-- | Perform aggregations on query results.
module Opaleye.Aggregate (module Opaleye.Aggregate, Aggregator) where

import qualified Opaleye.Internal.Aggregate as A
import           Opaleye.Internal.Aggregate (Aggregator)
import           Opaleye.QueryArr (Query)
import qualified Opaleye.Internal.QueryArr as Q
import qualified Opaleye.Column as C

import qualified Opaleye.Internal.HaskellDB.PrimQuery as HPQ

import           GHC.Int (Int64)

-- This page of Postgres documentation tell us what aggregate
-- functions are available
--
--   http://www.postgresql.org/docs/9.3/static/functions-aggregate.html

-- | Group the aggregation by equality on the input to 'groupBy'.
groupBy :: Aggregator (C.Column a) (C.Column a)
groupBy = A.makeAggr' Nothing

-- | Sum all rows in a group.
sum :: Aggregator (C.Column a) (C.Column a)
sum = A.makeAggr HPQ.AggrSum

-- | Count the number of non-null rows in a group.
count :: Aggregator (C.Column a) (C.Column Int64)
count = A.makeAggr HPQ.AggrCount

-- | Average of a group
avg :: Aggregator (C.Column Double) (C.Column Double)
avg = A.makeAggr HPQ.AggrAvg

{-|
Given a 'Query' producing rows of type @a@ and an 'Aggregator' accepting rows of
type @a@, apply the aggregator to the results of the query.
-}
aggregate :: Aggregator a b -> Query a -> Query b
aggregate agg q = Q.simpleQueryArr (A.aggregateU agg . Q.runSimpleQueryArr q)