react-spring/docs/scripts/mdx/rehype-highlight-line.js

/**
 * Credit to https://ped.ro/writing/code-blocks-but-better
 */

import { toHtml } from 'hast-util-to-html'
import { unified } from 'unified'
import parse from 'rehype-parse'

const lineNumberify = function lineNumberify(ast, lineNum = 1) {
  let lineNumber = lineNum
  return ast.reduce(
    (result, node) => {
      if (node.type === 'text') {
        if (node.value.indexOf('\n') === -1) {
          node.lineNumber = lineNumber
          result.nodes.push(node)
          return result
        }

        const lines = node.value.split('\n')
        for (let i = 0; i < lines.length; i++) {
          if (i !== 0) ++lineNumber
          if (i === lines.length - 1 && lines[i].length === 0) continue
          result.nodes.push({
            type: 'text',
            value: i === lines.length - 1 ? lines[i] : `${lines[i]}\n`,
            lineNumber: lineNumber,
          })
        }

        result.lineNumber = lineNumber
        return result
      }

      if (node.children) {
        node.lineNumber = lineNumber
        const processed = lineNumberify(node.children, lineNumber)
        node.children = processed.nodes
        result.lineNumber = processed.lineNumber
        result.nodes.push(node)
        return result
      }

      result.nodes.push(node)
      return result
    },
    { nodes: [], lineNumber: lineNumber }
  )
}

const wrapLines = (ast, linesToHighlight) => {
  const highlightAll =
    linesToHighlight.length === 1 && linesToHighlight[0] === 0
  const allLines = Array.from(new Set(ast.map(x => x.lineNumber)))
  let i = 0
  const wrapped = allLines.reduce((nodes, marker) => {
    const line = marker
    const children = []
    for (; i < ast.length; i++) {
      if (ast[i].lineNumber < line) {
        nodes.push(ast[i])
        continue
      }

      if (ast[i].lineNumber === line) {
        children.push(ast[i])
        continue
      }

      if (ast[i].lineNumber > line) {
        break
      }
    }

    nodes.push({
      type: 'element',
      tagName: 'div',
      properties: {
        dataLine: line,
        className: 'highlight-line',
        dataHighlighted:
          linesToHighlight.includes(line) || highlightAll ? 'true' : 'false',
      },
      children: children,
      lineNumber: line,
    })

    return nodes
  }, [])

  return wrapped
}

// https://github.com/gatsbyjs/gatsby/pull/26161/files
const MULTILINE_TOKEN_SPAN = /<span class="token ([^"]+)">[^<]*\n[^<]*<\/span>/g

const applyMultilineFix = function (ast) {
  // AST to HTML
  let html = toHtml(ast)

  // Fix JSX issue
  html = html.replace(MULTILINE_TOKEN_SPAN, (match, token) =>
    match.replace(/\n/g, `</span>\n<span class="token ${token}">`)
  )

  // HTML to AST
  const hast = unified()
    .use(parse, { emitParseErrors: true, fragment: true })
    .parse(html)

  return hast.children
}

export default function (ast, lines) {
  const formattedAst = applyMultilineFix(ast)
  const numbered = lineNumberify(formattedAst).nodes

  return wrapLines(numbered, lines)
}