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+/**
+ * Responsive images based on `srcset` and `window.devicePixelRatio` emulation where needed.
+ *
+ * Call `.hidpi()` on a document or part of a document to proces image srcsets within that section.
+ *
+ * `$.devicePixelRatio()` can be used as a substitute for `window.devicePixelRatio`.
+ * It provides a familiar interface to retrieve the pixel ratio for browsers that don't
+ * implement `window.devicePixelRatio` but do have a different way of getting it.
+ *
+ * @class jQuery.plugin.hidpi
+ */
+( function ( $ ) {
+
+	/**
+	 * Get reported or approximate device pixel ratio.
+	 *
+	 * - 1.0 means 1 CSS pixel is 1 hardware pixel
+	 * - 2.0 means 1 CSS pixel is 2 hardware pixels
+	 * - etc.
+	 *
+	 * Uses `window.devicePixelRatio` if available, or CSS media queries on IE.
+	 *
+	 * @static
+	 * @inheritable
+	 * @return {number} Device pixel ratio
+	 */
+	$.devicePixelRatio = function () {
+		if ( window.devicePixelRatio !== undefined ) {
+			// Most web browsers:
+			// * WebKit/Blink (Safari, Chrome, Android browser, etc)
+			// * Opera
+			// * Firefox 18+
+			// * Microsoft Edge (Windows 10)
+			return window.devicePixelRatio;
+		} else if ( window.msMatchMedia !== undefined ) {
+			// Windows 8 desktops / tablets, probably Windows Phone 8
+			//
+			// IE 10/11 doesn't report pixel ratio directly, but we can get the
+			// screen DPI and divide by 96. We'll bracket to [1, 1.5, 2.0] for
+			// simplicity, but you may get different values depending on zoom
+			// factor, size of screen and orientation in Metro IE.
+			if ( window.msMatchMedia( '(min-resolution: 192dpi)' ).matches ) {
+				return 2;
+			} else if ( window.msMatchMedia( '(min-resolution: 144dpi)' ).matches ) {
+				return 1.5;
+			} else {
+				return 1;
+			}
+		} else {
+			// Legacy browsers...
+			// Assume 1 if unknown.
+			return 1;
+		}
+	};
+
+	/**
+	 * Bracket a given device pixel ratio to one of [1, 1.5, 2].
+	 *
+	 * This is useful for grabbing images on the fly with sizes based on the display
+	 * density, without causing slowdown and extra thumbnail renderings on devices
+	 * that are slightly different from the most common sizes.
+	 *
+	 * The bracketed ratios match the default 'srcset' output on MediaWiki thumbnails,
+	 * so will be consistent with default renderings.
+	 *
+	 * @static
+	 * @inheritable
+	 * @param {number} baseRatio Base ratio
+	 * @return {number} Device pixel ratio
+	 */
+	$.bracketDevicePixelRatio = function ( baseRatio ) {
+		if ( baseRatio > 1.5 ) {
+			return 2;
+		} else if ( baseRatio > 1 ) {
+			return 1.5;
+		} else {
+			return 1;
+		}
+	};
+
+	/**
+	 * Get reported or approximate device pixel ratio, bracketed to [1, 1.5, 2].
+	 *
+	 * This is useful for grabbing images on the fly with sizes based on the display
+	 * density, without causing slowdown and extra thumbnail renderings on devices
+	 * that are slightly different from the most common sizes.
+	 *
+	 * The bracketed ratios match the default 'srcset' output on MediaWiki thumbnails,
+	 * so will be consistent with default renderings.
+	 *
+	 * - 1.0 means 1 CSS pixel is 1 hardware pixel
+	 * - 1.5 means 1 CSS pixel is 1.5 hardware pixels
+	 * - 2.0 means 1 CSS pixel is 2 hardware pixels
+	 *
+	 * @static
+	 * @inheritable
+	 * @return {number} Device pixel ratio
+	 */
+	$.bracketedDevicePixelRatio = function () {
+		return $.bracketDevicePixelRatio( $.devicePixelRatio() );
+	};
+
+	/**
+	 * Implement responsive images based on srcset attributes, if browser has no
+	 * native srcset support.
+	 *
+	 * @return {jQuery} This selection
+	 * @chainable
+	 */
+	$.fn.hidpi = function () {
+		var $target = this,
+			// TODO add support for dpi media query checks on Firefox, IE
+			devicePixelRatio = $.devicePixelRatio(),
+			testImage = new Image();
+
+		if ( devicePixelRatio > 1 && testImage.srcset === undefined ) {
+			// No native srcset support.
+			$target.find( 'img' ).each( function () {
+				var $img = $( this ),
+					srcset = $img.attr( 'srcset' ),
+					match;
+				if ( typeof srcset === 'string' && srcset !== '' ) {
+					match = $.matchSrcSet( devicePixelRatio, srcset );
+					if ( match !== null ) {
+						$img.attr( 'src', match );
+					}
+				}
+			} );
+		}
+
+		return $target;
+	};
+
+	/**
+	 * Match a srcset entry for the given device pixel ratio
+	 *
+	 * Exposed for testing.
+	 *
+	 * @private
+	 * @static
+	 * @param {number} devicePixelRatio
+	 * @param {string} srcset
+	 * @return {Mixed} null or the matching src string
+	 */
+	$.matchSrcSet = function ( devicePixelRatio, srcset ) {
+		var candidates,
+			candidate,
+			bits,
+			src,
+			i,
+			ratioStr,
+			ratio,
+			selectedRatio = 1,
+			selectedSrc = null;
+		candidates = srcset.split( / *, */ );
+		for ( i = 0; i < candidates.length; i++ ) {
+			candidate = candidates[ i ];
+			bits = candidate.split( / +/ );
+			src = bits[ 0 ];
+			if ( bits.length > 1 && bits[ 1 ].charAt( bits[ 1 ].length - 1 ) === 'x' ) {
+				ratioStr = bits[ 1 ].slice( 0, -1 );
+				ratio = parseFloat( ratioStr );
+				if ( ratio <= devicePixelRatio && ratio > selectedRatio ) {
+					selectedRatio = ratio;
+					selectedSrc = src;
+				}
+			}
+		}
+		return selectedSrc;
+	};
+
+	/**
+	 * @class jQuery
+	 * @mixins jQuery.plugin.hidpi
+	 */
+
+}( jQuery ) );