module Daybreak
  # Daybreak::DB contains the public api for Daybreak, you may extend it like
  # any other Ruby class (i.e. to overwrite serialize and parse). It includes
  # Enumerable for functional goodies like map, each, reduce and friends.
  class DB
    include Enumerable

    # Create a new Daybreak::DB. The second argument is the default value
    # to store when accessing a previously unset key, this follows the
    # Hash standard.
    # @param [String] file the path to the db file
    # @param default the default value to store and return when a key is
    #  not yet in the database.
    # @yield [key] blk a block that will return the default value to store.
    def initialize(file, default=nil, &blk)
      @file_name = file
      reset!
      @default = default
      @default = blk if block_given?
      read!
    end

    # Set a key in the database to be written at some future date. If the data
    # needs to be persisted immediately, call <tt>db.set(key, value, true)</tt>.
    # @param [#to_s] key the key of the storage slot in the database
    # @param value the value to store
    # @param [Boolean] sync if true, sync this value immediately
    def []=(key, value, sync = false)
      key = key.to_s
      @writer.write(Record.new(key, serialize(value)))
      flush! if sync
      @table[key] = value
    end
    alias_method :set, :"[]="

    # set! flushes data immediately to disk.
    # @param [#to_s] key the key of the storage slot in the database
    # @param value the value to store
    def set!(key, value)
      set key, value, true
    end

    # Retrieve a value at key from the database. If the default value was specified
    # when this database was created, that value will be set and returned. Aliased
    # as <tt>get</tt>.
    # @param [#to_s] key the value to retrieve from the database.
    def [](key)
      key = key.to_s
      if @table.has_key? key
        @table[key]
      elsif default?
        if @default.is_a? Proc
          value = @default.call(key)
        else
          value = @default
        end
        set key, value
      end
    end
    alias_method :get, :"[]"

    # Iterate over the key, value pairs in the database.
    def each(&blk)
      keys.each { |k| blk.call(k, get(k)) }
    end

    # Does this db have a default value.
    def default?
      !@default.nil?
    end

    # Does this db have a value for this key?
    def has_key?(key)
      @table.has_key? key.to_s
    end

    # Return the keys in the db.
    # @return [Array<String>]
    def keys
      @table.keys
    end

    # Return the number of stored items.
    # @return [Integer]
    def length
      @table.keys.length
    end

    # Serialize the data for writing to disk, if you don't want to use <tt>Marshal</tt>
    # overwrite this method.
    # @param value the value to be serialized
    # @return [String]
    def serialize(value)
      Marshal.dump(value)
    end

    # Parse the serialized value from disk, like serialize if you want to use a
    # different serialization method overwrite this method.
    # @param value the value to be parsed
    # @return [String]
    def parse(value)
      Marshal.load(value)
    end

    # Reset and empty the database file.
    def empty!
      reset!
      @writer.truncate!
    end

    # Force all queued commits to be written to disk.
    def flush!
      @writer.flush!
    end

    # Reset the state of the database, you should call <tt>read!</tt> after calling this.
    def reset!
      @table  = {}
      @writer = Daybreak::Writer.new(@file_name)
      @reader = Daybreak::Reader.new(@file_name)
    end

    # Close the database for reading and writing.
    def close!
      @writer.close!
      @reader.close!
    end

    # Compact the database to remove stale commits and reduce the file size.
    def compact!
      # Create a new temporary file
      tmp_file = Tempfile.new File.basename(@file_name)
      copy_db  = DB.new tmp_file.path

      # Copy the database key by key into the temporary table
      each do |key, i|
        copy_db.set(key, get(key))
      end
      copy_db.close!

      # Empty this database
      empty!

      # Move the copy into place
      tmp_file.close
      FileUtils.mv tmp_file.path, @file_name
      tmp_file.unlink

      # Reset this database
      close!
      reset!
      read!
    end

    # Read all values from the log file. If you want to check for changed data
    # call this again.
    def read!
      @reader.read do |record|
        @table[record.key] = parse record.data
      end
    end
  end
end