import { basename, extname } from 'pathe'
import grayMatter from 'gray-matter'
import { parse as parseSFC } from 'vue/compiler-sfc'

import type { RawPageMatter } from './types'

const dateRegex = /^(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})-(?<slug>.*)(?<ext>\.\w+)$/

export async function parsePageMatter (filename: string, content: string) {
  const parse = extname(filename) === '.vue' ? parsePageBlock : parseFrontmatter
  const matter = await parse(filename, content)
  return preparePageMatter(filename, matter)
}

/**
 * Parses a <page> block in a Vue SFC.
 * Supports extracting the layout from `<template layout="default">`.
 */
async function parsePageBlock (filename: string, content: string) {
  const parsed = await parseSFC(content, { pad: 'space' }).descriptor

  const block = parsed.customBlocks.find(block => block.type === 'page')
  const templateAttrs = parsed.template?.attrs

  const frontmatter = block
    && parseFrontmatter(filename, `---\n${block.content}\n---`, block.lang)

  return { templateAttrs, ...templateAttrs, ...frontmatter }
}

function parseFrontmatter (filename: string, content: string, language?: string) {
  try {
    return grayMatter(content, { language }).data || {}
  }
  catch (err: any) {
    err.message = `Invalid frontmatter for ${filename}\n${err.message}`
    throw err
  }
}

// Internal: Extracts layout, route, and meta data from the frontmatter.
function preparePageMatter (filename: string, matter: Record<string, any>): RawPageMatter {
  let { templateAttrs, layout, meta: rawMeta, route, ...frontmatter } = matter

  // Users can explicitly provide route to avoid unwanted behavior.
  if (!route) {
    const { name, path, redirect, alias } = frontmatter
    route = clearUndefined({ name, path, redirect, alias, templateAttrs })
  }

  // Skip layout for non-HTML files, such as RSS feeds.
  if (layout === undefined) {
    const ext = extname(route.path) || '.html'
    layout = ext !== '.html' ? false : undefined
  }

  // Add filename, date, and id.
  const meta = { filename, ...rawMeta }
  const { year, month, day, slug } = extractGroups(basename(filename), dateRegex)
  if (slug) {
    meta.date = new Date(year, month - 1, day)
    meta.slug = slug
  }

  return { layout, meta, route, ...frontmatter }
}

/**
 * Clear undefined fields from an object. It mutates the object
 */
export function clearUndefined<T extends object> (obj: T): T {
  for (const key in obj)
    if (obj[key] === undefined) delete obj[key]
  return obj
}

function extractGroups (val: string, regex: RegExp): any {
  return regex.exec(val)?.groups as any || {}
}