studio/src/utils/parseMessage.js

// Parse an assistant message into ordered segments: prose chunks and code
// chunks. Handles the streaming case where the trailing fence is still open.
//
// Each code segment carries: { type:'code', lang, filename, code, streaming }
// Each prose segment carries: { type:'prose', content }
//
// "streaming: true" marks a code block whose closing ``` hasn't arrived yet —
// the UI uses this to show a live indicator and auto-scroll.

const FENCE_OPEN_RE = /```([a-zA-Z0-9_-]*)?\n/
const FENCE_CLOSE_RE = /\n```/

const FILE_COMMENT_RE = /^\s*\/\/\s*[Ff]ile:\s*([^\n]+)\n/

function defaultFilename(lang) {
  const l = (lang || '').toLowerCase()
  if (l === 'css') return 'styles.css'
  if (l === 'html') return 'index.html'
  if (l === 'json') return 'data.json'
  return 'App.jsx'
}

export function parseMessageSegments(text) {
  const segments = []
  if (!text) return segments

  let cursor = 0
  while (cursor < text.length) {
    const rest = text.slice(cursor)
    const openMatch = rest.match(FENCE_OPEN_RE)

    if (!openMatch) {
      // Remaining is all prose
      const tail = rest
      if (tail.trim()) segments.push({ type: 'prose', content: tail })
      break
    }

    const openIdx = openMatch.index
    // Prose before this code block
    if (openIdx > 0) {
      const prose = rest.slice(0, openIdx)
      if (prose.trim()) segments.push({ type: 'prose', content: prose })
    }

    const lang = openMatch[1] || ''
    const afterOpen = rest.slice(openIdx + openMatch[0].length)
    const closeMatch = afterOpen.match(FENCE_CLOSE_RE)

    let body, streaming, advance
    if (closeMatch) {
      body = afterOpen.slice(0, closeMatch.index)
      streaming = false
      advance = openIdx + openMatch[0].length + closeMatch.index + closeMatch[0].length
    } else {
      // No closing fence yet — this block is still streaming.
      body = afterOpen
      streaming = true
      advance = rest.length
    }

    const fileComment = body.match(FILE_COMMENT_RE)
    const filename = fileComment
      ? fileComment[1].trim().replace(/^\//, '')
      : defaultFilename(lang)
    const code = fileComment ? body.slice(fileComment[0].length) : body

    segments.push({ type: 'code', lang, filename, code, streaming })
    cursor += advance
  }

  return segments
}