From 2243f4b6539577a6534e11bee66052ae5cda7e71 Mon Sep 17 00:00:00 2001
From: George Trudeau <9too@users.noreply.github.com>
Date: Sat, 5 Aug 2017 15:04:52 -0400
Subject: [PATCH] Improve relations initialization time (#924) (#926)

---
 CHANGELOG.md                 | 1 +
 src/PostgREST/DbStructure.hs | 6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 787b942da..a73d34b1a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - #877, Base64 secret read from a file ending with a newline - @eric-brechemier
 - #896, Boolean env var interpolation in config file - @begriffs
 - #885, OpenAPI repetition reduced by using more definitions- @ldesgoui
+- #924, Improve relations initialization time - @9too
 
 ## [0.4.2.0] - 2017-06-11
 
diff --git a/src/PostgREST/DbStructure.hs b/src/PostgREST/DbStructure.hs
index 5b59d609f..e877372ae 100644
--- a/src/PostgREST/DbStructure.hs
+++ b/src/PostgREST/DbStructure.hs
@@ -253,7 +253,11 @@ addManyToManyRelations rels = rels ++ addMirrorRelation (mapMaybe link2Relation
     links = join $ map (combinations 2) $ filter (not . null) $ groupWith groupFn $ filter ( (==Child). relType) rels
     groupFn :: Relation -> Text
     groupFn Relation{relTable=Table{tableSchema=s, tableName=t}} = s<>"_"<>t
-    combinations k ns = filter ((k==).length) (subsequences ns)
+    -- Reference : https://wiki.haskell.org/99_questions/Solutions/26
+    combinations :: Int -> [a] -> [[a]]
+    combinations 0 _  = [ [] ]
+    combinations n xs = [ y:ys | y:xs' <- tails xs
+                               , ys <- combinations (n-1) xs']
     addMirrorRelation [] = []
     addMirrorRelation (rel@(Relation t c ft fc _ lt lc1 lc2):rels') = Relation ft fc t c Many lt lc2 lc1 : rel : addMirrorRelation rels'
     link2Relation [