package com.rtnbleserver

import java.text.ParsePosition
import java.text.SimpleDateFormat
import java.util.Date
import java.util.Locale
import java.util.TimeZone

/**
 * ISO 8601 / RFC 3339 timestamps, matching the atproto `datetime` format.
 * `java.time` is avoided because it requires API 26 (or desugaring) and the
 * library supports API 24.
 */
internal object IsoDatetime {
  private val format = SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'", Locale.US).apply {
    timeZone = TimeZone.getTimeZone("UTC")
    isLenient = false
  }

  fun format(epochMillis: Long): String = synchronized(format) {
    format.format(Date(epochMillis))
  }

  /** returns [value] when it is a valid timestamp, otherwise `null`. */
  fun parse(value: String): String? = synchronized(format) {
    val position = ParsePosition(0)
    val parsed = format.parse(value, position)
    if (parsed == null || position.index != value.length) null else value
  }
}