Add rough memory size tracking with KllItemsSketch

When generating KllSketches, systems may need to have an idea
about how much memory utilization there is for a particular sketch.
For sketches with fixed-width types the answer can be computed
efficiently.

With the KllItemsSketch, this is more difficult because the sketch
can support String-types with variable widths. This commit adds
implementation support to expose the `getTotalItemsNumBytes`
method so that external systems can roughly track the memory
utilization of a particular sketch.

The change accomplishes this by intercepting the code where
a new item is added to the items array, or when a new array is
generated entirely. This will add a slight overhead due to the
sketch now needing to compute the length of inputs. For
fixed-width types the overhead is low. For string this will require
a call to encode the string as UTF-8/16 before adding it to the
array.

For fixed-width types, the calculations have little effective overhead
as the computation is a single array-access lookup + multiplication
with the type width.

Author: ZacBlanco

src/main/java/org/apache/datasketches/common/ArrayOfBooleansSerDe.java

@@ -122,4 +122,10 @@ public String toString(final Boolean item) {
 
   @Override
   public Class<Boolean> getClassOfT() { return Boolean.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return true;
+  }
 }

src/main/java/org/apache/datasketches/common/ArrayOfDoublesSerDe.java

@@ -101,4 +101,10 @@ public String toString(final Double item) {
 
   @Override
   public Class<Double> getClassOfT() { return Double.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return true;
+  }
 }

src/main/java/org/apache/datasketches/common/ArrayOfItemsSerDe.java

@@ -114,4 +114,9 @@ public int sizeOf(final T[] items) {
    * @return the concrete class of type T
    */
   public abstract Class<T> getClassOfT();
+
+  /**
+   * @return if this class serializes all types to a fixed width.
+   */
+  public abstract boolean isFixedWidth();
 }

src/main/java/org/apache/datasketches/common/ArrayOfLongsSerDe.java

@@ -100,4 +100,10 @@ public String toString(final Long item) {
 
   @Override
   public Class<Long> getClassOfT() { return Long.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return true;
+  }
 }

src/main/java/org/apache/datasketches/common/ArrayOfNumbersSerDe.java

@@ -240,4 +240,10 @@ public String toString(final Number item) {
 
   @Override
   public Class<Number> getClassOfT() { return Number.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return false;
+  }
 }

src/main/java/org/apache/datasketches/common/ArrayOfStringsSerDe.java

@@ -130,4 +130,10 @@ public String toString(final String item) {
 
   @Override
   public Class<String> getClassOfT() { return String.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return false;
+  }
 }

src/main/java/org/apache/datasketches/common/ArrayOfUtf16StringsSerDe.java

@@ -124,4 +124,10 @@ public String toString(final String item) {
 
   @Override
   public Class<String> getClassOfT() { return String.class; }
+
+  @Override
+  public boolean isFixedWidth()
+  {
+    return false;
+  }
 }

src/main/java/org/apache/datasketches/kll/KllDirectCompactItemsSketch.java

@@ -233,6 +233,12 @@ T[] getTotalItemsArray() {
     return capItems;
   }
 
+  @Override
+  int getTotalItemsNumBytesInternal()
+  {
+    return getRetainedItemsSizeBytes();
+  }
+
   @Override
   WritableMemory getWritableMemory() {
     return (WritableMemory)mem;

src/main/java/org/apache/datasketches/kll/KllDoublesSketch.java

@@ -514,7 +514,7 @@ int getSingleItemSizeBytes() {
   abstract byte[] getTotalItemsByteArr();
 
   @Override
-  int getTotalItemsNumBytes() {
+  public int getTotalItemsNumBytes() {
     return levelsArr[getNumLevels()] * Double.BYTES;
   }
 

src/main/java/org/apache/datasketches/kll/KllFloatsSketch.java

@@ -514,7 +514,7 @@ int getSingleItemSizeBytes() {
   abstract byte[] getTotalItemsByteArr();
 
   @Override
-  int getTotalItemsNumBytes() {
+  public int getTotalItemsNumBytes() {
     return levelsArr[getNumLevels()] * Float.BYTES;
   }