module Impala
  # Cursors are used to iterate over result sets without loading them all
  # into memory at once. This can be useful if you're dealing with lots of
  # rows. It implements Enumerable, so you can use each/select/map/etc.
  class Cursor
    BUFFER_SIZE = 1024
    include Enumerable

    def self.typecast_boolean(value)
      value == 'true'
    end

    def self.typecast_int(value)
      value.to_i
    end

    def self.typecast_float(value)
      value.to_f
    end

    def self.typecast_decimal(value)
      BigDecimal.new(value)
    end

    def self.typecast_timestamp(value)
      Time.parse(value)
    end

    TYPECAST_MAP = {
      'boolean'=>method(:typecast_boolean),
      'int'=>method(:typecast_int),
      'double'=>method(:typecast_float),
      'decimal'=>method(:typecast_decimal),
      'timestamp'=>method(:typecast_timestamp),
    }
    TYPECAST_MAP['tinyint'] = TYPECAST_MAP['smallint'] = TYPECAST_MAP['bigint'] = TYPECAST_MAP['int']
    TYPECAST_MAP['float'] = TYPECAST_MAP['double']
    TYPECAST_MAP.freeze

    NULL = 'NULL'.freeze

    attr_reader :columns

    attr_reader :typecast_map

    def initialize(handle, service)
      @handle = handle
      @service = service


      @row_buffer = []
      @done = false
      @open = true
      @typecast_map = TYPECAST_MAP.dup
      @columns = metadata.schema.fieldSchemas.map(&:name)
    end

    def inspect
      "#<#{self.class}#{open? ? '' : ' (CLOSED)'}>"
    end

    def each
      while row = fetch_row
        yield row
      end
    end

    # Returns the next available row as a hash, or nil if there are none left.
    # @return [Hash, nil] the next available row, or nil if there are none
    #    left
    # @see #fetch_all
    def fetch_row
      if @row_buffer.empty?
        if @done
          return nil
        else
          fetch_more
        end
      end

      @row_buffer.shift
    end

    # Returns all the remaining rows in the result set.
    # @return [Array<Hash>] the remaining rows in the result set
    # @see #fetch_one
    def fetch_all
      self.to_a
    end

    # Close the cursor on the remote server. Once a cursor is closed, you
    # can no longer fetch any rows from it.
    def close
      @open = false
      @service.close(@handle)
    end

    # Returns true if the cursor is still open.
    def open?
      @open
    end

    # Returns true if there are any more rows to fetch.
    def has_more?
      !@done || !@row_buffer.empty?
    end

    def runtime_profile
      @service.GetRuntimeProfile(@handle)
    end

    private

    def metadata
      @metadata ||= @service.get_results_metadata(@handle)
    end

    def fetch_more
      fetch_batch until @done || @row_buffer.count >= BUFFER_SIZE
    end

    def fetch_batch
      raise CursorError.new("Cursor has expired or been closed") unless @open

      begin
        res = @service.fetch(@handle, false, BUFFER_SIZE)
      rescue Protocol::Beeswax::BeeswaxException
        @open = false
        raise CursorError.new("Cursor has expired or been closed")
      end

      rows = res.data.map { |raw| parse_row(raw) }
      @row_buffer.concat(rows)

      unless res.has_more
        @done = true
        close
      end
    end

    def parse_row(raw)
      row = {}
      fields = raw.split(metadata.delim)

      row_convertor.each do |c, p, i|
        v = fields[i]
        row[c] = (p ? p.call(v) : v unless v == NULL)
      end

      row
    end

    def row_convertor
      @row_convertor ||= columns.zip(metadata.schema.fieldSchemas.map{|s| typecast_map[s.type]}, (0...(columns.length)).to_a)
    end
  end
end