Add a base RDB::Dumper module and implement an AOF dumper.

This version of `RDB::Dumpers::AOF` is actually a basic one, we should improve
it to leverage the variadic version of commands such as LPUSH, SADD and ZADD
to produce a more compact (and faster to ingest) AOF file when generating a
file for Redis >= 2.4.

Author: nrk

.gitignore

@@ -1 +1,2 @@
 *.gem
+*.aof

examples/aof_dumper.rb

@@ -0,0 +1,8 @@
+$:.unshift File.expand_path('../lib', File.dirname(__FILE__))
+
+require 'rdb'
+
+source = 'test/rdb/database_multiple_logical_dbs.rdb'
+destination = File.basename(source, '.rdb') + '.aof'
+
+RDB::Dumpers::AOF.new(source, destination).dump

lib/rdb.rb

@@ -6,3 +6,5 @@
 require 'rdb/callbacks'
 require 'rdb/reader-state'
 require 'rdb/reader'
+require 'rdb/dumper'
+require 'rdb/dumpers/aof'

lib/rdb/dumper.rb

@@ -0,0 +1,37 @@
+module RDB
+  module Dumper
+    include ReaderCallbacks
+
+    def initialize(source, destination, options = {})
+      @source = source
+      @destination = destination
+      @options = options
+      @output = nil
+    end
+
+    def <<(buffer)
+      @output << buffer unless @output.nil?; nil
+    end
+
+    def with_streams(&block)
+      input = open(@source, 'rb') unless @source.kind_of? IO
+      output = open(@destination, 'wb') unless @source.kind_of? IO
+
+      begin
+        block.call(input, output)
+      rescue
+        input.close
+        output.close
+      end
+    end
+
+    def dump
+      raise RuntimeException, 'Output stream already opened' if @output
+
+      with_streams do |input, output|
+        @output = output
+        Reader.read(input, callbacks: self)
+      end
+    end
+  end
+end

lib/rdb/dumpers/aof.rb

@@ -0,0 +1,47 @@
+module RDB
+  module Dumpers
+    #
+    # TODO: we should actually make the dumper configurable with the
+    # level of compatibility of the AOF file being produced against
+    # a specific Redis version. For example PEXPIREAT is unsupported
+    # on Redis <= 2.4. Also, Redis >= 2.4 can ingest AOF files using
+    # variadic LPUSH, SADD and ZADD.
+    #
+    class AOF
+      include Dumper
+
+      def start_database(database)
+        self << serialize_command(:select, [database])
+      end
+
+      def pexpireat(key, expiration, state)
+        self << serialize_command(:pexpireat, [key, expiration])
+      end
+
+      def set(key, value, state)
+        self << serialize_command(:set, [key, value])
+      end
+
+      def rpush(key, value, state)
+        self << serialize_command(:rpush, [key, value])
+      end
+
+      def sadd(key, value, state)
+        self << serialize_command(:sadd, [key, value])
+      end
+
+      def zadd(key, score, value, state)
+        self << serialize_command(:zadd, [key, score, value])
+      end
+
+      def hset(key, field, value, state)
+        self << serialize_command(:hset, [key, field, value])
+      end
+
+      def serialize_command(command, arguments)
+        buffer = "*#{arguments.length + 1}\r\n$#{command.length}\r\n#{command.upcase}\r\n"
+        buffer << arguments.map { |arg| "$#{arg.to_s.length}\r\n#{arg}\r\n" }.join
+      end
+    end
+  end
+end