interface SetPageMetaProps {
	/** Sets title, meta:og:title. Is postfixed by ` - {siteName}` **/
	title: string
	/** Sets meta:og:site_name **/
	siteName?: string
	/** Sets meta:description **/
	description?: string
	/** Sets meta:og:image **/
	image?: string
	/** Sets meta:og:locale **/
	locale?: string
}
interface SetPageMetaReturn {
	/** The current page title, meta:og:title. Is postfixed by ` - {siteName}` **/
	title: string
	/** The current meta:og:site_name **/
	siteName: string
	/** The current meta:description **/
	description: string
	/** The current meta:og:image **/
	image: string
	/** The current meta:og:locale **/
	locale: string
}

/**
 * Allows setting common page attrs.
 *
 * - Intelligently use the attrs, only setting if changed
 * - Resets back to initial if omitted, based on initial introspection
 * - Stores element handles in memory to remove need to query the dom
 *   on every update
 *
 * > Note: Set `setPageMeta.testMode=true` to disable setPageMeta for testing
 *
 * @param
 * object: an object of key/val pairs of meta attrs to set
 *
 * @returns
 * object: an object of key/val pairs of meta attrs that are currently set
 *
 * @dependency
 *
 * The page should already have default meta tags. Example:
 *
 * ```html
 * <title>React Template</title>
 * <meta property="og:title" content="React template" />
 * <meta property="og:site_name" content="React Template" />
 * <meta property="og:locale" content="en_US" />
 * <link rel="canonical" href="https://react-template.com" />
 * <meta
 *   name="description"
 *   content="A template to build tiny Preact applications"
 * />
 * <meta
 *   property="og:description"
 *   content="A template to build tiny React applications"
 * />
 * <meta
 *   property="og:url"
 *   content="https://github.com/bdombro/react-template"
 * />
 * <meta
 *   property="og:image"
 *   content="https://preact-template.com/apple-touch-icon.png"
 * />
 * ```
 *
 * @example
 *
 * ```typescript
 * const {description} = setPageMeta({
 *   title: `Hello World`,
 *   description: 'This page is awesome',
 * })
 * ```
 */
export function setPageMeta(p: SetPageMetaProps): SetPageMetaReturn {
	const {
		getLink,
		siteNameE,
		ogTitleMc,
		localeMc,
		descriptionMc,
		ogDescriptionMc,
		ogUrlMc,
		ogSiteNameMc,
		ogImageMc,
	} = getHeadHandles()
	const title = p.title ? `${p.title} - ${siteNameE}` : siteNameE
	if (title !== document.title) document.title = title

	const link = getLink()
	if (link.href !== location.href) link.href = location.href

	const locale = localeMc.upsert(p.locale)
	const description = descriptionMc.upsert(p.description)
	const siteName = ogSiteNameMc.upsert(p.siteName)
	const image = ogImageMc.upsert(p.image)

	ogTitleMc.upsert(p.title)
	ogDescriptionMc.upsert(p.description)
	ogUrlMc.upsert(location.href)

	return {
		description,
		image,
		locale,
		siteName,
		title: p.title || title,
	}
}
setPageMeta.testMode = !("document" in globalThis)

/** Wrapper class on meta elements to simplify usage and make more DRY **/
class MetaClass {
	get: () => string
	orig: string
	last?: string
	set: (val: string) => string
	constructor(getter: () => Element) {
		this.get = () =>
			setPageMeta.testMode
				? ""
				: this.last || getter().getAttribute("content") || throwError(`No content for ${getter}`)
		this.set = (v: string) => {
			if (setPageMeta.testMode) return v
			getter().setAttribute("content", v)
			return (this.last = v)
		}
		this.orig = this.last = this.get()
	}
	upsert(val?: string): string {
		if (setPageMeta.testMode) return val || ""
		if (!val) return (val = this.orig)
		if (this.last !== val) return this.set(val)
		return this.last
	}
}

const byName = (name: string) => {
	return find(`meta[name="${name}"]`)
}
const byProp = (prop: string) => {
	return find(`meta[property="${prop}"]`)
}
const find = (selector: string): HTMLMetaElement => {
	if (setPageMeta.testMode) {
		return {
			getAttribute: () => "",
			setAttribute: (v: any) => v,
		} as unknown as HTMLMetaElement
	}
	return document.head.querySelector(selector) || throwError(`Missing: ${selector}`)
}

interface HeadHandles {
	getLink: () => HTMLLinkElement
	siteNameE: string
	ogTitleMc: InstanceType<typeof MetaClass>
	localeMc: InstanceType<typeof MetaClass>
	descriptionMc: InstanceType<typeof MetaClass>
	ogDescriptionMc: InstanceType<typeof MetaClass>
	ogUrlMc: InstanceType<typeof MetaClass>
	ogSiteNameMc: InstanceType<typeof MetaClass>
	ogImageMc: InstanceType<typeof MetaClass>
}

/** Helper handles with cache  */
function getHeadHandles(): HeadHandles {
	if (getHeadHandles.last) return getHeadHandles.last
	return (getHeadHandles.last = {
		getLink: () => find('link[rel="canonical"]') as unknown as HTMLLinkElement,
		siteNameE: byProp("og:site_name").getAttribute("content") || "",
		ogTitleMc: new MetaClass(() => byProp("og:title")),
		localeMc: new MetaClass(() => byProp("og:locale")),
		descriptionMc: new MetaClass(() => byName("description")),
		ogDescriptionMc: new MetaClass(() => byProp("og:description")),
		ogUrlMc: new MetaClass(() => byProp("og:url")),
		ogSiteNameMc: new MetaClass(() => byProp("og:site_name")),
		ogImageMc: new MetaClass(() => byProp("og:image")),
	})
}
getHeadHandles.last = null as unknown as HeadHandles

/** Convenience function to throw an error */
function throwError(msg: string): never {
	throw new Error(msg)
}