codemirror-fold-by-indentation.js

import { foldService } from '@codemirror/language'

/** @param {string} text */
function indentLevel(text) {
  const mat = /** @type {RegExpMatchArray} */ (
    text.match(/^( *)([^ ]?)/))
  return mat[2] ? mat[1].length : -1
}

export const foldByIndentation = foldService.of(({ doc }, from, _to) => {
  const line = doc.lineAt(from)
  if (line.number == doc.lines) return null

  // do not make an empty line foldable
  const indThis = indentLevel(line.text)
  if (indThis < 0) return null

  // find the next non-empty line, recording the level
  // if it is indented more than the current line
  let indMin
  let extent = line.number
  for (let i = line.number + 1; i <= doc.lines; i++) {
    const ind = indentLevel(doc.line(i).text)
    if (ind < 0) continue
    if (ind <= indThis) return null
    indMin = ind
    extent = i
    break
  }

  // there might be no such line
  if (indMin == null) return null

  // skip until another non-empty line with less indentation level
  for (let i = extent + 1; i <= doc.lines; i++) {
    const ind = indentLevel(doc.line(i).text)
    if (ind < 0) continue
    if (ind < indMin) break
    extent = i
  }

  return extent == line.number ? null : {
    from: doc.line(line.number).to,
    to: doc.line(extent).to,
  }
})