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src/java.base/share/classes/java/util/stream/IntStream.java
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*** 541,550 ****
--- 541,691 ----
* @see #reduce(int, IntBinaryOperator)
*/
OptionalInt reduce(IntBinaryOperator op);
/**
+ * Folds the elements of this stream using the provided {@code seed} value and
+ * accumulation function, going left to right. This is equivalent to:
+ *
+ * <pre>{@code
+ * int result = seed;
+ * for (int element : this stream)
+ * result = accumulator.apply(result, element)
+ * return result;
+ * }
+ * </pre>
+ *
+ * <p>This is a <a href="package-summary.html#StreamOps">terminal
+ * operation</a>.
+ *
+ * <p>This method cannot take all the advantages of parallel streams as it must
+ * process elements strictly left to right. If your accumulator function
+ * is associative, and {@code seed} value has identity properties,
+ * consider using {@link #reduce(int, IntBinaryOperator)} method.
+ *
+ * @param seed the starting value
+ * @param accumulator a <a href="package-summary.html#NonInterference">non-interfering</a>,
+ * <a href="package-summary.html#Statelessness">stateless</a>
+ * function for incorporating an additional element into a result
+ * @return the result of the folding
+ * @see #reduce(int, IntBinaryOperator)
+ * @see #foldLeft(IntBinaryOperator)
+ * @since 10
+ */
+ default int foldLeft(int seed, IntBinaryOperator accumulator) {
+ ReduceOps.IntFoldLeftOp op = new ReduceOps.IntFoldLeftOp(accumulator);
+ op.accept(seed);
+ forEachOrdered(op);
+ return op.getAsInt();
+ }
+
+ /**
+ * Folds the elements of this stream using the provided accumulation
+ * function, going left to right. This is equivalent to:
+ *
+ * <pre>{@code
+ * boolean foundAny = false;
+ * int result = 0;
+ * for (int element : this stream) {
+ * if (!foundAny) {
+ * foundAny = true;
+ * result = element;
+ * }
+ * else
+ * result = accumulator.apply(result, element);
+ * }
+ * return foundAny ? OptionalInt.of(result) : OptionalInt.empty();
+ * }
+ * </pre>
+ *
+ * <p>This is a <a href="package-summary.html#StreamOps">terminal
+ * operation</a>.
+ *
+ * <p>This method cannot take all the advantages of parallel streams as it must
+ * process elements strictly left to right. If your accumulator function is
+ * associative, consider using {@link #reduce(IntBinaryOperator)} method.
+ *
+ * @param accumulator a <a href="package-summary.html#NonInterference">non-interfering</a>,
+ * <a href="package-summary.html#Statelessness">stateless</a>
+ * function for incorporating an additional element into a result
+ * @return the result of the folding
+ * @see #foldLeft(int, IntBinaryOperator)
+ * @see #reduce(IntBinaryOperator)
+ * @since 10
+ */
+ default OptionalInt foldLeft(IntBinaryOperator accumulator) {
+ ReduceOps.IntFoldLeftOp op = new ReduceOps.IntFoldLeftOp(accumulator);
+ forEachOrdered(op);
+ return op.get();
+ }
+
+ /**
+ * Folds the elements of this stream using the provided {@code seed} value and
+ * accumulation function, going right to left.
+ *
+ * <p>This is a <a href="package-summary.html#StreamOps">terminal
+ * operation</a>.
+ *
+ * <p>As this method must process elements strictly right to left, it cannot
+ * start processing till all the previous stream stages complete. Also it
+ * requires intermediate memory to store the whole content of the stream as
+ * the stream natural order is left to right. If your accumulator function
+ * is associative, and {@code seed} value has identity properties,
+ * consider using {@link #reduce(int, IntBinaryOperator)} method.
+ *
+ * @param seed the starting value
+ * @param accumulator a <a href="package-summary.html#NonInterference">non-interfering</a>,
+ * <a href="package-summary.html#Statelessness">stateless</a>
+ * function for incorporating an additional element into a result
+ * @return the result of the folding
+ * @see #foldLeft(int, IntBinaryOperator)
+ * @see #reduce(int, IntBinaryOperator)
+ * @since 10
+ */
+ default int foldRight(int seed, IntBinaryOperator accumulator) {
+ int[] array = toArray();
+ int result = seed;
+ for(int idx = array.length - 1; idx >= 0; idx--) {
+ result = accumulator.applyAsInt(array[idx], result);
+ }
+ return result;
+ }
+
+ /**
+ * Folds the elements of this stream using the provided accumulation
+ * function, going right to left.
+ *
+ * <p>This is a <a href="package-summary.html#StreamOps">terminal
+ * operation</a>.
+ *
+ * <p>As this method must process elements strictly right to left, it cannot
+ * start processing till all the previous stream stages complete. Also it
+ * requires intermediate memory to store the whole content of the stream as
+ * the stream natural order is left to right. If your accumulator function
+ * is associative, consider using {@link #reduce(IntBinaryOperator)} method.
+ *
+ * @param accumulator a <a href="package-summary.html#NonInterference">non-interfering</a>,
+ * <a href="package-summary.html#Statelessness">stateless</a>
+ * function for incorporating an additional element into a result
+ * @return the result of the folding
+ * @see #foldLeft(int, IntBinaryOperator)
+ * @see #reduce(int, IntBinaryOperator)
+ * @since 10
+ */
+ default OptionalInt foldRight(IntBinaryOperator accumulator) {
+ int[] array = toArray();
+ int idx = array.length;
+ if (idx == 0) return OptionalInt.empty();
+ int result = array[--idx];
+ while(idx > 0) {
+ int t = array[--idx];
+ result = accumulator.applyAsInt(t, result);
+ }
+ return OptionalInt.of(result);
+ }
+
+ /**
* Performs a <a href="package-summary.html#MutableReduction">mutable
* reduction</a> operation on the elements of this stream. A mutable
* reduction is one in which the reduced value is a mutable result container,
* such as an {@code ArrayList}, and elements are incorporated by updating
* the state of the result rather than by replacing the result. This
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