phaniarnab commented on code in PR #1650:
URL: https://github.com/apache/systemds/pull/1650#discussion_r932944938


##########
src/main/java/org/apache/sysds/runtime/matrix/data/LibMatrixCountDistinct.java:
##########
@@ -102,66 +114,255 @@ else if(in.getNonZeros() < minimumSize) {
         * 
         * Benefit: precise, but uses memory, on the scale of inputs number of 
distinct values.
         * 
-        * @param in The input matrix to count number distinct values in
-        * @return The absolute distinct count
+        * @param blkIn The input matrix to count number distinct values in
+        * @return A matrix block containing the absolute distinct count for 
the entire input or along given row/col axis
         */
-       private static int countDistinctValuesNaive(MatrixBlock in) {
+       private static MatrixBlock countDistinctValuesNaive(MatrixBlock blkIn, 
CountDistinctOperator op) {
+
+               if (blkIn.isEmpty()) {
+                       return new MatrixBlock(1);
+               }
+               else if(blkIn instanceof CompressedMatrixBlock) {
+                       throw new NotImplementedException("countDistinct() does 
not support CompressedMatrixBlock");
+               }
+
                Set<Double> distinct = new HashSet<>();
+               MatrixBlock blkOut;
                double[] data;
-               if(in.isEmpty())
-                       return 1;
-               else if(in instanceof CompressedMatrixBlock)
-                       throw new NotImplementedException();
 
-               long nonZeros = in.getNonZeros();
+               if (op.getDirection().isRowCol()) {
+                       blkOut = new MatrixBlock(1, 1, false);
 
-               if(nonZeros != -1 && nonZeros < in.getNumColumns() * 
in.getNumRows()) {
-                       distinct.add(0d);
-               }
+                       long distinctCount = 0;
+                       long nonZeros = blkIn.getNonZeros();
 
-               if(in.sparseBlock != null) {
-                       SparseBlock sb = in.sparseBlock;
+                       // Check if input matrix contains any 0 values for 
RowCol case.
+                       // This does not apply to row/col case, where we count 
nnz per row or col during iteration.
+                       if(nonZeros != -1 && nonZeros < blkIn.getNumColumns() * 
blkIn.getNumRows()) {
+                               distinct.add(0d);
+                       }
 
-                       if(in.sparseBlock.isContiguous()) {
-                               data = sb.values(0);
-                               countDistinctValuesNaive(data, distinct);
+                       if(blkIn.getSparseBlock() != null) {
+                               SparseBlock sb = blkIn.getSparseBlock();
+                               if(blkIn.getSparseBlock().isContiguous()) {
+                                       // COO, CSR
+                                       data = sb.values(0);
+                                       distinctCount = 
countDistinctValuesNaive(data, distinct);
+                               } else {
+                                       // MCSR
+                                       for(int i = 0; i < blkIn.getNumRows(); 
i++) {
+                                               if(!sb.isEmpty(i)) {
+                                                       data = 
blkIn.getSparseBlock().values(i);
+                                                       distinctCount = 
countDistinctValuesNaive(data, distinct);
+                                               }
+                                       }
+                               }
+                       } else if(blkIn.getDenseBlock() != null) {
+                               DenseBlock db = blkIn.getDenseBlock();
+                               for (int i = 0; i <= db.numBlocks(); i++) {
+                                       data = db.valuesAt(i);
+                                       distinctCount = 
countDistinctValuesNaive(data, distinct);
+                               }
                        }
-                       else {
-                               for(int i = 0; i < in.getNumRows(); i++) {
-                                       if(!sb.isEmpty(i)) {
-                                               data = in.sparseBlock.values(i);
+
+                       blkOut.setValue(0, 0, distinctCount);
+               } else if (op.getDirection().isRow()) {
+                       blkOut = blkIn.slice(0, blkIn.getNumRows() - 1, 0, 0);

Review Comment:
   Why deep copying (slice) the first column is required? You are anyway going 
to set individual values. Won't just allocating a dense column vector work? 
Something like below.
   `blkOut = new MatrixBlock(blkIn.getNumRows(), 1, false, blkIn.getNumRows());
    blkOut.allocateBlock();`



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