"An iterable collection of elements of finite [[size]], with a well-defined notion of [[value equality|equals]]. `Collection` is the abstract supertype of [[List]], [[Map]], and [[Set]]. A `Collection` forms a [[Category]] of its elements, and is [[Iterable]]. The elements of a collection are not necessarily distinct when compared using [[Object.equals]]. A `Collection` may be [[cloned|clone]]. If a collection is immutable, it is acceptable that `clone()` produce a reference to the collection itself. If a collection is mutable, `clone()` should produce a collection containing references to the same elements, with the same structure as the original collection—that is, it should produce a shallow copy of the collection. All `Collection`s are required to support a well-defined notion of [[value equality|Object.equals]], but the definition of equality depends upon the kind of collection. Equality for `Map`s and `Set`s has a quite different definition to equality for `List`s. Instances of two different kinds of collection are never equal—for example, a `Map` is never equal to a `List`." see (`interface List`, `interface Map`, `interface Set`) tagged("Collections") shared interface Collection<out Element=Anything> satisfies {Element*} { "A shallow copy of this collection, that is, a collection with identical elements which does not change if this collection changes. If this collection is immutable, it is acceptable to return a reference to this collection. If this collection is mutable, a newly instantiated collection must be returned." shared formal Collection<Element> clone(); "Determine if the collection is empty, that is, if it has no elements." shared actual default Boolean empty => size==0; "Return `true` if the given object is an element of this collection. In this default implementation, and in most refining implementations, return `false` otherwise. An acceptable refining implementation may return `true` for objects which are not elements of the collection, but this is not recommended. (For example, the `contains()` method of `String` returns `true` for any substring of the string.)" shared actual default Boolean contains(Object element) { for (elem in this) { if (exists elem, elem==element) { return true; } } else { return false; } } "A string of form `\"{ x, y, z }\"` where `x`, `y`, and `z` are the `string` representations of the elements of this collection, as produced by the iterator of the collection, or the string `\"{}\"` if this collection is empty. If the collection iterator produces the value `null`, the string representation contains the string `\"<null>\"`." shared actual default String string => empty then "{}" else "{ ``commaList(this)`` }"; "The permutations of this collection, as a stream of nonempty [[sequences|Sequence]]. That is, a stream producing every distinct ordering of the elements of this collection. For example, \"ABC\".permutations.map(String) is the stream of strings `{ \"ABC\", \"ACB\", \"BAC\", \"BCA\", \"CAB\", \"CBA\" }`. If this collection is empty, the resulting stream is empty. The permutations are enumerated lexicographically according to the order in which each distinct element of this collection is first produced by its iterator. No permutation is repeated. Two elements are considered distinct if either: - they are both instances of `Object`, and are [[unequal|Object.equals]], or - one element is an `Object` and the other is `null`." since("1.2.0") shared {[Element+]*} permutations => object satisfies {[Element+]*} { value multiset = outer .indexed .group((_->item) => item else nullElement) .items .sort((x,y) => x.first.key<=>y.first.key) .indexed .flatMap((index->entries) => entries.map((_->item) => index->item)); empty => multiset.empty; iterator() => object satisfies Iterator<[Element+]> { value elements = Array(multiset); value size = elements.size; variable value initial = true; shared actual [Element+]|Finished next() { if (initial) { initial = false; } else if (exists i -> [key->_, __] = elements.paired.locateLast( ([m->_, n->__]) => m < n)) { assert (exists j = elements.lastIndexWhere( (k->_) => k > key)); elements.swap(i,j); for (k in 0 : (size-i-1)/2) { elements.swap(i+1+k, size-1-k); } } else { return finished; } return if (nonempty permutation = elements*.item) then permutation else finished; } }; }; "The combinations of elements of this collection, of the given positive [[size|length]], as a stream of nonempty [[sequences|Sequence]]. That is, a stream producing every distinct selection of `length` elements of this collection. For example, \"ABCD\".combinations(2).map(String) is the stream of strings `{ \"AB\", \"AC\", \"AD\", \"BC\", \"BD\", \"CD\" }`. If this collection is empty, the resulting stream is empty. The combinations are enumerated lexicographically according to the order in which each distinct element of this collection is first produced by its iterator. No combination is repeated. Two elements are considered distinct if either: - they are both instances of `Object`, and are [[unequal|Object.equals]], or - one element is an `Object` and the other is `null`." throws (`class AssertionError`, "if [[length]] is nonpositive or if `length` is larger than the number of distinct elements of this collection") since("1.3.0") shared {[Element+]*} combinations( "The number of distinct elements in each combination" Integer length) { "length must be strictly positive" assert (length>0); return object satisfies {[Element+]*} { value distinctElements = outer.distinct; empty => outer.empty; iterator() => object satisfies Iterator<[Element+]> { value elements = distinctElements.sequence(); value size = elements.size; "length is larger than the number of distinct elements" assert (length <= size); value selection = Array(0:length); variable value done = elements.empty; shared actual [Element+]|Finished next() { if (done) { return finished; } value current = selection.collect((i) { if (exists e = elements.getFromFirst(i)) { return e; } else { assert (is Element null); return null; } }); assert (nonempty current); variable value i = length-1; while (true) { if (i<0) { done = true; break; } assert (exists s = selection[i]); if (s == size-length+i) { i--; } else { variable value j = s; while (i<length) { selection[i++] = ++j; } break; } } return current; } }; }; } } "Used by [[Collection.permutations]] to group nulls together." object nullElement {}