xduce
Version:
Composable algorithmic transformations library for JavaScript
246 lines (227 loc) • 9.57 kB
JavaScript
/*
* Copyright (c) 2017 Thomas Otterson
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
/**
* Transducers for dropping some number of elements at the end of a collection.
*
* @module take
* @private
*/
const { protocols } = require('../modules/protocol');
const { ensureCompleted } = require('../modules/reduction');
const { sequence } = require('../modules/transformation');
const { isNumber, isFunction } = require('../modules/util');
const p = protocols;
/**
* A transducer function that is returned by the `{@link module:xduce.transducers.take|take}` transducer.
*
* @private
*
* @param {module:xduce~transducerObject} xform The transducer object that the new one should be chained to.
* @return {module:xduce~transducerObject} A new transducer object, performing no transformation and chaining to the
* provided transducer object.
*/
function takeTransducer(n, xform) {
let i = 0;
return {
[p.init]() {
return xform[p.init]();
},
[p.step](acc, input) {
let result = acc;
if (i < n) {
result = xform[p.step](acc, input);
if (i === n - 1) {
result = ensureCompleted(result);
}
}
i++;
return result;
},
[p.result](value) {
return xform[p.result](value);
}
};
}
/**
* **Creates a new collection containing only the first `n` elements of the input collection.**
*
* Note that this is an excellent way to turn an 'infinite' collection - one that doesn't have a well-defined end, like
* a stream, channel, or infinite generator - into a finite collection.
*
* If no collection is provided, a function is returned that can be passed to `{@link module:xduce.sequence|sequence}`,
* et al.
*
* ```
* // An iterator that will return every positive integer, one at a time per next() call
* function* naturals() {
* let x = 1;
* while (true) {
* yield x++;
* }
* }
*
* const result = take(naturals(), 3);
* // result is now an iterator that has only three values in it
* result.next().value === 1; // true
* result.next().value === 2; // true
* result.next().value === 3; // true
* result.next().done; // true
* ```
*
* @memberof module:xduce.transducers
*
* @param {*} [collection] An optional input collection that is to be transduced.
* @param {number} n The number of elements at the beginning of the input collection that should be kept in the
* output collection.
* @return {(*|module:xduce~transducerFunction)} If a collection is supplied, then the function returns a new
* collection of the same type containing only the first `n` elements. If no collection is supplied, a transducer
* function, suitable for passing to `{@link module:xduce.sequence|sequence}`, `{@link module:xduce.into|into}`,
* etc. is returned.
*/
function take(collection, n) {
const [col, num] = isNumber(collection) ? [null, collection] : [collection, n];
return col ? sequence(col, take(num)) : xform => takeTransducer(num, xform);
}
/**
* A transducer function that is returned by the `{@link module:xduce.transducers.takeWhile|takeWhile}` transducer.
*
* @private
*
* @param {function} fn A single-parameter predicate function that determines which is the first element to be rejected
* in the output collection.
* @param {module:xduce~transducerObject} xform The transducer object that the new one should be chained to.
* @return {module:xduce~transducerObject} A new transducer object, performing no transformation and chaining to the
* provided transducer object.
*/
function takeWhileTransducer(fn, xform) {
return {
[p.init]() {
return xform[p.init]();
},
[p.step](acc, input) {
return fn(input) ? xform[p.step](acc, input) : ensureCompleted(acc);
},
[p.result](value) {
return xform[p.result](value);
}
};
}
/**
* **Creates a new collection containing the elements of the input collection up until the first one that causes a
* predicate function to return `false`.**
*
* While this is similar to `{@link module:xduce.transducers.filter|filter}`, there is one key difference. `takeWhile`
* will not add any further elements to a collection once the first fails the predicate, including later elements that
* might pass the predicate. `{@link module:xduce.transducers.filter|filter}`, on the other hand, will continue to add
* those later elements. Therefore `takeWhile` will convert an infinite collection to a finite one while
* `{@link module:xduce.transducers.filter|filter}` cannot.
*
* If no collection is provided, a function is returned that can be passed to `{@link module:xduce.sequence|sequence}`,
* et al.
*
* ```
* const array = [2, 4, 6, 8, 1, 3, 5, 7, 9, 10];
* const even = x => x % 2 === 0;
*
* let result = takeWhile(array, even);
* // result = [2, 4, 6, 8];
*
* // This shows the difference between takeWhile and filter with the same parameters
* result = filter(array, even);
* // result = [2, 4, 6, 8, 10];
* ```
*
* @memberof module:xduce.transducers
*
* @param {*} [collection] An optional input collection that is to be transduced.
* @param {function} fn A predicate function. This takes each element of the input collection and returns `true` or
* `false` based on that element. The first one to return `false` is the first element of the input collection that
* does *not* appear in the output collection.
* @param {object} [ctx] An optional context object which is set to `this` for the function `fn`. This does not work if
* `fn` is an arrow function, as they cannot be bound to contexts.
* @return {(*|module:xduce~transducerFunction)} If a collection is supplied, then the function returns a new
* collection of the same type with some of the elements of the input collection dropped. If no collection is
* supplied, a transducer function, suitable for passing to `{@link module:xduce.sequence|sequence}`,
* `{@link module:xduce.into|into}`, etc. is returned.
*/
function takeWhile(collection, fn, ctx) {
const [col, func] = isFunction(collection) ? [null, collection.bind(fn)] : [collection, fn.bind(ctx)];
return col ? sequence(col, takeWhile(func)) : xform => takeWhileTransducer(func, xform);
}
/**
* A transducer function that is returned by the `{@link module:xduce.transducers.takeNth|takeNth}` transducer.
*
* @private
*
* @param {number} n The skip value, meaning that only every `n`th element is retained.
* @param {module:xduce~transducerObject} xform The transducer object that the new one should be chained to.
* @return {module:xduce~transducerObject} A new transducer object, performing no transformation and chaining to the
* provided transducer object.
*/
function takeNthTransducer(n, xform) {
let i = -1;
return {
[p.init]() {
return xform[p.init]();
},
[p.step](acc, input) {
return ++i % n === 0 ? xform[p.step](acc, input) : acc;
},
[p.result](value) {
return xform[p.result](value);
}
};
}
/**
* **Creates a new collection consisting of the first element of the input collection, and then every `n`th element
* after that.**
*
* Note that unlike `{@link module:xduce.transducers.take|take}` and
* `{@link module:xduce.transducers.takeWhile|takeWhile}`, this function is not capable of returning a finite collection
* when given an infinite collection.
*
* If no collection is provided, a function is returned that can be passed to `{@link module:xduce.sequence|sequence}`,
* et al.
*
* ```
* const result = takeNth([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 3);
* // result = [1, 4, 7, 10]
* ```
*
* @memberof module:xduce.transducers
*
* @param {*} [collection] An optional input collection that is to be transduced.
* @param {number} n The skip value. Every `n`th element of the input collection, after the first, will be a part of
* the output collection.
* @return {(*|module:xduce~transducerFunction)} If a collection is supplied, then the function returns a new
* collection of the same type containing only every `n` elements. If no collection is supplied, a transducer
* function, suitable for passing to `{@link module:xduce.sequence|sequence}`, `{@link module:xduce.into|into}`,
* etc. is returned.
*/
function takeNth(collection, n) {
const [col, num] = isNumber(collection) ? [null, collection] : [collection, n];
return col ? sequence(col, takeNth(num)) : xform => takeNthTransducer(num, xform);
}
module.exports = {
take,
takeWhile,
takeNth
};