require 'logger'
require 'stringio'
require 'singleton'
require 'activerecord'
require 'active_record/connection_adapters/abstract_adapter'

class ActiveRecord::Base
  # Instantiate a new NullDB connection.  Used by ActiveRecord internally.
  def self.nulldb_connection(config)
    ActiveRecord::ConnectionAdapters::NullDBAdapter.new(config)
  end
end

class ActiveRecord::ConnectionAdapters::NullDBAdapter <
    ActiveRecord::ConnectionAdapters::AbstractAdapter

  class Statement
    attr_reader :entry_point, :content

    def initialize(entry_point, content = "")
      @entry_point, @content = entry_point, content
    end

    def ==(other)
      self.entry_point == other.entry_point
    end
  end

  class Checkpoint < Statement
    def initialize
      super(:checkpoint, "")
    end

    def ==(other)
      self.class == other.class
    end
  end

  TableDefinition = ActiveRecord::ConnectionAdapters::TableDefinition

  class NullObject
    def method_missing(*args, &block)
      nil
    end
  end

  # A convenience method for integratinginto RSpec.  See README for example of
  # use.
  def self.insinuate_into_spec(config)
    config.before :all do
      ActiveRecord::Base.establish_connection(:adapter => :nulldb)
    end

    config.after :all do
      ActiveRecord::Base.establish_connection(:test)
    end
  end

  # Recognized options:
  #
  # [+:schema+] path to the schema file, relative to RAILS_ROOT
  def initialize(config={})
    @log            = StringIO.new
    @logger         = Logger.new(@log)
    @last_unique_id = 0
    @tables         = {'schema_info' =>  TableDefinition.new(nil)}
    @schema_path    = config.fetch(:schema){ "db/schema.rb" }
    super(nil, @logger)
  end

  # A log of every statement that has been "executed" by this connection adapter
  # instance.
  def execution_log
    (@execution_log ||= [])
  end

  # A log of every statement that has been "executed" since the last time
  # #checkpoint! was called, or since the connection was created.
  def execution_log_since_checkpoint
    checkpoint_index = @execution_log.rindex(Checkpoint.new)
    checkpoint_index = checkpoint_index ? checkpoint_index + 1 : 0
    @execution_log[(checkpoint_index..-1)]
  end

  # Inserts a checkpoint in the log.  See also #execution_log_since_checkpoint.
  def checkpoint!
    self.execution_log << Checkpoint.new
  end

  def adapter_name
    "NullDB"
  end

  def supports_migrations?
    true
  end

  def create_table(table_name, options = {})
    table_definition = ActiveRecord::ConnectionAdapters::TableDefinition.new(self)
    unless options[:id] == false
      table_definition.primary_key(options[:primary_key] || "id")
    end

    yield table_definition

    @tables[table_name] = table_definition
  end

  def add_fk_constraint(*args)
    # NOOP
  end

  def add_pk_constraint(*args)
    # NOOP
  end

  # Retrieve the table names defined by the schema
  def tables
    @tables.keys.map(&:to_s)
  end

  # Retrieve table columns as defined by the schema
  def columns(table_name, name = nil)
    if @tables.size <= 1
      ActiveRecord::Migration.verbose = false
      Kernel.load(File.join(@schema_path))
    end
    table = @tables[table_name]
    table.columns.map do |col_def|
      ActiveRecord::ConnectionAdapters::Column.new(col_def.name.to_s,
                                                    col_def.default,
                                                    col_def.type,
                                                    col_def.null)
    end
  end

  def execute(statement, name = nil)
    self.execution_log << Statement.new(entry_point, statement)
    NullObject.new
  end

  def insert(statement, name, primary_key, object_id, sequence_name)
    returning(object_id || next_unique_id) do
      with_entry_point(:insert) do
        super(statement, name, primary_key, object_id, sequence_name)
      end
    end
  end

  def update(statement, name=nil)
    with_entry_point(:update) do
      super(statement, name)
    end
  end

  def delete(statement, name=nil)
    with_entry_point(:delete) do
      super(statement, name)
    end
  end

  def select_all(statement, name=nil)
    with_entry_point(:select_all) do
      super(statement, name)
    end
  end

  def select_one(statement, name=nil)
    with_entry_point(:select_one) do
      super(statement, name)
    end
  end

  def select_value(statement, name=nil)
    with_entry_point(:select_value) do
      super(statement, name)
    end
  end

  protected

  def select(statement, name)
    returning([]) do
      self.execution_log << Statement.new(entry_point, statement)
    end
  end

  private

  def next_unique_id
    @last_unique_id += 1
  end

  def with_entry_point(method)
    if entry_point.nil?
      with_thread_local_variable(:entry_point, method) do
        yield
      end
    else
      yield
    end
  end

  def entry_point
    Thread.current[:entry_point]
  end

  def with_thread_local_variable(name, value)
    old_value = Thread.current[name]
    Thread.current[name] = value
    begin
      yield
    ensure
      Thread.current[name] = old_value
    end
  end
end