Username.kt

@JvmInline
@Serializable
value class Username private constructor(@JvmField val value: String) {
  companion object {
    private const val MIN_LENGTH = 5
    private const val MAX_LENGTH = 20

    fun getOrNull(value: String): Username? = if (value.isValid()) Username(value) else null
    private fun String.isValid(): Boolean {
      if (length !in MIN_LENGTH..MAX_LENGTH) return false
      if (this[0] == '_' || this[lastIndex] == '_' || this[0].isDigit()) return false
      for (i in 0..lastIndex) {
        val char = this[i]
        if (!char.isDigit() && !char.isAlphabet() && char != '_') return false
        if (i != lastIndex && char == '_' && this[i + 1] == '_') return false
      }
      return true
    }

    private fun Char.isAlphabet() = this in 'a'..'z' || this in 'A'..'Z'
    private fun Char.isDigit() = this in '0'..'9'
  }
}