/**
 * @namespace WPGMZA
 * @module EliasFano
 * @requires WPGMZA
 */
jQuery(function($) {
	
	WPGMZA.EliasFano = function()
	{
		if(!WPGMZA.EliasFano.isSupported)
			throw new Error("Elias Fano encoding is not supported on browsers without Uint8Array");
		
		if(!WPGMZA.EliasFano.decodingTablesInitialised)
			WPGMZA.EliasFano.createDecodingTable();
	}
	
	WPGMZA.EliasFano.isSupported = ("Uint8Array" in window);
	
	WPGMZA.EliasFano.decodingTableHighBits			= [];
	WPGMZA.EliasFano.decodingTableDocIDNumber		= null;
	WPGMZA.EliasFano.decodingTableHighBitsCarryover = null;
	
	WPGMZA.EliasFano.createDecodingTable = function()
	{
		WPGMZA.EliasFano.decodingTableDocIDNumber = new Uint8Array(256);
		WPGMZA.EliasFano.decodingTableHighBitsCarryover = new Uint8Array(256);
		
		var decodingTableHighBits = WPGMZA.EliasFano.decodingTableHighBits;
		var decodingTableDocIDNumber = WPGMZA.EliasFano.decodingTableDocIDNumber;
		var decodingTableHighBitsCarryover = WPGMZA.EliasFano.decodingTableHighBitsCarryover;
		
		for(var i = 0; i < 256; i++)
		{
			var zeroCount = 0;
			
			decodingTableHighBits[i] = [];
			
			for(var j = 7; j >= 0; j--)
			{
				if((i & (1 << j)) > 0)
				{
					decodingTableHighBits[i][decodingTableDocIDNumber[i]] = zeroCount;
					
					decodingTableDocIDNumber[i]++;
					zeroCount = 0;
				}
				else
					zeroCount = (zeroCount + 1) % 0xFF;
			}
			
			decodingTableHighBitsCarryover[i] = zeroCount;
		}
		
		WPGMZA.EliasFano.decodingTablesInitialised = true;
	}
	
	WPGMZA.EliasFano.prototype.encode = function(list)
	{
		var lastDocID		= 0,
			buffer1 		= 0,
			bufferLength1 	= 0,
			buffer2 		= 0,
			bufferLength2 	= 0;
		
		if(list.length == 0)
			return result;
		
		function toByte(n)
		{
			return n & 0xFF;
		}
		
		var compressedBufferPointer1 = 0;
		var compressedBufferPointer2 = 0;
		var largestBlockID = list[list.length - 1];
		var averageDelta = largestBlockID / list.length;
		var averageDeltaLog = Math.log2(averageDelta);
		var lowBitsLength = Math.floor(averageDeltaLog);
		var lowBitsMask = (1 << lowBitsLength) - 1;
		var prev = null;
		
		var maxCompressedSize = Math.floor(
			(
				2 + Math.ceil(
					Math.log2(averageDelta)
				)
			) * list.length / 8
		) + 6;
		
		var compressedBuffer = new Uint8Array(maxCompressedSize);
		
		if(lowBitsLength < 0)
			lowBitsLength = 0;
		
		compressedBufferPointer2 = Math.floor(lowBitsLength * list.length / 8 + 6);
		
		compressedBuffer[compressedBufferPointer1++] = toByte( list.length );
		compressedBuffer[compressedBufferPointer1++] = toByte( list.length >> 8 );
		compressedBuffer[compressedBufferPointer1++] = toByte( list.length >> 16 );
		compressedBuffer[compressedBufferPointer1++] = toByte( list.length >> 24 );
		
		compressedBuffer[compressedBufferPointer1++] = toByte( lowBitsLength );
		
		list.forEach(function(docID) {
			
			var docIDDelta = (docID - lastDocID - 1);
			
			if(!$.isNumeric(docID))
				throw new Error("Value is not numeric");
			
			// NB: Force docID to an integer in case it's a string
			docID = parseInt(docID);
			
			if(prev !== null && docID <= prev)
				throw new Error("Elias Fano encoding can only be used on a sorted, ascending list of unique integers.");
			
			prev = docID;
			
			buffer1 <<= lowBitsLength;
			buffer1 |= (docIDDelta & lowBitsMask);
			bufferLength1 += lowBitsLength;
			
			// Flush buffer 1
			while(bufferLength1 > 7)
			{
				bufferLength1 -= 8;
				compressedBuffer[compressedBufferPointer1++] = toByte( buffer1 >> bufferLength1 );
			}
			
			var unaryCodeLength = (docIDDelta >> lowBitsLength) + 1;
			
			buffer2 <<= unaryCodeLength;
			buffer2 |= 1;
			bufferLength2 += unaryCodeLength;
			
			// Flush buffer 2
			while(bufferLength2 > 7)
			{
				bufferLength2 -= 8;
				compressedBuffer[compressedBufferPointer2++] = toByte( buffer2 >> bufferLength2 );
			}
			
			lastDocID = docID;
		});
		
		if(bufferLength1 > 0)
			compressedBuffer[compressedBufferPointer1++] = toByte( buffer1 << (8 - bufferLength1) );
		
		if(bufferLength2 > 0)
			compressedBuffer[compressedBufferPointer2++] = toByte( buffer2 << (8 - bufferLength2) );
		
		var result = new Uint8Array(compressedBuffer);
		
		result.pointer = compressedBufferPointer2;
		
		return result;
	}
	
	WPGMZA.EliasFano.prototype.decode = function(compressedBuffer)
	{
		var resultPointer = 0;
		var list = [];
		
		//console.log("Decoding buffer from pointer " + compressedBuffer.pointer);
		//console.log(compressedBuffer);
		
		var decodingTableHighBits = WPGMZA.EliasFano.decodingTableHighBits;
		var decodingTableDocIDNumber = WPGMZA.EliasFano.decodingTableDocIDNumber;
		var decodingTableHighBitsCarryover = WPGMZA.EliasFano.decodingTableHighBitsCarryover;
		
		var lowBitsPointer = 0,
			lastDocID = 0,
			docID = 0,
			docIDNumber = 0;
		
		var listCount = compressedBuffer[lowBitsPointer++];
		
		//console.log("listCount is now " + listCount);
		
		listCount |= compressedBuffer[lowBitsPointer++] << 8;
		
		//console.log("listCount is now " + listCount);
		
		listCount |= compressedBuffer[lowBitsPointer++] << 16;
		
		//console.log("listCount is now " + listCount);
		
		listCount |= compressedBuffer[lowBitsPointer++] << 24;
		
		//console.log("Read list count " + listCount);
		
		var lowBitsLength = compressedBuffer[lowBitsPointer++];
		
		//console.log("lowBitsLength = " + lowBitsLength);
		
		var highBitsPointer,
			lowBitsCount = 0,
			lowBits = 0,
			cb = 1;
		
		for(
			highBitsPointer = Math.floor(lowBitsLength * listCount / 8 + 6);
			highBitsPointer < compressedBuffer.pointer;
			highBitsPointer++
			)
		{
			docID += decodingTableHighBitsCarryover[cb];
			cb = compressedBuffer[highBitsPointer];
			
			docIDNumber = decodingTableDocIDNumber[cb];
			
			for(var i = 0; i < docIDNumber; i++)
			{
				docID <<= lowBitsCount;
				docID |= lowBits & ((1 << lowBitsCount) - 1);
				
				while(lowBitsCount < lowBitsLength)
				{
					docID <<= 8;
					
					lowBits = compressedBuffer[lowBitsPointer++];
					docID |= lowBits;
					lowBitsCount += 8;
				}
				
				lowBitsCount -= lowBitsLength;
				docID >>= lowBitsCount;
				
				docID += (decodingTableHighBits[cb][i] << lowBitsLength) + lastDocID + 1;
				
				list[resultPointer++] = docID;
				
				lastDocID = docID;
				docID = 0;
			}
		}
		
		return list;
	}
	
});