/**
 * This splits a string on a top-level character.
 *
 * Regex doesn't support recursion (at least not the JS-flavored version).
 * So we have to use a tiny state machine to keep track of paren placement.
 *
 * Expected behavior using commas:
 * var(--a, 0 0 1px rgb(0, 0, 0)), 0 0 1px rgb(0, 0, 0)
 *       ─┬─             ┬  ┬    ┬
 *        x              x  x    ╰──────── Split because top-level
 *        ╰──────────────┴──┴───────────── Ignored b/c inside >= 1 levels of parens
 *
 * @param {string} input
 * @param {string} separator
 */
export function splitAtTopLevelOnly(input, separator) {
  let stack = []
  let parts = []
  let lastPos = 0
  let isEscaped = false

  for (let idx = 0; idx < input.length; idx++) {
    let char = input[idx]

    if (stack.length === 0 && char === separator[0] && !isEscaped) {
      if (separator.length === 1 || input.slice(idx, idx + separator.length) === separator) {
        parts.push(input.slice(lastPos, idx))
        lastPos = idx + separator.length
      }
    }

    if (isEscaped) {
      isEscaped = false
    } else if (char === '\\') {
      isEscaped = true
    }

    if (char === '(' || char === '[' || char === '{') {
      stack.push(char)
    } else if (
      (char === ')' && stack[stack.length - 1] === '(') ||
      (char === ']' && stack[stack.length - 1] === '[') ||
      (char === '}' && stack[stack.length - 1] === '{')
    ) {
      stack.pop()
    }
  }

  parts.push(input.slice(lastPos))

  return parts
}