simplify fromCOO implementation

Author: mengxr

mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala

@@ -362,46 +362,46 @@ object SparseMatrix {
    * @return The corresponding `SparseMatrix`
    */
   def fromCOO(numRows: Int, numCols: Int, entries: Array[(Int, Int, Double)]): SparseMatrix = {
+    val numEntries = entries.size
     val sortedEntries = entries.sortBy(v => (v._2, v._1))
-    val colPtrs = new Array[Int](numCols + 1)
-    var nnz = 0
-    var lastCol = -1
-    var lastIndex = -1
-    sortedEntries.foreach { case (i, j, v) =>
-      require(i >= 0 && j >= 0, "Negative indices given. Please make sure all indices are " +
-        s"greater than or equal to zero. i: $i, j: $j, value: $v")
-      if (v != 0.0) {
-        while (j != lastCol) {
-          colPtrs(lastCol + 1) = nnz
-          lastCol += 1
-        }
-        val index = j * numRows + i
-        if (lastIndex != index) {
-          nnz += 1
-          lastIndex = index
-        }
+    if (sortedEntries.nonEmpty) {
+      // Since the entries are sorted by column index, we only need to check the first and the last.
+      for (col <- Seq(sortedEntries.head._2, sortedEntries.last._2)) {
+        require(col >= 0 && col < numCols, s"Column index out of range [0, $numCols): $col.")
       }
     }
-    while (numCols > lastCol) {
-      colPtrs(lastCol + 1) = nnz
-      lastCol += 1
-    }
-    val values = new Array[Double](nnz)
-    val rowIndices = new Array[Int](nnz)
-    lastIndex = -1
-    var cnt = -1
-    sortedEntries.foreach { case (i, j, v) =>
-      if (v != 0.0) {
-        val index = j * numRows + i
-        if (lastIndex != index) {
-          cnt += 1
-          lastIndex = index
+    val colPtrs = new Array[Int](numCols + 1)
+    val rowIndices = MArrayBuilder.make[Int]
+    rowIndices.sizeHint(numEntries)
+    val values = MArrayBuilder.make[Double]
+    values.sizeHint(numEntries)
+    var nnz = 0
+    var prevCol = 0
+    var prevRow = -1
+    var prevVal = 0.0
+    // Append a dummy entry to include the last one at the end of the loop.
+    (sortedEntries.view :+ (numRows, numCols, 1.0)).foreach { case (i, j, v) =>
+      if (v != 0) {
+        if (i == prevRow && j == prevCol) {
+          prevVal += v
+        } else {
+          if (prevVal != 0) {
+            require(prevRow >= 0 && prevRow < numRows,
+              s"Row index out of range [0, $numRows): $prevRow.")
+            nnz += 1
+            rowIndices += prevRow
+            values += prevVal
+          }
+          prevRow = i
+          prevVal = v
+          while (prevCol < j) {
+            colPtrs(prevCol + 1) = nnz
+            prevCol += 1
+          }
         }
-        values(cnt) += v
-        rowIndices(cnt) = i
       }
     }
-    new SparseMatrix(numRows, numCols, colPtrs.toArray, rowIndices, values)
+    new SparseMatrix(numRows, numCols, colPtrs, rowIndices.result(), values.result())
   }
 
   /**