//------------------------------------------------------------------------------
// <copyright file="Base64Decoder.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
// <owner current="true" primary="true">Microsoft</owner>
//------------------------------------------------------------------------------

using System;
using System.Diagnostics;

namespace System.Xml {

    internal class Base64Decoder : IncrementalReadDecoder {
//
// Fields
//
        byte[]  buffer;
        int     startIndex;
        int     curIndex;
        int     endIndex;

        int     bits;
        int     bitsFilled;

        private static readonly String CharsBase64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
        private static readonly byte[] MapBase64 = ConstructMapBase64();
        private const int MaxValidChar = (int)'z';
        private const byte Invalid = unchecked((byte)-1);

//
// IncrementalReadDecoder interface
//
        internal override int DecodedCount { 
            get { 
                return curIndex - startIndex; 
            } 
        }

        internal override bool IsFull { 
            get { 
                return curIndex == endIndex; 
            } 
        }

#if SILVERLIGHT && !SILVERLIGHT_DISABLE_SECURITY
        [System.Security.SecuritySafeCritical]
#endif
        internal override unsafe int Decode( char[] chars, int startPos, int len ) {
            if ( chars == null ) {
                throw new ArgumentNullException( "chars" );
            }
            if ( len < 0 ) {
                throw new ArgumentOutOfRangeException( "len" );
            }
            if ( startPos < 0 ) {
                throw new ArgumentOutOfRangeException( "startPos" );
            }
            if ( chars.Length - startPos < len ) {
                throw new ArgumentOutOfRangeException( "len" );
            }

            if ( len == 0 ) {
                return 0;
            }
            int bytesDecoded, charsDecoded;
            fixed ( char* pChars = &chars[startPos] ) {
                fixed ( byte* pBytes = &buffer[curIndex] ) {
                    Decode( pChars, pChars + len, pBytes, pBytes + ( endIndex - curIndex ), out charsDecoded, out bytesDecoded );
                }
            }
            curIndex += bytesDecoded;
            return charsDecoded;
        }

#if SILVERLIGHT && !SILVERLIGHT_DISABLE_SECURITY
        [System.Security.SecuritySafeCritical]
#endif
        internal override unsafe int Decode(string str, int startPos, int len) {
            if ( str == null ) {
                throw new ArgumentNullException( "str" );
            }
            if ( len < 0 ) {
                throw new ArgumentOutOfRangeException( "len" );
            }
            if ( startPos < 0 ) {
                throw new ArgumentOutOfRangeException( "startPos" );
            }
            if ( str.Length - startPos < len ) {
                throw new ArgumentOutOfRangeException( "len" );
            }

            if ( len == 0 ) {
                return 0;
            }
            int bytesDecoded, charsDecoded;
            fixed ( char* pChars = str ) {
                fixed ( byte* pBytes = &buffer[curIndex] ) {
                    Decode( pChars + startPos, pChars + startPos + len, pBytes, pBytes + ( endIndex - curIndex ), out charsDecoded, out bytesDecoded );
                }
            }
            curIndex += bytesDecoded;
            return charsDecoded;
        }

        internal override void Reset() {
            bitsFilled = 0;
            bits = 0;
        }

        internal override void SetNextOutputBuffer( Array buffer, int index, int count ) {
            Debug.Assert( buffer != null );
            Debug.Assert( count >= 0 );
            Debug.Assert( index >= 0 );
            Debug.Assert( buffer.Length - index >= count );
            Debug.Assert( ( buffer as byte[] ) != null );

            this.buffer = (byte[])buffer;
            this.startIndex = index;
            this.curIndex = index;
            this.endIndex = index + count;
        }

//
// Private methods
//
        private static byte[] ConstructMapBase64() {
            byte[] mapBase64 = new byte[MaxValidChar + 1];
            for ( int i = 0; i < mapBase64.Length; i++ ) {
                mapBase64[i]= Invalid;
            }
            for ( int i = 0; i < CharsBase64.Length; i++ ) {
                mapBase64[(int)CharsBase64[i]] = (byte)i;
            }
            return mapBase64;
        }

#if SILVERLIGHT && !SILVERLIGHT_DISABLE_SECURITY
        [System.Security.SecurityCritical]
#endif
        private unsafe void Decode(char* pChars, char* pCharsEndPos, 
                             byte* pBytes, byte* pBytesEndPos, 
                             out int charsDecoded, out int bytesDecoded ) {
#if DEBUG
            Debug.Assert( pCharsEndPos - pChars >= 0 );
            Debug.Assert( pBytesEndPos - pBytes >= 0 );
#endif

            // walk hex digits pairing them up and shoving the value of each pair into a byte
            byte *pByte = pBytes;
            char *pChar = pChars;
            int b = bits;
            int bFilled = bitsFilled;
            XmlCharType xmlCharType = XmlCharType.Instance;
            while ( pChar < pCharsEndPos && pByte < pBytesEndPos ) {
                char ch = *pChar;
                // end?
                if ( ch == '=' ) { 
                    break;
                }
                pChar++;

                // ignore white space
                if ( ( xmlCharType.charProperties[ch] & XmlCharType.fWhitespace ) != 0 ) {  // if ( xmlCharType.IsWhiteSpace(ch) ) {
                    continue;
                }

                int digit;
                if ( ch > 122 || ( digit = MapBase64[ch] ) == Invalid ) {
                    throw new XmlException( Res.Xml_InvalidBase64Value, new string( pChars, 0, (int)( pCharsEndPos - pChars ) ) );
                }

                b = ( b << 6 ) | digit;
                bFilled += 6;

                if ( bFilled >= 8 ) {
                    // get top eight valid bits
                    *pByte++ = (byte)( ( b >> ( bFilled - 8 ) ) & 0xFF ); 
                    bFilled -= 8;

                    if ( pByte == pBytesEndPos ) {
                        goto Return;
                    }
                }
            }

            if ( pChar < pCharsEndPos && *pChar == '=' ) {
                bFilled = 0;
                // ignore padding chars
                do {
                    pChar++;
                } while ( pChar < pCharsEndPos && *pChar == '=' );

                // ignore whitespace after the padding chars
                if ( pChar < pCharsEndPos ) {
                    do {
                        if ( !( ( xmlCharType.charProperties[*pChar++] & XmlCharType.fWhitespace ) != 0 ) ) {  // if ( !( xmlCharType.IsWhiteSpace( chars[charPos++] ) ) ) {
                            throw new XmlException( Res.Xml_InvalidBase64Value, new string( pChars, 0, (int)( pCharsEndPos - pChars ) ) );
                        }
                    } while ( pChar < pCharsEndPos );
                }
            }

        Return:
            bits = b;
            bitsFilled = bFilled;

            bytesDecoded = (int)(pByte - pBytes);
            charsDecoded = (int)(pChar - pChars);
        }
    }
}