module Algo.Tridiag ( tridiag ) where import Data.Vector.Unboxed as V tridiag :: (Vector Double, Vector Double, Vector Double, Vector Double) -> Vector Double {-# NOINLINE tridiag #-} tridiag (as,bs,cs,ds) = V.prescanr' (\(c,d) x' -> d - c*x') 0 $ V.prescanl' modify (0,0) $ V.zip (V.zip as bs) (V.zip cs ds) where modify (c',d') ((a,b),(c,d)) = let id = 1 / (b - c'*a) in id `seq` (c*id, (d-d'*a)*id)