type error = Malformed of string

let pp_error ppf = function
  | Malformed detail -> Format.fprintf ppf "malformed timestamp: %s" detail

let encode timestamp =
  let time =
    Unix.gmtime (float_of_int (Recovery.timestamp_to_unix_seconds timestamp))
  in
  Printf.sprintf "%04d-%02d-%02dT%02d:%02d:%02dZ" (time.Unix.tm_year + 1900)
    (time.Unix.tm_mon + 1) time.Unix.tm_mday time.Unix.tm_hour time.Unix.tm_min
    time.Unix.tm_sec

let digit text index =
  let value = Char.code text.[index] - Char.code '0' in
  if value >= 0 && value <= 9 then Some value else None

let number text start length =
  let rec loop index value =
    if index = start + length then Some value
    else
      match digit text index with
      | None -> None
      | Some value' -> loop (index + 1) ((value * 10) + value')
  in
  loop start 0

(* Return the number of days from 1970-01-01 for a Gregorian date. *)
let days_from_civil year month day =
  let year = year - if month <= 2 then 1 else 0 in
  let era = if year >= 0 then year / 400 else (year - 399) / 400 in
  let year_of_era = year - (era * 400) in
  let month = month + if month > 2 then -3 else 9 in
  let day_of_year = (((153 * month) + 2) / 5) + day - 1 in
  let day_of_era =
    (year_of_era * 365) + (year_of_era / 4) - (year_of_era / 100) + day_of_year
  in
  (era * 146097) + day_of_era - 719468

let decode text =
  let malformed detail = Error (Malformed detail) in
  if String.length text <> 20 then malformed "expected YYYY-MM-DDTHH:MM:SSZ"
  else if
    text.[4] <> '-'
    || text.[7] <> '-'
    || text.[10] <> 'T'
    || text.[13] <> ':'
    || text.[16] <> ':'
    || text.[19] <> 'Z'
  then malformed "expected YYYY-MM-DDTHH:MM:SSZ"
  else
    match
      ( number text 0 4,
        number text 5 2,
        number text 8 2,
        number text 11 2,
        number text 14 2,
        number text 17 2 )
    with
    | Some year, Some month, Some day, Some hour, Some minute, Some second ->
        if
          month < 1 || month > 12 || day < 1 || day > 31 || hour > 23
          || minute > 59 || second > 59
        then malformed "timestamp component out of range"
        else
          let seconds =
            (days_from_civil year month day * 86_400)
            + (hour * 3_600) + (minute * 60) + second
          in
          let timestamp = Recovery.timestamp_of_unix_seconds seconds in
          if String.equal (encode timestamp) text then Ok timestamp
          else malformed "invalid calendar date"
    | _ -> malformed "timestamp contains a non-digit"