module DynamodbHelpers

# File lib/dynamodb_helpers.rb, line 4
def scan_and_find_by(query_opts, opts = {segments: false, select: false, table_name: false})

  query = {table_name: opts[:table_name] ? opts[:table_name] : self.table_name, select: 'ALL_ATTRIBUTES', scan_filter: {}}

  query_opts.each do | key, value |
    if value.nil?
      # raise "Detected nil value for #{key}"
      return []
    else
      query[:scan_filter].merge!(key.to_s => {:comparison_operator => "EQ", :attribute_value_list => [value]})
    end
  end

  if query[:scan_filter].length >= 2
    query.merge!(:conditional_operator => "AND")
  end

  # field selection
  query.merge!(select: "SPECIFIC_ATTRIBUTES", attributes_to_get: opts[:select]) if opts[:select]

  return scan_in_parallel(query, opts) if opts[:segments]
  return scan_sequentialy(query)
end

# File lib/dynamodb_helpers.rb, line 46
def scan_in_parallel(query, opts)
  segments = Integer(opts[:segments])
  items = [] # no need for mutex, we are just appending to this. ordering is not guaranteed !!!

  # do sequential scan for each segment
  segments.times do |n|
    Thread.new{items = items + scan_sequentialy(query.merge(segment: n, total_segments: segments))}.join
  end

  items
end

# File lib/dynamodb_helpers.rb, line 30
def scan_sequentialy(query)
  client =  ::Aws::DynamoDB::Client.new

  # first query
  result = client.scan(query)
  items = result.items

  # extra queries if data > 1 MB
  while result.last_evaluated_key
    result = client.scan(query.merge(exclusive_start_key: result.last_evaluated_key))
    items = items + result.items
  end

  items
end