flow like the river

BACK_BACK/node_modules/srcset/index.js

@@ -0,0 +1,93 @@
+'use strict';
+
+/**
+This regex represents a loose rule of an “image candidate string”.
+
+@see https://html.spec.whatwg.org/multipage/images.html#srcset-attribute
+
+An “image candidate string” roughly consists of the following:
+1. Zero or more whitespace characters.
+2. A non-empty URL that does not start or end with `,`.
+3. Zero or more whitespace characters.
+4. An optional “descriptor” that starts with a whitespace character.
+5. Zero or more whitespace characters.
+6. Each image candidate string is separated by a `,`.
+
+We intentionally implement a loose rule here so that we can perform more aggressive error handling and reporting in the below code.
+*/
+const imageCandidateRegex = /\s*([^,]\S*[^,](?:\s+[^,]+)?)\s*(?:,|$)/;
+
+function deepUnique(array) {
+	return array.sort().filter((element, index) => {
+		return JSON.stringify(element) !== JSON.stringify(array[index - 1]);
+	});
+}
+
+exports.parse = string => {
+	return deepUnique(
+		string.split(imageCandidateRegex)
+			.filter((part, index) => index % 2 === 1)
+			.map(part => {
+				const [url, ...elements] = part.trim().split(/\s+/);
+
+				const result = {url};
+
+				const descriptors = elements.length > 0 ? elements : ['1x'];
+
+				for (const descriptor of descriptors) {
+					const postfix = descriptor[descriptor.length - 1];
+					const value = Number.parseFloat(descriptor.slice(0, -1));
+
+					if (Number.isNaN(value)) {
+						throw new TypeError(`${descriptor.slice(0, -1)} is not a valid number`);
+					}
+
+					if (postfix === 'w') {
+						if (value <= 0) {
+							throw new Error('Width descriptor must be greater than zero');
+						} else if (!Number.isInteger(value)) {
+							throw new TypeError('Width descriptor must be an integer');
+						}
+
+						result.width = value;
+					} else if (postfix === 'x') {
+						if (value <= 0) {
+							throw new Error('Pixel density descriptor must be greater than zero');
+						}
+
+						result.density = value;
+					} else {
+						throw new Error(`Invalid srcset descriptor: ${descriptor}`);
+					}
+
+					if (result.width && result.density) {
+						throw new Error('Image candidate string cannot have both width descriptor and pixel density descriptor');
+					}
+				}
+
+				return result;
+			})
+	);
+};
+
+exports.stringify = array => {
+	return [...new Set(
+		array.map(element => {
+			if (!element.url) {
+				throw new Error('URL is required');
+			}
+
+			const result = [element.url];
+
+			if (element.width) {
+				result.push(`${element.width}w`);
+			}
+
+			if (element.density) {
+				result.push(`${element.density}x`);
+			}
+
+			return result.join(' ');
+		})
+	)].join(', ');
+};