add Streams.skipLast operator (#77)

davidmoten · Aug 3, 2023 · 09943f7 · 09943f7
1 parent 90acee8
commit 09943f7
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diff --git a/kool/src/main/java/org/davidmoten/kool/Stream.java b/kool/src/main/java/org/davidmoten/kool/Stream.java
@@ -89,6 +89,7 @@
 import org.davidmoten.kool.internal.operators.stream.RetryWhen;
 import org.davidmoten.kool.internal.operators.stream.Reverse;
 import org.davidmoten.kool.internal.operators.stream.Skip;
+import org.davidmoten.kool.internal.operators.stream.SkipLast;
 import org.davidmoten.kool.internal.operators.stream.SkipUntil;
 import org.davidmoten.kool.internal.operators.stream.Sorted;
 import org.davidmoten.kool.internal.operators.stream.Split;
@@ -844,6 +845,11 @@ default Stream<List<T>> buffer(int size, int step) {
     default Stream<T> skip(int size) {
         return new Skip<T>(size, this);
     }
+
+    default Stream<T> skipLast(int size) {
+        Preconditions.checkArgument(size >=0, "size must be non-negative");
+        return new SkipLast<T>(size, this);
+    }
 
     default Stream<T> skipUntil(Predicate<? super T> predicate) {
         return new SkipUntil<T>(this, predicate, false);

diff --git a/kool/src/main/java/org/davidmoten/kool/internal/operators/stream/SkipLast.java b/kool/src/main/java/org/davidmoten/kool/internal/operators/stream/SkipLast.java
@@ -0,0 +1,53 @@
+package org.davidmoten.kool.internal.operators.stream;
+
+import java.util.NoSuchElementException;
+
+import org.davidmoten.kool.Stream;
+import org.davidmoten.kool.StreamIterator;
+import org.davidmoten.kool.internal.util.RingBuffer;
+
+public final class SkipLast<T> implements Stream<T> {
+
+    private final Stream<T> stream;
+    private final int size;
+
+    public SkipLast(int size, Stream<T> stream) {
+        this.stream = stream;
+        this.size = size;
+    }
+
+    @Override
+    public StreamIterator<T> iterator() {
+        RingBuffer<T> buffer = new RingBuffer<T>(size + 1);
+        StreamIterator<T> it = stream.iterator();
+        return new StreamIterator<T>() {
+
+            @Override
+            public boolean hasNext() {
+                loadNext();
+                return buffer.size() == size + 1;
+            }
+
+            @Override
+            public T next() {
+                loadNext();
+                if (buffer.size() == size + 1) {
+                    return buffer.poll();
+                } else {
+                    throw new NoSuchElementException();
+                }
+            }
+
+            @Override
+            public void dispose() {
+                it.dispose();
+            }
+
+            private void loadNext() {
+                while (buffer.size() < size + 1 && it.hasNext()) {
+                    buffer.add(it.next());
+                }
+            }
+        };
+    }
+}
diff --git a/kool/src/test/java/org/davidmoten/kool/StreamTest.java b/kool/src/test/java/org/davidmoten/kool/StreamTest.java
@@ -573,6 +573,31 @@ public void testSkip() {
     public void testSkipMoreThanAvailable() {
         Stream.of(1, 2, 3, 4).skip(5).test().assertNoValuesOnly();
     }
+
+    @Test
+    public void testSkipLast() {
+        Stream.of(1, 2, 3, 4).skipLast(2).test().assertValuesOnly(1, 2);
+    }
+
+    @Test
+    public void testSkipLastZero() {
+        Stream.of(1, 2, 3, 4).skipLast(0).test().assertValuesOnly(1, 2, 3, 4);
+    }
+
+    @Test
+    public void testSkipLastMoreThanAvailable() {
+        Stream.of(1, 2, 3, 4).skipLast(5).test().assertNoValuesOnly();
+    }
+
+    @Test(expected = NoSuchElementException.class)
+    public void testSkipLastNoSuchElement() {
+        Stream.empty().skipLast(5).iterator().next();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testSkipLastNegative() {
+        Stream.of(1, 2, 3, 4).skipLast(-1);
+    }
 
     @Test
     public void testTakeDisposes() {