amaliujia commented on a change in pull request #2035:
URL: https://github.com/apache/calcite/pull/2035#discussion_r445212926
##########
File path:
linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
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@@ -817,6 +817,112 @@ public void remove() {
resultSelector);
}
+ /**
+ * Group keys are sorted already. Key values are compared by using a
+ * specified comparator. Groups the elements of a sequence according to a
+ * specified key selector function and initializing one accumulator at a
time.
+ * Go over elements sequentially, adding to accumulator each time an element
+ * with the same key is seen. When key changes, creates a result value from
the
+ * accumulator and then re-initializes the accumulator.
+ */
+ public static <TSource, TKey, TAccumulate, TResult> Enumerable<TResult>
sortedGroupBy(
+ Enumerable<TSource> enumerable,
+ Function1<TSource, TKey> keySelector,
+ Function0<TAccumulate> accumulatorInitializer,
+ Function2<TAccumulate, TSource, TAccumulate> accumulatorAdder,
+ final Function2<TKey, TAccumulate, TResult> resultSelector,
+ final Comparator<TKey> comparator) {
+ return new AbstractEnumerable<TResult>() {
+ public Enumerator<TResult> enumerator() {
+ return new SortedAggregateEnumerator(
+ enumerable, keySelector, accumulatorInitializer,
+ accumulatorAdder, resultSelector, comparator);
+ }
+ };
+ }
+
+ private static class SortedAggregateEnumerator<TSource, TKey, TAccumulate,
TResult>
+ implements Enumerator<TResult> {
+ private final Enumerable<TSource> enumerable;
+ private final Function1<TSource, TKey> keySelector;
+ private final Function0<TAccumulate> accumulatorInitializer;
+ private final Function2<TAccumulate, TSource, TAccumulate>
accumulatorAdder;
+ private final Function2<TKey, TAccumulate, TResult> resultSelector;
+ private final Comparator<TKey> comparator;
+ private boolean isInitialized;
+ private TAccumulate curAccumulator;
+ private Enumerator<TSource> enumerator;
+
+ SortedAggregateEnumerator(
+ Enumerable<TSource> enumerable,
+ Function1<TSource, TKey> keySelector,
+ Function0<TAccumulate> accumulatorInitializer,
+ Function2<TAccumulate, TSource, TAccumulate> accumulatorAdder,
+ final Function2<TKey, TAccumulate, TResult> resultSelector,
+ final Comparator<TKey> comparator) {
+ this.enumerable = enumerable;
+ this.keySelector = keySelector;
+ this.accumulatorInitializer = accumulatorInitializer;
+ this.accumulatorAdder = accumulatorAdder;
+ this.resultSelector = resultSelector;
+ this.comparator = comparator;
+ isInitialized = false;
+ curAccumulator = null;
+ enumerator = enumerable.enumerator();
+ }
+
+ @Override public TResult current() {
Review comment:
Thanks for the example!
I doubled check the contract and have the following conclusion:
```
* <p>After an enumerator is created or after the {@link #reset} method is
* called, the {@link #moveNext} method must be called to advance the
* enumerator to the first element of the collection before reading the
* value of the {@code current} property; otherwise, {@code current} is
* undefined.
```
So in this case, return `null` is right as the result is undefined.
```
* <p>This method also throws {@link java.util.NoSuchElementException} if
* the last call to {@code moveNext} returned {@code false}, which
indicates
* the end of the collection.
```
I updated the code to make sure it throws `NoSuchElementException` in this
case.
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