streetsidesoftwareGenSequence
Small library to simplify working with Generators and Iterators in Javascript / Typescript
Javascript Iterators and Generators are very exciting and provide some powerful new ways to solve programming problems.
The purpose of this library is to make using the results of a generator function easier.
It is not intended as a replacement for arrays and the convenient [...genFn()] notation.
GenSequence is useful for cases where you might not want an array of all possible values.
GenSequence deals efficiently with large sequences because only one element at a time is evaluated.
Intermediate arrays are not created, saving memory and cpu cycles.
Installation
npm install -S gensequenceUsage
import { genSequence } from 'gensequence';Examples
Fibonacci
The Fibonacci sequence can be very simply expressed using a generator. Yet using the result of a generator can be a bit convoluted. GenSequence provides a wrapper to add familiar functionality similar to arrays.
function fibonacci() {
function* fib() {
let [a, b] = [0, 1];
while (true) {
yield b;
[a, b] = [b, a + b];
}
}
// Wrapper the Iterator result from calling the generator.
return genSequence(fib);
}
let fib5 = fibonacci()
.take(5) // Take the first 5 from the fibonacci sequence
.toArray(); // Convert it into an array
// fib5 == [1, 1, 2, 3, 5]
let fib6n7seq = fibonacci().skip(5).take(2);
let fib6n7arr = [...fib6n7seq]; // GenSequence are easily converted into arrays.
let fib5th = fibonacci()
.skip(4) // Skip the first 4
.first(); // Return the next one.RegEx Match
Regular expressions are wonderfully powerful. Yet, working with the results can sometimes be a bit of a pain.
function* execRegEx(reg: RegExp, text: string) {
const regLocal = new RegExp(reg);
let r;
while ((r = regLocal.exec(text))) {
yield r;
}
}
/* return a sequence of matched text */
function match(reg: RegExp, text: string) {
return (
genSequence(execRegEx(reg, text))
// extract the full match
.map((a) => a[0])
);
}
/* extract words out of a string of text */
function matchWords(text: string) {
return genSequence(match(/\w+/g, text));
}
/* convert some text into a set of unique words */
function toSetOfWords(text: string) {
// Sequence can be used directly with a Set or Match
return new Set(matchWords(text));
}
const text = 'Some long bit of text with many words, duplicate words...';
const setOfWords = toSetOfWords(text);
// Walk through the set of words and pull out the 4 letter one.
const setOf4LetterWords = new Set(genSequence(setOfWords).filter((a) => a.length === 4));Reference
genSequence(Iterable|Array|()=>Iterable)-- generate a new Iterable from an Iterable, Array or function with the following functions.
Filters
.filter(fn)-- just like array.filter, filters the sequence.skip(n)-- skip n entries in the sequence.take(n)-- take the next n entries in the sequence.
Extenders
.concat(iterable)-- this will extend the current sequence with the values from iterable.concatMap(fnMap)-- this is used to flatten the result of a map function.
Mappers
.combine(fnCombiner, iterable)-- is used to combine values from two different lists..map(fn)-- just like array.map, allows you to convert the values in a sequence..pipe(...operatorFns)-- pipe any amount of operators in sequence..scan(fn, init?)-- similar to reduce, but returns a sequence of all the results of fn.
Reducers
.all(fn)-- true if all values in the sequence return true for fn(value) or the sequence is empty..any(fn)-- true if any value in the sequence exists where fn(value) returns true..count()-- return the number of values in the sequence..first()-- return the next value in the sequence..first(fn)-- return the next value in the sequence where fn(value) return true..forEach(fn)-- apply fn(value, index) to all values..max()-- return the largest value in the sequence..max(fn)-- return the largest value of fn(value) in the sequence..min()-- return the smallest value in the sequence..min(fn)-- return the smallest value of fn(value) in the sequence..reduce(fn, init?)-- just like array.reduce, reduces the sequence into a single result..reduceAsync(fn, init?)-- just like array.reduce, reduces promises into the sequence into a single result chaining the promises, fn/init can be async or not, it will work, the previousValue, and currentValue will never be a promise..reduceToSequence(fn, init)-- return a sequence of values that fn creates from looking at all the values and the initial sequence.
Cast
.toArray()-- convert the sequence into an array. This is the same as [...iterable]..toIterable()-- Casts a Sequence into an IterableIterator - used in cases where type checking is too strict.