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import 'dart:io';
// Generates a stream of n fibonacci numbers, sleeping 1 second every iteration
Stream<int> Function() fibonnaciNumbers(int n) {
final Stream<int> Function() func = (() async* {
var x = 0, y = 1;
for (int i = 0; i <= n; i++) {
yield x;
sleep(new Duration(seconds: 1));
y += x;
x = y - x;
}
});
return func;
}
// Filters a stream of integers with a function predicate determining to each of the elements in the stream.
Stream<int> Function() streamFilter(Stream<int> stream, bool Function(int) f) {
final Stream<int> Function() func = (() async* {
await for (var i in stream) {
if (f(i)) {
yield i;
}
}
});
return func;
}
// Iterates over a stream, accumulatively applying a binary function to a stream of integers until the last
// element and returns the accumulation.
Stream<int> Function() streamAccumulation(
Stream<int> stream, int Function(int, int) f, initial) {
final Stream<int> Function() func = (() async* {
int a = initial;
await for (var i in stream) {
a = f(a, i);
yield a;
}
});
return func;
}
Stream<int> Function() generateNumbers(int n) {
final Stream<int> Function() func = (() async* {
for (int i = 1; i <= n; i++) {
await Future<void>.delayed(const Duration(seconds: 1));
yield i;
}
});
return func;
}
// Given a list of lists, returns a list containing all the non-null elements of each list into a single list
List? flatten(List<List?>? aList) {
if (aList == null) return null;
return aList
.where((i) => i != null)
.expand((i) => i!)
.where((i) => i != null)
.toList();
}
// Generates a nested list where each element's index i is in a sublist of depth i+1.
// For example, deepen([1,2,3]) => [1, [2, [3]]]
List? deepen(List? aList) {
if (aList == null) return null;
return aList.reversed
.fold([], (a, x) => (a != null && a.isEmpty) ? [x] : [x, a]);
}
void main(List<String> arguments) async {
print('flattening [[0,1], [2]] yields ${flatten([
[0, 1],
[2]
])}');
print('flattening [[0,1], [2], null] yields ${flatten([
[0, 1],
[2],
null
])}');
print('flattening [[0,1, null], [2]] yields ${flatten([
[0, 1, null],
[2]
])}');
print('flattening null yields ${flatten(null)}');
print('flattening [null] yields ${flatten([null])}');
print('deepening [0,1,2] yields ${deepen([0, 1, 2])}');
print('deepening [0,null,2] yields ${deepen([0, null, 2])}');
print('deepening [0] yields ${deepen([0])}');
print('deepening [] yields ${deepen([])}');
print('deepening null yields ${deepen(null)}');
await for (final number in fibonnaciNumbers(7)()) {
print('fibonnaci number is ${number}');
}
await for (final number in streamFilter(generateNumbers(10)(), ((a) {
return a % 2 == 0;
}))()) {
print('filtered number is $number');
}
await for (final number in streamAccumulation(generateNumbers(10)(), ((a, b) {
return a + b;
}), 0)()) {
print('cumulative number is $number.');
}
}
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