// Minimal, single-method evaluator to avoid parser seam issues.

static box Main {
  main() {
    local j = "{\"a\":{\"b\":[1,2,3]}}"
    local path = ".a.b[1]"
    local out = this.eval_path_text(j, path)
    if out == null { print("null") } else { print(out) }
    return 0
  }

  // Minimal evaluator for the demo path shape: .a.b[<int>]
  eval_path_text(json_text, path) {
    // Find key "a"
    local k1 = json_text.indexOf("\"a\"")
    if k1 < 0 { return null }
    // find ':' after k1
    local c1 = k1
    loop(c1 < json_text.length() and json_text.substring(c1, c1+1) != ":") { c1 = c1 + 1 }
    if c1 >= json_text.length() { return null }
    if c1 < 0 { return null }
    // Find key "b" after a's value colon
    // find '"b"' after c1
    local k2 = c1
    loop(k2 + 2 < json_text.length()) {
      if json_text.substring(k2, k2+1) == "\"" and json_text.substring(k2+1, k2+2) == "b" and json_text.substring(k2+2, k2+3) == "\"" { break }
      k2 = k2 + 1
    }
    if k2 >= json_text.length() { return null }
    if k2 < 0 { return null }
    // find ':' after k2
    local c2 = k2
    loop(c2 < json_text.length() and json_text.substring(c2, c2+1) != ":") { c2 = c2 + 1 }
    if c2 >= json_text.length() { return null }
    if c2 < 0 { return null }
    // Find '[' starting the array
    // find '[' after c2
    local arr = c2
    loop(arr < json_text.length() and json_text.substring(arr, arr+1) != "[") { arr = arr + 1 }
    if arr >= json_text.length() { return null }
    if arr < 0 { return null }

    // Parse index from path
    local p_lb = path.indexOf("[")
    if p_lb < 0 { return null }
    // find ']' after p_lb
    local p_rb = p_lb
    loop(p_rb < path.length() and path.substring(p_rb, p_rb+1) != "]") { p_rb = p_rb + 1 }
    if p_rb < 0 { return null }
    local idx_text = path.substring(p_lb + 1, p_rb)
    // parse integer
    local k = 0
    local idx = 0
    loop(k < idx_text.length()) {
      local ch = idx_text.substring(k, k + 1)
      if ch == "0" { idx = idx * 10 + 0 }
      else { if ch == "1" { idx = idx * 10 + 1 }
      else { if ch == "2" { idx = idx * 10 + 2 }
      else { if ch == "3" { idx = idx * 10 + 3 }
      else { if ch == "4" { idx = idx * 10 + 4 }
      else { if ch == "5" { idx = idx * 10 + 5 }
      else { if ch == "6" { idx = idx * 10 + 6 }
      else { if ch == "7" { idx = idx * 10 + 7 }
      else { if ch == "8" { idx = idx * 10 + 8 }
      else { if ch == "9" { idx = idx * 10 + 9 } }}}}}}}}
      k = k + 1
    }

    // Iterate array to the idx-th element (numbers only in this demo)
    local pos = arr + 1
    local cur = 0
    loop(pos < json_text.length()) {
      // skip whitespace
      loop(pos < json_text.length() and (json_text.substring(pos, pos+1) == " " or json_text.substring(pos, pos+1) == "\n" or json_text.substring(pos, pos+1) == "\t" or json_text.substring(pos, pos+1) == "\r")) { pos = pos + 1 }
      if json_text.substring(pos, pos+1) == "]" { return null }
      // read token
      local start = pos
      loop(pos < json_text.length()) {
        local c = json_text.substring(pos, pos+1)
        if c == "," or c == "]" or c == " " or c == "\n" or c == "\t" or c == "\r" { break }
        pos = pos + 1
      }
      if cur == idx { return json_text.substring(start, pos) }
      // advance to next
      loop(pos < json_text.length() and json_text.substring(pos, pos+1) != "," and json_text.substring(pos, pos+1) != "]") { pos = pos + 1 }
      if pos < json_text.length() and json_text.substring(pos, pos+1) == "," { pos = pos + 1 cur = cur + 1 continue }
      return null
    }
    return null
  }
}
}