add function for generating sequence of pairs (value, previousValue)

java/java-impl/src/com/intellij/util/collections.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2000-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.util
+
+typealias LookbackValue<T> = Pair<T, T?>
+
+/**
+ * @return sequence of `Pair(value, previousValue)`. First element of sequence has `null` previous value.
+ */
+fun <T> Sequence<T>.withPrevious(): Sequence<LookbackValue<T>> = LookbackSequence(this)
+
+
+private class LookbackSequence<T>(private val sequence: Sequence<T>) : Sequence<LookbackValue<T>> {
+
+  override fun iterator(): Iterator<LookbackValue<T>> = LookbackIterator(sequence.iterator())
+}
+
+private class LookbackIterator<T>(private val iterator: Iterator<T>) : Iterator<LookbackValue<T>> {
+
+  private var previous: T? = null
+
+  override fun hasNext() = iterator.hasNext()
+
+  override fun next(): LookbackValue<T> {
+    val next = iterator.next()
+    val result = LookbackValue(next, previous)
+    previous = next
+    return result
+  }
+}