// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

//
// This is used internally to create best fit behavior as per the original windows best fit behavior.
//

using System;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Text;
using System.Threading;

namespace System.Text
{
    internal sealed class InternalDecoderBestFitFallback : DecoderFallback
    {
        // Our variables
        internal BaseCodePageEncoding encoding;
        internal char[]? arrayBestFit;
        internal char cReplacement = '?';

        internal InternalDecoderBestFitFallback(BaseCodePageEncoding _encoding)
        {
            // Need to load our replacement characters table.
            encoding = _encoding;
        }

        public override DecoderFallbackBuffer CreateFallbackBuffer() =>
            new InternalDecoderBestFitFallbackBuffer(this);

        // Maximum number of characters that this instance of this fallback could return
        public override int MaxCharCount => 1;

        public override bool Equals([NotNullWhen(true)] object? value) =>
            value is InternalDecoderBestFitFallback that && encoding.CodePage == that.encoding.CodePage;

        public override int GetHashCode() => encoding.CodePage;
    }

    internal sealed class InternalDecoderBestFitFallbackBuffer : DecoderFallbackBuffer
    {
        // Our variables
        internal char cBestFit;
        internal int iCount = -1;
        internal int iSize;
        private readonly InternalDecoderBestFitFallback _oFallback;

        // Private object for locking instead of locking on a public type for SQL reliability work.
        private static object InternalSyncObject =>
            field ?? Interlocked.CompareExchange(ref field, new object(), null) ?? field;

        // Constructor
        public InternalDecoderBestFitFallbackBuffer(InternalDecoderBestFitFallback fallback)
        {
            _oFallback = fallback;

            if (_oFallback.arrayBestFit == null)
            {
                // Lock so we don't confuse ourselves.
                lock (InternalSyncObject)
                {
                    // Double check before we do it again.
                    _oFallback.arrayBestFit ??= fallback.encoding.GetBestFitBytesToUnicodeData();
                }
            }
        }

        // Fallback methods
        public override bool Fallback(byte[] bytesUnknown, int index)
        {
            // We expect no previous fallback in our buffer
            Debug.Assert(iCount < 1, "[DecoderReplacementFallbackBuffer.Fallback] Calling fallback without a previously empty buffer");

            cBestFit = TryBestFit(bytesUnknown);
            if (cBestFit == '\0')
                cBestFit = _oFallback.cReplacement;

            iCount = iSize = 1;

            return true;
        }

        // Default version is overridden in DecoderReplacementFallback.cs
        public override char GetNextChar()
        {
            // We want it to get < 0 because == 0 means that the current/last character is a fallback
            // and we need to detect recursion.  We could have a flag but we already have this counter.
            iCount--;

            // Do we have anything left? 0 is now last fallback char, negative is nothing left
            if (iCount < 0)
                return '\0';

            // Need to get it out of the buffer.
            // Make sure it didn't wrap from the fast count-- path
            if (iCount == int.MaxValue)
            {
                iCount = -1;
                return '\0';
            }

            // Return the best fit character
            return cBestFit;
        }

        public override bool MovePrevious()
        {
            // Exception fallback doesn't have anywhere to back up to.
            if (iCount >= 0)
                iCount++;

            // Return true if we could do it.
            return (iCount >= 0 && iCount <= iSize);
        }

        // How many characters left to output?
        public override int Remaining
        {
            get
            {
                return (iCount > 0) ? iCount : 0;
            }
        }

        // Clear the buffer
        public override void Reset()
        {
            iCount = -1;
        }

        // private helper methods
        private char TryBestFit(byte[] bytesCheck)
        {
            // Need to figure out our best fit character, low is beginning of array, high is 1 AFTER end of array
            int lowBound = 0;
            int highBound = _oFallback.arrayBestFit!.Length;
            int index;
            char cCheck;

            // Check trivial case first (no best fit)
            if (highBound == 0)
                return '\0';

            // If our array is too small or too big we can't check
            if (bytesCheck.Length == 0 || bytesCheck.Length > 2)
                return '\0';

            if (bytesCheck.Length == 1)
                cCheck = unchecked((char)bytesCheck[0]);
            else
                cCheck = unchecked((char)((bytesCheck[0] << 8) + bytesCheck[1]));

            // Check trivial out of range case
            if (cCheck < _oFallback.arrayBestFit[0] || cCheck > _oFallback.arrayBestFit[highBound - 2])
                return '\0';

            // Binary search the array
            int iDiff;
            while ((iDiff = (highBound - lowBound)) > 6)
            {
                // Look in the middle, which is complicated by the fact that we have 2 #s for each pair,
                // so we don't want index to be odd because it must be word aligned.
                // Also note that index can never == highBound (because diff is rounded down)
                index = ((iDiff / 2) + lowBound) & 0xFFFE;

                char cTest = _oFallback.arrayBestFit[index];
                if (cTest == cCheck)
                {
                    // We found it
                    Debug.Assert(index + 1 < _oFallback.arrayBestFit.Length,
                        "[InternalDecoderBestFitFallbackBuffer.TryBestFit]Expected replacement character at end of array");
                    return _oFallback.arrayBestFit[index + 1];
                }
                else if (cTest < cCheck)
                {
                    // We weren't high enough
                    lowBound = index;
                }
                else
                {
                    // We weren't low enough
                    highBound = index;
                }
            }

            for (index = lowBound; index < highBound; index += 2)
            {
                if (_oFallback.arrayBestFit[index] == cCheck)
                {
                    // We found it
                    Debug.Assert(index + 1 < _oFallback.arrayBestFit.Length,
                        "[InternalDecoderBestFitFallbackBuffer.TryBestFit]Expected replacement character at end of array");
                    return _oFallback.arrayBestFit[index + 1];
                }
            }

            // Char wasn't in our table
            return '\0';
        }
    }
}