[utils] Move atMostOne to core/util.runtime

There is nothing backend-specific about this helper function.

core/util.runtime/src/org/jetbrains/kotlin/utils/CollectionUtil.kt

@@ -47,3 +47,31 @@ inline fun <reified T, reified R, C : MutableCollection<in R>> Iterable<*>.filte
     }
     return destination
 }
+
+/**
+ * Returns the single element of the collection if it contains at most one element.
+ *
+ * If the collection is empty, returns `null`.
+ *
+ * If the collection contains exactly one element, returns that element.
+ *
+ * If the collection contains more than one element, throws an exception.
+ */
+fun <T> Collection<T>.atMostOne(): T? {
+    return when (size) {
+        0 -> null
+        1 -> this.iterator().next()
+        else -> throw IllegalArgumentException("Collection has more than one element.")
+    }
+}
+
+/**
+ * Returns at most one element from the iterable that satisfies the given predicate.
+ *
+ * If there are no elements that satisfy [predicate], returns `null`.
+ *
+ * If there is exactly one element that satisfies [predicate], returns that element.
+ *
+ * If there are more such elements, throws an exception.
+ */
+inline fun <T> Iterable<T>.atMostOne(predicate: (T) -> Boolean): T? = this.filter(predicate).atMostOne()

core/util.runtime/src/org/jetbrains/kotlin/utils/MemoryOptimizedCollectionUtil.kt

@@ -104,12 +104,13 @@ infix fun <T, R> Collection<T>.memoryOptimizedZip(other: Collection<R>): List<Pa
 }
 
 /**
- * [Sequence] variant of [org.jetbrains.kotlin.backend.common.atMostOne]
+ * [Sequence] variant of [org.jetbrains.kotlin.utils.atMostOne]
+ *
  * So, when:
  * - there is no element then `null` will be returned
  * - there is a single element then the element will be returned
  * - there is more than one element then the error will be thrown
- * @see org.jetbrains.kotlin.backend.common.atMostOne
+ * @see org.jetbrains.kotlin.utils.atMostOne
  */
 fun <T> Sequence<T>.atMostOne(): T? {
     val iterator = iterator()
@@ -143,4 +144,4 @@ inline fun <reified T> Iterable<*>.findIsInstanceAnd(predicate: (T) -> Boolean):
  * The same as [Collection.toMutableList] extension function, but it returns a SmartList which is better with in sense of memory consumption
  * @see Collection.toMutableList
  */
-fun <T> Collection<T>.toSmartList(): List<T> = SmartList<T>(this)
+fun <T> Collection<T>.toSmartList(): List<T> = SmartList<T>(this)