namespace System.IO.Compression
{
    using System;
    using System.Diagnostics;
    
    // This class can be used to read bits from an byte array quickly.
    // Normally we get bits from 'bitBuffer' field and bitsInBuffer stores
    // the number of bits available in 'BitBuffer'.
    // When we used up the bits in bitBuffer, we will try to get byte from
    // the byte array and copy the byte to appropiate position in bitBuffer.
    //
    // The byte array is not reused. We will go from 'start' to 'end'. 
    // When we reach the end, most read operations will return -1, 
    // which means we are running out of input.

    internal class InputBuffer {

        private byte[] buffer;           // byte array to store input
        private int start;               // start poisition of the buffer
        private int end;                 // end position of the buffer
        private uint bitBuffer = 0;      // store the bits here, we can quickly shift in this buffer
        private int bitsInBuffer = 0;    // number of bits available in bitBuffer

        // Total bits available in the input buffer
        public int AvailableBits {   
            get{
                return bitsInBuffer;
            }
        }

        // Total bytes available in the input buffer
        public int AvailableBytes {   
            get{
                return(end - start)  + (bitsInBuffer / 8);
            }
        }

        // Ensure that count bits are in the bit buffer.
        // Returns false if input is not sufficient to make this true.
        // Count can be up to 16.
        public bool EnsureBitsAvailable(int count) {
            Debug.Assert( 0 < count && count <= 16, "count is invalid.");
            
            // manual inlining to improve perf
            if (bitsInBuffer < count) {
                if (NeedsInput()) {
                    return false;
                }
                // insert a byte to bitbuffer
                bitBuffer |= (uint)buffer[start++] << bitsInBuffer; 
                bitsInBuffer += 8; 

                if (bitsInBuffer < count) {
                    if (NeedsInput()) {
                        return false;
                    }
                    // insert a byte to bitbuffer
                    bitBuffer |= (uint)buffer[start++] << bitsInBuffer; 
                    bitsInBuffer += 8; 
                }
            }

            return true;
        }

        // This function will try to load 16 or more bits into bitBuffer.
        // It returns whatever is contained in bitBuffer after loading.
        // The main difference between this and GetBits is that this will
        // never return -1. So the caller needs to check AvailableBits to 
        // see how many bits are available. 
        public uint TryLoad16Bits() {
            if(bitsInBuffer < 8) {
                if( start < end) {
                    bitBuffer |= (uint)buffer[start++] << bitsInBuffer; 
                    bitsInBuffer += 8; 
                }
                
                if( start < end) {
                    bitBuffer |= (uint)buffer[start++] << bitsInBuffer; 
                    bitsInBuffer += 8; 
                }
            }
            else if(bitsInBuffer < 16) {
                if( start < end) {
                    bitBuffer |= (uint)buffer[start++] << bitsInBuffer; 
                    bitsInBuffer += 8; 
                }
            }

            return bitBuffer;
        }

        private uint GetBitMask(int count) {
            return ((uint)1 << count) - 1;
        }

        // Gets count bits from the input buffer. Returns -1 if not enough bits available.
        public int GetBits(int count) {
            Debug.Assert( 0 < count && count <= 16, "count is invalid.");

            if ( !EnsureBitsAvailable(count) ) {
                return -1;
            } 

            int result = (int)(bitBuffer & GetBitMask(count));
            bitBuffer >>= count; 
            bitsInBuffer -= count; 
            return result;
        }

        /// Copies length bytes from input buffer to output buffer starting
        /// at output[offset].  You have to make sure, that the buffer is
        /// byte aligned.  If not enough bytes are available, copies fewer
        /// bytes.
        /// Returns the number of bytes copied, 0 if no byte is available.
        public int CopyTo(byte[] output, int offset, int length) {
            Debug.Assert( output != null, "");
            Debug.Assert( offset >=0 , "");
            Debug.Assert( length >=0 , "");
            Debug.Assert( offset <= output.Length - length, "");
            Debug.Assert( (bitsInBuffer % 8) ==0, "");

            // Copy the bytes in bitBuffer first.
            int bytesFromBitBuffer = 0;
            while (bitsInBuffer > 0 && length > 0) {
                output[offset++] = (byte) bitBuffer;
                bitBuffer >>= 8;
                bitsInBuffer -= 8;
                length--;
                bytesFromBitBuffer++;
            }

            if (length == 0) {
                return bytesFromBitBuffer;
            }

            int avail = end - start;
            if (length > avail) {    
                length = avail;
            }

            Array.Copy(buffer, start, output, offset, length);
            start += length;
            return bytesFromBitBuffer + length;
        }

        // Return true is all input bytes are used. 
        // This means the caller can call SetInput to add more input.        
        public bool NeedsInput() {
            return start == end;
        }

        // Set the byte array to be processed.
        // All the bits remained in bitBuffer will be processed before the new bytes.
        // We don't clone the byte array here since it is expensive.
        // The caller should make sure after a buffer is passed in. 
        // It will not be changed before calling this function again.

        public void SetInput(byte[] buffer, int offset, int length) {
            Debug.Assert( buffer != null, "");
            Debug.Assert( offset >=0 , "");
            Debug.Assert( length >=0 , "");
            Debug.Assert( offset <= buffer.Length - length, "");
            Debug.Assert( start == end, "");

            this.buffer = buffer; 
            start = offset;
            end = offset + length;
        }

        // Skip n bits in the buffer
        public void SkipBits(int n) {
            Debug.Assert(bitsInBuffer >= n, "No enough bits in the buffer, Did you call EnsureBitsAvailable?");
            bitBuffer >>= n;
            bitsInBuffer -= n;
        }

        // Skips to the next byte boundary.
        public void SkipToByteBoundary() {
            bitBuffer >>= (bitsInBuffer % 8);
            bitsInBuffer = bitsInBuffer - (bitsInBuffer % 8);
        }
    }
}