elk/composables/content-parse.ts

import type { Emoji } from 'masto'
import type { Node } from 'ultrahtml'
import { TEXT_NODE, parse, render, walkSync } from 'ultrahtml'

const decoder = process.client ? document.createElement('textarea') : null as any as HTMLTextAreaElement
function decode(text: string) {
  decoder.innerHTML = text
  return decoder.value
}

/**
 * Parse raw HTML form Mastodon server to AST,
 * with interop of custom emojis and inline Markdown syntax
 */
export function parseMastodonHTML(html: string, customEmojis: Record<string, Emoji> = {}) {
  let processed = html
    // custom emojis
    .replace(/:([\w-]+?):/g, (_, name) => {
      const emoji = customEmojis[name]
      if (emoji)
        return `<img src="${emoji.url}" alt=":${name}:" class="custom-emoji" data-emoji-id="${name}" />`
      return `:${name}:`
    })
    // handle code blocks
    .replace(/>(```|~~~)(\w*)([\s\S]+?)\1/g, (_1, _2, lang, raw) => {
      const code = htmlToText(raw)
      const classes = lang ? ` class="language-${lang}"` : ''
      return `><pre><code${classes}>${code}</code></pre>`
    })

  walkSync(parse(processed), (node) => {
    if (node.type !== TEXT_NODE)
      return
    const replacements = [
      [/\*\*\*(.*?)\*\*\*/g, '<b><em>$1</em></b>'],
      [/\*\*(.*?)\*\*/g, '<b>$1</b>'],
      [/\*(.*?)\*/g, '<em>$1</em>'],
      [/~~(.*?)~~/g, '<del>$1</del>'],
      [/`([^`]+?)`/g, '<code>$1</code>'],
      [/&[^;]+;/g, (val: string) => decode(val)],
    ] as any

    for (const [re, replacement] of replacements) {
      for (const match of node.value.matchAll(re)) {
        if (node.loc) {
          const start = match.index! + node.loc[0].start
          const end = start + match[0].length + node.loc[0].start
          processed = processed.slice(0, start) + match[0].replace(re, replacement) + processed.slice(end)
        }
        else {
          processed = processed.replace(match[0], match[0].replace(re, replacement))
        }
      }
    }
  })

  return parse(processed)
}

export async function convertMastodonHTML(html: string, customEmojis: Record<string, Emoji> = {}) {
  const tree = parseMastodonHTML(html, customEmojis)
  return await render(tree)
}

export function htmlToText(html: string) {
  const tree = parse(html)
  return (tree.children as Node[]).map(n => treeToText(n)).join('').trim()
}

export function treeToText(input: Node): string {
  let pre = ''
  let body = ''
  let post = ''

  if (input.type === TEXT_NODE)
    return input.value

  if (input.name === 'br')
    return '\n'

  if (['p', 'pre'].includes(input.name))
    pre = '\n'

  if (input.name === 'code') {
    if (input.parent?.name === 'pre') {
      const lang = input.attributes.class?.replace('language-', '')

      pre = `\`\`\`${lang || ''}\n`
      post = '\n```'
    }
    else {
      pre = '`'
      post = '`'
    }
  }
  else if (input.name === 'b' || input.name === 'strong') {
    pre = '**'
    post = '**'
  }
  else if (input.name === 'i' || input.name === 'em') {
    pre = '*'
    post = '*'
  }
  else if (input.name === 'del') {
    pre = '~~'
    post = '~~'
  }

  if ('children' in input)
    body = (input.children as Node[]).map(n => treeToText(n)).join('')

  if (input.name === 'img' && input.attributes.class?.includes('custom-emoji'))
    return `:${input.attributes['data-emoji-id']}:`

  return pre + body + post
}