// array_prepend_zero_macro.nyash
// MacroBoxSpec.expand: prepend 0 to every Array elements list in AST JSON v0
// Contract: expand(json: string) -> string (AST JSON v0)

static box MacroBoxSpec {
  expand(json) {
    local s, out, i, pat, plen, ins
    s = json
    out = ""
    i = 0
    pat = "\"kind\":\"Array\",\"elements\":["
    plen = pat.length()
    ins = "{\"kind\":\"Literal\",\"value\":{\"type\":\"int\",\"value\":0}}"

    loop(i < s.length()) {
      if i + plen <= s.length() && s.substring(i, i + plen) == pat {
        // Copy the Array header
        out = out + pat
        // Look ahead: if next is ']' it's empty array; otherwise add comma after inserted element
        if i + plen < s.length() && s.substring(i + plen, i + plen + 1) != "]" {
          out = out + ins + ","
        } else {
          out = out + ins
        }
        i = i + plen
        continue
      }
      // Also support reversed key order: ..."elements":[ ... ],"kind":"Array"
      if i + 12 <= s.length() && s.substring(i, i + 12) == "\"elements\":[" {
        // We will inject blindly here assuming this 'elements' belongs to Array (per schema it does)
        out = out + "\"elements\":["
        if i + 12 < s.length() && s.substring(i + 12, i + 13) != "]" {
          out = out + ins + ","
        } else {
          out = out + ins
        }
        i = i + 12
        continue
      }
      // Copy-through
      out = out + s.substring(i, i + 1)
      i = i + 1
    }
    return out
  }
}