require "ecr/macros"
require "html"
require "uri"
require "mime"

# A handler that lists directories and serves files under a given public directory.
#
# This handler can send precompressed content, if the client accepts it, and a file
# with the same name and `.gz` extension appended is found in the same directory.
# Precompressed files are only served if they are newer than the original file.
#
# NOTE: To use `StaticFileHandler`, you must explicitly import it with `require "http"`
class HTTP::StaticFileHandler
  include HTTP::Handler

  # In some file systems, using `gz --keep` to compress the file will keep the
  # modification time of the original file but truncating some decimals. We
  # serve the gzipped file nonetheless if the .gz file is modified by a duration
  # of `TIME_DRIFT` before the original file. This value should match the
  # granularity of the underlying file system's modification times
  private TIME_DRIFT = 10.milliseconds

  @public_dir : Path

  # Creates a handler that will serve files in the given *public_dir*, after
  # expanding it (using `File#expand_path`).
  #
  # If *fallthrough* is `false`, this handler does not call next handler when
  # request method is neither GET or HEAD, then serves `405 Method Not Allowed`.
  # Otherwise, it calls next handler.
  #
  # If *directory_listing* is `false`, directory listing is disabled. This means that
  # paths matching directories are ignored and next handler is called.
  def initialize(public_dir : String, @fallthrough : Bool = true, @directory_listing : Bool = true)
    @public_dir = Path.new(public_dir).expand
  end

  # :ditto:
  @[Deprecated]
  def self.new(public_dir : String, fallthrough = true, directory_listing = true)
    new(public_dir, fallthrough: !!fallthrough, listing: !!listing)
  end

  def call(context) : Nil
    unless context.request.method.in?("GET", "HEAD")
      if @fallthrough
        call_next(context)
      else
        context.response.status = :method_not_allowed
        context.response.headers.add("Allow", "GET, HEAD")
      end
      return
    end

    original_path = context.request.path.not_nil!
    is_dir_path = original_path.ends_with?("/")
    request_path = self.request_path(URI.decode(original_path))

    # File path cannot contains '\0' (NUL) because all filesystem I know
    # don't accept '\0' character as file name.
    if request_path.includes? '\0'
      context.response.respond_with_status(:bad_request)
      return
    end

    request_path = Path.posix(request_path)
    expanded_path = request_path.expand("/")

    file_path = @public_dir.join(expanded_path.to_kind(Path::Kind.native))
    file_info = File.info? file_path
    is_dir = @directory_listing && file_info && file_info.directory?
    is_file = file_info && file_info.file?

    if request_path != expanded_path || is_dir && !is_dir_path
      redirect_path = expanded_path
      if is_dir && !is_dir_path
        # Append / to path if missing
        redirect_path = expanded_path.join("")
      end
      redirect_to context, redirect_path
      return
    end

    return call_next(context) unless file_info

    context.response.headers["Accept-Ranges"] = "bytes"

    if is_dir
      context.response.content_type = "text/html; charset=utf-8"
      directory_listing(context.response, request_path, file_path)
    elsif is_file
      last_modified = file_info.modification_time
      add_cache_headers(context.response.headers, last_modified)

      if cache_request?(context, last_modified)
        context.response.status = :not_modified
        return
      end

      context.response.content_type = MIME.from_filename(file_path.to_s, "application/octet-stream")

      # Checks if pre-gzipped file can be served
      if context.request.headers.includes_word?("Accept-Encoding", "gzip")
        gz_file_path = "#{file_path}.gz"

        if (gz_file_info = File.info?(gz_file_path)) &&
           last_modified - gz_file_info.modification_time < TIME_DRIFT
          file_path = gz_file_path
          file_info = gz_file_info
          context.response.headers["Content-Encoding"] = "gzip"
        end
      end

      File.open(file_path) do |file|
        if range_header = context.request.headers["Range"]?
          range_header = range_header.lchop?("bytes=")
          unless range_header
            context.response.headers["Content-Range"] = "bytes */#{file_info.size}"
            context.response.status = :range_not_satisfiable
            context.response.close
            return
          end

          ranges = parse_ranges(range_header, file_info.size)
          unless ranges
            context.response.respond_with_status :bad_request
            return
          end

          if file_info.size.zero? && ranges.size == 1 && ranges[0].begin.zero?
            context.response.status = :ok
            return
          end

          # If any of the ranges start beyond the end of the file, we return an
          # HTTP 416 Range Not Satisfiable.
          # See https://www.rfc-editor.org/rfc/rfc9110.html#section-14.1.2-11.1
          if ranges.any? { |range| range.begin >= file_info.size }
            context.response.headers["Content-Range"] = "bytes */#{file_info.size}"
            context.response.status = :range_not_satisfiable
            context.response.close
            return
          end

          ranges.map! { |range| range.begin..(Math.min(range.end, file_info.size - 1)) }

          context.response.status = :partial_content

          if ranges.size == 1
            range = ranges.first
            file.seek range.begin
            context.response.headers["Content-Range"] = "bytes #{range.begin}-#{range.end}/#{file_info.size}"
            IO.copy file, context.response, range.size
          else
            MIME::Multipart.build(context.response) do |builder|
              content_type = context.response.headers["Content-Type"]?
              context.response.headers["Content-Type"] = builder.content_type("byterange")

              ranges.each do |range|
                file.seek range.begin
                headers = HTTP::Headers{
                  "Content-Range"  => "bytes #{range.begin}-#{range.end}/#{file_info.size}",
                  "Content-Length" => range.size.to_s,
                }
                headers["Content-Type"] = content_type if content_type
                chunk_io = IO::Sized.new(file, range.size)
                builder.body_part headers, chunk_io
              end
            end
          end
        else
          context.response.status = :ok
          context.response.content_length = file_info.size
          IO.copy(file, context.response)
        end
      end
    else # Not a normal file (FIFO/device/socket)
      call_next(context)
    end
  end

  # TODO: Optimize without lots of intermediary strings
  private def parse_ranges(header, file_size)
    ranges = [] of Range(Int64, Int64)
    header.split(",") do |range|
      start_string, dash, finish_string = range.lchop(' ').partition("-")
      return if dash.empty?
      start = start_string.to_i64?
      return if start.nil? && !start_string.empty?
      if finish_string.empty?
        return if start_string.empty?
        finish = file_size
      else
        finish = finish_string.to_i64? || return
      end
      if file_size.zero?
        # > When a selected representation has zero length, the only satisfiable
        # > form of range-spec in a GET request is a suffix-range with a non-zero suffix-length.

        if start
          # This return value signals an unsatisfiable range.
          return [1_i64..0_i64]
        elsif finish <= 0
          return
        else
          start = finish = 0_i64
        end
      elsif !start
        # suffix-range
        start = {file_size - finish, 0_i64}.max
        finish = file_size - 1
      end

      range = (start..finish)
      return unless 0 <= range.begin <= range.end
      ranges << range
    end
    ranges unless ranges.empty?
  end

  # given a full path of the request, returns the path
  # of the file that should be expanded at the public_dir
  protected def request_path(path : String) : String
    path
  end

  private def redirect_to(context, url)
    context.response.redirect url.to_s
  end

  private def add_cache_headers(response_headers : HTTP::Headers, last_modified : Time) : Nil
    response_headers["Etag"] = etag(last_modified)
    response_headers["Last-Modified"] = HTTP.format_time(last_modified)
  end

  private def cache_request?(context : HTTP::Server::Context, last_modified : Time) : Bool
    # According to RFC 7232:
    # A recipient must ignore If-Modified-Since if the request contains an If-None-Match header field
    if if_none_match = context.request.if_none_match
      match = {"*", context.response.headers["Etag"]}
      if_none_match.any? { |etag| match.includes?(etag) }
    elsif if_modified_since = context.request.headers["If-Modified-Since"]?
      header_time = HTTP.parse_time(if_modified_since)
      # File mtime probably has a higher resolution than the header value.
      # An exact comparison might be slightly off, so we add 1s padding.
      # Static files should generally not be modified in subsecond intervals, so this is perfectly safe.
      # This might be replaced by a more sophisticated time comparison when it becomes available.
      !!(header_time && last_modified <= header_time + 1.second)
    else
      false
    end
  end

  private def etag(modification_time)
    %{W/"#{modification_time.to_unix}"}
  end

  record DirectoryListing, request_path : String, path : String do
    def each_entry(&)
      Dir.each_child(path) do |entry|
        yield entry
      end
    end

    ECR.def_to_s "#{__DIR__}/static_file_handler.html"
  end

  private def directory_listing(io, request_path, path)
    DirectoryListing.new(request_path.to_s, path.to_s).to_s(io)
  end
end
