/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.sysds.runtime.compress;

import java.util.Arrays;

import org.apache.sysds.runtime.compress.utils.DblArray;
import org.apache.sysds.runtime.data.SparseRow;
import org.apache.sysds.runtime.matrix.data.MatrixBlock;

/**
 * Used to extract the values at certain indexes from each row in a sparse matrix
 * 
 * Keeps returning all-zeros arrays until reaching the last possible index. The current compression algorithm treats the
 * zero-value in a sparse matrix like any other value.
 */
public class ReaderColumnSelectionSparse extends ReaderColumnSelection {
	private final DblArray ZERO_DBL_ARRAY;
	private DblArray nonZeroReturn;

	// reusable return
	private DblArray reusableReturn;
	private double[] reusableArr;

	// current sparse row positions
	private SparseRow[] sparseCols = null;
	private int[] sparsePos = null;

	public ReaderColumnSelectionSparse(MatrixBlock data, int[] colIndexes, CompressionSettings compSettings) {
		super(colIndexes, compSettings.transposeInput ? data.getNumColumns() : data.getNumRows(), compSettings);
		ZERO_DBL_ARRAY = new DblArray(new double[colIndexes.length], true);
		reusableArr = new double[colIndexes.length];
		reusableReturn = new DblArray(reusableArr);

		if(!_compSettings.transposeInput) {
			throw new RuntimeException("SparseColumnSelectionReader should not be used without transposed input.");
		}

		sparseCols = new SparseRow[colIndexes.length];
		sparsePos = new int[colIndexes.length];
		if(data.getSparseBlock() != null)
			for(int i = 0; i < colIndexes.length; i++)
				sparseCols[i] = data.getSparseBlock().get(colIndexes[i]);
	}

	@Override
	public DblArray nextRow() {
		// if(_skipZeros) {
			while((nonZeroReturn = getNextRow()) != null && nonZeroReturn == ZERO_DBL_ARRAY) {
			}
			return nonZeroReturn;
		// }
		// else {
			// return getNextRow();
		// }
	}

	private DblArray getNextRow() {
		if(_lastRow == _numRows - 1)
			return null;
		_lastRow++;

		if(!_compSettings.transposeInput) {
			throw new RuntimeException("SparseColumnSelectionReader should not be used without transposed input.");
		}

		// move pos to current row if necessary (for all columns)
		for(int i = 0; i < _colIndexes.length; i++)
			if(sparseCols[i] != null &&
				(sparseCols[i].indexes().length <= sparsePos[i] || sparseCols[i].indexes()[sparsePos[i]] < _lastRow)) {
				sparsePos[i]++;
			}

		// extract current values
		Arrays.fill(reusableArr, 0);
		boolean zeroResult = true;
		for(int i = 0; i < _colIndexes.length; i++)
			if(sparseCols[i] != null && sparseCols[i].indexes().length > sparsePos[i] &&
				sparseCols[i].indexes()[sparsePos[i]] == _lastRow) {
				reusableArr[i] = sparseCols[i].values()[sparsePos[i]];
				zeroResult = false;
			}

		return zeroResult ? ZERO_DBL_ARRAY : reusableReturn;
	}
}