ICU-22789 Replace Stream of boundary Integer with efficient IntStream

icu4j/main/core/src/main/java/com/ibm/icu/text/segmenter/Segments.java

@@ -2,7 +2,9 @@ package com.ibm.icu.text.segmenter;
 
 import com.ibm.icu.text.BreakIterator;
 import java.util.Iterator;
+import java.util.Spliterator;
 import java.util.function.Function;
+import java.util.function.IntConsumer;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
 
@@ -158,29 +160,12 @@ public interface Segments {
   // Inner classes for BoundaryIterable and BoundaryIterator
   //
 
-  class BoundaryIterable implements Iterable<Integer> {
-    BreakIterator breakIter;
-    IterationDirection direction;
-    int startIdx;
-
-    BoundaryIterable(BreakIterator breakIter, IterationDirection direction, int startIdx) {
-      this.breakIter = breakIter;
-      this.direction = direction;
-      this.startIdx = startIdx;
-    }
-
-    @Override
-    public Iterator<Integer> iterator() {
-      return new BoundaryIterator(this.breakIter, this.direction, this.startIdx);
-    }
-  }
-
-  class BoundaryIterator implements Iterator<Integer> {
+  class BoundaryIteratorOfInts {
     BreakIterator breakIter;
     IterationDirection direction;
     int currIdx;
 
-    BoundaryIterator(BreakIterator breakIter, IterationDirection direction, int startIdx) {
+    BoundaryIteratorOfInts(BreakIterator breakIter, IterationDirection direction, int startIdx) {
       this.breakIter = breakIter;
       this.direction = direction;
 
@@ -198,12 +183,10 @@ public interface Segments {
       }
     }
 
-    @Override
     public boolean hasNext() {
       return this.currIdx != BreakIterator.DONE;
     }
 
-    @Override
     public Integer next() {
       int result = this.currIdx;
 
@@ -218,4 +201,55 @@ public interface Segments {
     }
   }
 
+  class SegmentSpliterator implements Spliterator.OfInt {
+
+    private final BoundaryIteratorOfInts iter;
+
+    SegmentSpliterator(BreakIterator breakIter, IterationDirection direction, int startIdx) {
+      iter = new BoundaryIteratorOfInts(breakIter, direction, startIdx);
+    }
+
+    @Override
+    public OfInt trySplit() {
+      // The elements of the Stream represent an iteration through a string, and is thus inherently
+      // stateful. Therefore, splitting this Stream does not make sense. Ex: splitting the Stream
+      // is tantamount to discarding the segment subtended by the end value (index into the input
+      // string) of one substream and the beginning value of the next substream.
+      return null;
+    }
+
+    @Override
+    public long estimateSize() {
+      // The number of segments per input size depends on language, script, and
+      // the content of the input string, and thus is hard to estimate without
+      // sacrificing performance. Thus, returning `Long.MAX_VALUE`, according
+      // to the API, to mean "unknown, or too expensive to compute".
+      return Long.MAX_VALUE;
+    }
+
+    @Override
+    public int characteristics() {
+      return Spliterator.DISTINCT  // BreakIterator always advances
+          | Spliterator.IMMUTABLE // design of Segmenter API is to provide an immutable view of
+          // segmentation by preventing the input string from mutating
+          // in the underlying BreakIterator
+          | Spliterator.NONNULL   // primtive int is non-null
+          | Spliterator.ORDERED   // BreakIterator always advances, and in a single direction
+          ;
+    }
+
+    @Override
+    public boolean tryAdvance(IntConsumer action) {
+      if (action == null) {
+        throw new NullPointerException();
+      }
+      if (iter.hasNext()) {
+        action.accept(iter.next());
+        return true;
+      } else {
+        return false;
+      }
+    }
+  }
+
 }

icu4j/main/core/src/main/java/com/ibm/icu/text/segmenter/SegmentsImplUtils.java

@@ -1,10 +1,10 @@
 package com.ibm.icu.text.segmenter;
 
 import com.ibm.icu.text.BreakIterator;
-import com.ibm.icu.text.segmenter.Segments.BoundaryIterable;
 import com.ibm.icu.text.segmenter.Segments.IterationDirection;
 import com.ibm.icu.text.segmenter.Segments.Segment;
 import com.ibm.icu.text.segmenter.Segments.SegmentIterable;
+import com.ibm.icu.text.segmenter.Segments.SegmentSpliterator;
 import java.util.function.Function;
 import java.util.stream.IntStream;
 import java.util.stream.Stream;
@@ -86,6 +86,8 @@ public class SegmentsImplUtils {
     return new Segment(start, limit, sourceSequence);
   }
 
+  // TODO(ICU-22987): Remove unused segmentBeforeIndex / segmentAfterIndex after
+  //  ensuring fix for preceding(int) to return `DONE` for negative inputs
   public static Segment segmentBeforeIndex(BreakIterator breakIter, CharSequence sourceSequence, int i) {
     breakIter.setText(sourceSequence);
 
@@ -120,10 +122,7 @@ public class SegmentsImplUtils {
     breakIter.setText(sourceSequence);
 
     // create a Stream from a Spliterator of an Iterable so that the Stream can be lazy, not eager
-    // TODO: optimize IntStream creation to avoid autoboxing
-    BoundaryIterable iterable = new BoundaryIterable(breakIter, IterationDirection.FORWARDS, i);
-    Stream<Integer> boundariesAsIntegers =  StreamSupport.stream(iterable.spliterator(), false);
-    return boundariesAsIntegers.mapToInt(Integer::intValue);
+    return StreamSupport.intStream(new SegmentSpliterator(breakIter, IterationDirection.FORWARDS, i), false);
   }
 
   public static IntStream boundariesBackFrom(BreakIterator breakIter, CharSequence sourceSequence, int i) {
@@ -139,10 +138,7 @@ public class SegmentsImplUtils {
     int backFromIdx = isOnBoundary ? i + 1 : i;
 
     // create a Stream from a Spliterator of an Iterable so that the Stream can be lazy, not eager
-    // TODO: optimize IntStream creation to avoid autoboxing
-    BoundaryIterable iterable = new BoundaryIterable(breakIter, IterationDirection.BACKWARDS, backFromIdx);
-    Stream<Integer> boundariesAsIntegers =  StreamSupport.stream(iterable.spliterator(), false);
-    return boundariesAsIntegers.mapToInt(Integer::intValue);
+    return StreamSupport.intStream(new SegmentSpliterator(breakIter, IterationDirection.BACKWARDS, backFromIdx), false);
   }
 
 }