class FormatParser::RemoteIO
  # Represents a failure that might be retried
  # (like a 5xx response or a timeout)
  class IntermittentFailure < StandardError
  end

  # Represents a failure that should not be retried
  # (like a 4xx response or a DNS resolution error)
  class InvalidRequest < StandardError
  end

  # @param uri[URI, String] the remote URL to obtain
  def initialize(uri)
    require 'faraday'
    @uri = uri
    @pos = 0
    @remote_size = false
  end

  # Emulates IO#seek
  def seek(offset)
    @pos = offset
    0 # always return 0
  end

  # Emulates IO#pos
  def pos
    @pos
  end

  # Emulates IO#size.
  #
  # @return [Integer] the size of the remote resource
  def size
    raise 'Remote size not yet obtained, need to perform at least one read() to retrieve it' unless @remote_size
    @remote_size
  end

  # Emulates IO#read, but requires the number of bytes to read
  # The read will be limited to the
  # size of the remote resource relative to the current offset in the IO,
  # so if you are at offset 0 in the IO of size 10, doing a `read(20)`
  # will only return you 10 bytes of result, and not raise any exceptions.
  #
  # @param n_bytes[Fixnum, nil] how many bytes to read, or `nil` to read all the way to the end
  # @return [String] the read bytes
  def read(n_bytes)
    http_range = (@pos..(@pos + n_bytes - 1))
    maybe_size, maybe_body = request_range(http_range)
    if maybe_size && maybe_body
      @remote_size = maybe_size
      @pos += maybe_body.bytesize
      maybe_body.force_encoding(Encoding::ASCII_8BIT)
    end
  end

  protected

  # Only used internally when reading the remote file
  #
  # @param range[Range] the HTTP range of data to fetch from remote
  # @return [String] the response body of the ranged request
  def request_range(range)
    # We use a GET and not a HEAD request followed by a GET because
    # S3 does not allow HEAD requests if you only presigned your URL for GETs, so we
    # combine the first GET of a segment and retrieving the size of the resource
    response = Faraday.get(@uri, nil, range: 'bytes=%d-%d' % [range.begin, range.end])

    case response.status
    when 200, 206
      # Figure out of the server supports content ranges, if it doesn't we have no
      # business working with that server
      range_header = response.headers['Content-Range']
      raise InvalidRequest, "No range support at #{@uri}" unless range_header

      # "Content-Range: bytes 0-0/307404381" is how the response header is structured
      size = range_header[/\/(\d+)$/, 1].to_i

      # S3 returns 200 when you request a Range that is fully satisfied by the entire object,
      # we take that into account here. For other servers, 206 is the expected response code.
      # Also, if we request a _larger_ range than what can be satisfied by the server,
      # the response is going to only contain what _can_ be sent and the status is also going
      # to be 206
      return [size, response.body]
    when 416
      # We return `nil` if we tried to read past the end of the IO,
      # which satisfies the Ruby IO convention. The caller should deal with `nil` being the result of a read()
      # S3 will also handily _not_ supply us with the Content-Range of the actual resource, so we
      # cannot hint size with this response - at lease not when working with S3
      return
    when 500..599
      raise IntermittentFailure, "Server at #{@uri} replied with a #{response.status} and we might want to retry"
    else
      raise InvalidRequest, "Server at #{@uri} replied with a #{response.status} and refused our request"
    end
  end
end