module Sequel
  class Dataset
    # ---------------------
    # :section: 5 - Methods related to dataset graphing
    # Dataset graphing changes the dataset to yield hashes where keys are table
    # name symbols and values are hashes representing the columns related to
    # that table.  All of these methods return modified copies of the receiver.
    # ---------------------
    
    # Adds the given graph aliases to the list of graph aliases to use,
    # unlike +set_graph_aliases+, which replaces the list (the equivalent
    # of +select_more+ when graphing).  See +set_graph_aliases+.
    #
    #   DB[:table].add_graph_aliases(:some_alias=>[:table, :column])
    #   # SELECT ..., table.column AS some_alias
    #   # => {:table=>{:column=>some_alias_value, ...}, ...}
    def add_graph_aliases(graph_aliases)
      columns, graph_aliases = graph_alias_columns(graph_aliases)
      ds = select_more(*columns)
      ds.opts[:graph_aliases] = (ds.opts[:graph_aliases] || (ds.opts[:graph][:column_aliases] rescue {}) || {}).merge(graph_aliases)
      ds
    end

    # Allows you to join multiple datasets/tables and have the result set
    # split into component tables.
    #
    # This differs from the usual usage of join, which returns the result set
    # as a single hash.  For example:
    #
    #   # CREATE TABLE artists (id INTEGER, name TEXT);
    #   # CREATE TABLE albums (id INTEGER, name TEXT, artist_id INTEGER);
    #
    #   DB[:artists].left_outer_join(:albums, :artist_id=>:id).first
    #   #=> {:id=>albums.id, :name=>albums.name, :artist_id=>albums.artist_id}
    #
    #   DB[:artists].graph(:albums, :artist_id=>:id).first
    #   #=> {:artists=>{:id=>artists.id, :name=>artists.name}, :albums=>{:id=>albums.id, :name=>albums.name, :artist_id=>albums.artist_id}}
    #
    # Using a join such as left_outer_join, the attribute names that are shared between
    # the tables are combined in the single return hash.  You can get around that by
    # using +select+ with correct aliases for all of the columns, but it is simpler to
    # use +graph+ and have the result set split for you.  In addition, +graph+ respects
    # any +row_proc+ of the current dataset and the datasets you use with +graph+.
    #
    # If you are graphing a table and all columns for that table are nil, this
    # indicates that no matching rows existed in the table, so graph will return nil
    # instead of a hash with all nil values:
    #
    #   # If the artist doesn't have any albums
    #   DB[:artists].graph(:albums, :artist_id=>:id).first
    #   => {:artists=>{:id=>artists.id, :name=>artists.name}, :albums=>nil}
    #
    # Arguments:
    # dataset :: Can be a symbol (specifying a table), another dataset,
    #            or an object that responds to +dataset+ and returns a symbol or a dataset
    # join_conditions :: Any condition(s) allowed by +join_table+.
    # block :: A block that is passed to +join_table+.
    #
    # Options:
    # :from_self_alias :: The alias to use when the receiver is not a graphed
    #                     dataset but it contains multiple FROM tables or a JOIN.  In this case,
    #                     the receiver is wrapped in a from_self before graphing, and this option
    #                     determines the alias to use.
    # :implicit_qualifier :: The qualifier of implicit conditions, see #join_table.
    # :join_type :: The type of join to use (passed to +join_table+).  Defaults to :left_outer.
    # :select :: An array of columns to select.  When not used, selects
    #            all columns in the given dataset.  When set to false, selects no
    #            columns and is like simply joining the tables, though graph keeps
    #            some metadata about the join that makes it important to use +graph+ instead
    #            of +join_table+.
    # :table_alias :: The alias to use for the table.  If not specified, doesn't
    #                 alias the table.  You will get an error if the the alias (or table) name is
    #                 used more than once.
    def graph(dataset, join_conditions = nil, options = {}, &block)
      # Allow the use of a model, dataset, or symbol as the first argument
      # Find the table name/dataset based on the argument
      dataset = dataset.dataset if dataset.respond_to?(:dataset)
      table_alias = options[:table_alias]
      case dataset
      when Symbol
        table = dataset
        dataset = @db[dataset]
        table_alias ||= table
      when ::Sequel::Dataset
        if dataset.simple_select_all?
          table = dataset.opts[:from].first
          table_alias ||= table
        else
          table = dataset
          table_alias ||= dataset_alias((@opts[:num_dataset_sources] || 0)+1)
        end
      else
        raise Error, "The dataset argument should be a symbol, dataset, or model"
      end

      # Raise Sequel::Error with explanation that the table alias has been used
      raise_alias_error = lambda do
        raise(Error, "this #{options[:table_alias] ? 'alias' : 'table'} has already been been used, please specify " \
          "#{options[:table_alias] ? 'a different alias' : 'an alias via the :table_alias option'}") 
      end

      # Only allow table aliases that haven't been used
      raise_alias_error.call if @opts[:graph] && @opts[:graph][:table_aliases] && @opts[:graph][:table_aliases].include?(table_alias)
      
      # Use a from_self if this is already a joined table
      ds = (!@opts[:graph] && (@opts[:from].length > 1 || @opts[:join])) ? from_self(:alias=>options[:from_self_alias] || first_source) : self
      
      # Join the table early in order to avoid cloning the dataset twice
      ds = ds.join_table(options[:join_type] || :left_outer, table, join_conditions, :table_alias=>table_alias, :implicit_qualifier=>options[:implicit_qualifier], &block)
      opts = ds.opts

      # Whether to include the table in the result set
      add_table = options[:select] == false ? false : true
      # Whether to add the columns to the list of column aliases
      add_columns = !ds.opts.include?(:graph_aliases)
      # columns to select
      select = (opts[:select] || []).dup

      # Setup the initial graph data structure if it doesn't exist
      unless graph = opts[:graph]
        master = alias_symbol(ds.first_source_alias)
        raise_alias_error.call if master == table_alias
        # Master hash storing all .graph related information
        graph = opts[:graph] = {}
        # Associates column aliases back to tables and columns
        column_aliases = graph[:column_aliases] = {}
        # Associates table alias (the master is never aliased)
        table_aliases = graph[:table_aliases] = {master=>self}
        # Keep track of the alias numbers used
        ca_num = graph[:column_alias_num] = Hash.new(0)
        # All columns in the master table are never
        # aliased, but are not included if set_graph_aliases
        # has been used.
        if add_columns
          columns.each do |column|
            column_aliases[column] = [master, column]
            select.push(SQL::QualifiedIdentifier.new(master, column))
          end
        end
      end

      # Add the table alias to the list of aliases
      # Even if it isn't been used in the result set,
      # we add a key for it with a nil value so we can check if it
      # is used more than once
      table_aliases = graph[:table_aliases]
      table_aliases[table_alias] = add_table ? dataset : nil

      # Add the columns to the selection unless we are ignoring them
      if add_table && add_columns
        column_aliases = graph[:column_aliases]
        ca_num = graph[:column_alias_num]
        # Which columns to add to the result set
        cols = options[:select] || dataset.columns
        # If the column hasn't been used yet, don't alias it.
        # If it has been used, try table_column.
        # If that has been used, try table_column_N 
        # using the next value of N that we know hasn't been
        # used
        cols.each do |column|
          col_alias, identifier = if column_aliases[column]
            column_alias = :"#{table_alias}_#{column}"
            if column_aliases[column_alias]
              column_alias_num = ca_num[column_alias]
              column_alias = :"#{column_alias}_#{column_alias_num}" 
              ca_num[column_alias] += 1
            end
            [column_alias, SQL::AliasedExpression.new(SQL::QualifiedIdentifier.new(table_alias, column), column_alias)]
          else
            ident = SQL::QualifiedIdentifier.new(table_alias, column)
            [column, ident]
          end
          column_aliases[col_alias] = [table_alias, column]
          select.push(identifier)
        end
      end
      ds.select(*select)
    end

    # This allows you to manually specify the graph aliases to use
    # when using graph.  You can use it to only select certain
    # columns, and have those columns mapped to specific aliases
    # in the result set.  This is the equivalent of +select+ for a
    # graphed dataset, and must be used instead of +select+ whenever
    # graphing is used.
    #
    # graph_aliases :: Should be a hash with keys being symbols of
    #                  column aliases, and values being either symbols or arrays with one to three elements.
    #                  If the value is a symbol, it is assumed to be the same as a one element
    #                  array containing that symbol.
    #                  The first element of the array should be the table alias symbol.
    #                  The second should be the actual column name symbol.  If the array only
    #                  has a single element the column name symbol will be assumed to be the
    #                  same as the corresponding hash key. If the array
    #                  has a third element, it is used as the value returned, instead of
    #                  table_alias.column_name.
    #
    #   DB[:artists].graph(:albums, :artist_id=>:id).
    #     set_graph_aliases(:name=>:artists,
    #                       :album_name=>[:albums, :name],
    #                       :forty_two=>[:albums, :fourtwo, 42]).first
    #   # SELECT artists.name, albums.name AS album_name, 42 AS forty_two ...
    #   # => {:artists=>{:name=>artists.name}, :albums=>{:name=>albums.name, :fourtwo=>42}}
    def set_graph_aliases(graph_aliases)
      columns, graph_aliases = graph_alias_columns(graph_aliases)
      ds = select(*columns)
      ds.opts[:graph_aliases] = graph_aliases
      ds
    end

    # Remove the splitting of results into subhashes, and all metadata
    # related to the current graph (if any).
    def ungraphed
      clone(:graph=>nil, :graph_aliases=>nil)
    end

    private

    # Transform the hash of graph aliases and return a two element array
    # where the first element is an array of identifiers suitable to pass to
    # a select method, and the second is a new hash of preprocessed graph aliases.
    def graph_alias_columns(graph_aliases)
      gas = {}
      identifiers = graph_aliases.collect do |col_alias, tc| 
        table, column, value = Array(tc)
        column ||= col_alias
        gas[col_alias] = [table, column]
        identifier = value || SQL::QualifiedIdentifier.new(table, column)
        identifier = SQL::AliasedExpression.new(identifier, col_alias) if value || column != col_alias
        identifier
      end
      [identifiers, gas]
    end

    # Fetch the rows, split them into component table parts,
    # tranform and run the row_proc on each part (if applicable),
    # and yield a hash of the parts.
    def graph_each
      # Reject tables with nil datasets, as they are excluded from
      # the result set
      datasets = @opts[:graph][:table_aliases].to_a.reject{|ta,ds| ds.nil?}
      # Get just the list of table aliases into a local variable, for speed
      table_aliases = datasets.collect{|ta,ds| ta}
      # Get an array of arrays, one for each dataset, with
      # the necessary information about each dataset, for speed
      datasets = datasets.collect{|ta, ds| [ta, ds, ds.row_proc]}
      # Use the manually set graph aliases, if any, otherwise
      # use the ones automatically created by .graph
      column_aliases = @opts[:graph_aliases] || @opts[:graph][:column_aliases]
      fetch_rows(select_sql) do |r|
        graph = {}
        # Create the sub hashes, one per table
        table_aliases.each{|ta| graph[ta]={}}
        # Split the result set based on the column aliases
        # If there are columns in the result set that are
        # not in column_aliases, they are ignored
        column_aliases.each do |col_alias, tc|
          ta, column = tc
          graph[ta][column] = r[col_alias]
        end
        # For each dataset run the row_proc if applicable
        datasets.each do |ta,ds,rp|
          g = graph[ta]
          graph[ta] = if g.values.any?{|x| !x.nil?}
            rp ? rp.call(g) : g
          else
            nil
          end
        end

        yield graph
      end
      self
    end
  end
end
