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/**
* Copyright 2015 The AMP HTML Authors. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS-IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import {dev, user} from './log';
/**
* A single source within a srcset. Only one: width or DPR can be specified at
* a time.
* @typedef {{
* url: string,
* width: (number|undefined),
* dpr: (number|undefined)
* }}
*/
let SrcsetSourceDef;
/**
* Extracts `srcset` and fallbacks to `src` if not available.
* @param {!Element} element
* @return {!Srcset}
*/
export function srcsetFromElement(element) {
const srcsetAttr = element.getAttribute('srcset');
if (srcsetAttr) {
return parseSrcset(srcsetAttr);
}
// We can't push `src` via `parseSrcset` because URLs in `src` are not always
// RFC compliant and can't be easily parsed as an `srcset`. For instance,
// they sometimes contain space characters.
const srcAttr = user.assert(element.getAttribute('src'),
'Either non-empty "srcset" or "src" attribute must be specified: %s',
element);
return new Srcset([{url: srcAttr, width: undefined, dpr: 1}]);
}
/**
* Parses the text representation of srcset into Srcset object.
* See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#Attributes.
* See http://www.w3.org/html/wg/drafts/html/master/semantics.html#attr-img-srcset.
* @param {string} s
* @param {!Element=} opt_element
* @return {!Srcset}
*/
export function parseSrcset(s, opt_element) {
// General grammar: (URL [NUM[w|x]],)*
// Example 1: "image1.png 100w, image2.png 50w"
// Example 2: "image1.png 2x, image2.png"
// Example 3: "image1,100w.png 100w, image2.png 50w"
const sSources = s.match(
/\s*(?:[\S]*)(?:\s+(?:-?(?:\d+(?:\.(?:\d+)?)?|\.\d+)[a-zA-Z]))?(?:\s*,)?/g
);
user.assert(sSources.length > 0,
'srcset has to have at least one source: %s',
opt_element);
const sources = [];
for (let i = 0; i < sSources.length; i++) {
let sSource = sSources[i].trim();
if (sSource.substr(-1) == ',') {
sSource = sSource.substr(0, sSource.length - 1).trim();
}
const parts = sSource.split(/\s+/, 2);
if (parts.length == 0 ||
parts.length == 1 && !parts[0] ||
parts.length == 2 && !parts[0] && !parts[1]) {
continue;
}
const url = parts[0];
if (parts.length == 1 || parts.length == 2 && !parts[1]) {
// If no "w" or "x" specified, we assume it's "1x".
sources.push({url: url, width: undefined, dpr: 1});
} else {
const spec = parts[1].toLowerCase();
const lastChar = spec.substring(spec.length - 1);
if (lastChar == 'w') {
sources.push({url: url, width: parseFloat(spec), dpr: undefined});
} else if (lastChar == 'x') {
sources.push({url: url, width: undefined, dpr: parseFloat(spec)});
}
}
}
return new Srcset(sources);
}
/**
* A srcset object contains one or more sources.
*
* There are two types of sources: width-based and DPR-based. Only one type
* of sources allowed to be specified within a single srcset. Depending on a
* usecase, the components are free to choose any source that best corresponds
* to the required rendering quality and network and CPU conditions. See
* "select" method for details on how this selection is performed.
*
* See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img#Attributes
*/
export class Srcset {
/**
* @param {!Array<!SrcsetSourceDef>} sources
*/
constructor(sources) {
user.assert(sources.length > 0, 'Srcset must have at least one source');
/** @private @const {!Array<!SrcsetSourceDef>} */
this.sources_ = sources;
// Only one type of source specified can be used - width or DPR.
let hasWidth = false;
let hasDpr = false;
for (let i = 0; i < this.sources_.length; i++) {
const source = this.sources_[i];
user.assert(
(source.width || source.dpr) && (!source.width || !source.dpr),
'Either dpr or width must be specified');
hasWidth = hasWidth || !!source.width;
hasDpr = hasDpr || !!source.dpr;
}
user.assert(!hasWidth || !hasDpr,
'Srcset cannot have both width and dpr sources');
// Source and assert duplicates.
if (hasWidth) {
this.sources_.sort(sortByWidth);
} else {
this.sources_.sort(sortByDpr);
}
/** @private @const {boolean} */
this.widthBased_ = hasWidth;
/** @private @const {boolean} */
this.dprBased_ = hasDpr;
}
/**
* Performs selection for specified width and DPR. Here, width is the width
* in screen pixels and DPR is the device-pixel-ratio or pixel density of
* the device. Depending on the circumstances, such as low network conditions,
* it's possible to manipulate the result of this method by passing a lower
* DPR value.
*
* The source selection depends on whether this is width-based or DPR-based
* srcset.
*
* In a width-based source, the source's width is the physical width of a
* resource (e.g. an image). Depending on the provided DPR, this width is
* converted to the screen pixels as following:
* pixelWidth = sourceWidth / DPR
*
* Then, the source closest to the requested "width" is selected using
* the "pixelWidth". The slight preference is given to the bigger sources to
* ensure the most optimal quality.
*
* In a DPR-based source, the source's DPR is used to return the source that
* is closest to the requested DPR.
*
* Based on
* http://www.w3.org/html/wg/drafts/html/master/semantics.html#attr-img-srcset.
* @param {number} width
* @param {number} dpr
* @return {!SrcsetSourceDef}
*/
select(width, dpr) {
dev.assert(width, 'width=%s', width);
dev.assert(dpr, 'dpr=%s', dpr);
let index = -1;
if (this.widthBased_) {
index = this.selectByWidth_(width, dpr);
} else if (this.dprBased_) {
index = this.selectByDpr_(width, dpr);
}
if (index != -1) {
return this.sources_[index];
}
return this.getLast();
}
/**
* @param {number} width
* @param {number} dpr
* @return {number}
* @private
*/
selectByWidth_(width, dpr) {
let minIndex = -1;
let minWidth = 1000000;
let minScore = 1000000;
for (let i = 0; i < this.sources_.length; i++) {
const source = this.sources_[i];
let sourceWidth;
if (source.width) {
sourceWidth = source.width / dpr;
} else {
// Default source: no width: assume values are half of of the
// minimum values seen.
sourceWidth = minWidth / 2;
}
minWidth = Math.min(minWidth, sourceWidth);
// The calculation is slightly biased toward higher width by offsetting
// score by negative 0.2.
const score = Math.abs((sourceWidth - width) / width - 0.2);
if (score < minScore) {
minScore = score;
minIndex = i;
}
}
return minIndex;
}
/**
* @param {number} width
* @param {number} dpr
* @return {number}
* @private
*/
selectByDpr_(_width, dpr) {
let minIndex = -1;
let minScore = 1000000;
for (let i = 0; i < this.sources_.length; i++) {
const source = this.sources_[i];
// Default DPR = 1.
const sourceDpr = source.dpr || 1;
const score = Math.abs(sourceDpr - dpr);
if (score < minScore) {
minScore = score;
minIndex = i;
}
}
return minIndex;
}
/**
* Returns the last source in the srcset, which is the default source.
* @return {!SrcsetSourceDef}
*/
getLast() {
return this.sources_[this.sources_.length - 1];
}
/**
* Returns all sources in the srcset.
* @return {!Array<!SrcsetSourceDef>}
*/
getSources() {
return this.sources_;
}
/**
* Reconstructs the string expression for this srcset.
* @return {string}
*/
stringify() {
const res = [];
for (let i = 0; i < this.sources_.length; i++) {
const source = this.sources_[i];
if (source.width) {
res.push(`${source.url} ${source.width}w`);
} else if (source.dpr) {
res.push(`${source.url} ${source.dpr}x`);
} else {
res.push(`${source.url}`);
}
}
return res.join(', ');
}
}
function sortByWidth(s1, s2) {
user.assert(s1.width != s2.width, 'Duplicate width: %s', s1.width);
return s2.width - s1.width;
}
function sortByDpr(s1, s2) {
user.assert(s1.dpr != s2.dpr, 'Duplicate dpr: %s', s1.dpr);
return s2.dpr - s1.dpr;
}
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