.Successor = CreateSuccessors(true, state, minimumContext);
            if (_foundState.Successor == PpmContext.ZERO)
            {
                goto RestartModel;
            }
            _maximumContext = _foundState.Successor;
            return;
        }

        _allocator._text[0] = foundStateSymbol;
        _allocator._text++;
        successor = _allocator._text;

        if (_allocator._text >= _allocator._baseUnit)
        {
            goto RestartModel;
        }

        if (foundStateSuccessor != PpmContext.ZERO)
        {
            if (foundStateSuccessor < _allocator._baseUnit)
            {
                foundStateSuccessor = CreateSuccessors(false, state, minimumContext);
            }
        }
        else
        {
            foundStateSuccessor = ReduceOrder(state, minimumContext);
        }

        if (foundStateSuccessor == PpmContext.ZERO)
        {
            goto RestartModel;
        }

        if (--_orderFall == 0)
        {
            successor = foundStateSuccessor;
            _allocator._text -= (_maximumContext != minimumContext) ? 1 : 0;
        }
        else if (_method > ModelRestorationMethod.Freeze)
        {
            successor = foundStateSuccessor;
            _allocator._text = _allocator._heap;
            _orderFall = 0;
        }

        numberStatistics = minimumContext.NumberStatistics;
        s0 = minimumContext.SummaryFrequency - numberStatistics - foundStateFrequency;
        flag = (byte)((foundStateSymbol >= 0x40) ? 0x08 : 0x00);
        for (; currentContext != minimumContext; currentContext = currentContext.Suffix)
        {
            ns1 = currentContext.NumberStatistics;
            if (ns1 != 0)
            {
                if ((ns1 & 1) != 0)
                {
                    state = _allocator.ExpandUnits(currentContext.Statistics, (ns1 + 1) >> 1);
                    if (state == PpmState.ZERO)
                    {
                        goto RestartModel;
                    }
                    currentContext.Statistics = state;
                }
                currentContext.SummaryFrequency += (ushort)(
                    ((3 * ns1) + 1 < numberStatistics) ? 1 : 0
                );
            }
            else
            {
                state = _allocator.AllocateUnits(1);
                if (state == PpmState.ZERO)
                {
                    goto RestartModel;
                }
                Copy(state, currentContext.FirstState);
                currentContext.Statistics = state;
                if (state.Frequency < (MAXIMUM_FREQUENCY / 4) - 1)
                {
                    state.Frequency += state.Frequency;
                }
                else
                {
                    state.Frequency = (byte)(MAXIMUM_FREQUENCY - 4);
                }
                currentContext.SummaryFrequency = (ushort)(
                    state.Frequency + _initialEscape + ((numberStatistics > 2) ? 1 : 0)
                );
            }

            cf = (uint)(2 * foundStateFrequency * (currentContext.SummaryFrequency + 6));
            sf = s0 + currentContext.SummaryFrequency;

            if (cf < 6 * sf)
            {
                cf = (uint)(1 + ((cf > sf) ? 1 : 0) + ((cf >= 4 * sf) ? 1 : 0));
                currentContext.SummaryFrequency += 4;
            }
            else
            {
                cf = (uint)(
                    4
                    + ((cf > 9 * sf) ? 1 : 0)
                    + ((cf > 12 * sf) ? 1 : 0)
                    + ((cf > 15 * sf) ? 1 : 0)
                );
                currentContext.SummaryFrequency += (ushort)cf;
            }

            state = currentContext.Statistics + (++currentContext.NumberStatistics);
            state.Successor = successor;
            state.Symbol = foundStateSymbol;
            state.Frequency = (byte)cf;
            currentContext.Flags |= flag;
        }

        _maximumContext = foundStateSuccessor;
        return;

        RestartModel:
        RestoreModel(currentContext, minimumContext, foundStateSuccessor);
    }

    private PpmContext CreateSuccessors(bool skip, PpmState state, PpmContext context)
    {
        var upBranch = _foundState.Successor;
        var states = new PpmState[MAXIMUM_ORDER];
        uint stateIndex = 0;
        var symbol = _foundState.Symbol;

        if (!skip)
        {
            states[stateIndex++] = _foundState;
            if (context.Suffix == PpmContext.ZERO)
            {
                goto NoLoop;
            }
        }

        var gotoLoopEntry = false;
        if (state != PpmState.ZERO)
        {
            context = context.Suffix;
            gotoLoopEntry = true;
        }

        do
        {
            if (gotoLoopEntry)
            {
                gotoLoopEntry = false;
                goto LoopEntry;
            }

            context = context.Suffix;
            if (context.NumberStatistics != 0)
            {
                byte temporary;
                state = context.Statistics;
                if (state.Symbol != symbol)
                {
                    do
                    {
                        temporary = state[1].Symbol;
                        state++;
                    } while (temporary != symbol);
                }
                temporary = (byte)((state.Frequency < MAXIMUM_FREQUENCY - 9) ? 1 : 0);
                state.Frequency += temporary;
                context.SummaryFrequency += temporary;
            }
            else
            {
                state = context.FirstState;
                state.Frequency += (byte)(
                    ((context.Suffix.NumberStatistics == 0) ? 1 : 0)
                    & ((state.Frequency < 24) ? 1 : 0)
                );
            }

            LoopEntry:
            if (state.Successor != upBranch)
            {
                context = state.Successor;
                break;
            }
            states[stateIndex++] = state;
        } while (context.Suffix != PpmContext.ZERO);

        NoLoop:
        if (stateIndex == 0)
        {
            return context;
        }

        byte localNumberStatistics = 0;
        var localFlags = (byte)((symbol >= 0x40) ? 0x10 : 0x00);
        symbol = upBranch.NumberStatistics;
        var localSymbol = symbol;
        byte localFrequency;
        PpmContext localSuccessor = ((Pointer)upBranch) + 1;
        localFlags |= (byte)((symbol >= 0x40) ? 0x08 : 0x00);

        if (context.NumberStatistics != 0)
        {
            state = context.Statistics;
            if (state.Symbol != symbol)
            {
                byte temporary;
                do
                {
                    temporary = state[1].Symbol;
                    state++;
                } while (temporary != symbol);
            }
            var cf = (uint)(state.Frequency - 1);
            var s0 = (uint)(context.SummaryFrequency - context.NumberStatistics - cf);
            localFrequency = (byte)(
                1 + ((2 * cf <= s0) ? (uint)((5 * cf > s0) ? 1 : 0) : ((cf + (2 * s0) - 3) / s0))
            );
        }
        else
        {
            localFrequency = context.FirstStateFrequency;
        }

        do
        {
            PpmContext currentContext = _allocator.AllocateContext();
            if (currentContext == PpmContext.ZERO)
            {
                return PpmContext.ZERO;
            }
            currentContext.NumberStatistics = localNumberStatistics;
            currentContext.Flags = localFlags;
            currentContext.FirstStateSymbol = localSymbol;
            currentContext.FirstStateFrequency = localFrequency;
            currentContext.FirstStateSuccessor = localSuccessor;
            currentContext.Suffix = context;
            context = currentContext;
            states[--stateIndex].Successor = context;
        } while (stateIndex != 0);

        return context;
    }

    private PpmContext ReduceOrder(PpmState state, PpmContext context)
    {
        PpmState currentState;
        var states = new PpmState[MAXIMUM_ORDER];
        uint stateIndex = 0;
        var currentContext = context;
        PpmContext upBranch = _allocator._text;
        byte temporary;
        var symbol = _foundState.Symbol;

        states[stateIndex++] = _foundState;
        _foundState.Successor = upBranch;
        _orderFall++;

        var gotoLoopEntry = false;
        if (state != PpmState.ZERO)
        {
            context = context.Suffix;
            gotoLoopEntry = true;
        }

        while (true)
        {
            if (gotoLoopEntry)
            {
                gotoLoopEntry = false;
                goto LoopEntry;
            }

            if (context.Suffix == PpmContext.ZERO)
            {
                if (_method > ModelRestorationMethod.Freeze)
                {
                    do
                    {
                        states[--stateIndex].Successor = context;
                    } while (stateIndex != 0);
                    _allocator._text = _allocator._heap + 1;
                    _orderFall = 1;
                }
                return context;
            }

            context = context.Suffix;
            if (context.NumberStatistics != 0)
            {
                state = context.Statistics;
                if (state.Symbol != symbol)
                {
                    do
                    {
                        temporary = state[1].Symbol;
                        state++;
                    } while (temporary != symbol);
                }
                temporary = (byte)((state.Frequency < MAXIMUM_FREQUENCY - 9) ? 2 : 0);
                state.Frequency += temporary;
                context.SummaryFrequency += temporary;
            }
            else
            {
                state = context.FirstState;
                state.Frequency += (byte)((state.Frequency < 32) ? 1 : 0);
            }

            LoopEntry:
            if (state.Successor != PpmContext.ZERO)
            {
                break;
            }
            states[stateIndex++] = state;
            state.Successor = upBranch;
            _orderFall++;
        }

        if (_method > ModelRestorationMethod.Freeze)
        {
            context = state.Successor;
            do
            {
                states[--stateIndex].Successor = context;
            } while (stateIndex != 0);
            _allocator._text = _allocator._heap + 1;
            _orderFall = 1;
            return context;
        }
        if (state.Successor <= upBranch)
        {
            currentState = _foundState;
            _foundState = state;
            state.Successor = CreateSuccessors(false, PpmState.ZERO, context);
            _foundState = currentState;
        }

        if (_orderFall == 1 && currentContext == _maximumContext)
        {
            _foundState.Successor = state.Successor;
            _allocator._text--;
        }

        return state.Successor;
    }

    private void RestoreModel(
        PpmContext context,
        PpmContext minimumContext,
        PpmContext foundStateSuccessor
    )
    {
        PpmContext currentContext;

        _allocator._text = _allocator._heap;
        for (
            currentContext = _maximumContext;
            currentContext != context;
            currentContext = currentContext.Suffix
        )
        {
            if (--currentContext.NumberStatistics == 0)
            {
                currentContext.Flags = (byte)(
                    (currentContext.Flags & 0x10)
                    + ((currentContext.Statistics.Symbol >= 0x40) ? 0x08 : 0x00)
                );
                var state = currentContext.Statistics;
                Copy(currentContext.FirstState, state);
                _allocator.SpecialFreeUnits(state);
                currentContext.FirstStateFrequency = (byte)(
                    (currentContext.FirstStateFrequency + 11) >> 3
                );
            }
            else
            {
                Refresh((uint)((currentContext.NumberStatistics + 3) >> 1), false, currentContext);
            }
        }

        for (; currentContext != minimumContext; currentContext = currentContext.Suffix)
        {
            if (currentContext.NumberStatistics == 0)
            {
                currentContext.FirstStateFrequency -= (byte)(
                    currentContext.FirstStateFrequency >> 1
                );
            }
            else if (
                (currentContext.SummaryFrequency += 4)
                > 128 + (4 * currentContext.NumberStatistics)
            )
            {
                Refresh((uint)((currentContext.NumberStatistics + 2) >> 1), true, currentContext);
            }
        }

        if (_method > ModelRestorationMethod.Freeze)
        {
            _maximumContext = foundStateSuccessor;
            _allocator._glueCount += (uint)(((_allocator._memoryNodes[1].Stamp & 1) == 0) ? 1 : 0);
        }
        else if (_method == ModelRestorationMethod.Freeze)
        {
            while (_maximumContext.Suffix != PpmContext.ZERO)
            {
                _maximumContext = _maximumContext.Suffix;
            }

            RemoveBinaryContexts(0, _maximumContext);
            _method++;
            _allocator._glueCount = 0;
            _orderFall = _modelOrder;
        }
        else if (
            _method == ModelRestorationMethod.Restart
            || _allocator.GetMemoryUsed() < (_allocator._allocatorSize >> 1)
        )
        {
            StartModel(_modelOrder, _method);
            _escapeCount = 0;
        }
        else
        {
            while (_maximumContext.Suffix != PpmContext.ZERO)
            {
                _maximumContext = _maximumContext.Suffix;
            }

            do
            {
                CutOff(0, _maximumContext);
                _allocator.ExpandText();
            } while (_allocator.GetMemoryUsed() > 3 * (_allocator._allocatorSize >> 2));

            _allocator._glueCount = 0;
            _orderFall = _modelOrder;
        }
    }

    private static void Swap(PpmState state1, PpmState state2)
    {
        var swapSymbol = state1.Symbol;
        var swapFrequency = state1.Frequency;
        var swapSuccessor = state1.Successor;

        state1.Symbol = state2.Symbol;
        state1.Frequency = state2.Frequency;
        state1.Successor = state2.Successor;

        state2.Symbol = swapSymbol;
        state2.Frequency = swapFrequency;
        state2.Successor = swapSuccessor;
    }

    private static void Copy(PpmState state1, PpmState state2)
    {
        state1.Symbol = state2.Symbol;
        state1.Frequency = state2.Frequency;
        state1.Successor = state2.Successor;
    }

    private static int Mean(int sum, int shift, int round) =>
        (sum + (1 << (shift - round))) >> shift;

    private void ClearMask()
    {
        _escapeCount = 1;
        Array.Clear(_characterMask, 0, _characterMask.Length);
    }

    #endregion
}
