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

using System.Buffers.Binary;
using System.Diagnostics;
using System.IO;
using System.Runtime.CompilerServices;
using System.Runtime.Serialization;

namespace System.Text
{
    internal abstract partial class BaseCodePageEncoding : EncodingNLS, ISerializable
    {
        private static unsafe void ReadCodePageIndex(Stream stream, Span<byte> codePageIndex)
        {
            Debug.Assert(stream is UnmanagedMemoryStream, "UnmanagedMemoryStream will read a full buffer on one call to Read.");
            int bytesRead = stream.Read(codePageIndex);
            Debug.Assert(bytesRead == codePageIndex.Length);

            if (!BitConverter.IsLittleEndian)
            {
                fixed (byte* pBytes = &codePageIndex[0])
                {
                    CodePageIndex* p = (CodePageIndex*)pBytes;
                    char *pCodePageName = &p->CodePageName;
                    for (int i = 0; i < 16; i++)
                    {
                        pCodePageName[i] = (char)BinaryPrimitives.ReverseEndianness((ushort)pCodePageName[i]);
                    }
                    p->CodePage = BinaryPrimitives.ReverseEndianness(p->CodePage);
                    p->ByteCount = BinaryPrimitives.ReverseEndianness(p->ByteCount);
                    p->Offset = BinaryPrimitives.ReverseEndianness(p->Offset);
                }
            }
        }

        internal static unsafe EncodingInfo [] GetEncodings(CodePagesEncodingProvider provider)
        {
            lock (s_streamLock)
            {
                s_codePagesEncodingDataStream.Seek(CODEPAGE_DATA_FILE_HEADER_SIZE, SeekOrigin.Begin);

                int codePagesCount;
                fixed (byte* pBytes = &s_codePagesDataHeader[0])
                {
                    CodePageDataFileHeader* pDataHeader = (CodePageDataFileHeader*)pBytes;
                    codePagesCount = pDataHeader->CodePageCount;
                }

                EncodingInfo [] encodingInfoList = new EncodingInfo[codePagesCount];

                CodePageIndex codePageIndex = default;
                Span<byte> pCodePageIndex = new Span<byte>(&codePageIndex, sizeof(CodePageIndex));

                for (int i = 0; i < codePagesCount; i++)
                {
                    ReadCodePageIndex(s_codePagesEncodingDataStream, pCodePageIndex);

                    string codePageName;
                    switch (codePageIndex.CodePage)
                    {
                        // Fixup some encoding names.
                        case 950:   codePageName = "big5"; break;
                        case 10002: codePageName = "x-mac-chinesetrad"; break;
                        case 20833: codePageName = "x-ebcdic-koreanextended"; break;
                        default:    codePageName = new string(&codePageIndex.CodePageName); break;
                    }

                    string? displayName = null;
                    if (!SR.UsingResourceKeys())
                    {
                        displayName = EncodingNLS.GetLocalizedEncodingNameResource(codePageIndex.CodePage);
                    }

                    encodingInfoList[i] = new EncodingInfo(provider, codePageIndex.CodePage, codePageName, displayName ?? codePageName);
                }

                return encodingInfoList;
            }
        }
    }
}