diff --git a/nurbs.cabal b/nurbs.cabal
--- a/nurbs.cabal
+++ b/nurbs.cabal
@@ -1,7 +1,71 @@
 name:                nurbs
-version:             0.1.0.0
+version:             0.1.1.0
 synopsis:            NURBS
-description:         NURBS library
+description:
+  Simple NURBS library with support of NURBS, periodic NURBS, knot insertion-removal, NURBS split-joint
+  .
+  > import Control.Lens
+  > import Linear.NURBS
+  > import Linear.V2
+  > import Test.Hspec
+  > 
+  > -- | Simple NURBS of degree 3
+  > test₁ ∷ NURBS V2 Double
+  > test₁ = nurbs 3 [
+  >   V2 0.0 0.0,
+  >   V2 10.0 0.0,
+  >   V2 10.0 10.0,
+  >   V2 20.0 20.0,
+  >   V2 0.0 20.0,
+  >   V2 (-20.0) 0.0]
+  > 
+  > -- | Another NURBS of degree 3
+  > test₂ ∷ NURBS V2 Double
+  > test₂ = nurbs 3 [
+  >   V2 (-20.0) 0.0,
+  >   V2 (-20.0) (-20.0),
+  >   V2 0.0 (-40.0),
+  >   V2 20.0 20.0]
+  > 
+  > -- | Make test₁ periodic
+  > testₒ ∷ NURBS V2 Double
+  > testₒ = set periodic True test₁
+  > 
+  > main ∷ IO ()
+  > main = hspec $ do
+  >   describe "evaluate point" $ do
+  >     it "should start from first control point" $
+  >       (test₁ `eval` 0.0) ≃ (test₁ ^?! wpoints . _head . wpoint)
+  >     it "should end at last control point" $
+  >       (test₁ `eval` 1.0) ≃ (test₁ ^?! wpoints . _last . wpoint)
+  >   describe "insert knot" $ do
+  >     it "should not change nurbs curve" $
+  >       insertKnots [(1, 0.1), (2, 0.3)] test₁ ≃ test₁
+  >   describe "remove knots" $ do
+  >     it "should not change nurbs curve" $
+  >       removeKnots [(1, 0.1), (2, 0.3)] (insertKnots [(1, 0.1), (2, 0.3)] test₁) ≃ test₁
+  >   describe "purge knots" $ do
+  >     it "should not change nurbs curve" $
+  >       purgeKnots (insertKnots [(1, 0.1), (2, 0.6)] test₁) ≃ test₁
+  >   describe "split" $ do
+  >     it "should work as cut" $
+  >       snd (split 0.4 test₁) ≃ cut (Span 0.4 1.0) test₁
+  >   describe "normalize" $ do
+  >     it "should not affect curve" $
+  >       cut (Span 0.2 0.8) test₁ ≃ normalizeKnot (cut (Span 0.2 0.8) test₁)
+  >   describe "joint" $ do
+  >     it "should joint cutted nurbs" $
+  >       uncurry joint (split 0.3 test₁) ≃ Just test₁
+  >     it "should cut jointed nurbs" $
+  >       (cut (Span 0.0 1.0) <$> (test₁ ⊕ test₂)) ≃ Just test₁
+  >     it "should cut jointed nurbs" $
+  >       (cut (Span 1.0 2.0) <$> (test₁ ⊕ test₂)) ≃ Just test₂
+  >   describe "periodic" $ do
+  >     it "can be broken into simple nurbs" $
+  >       breakLoop 0.0 testₒ ≃ testₒ
+  >     it "can be broken in any place" $
+  >       uncurry (flip (⊕)) (split 0.5 (breakLoop 0.0 testₒ)) ≃ Just (breakLoop 0.5 testₒ)
+
 homepage:            https://github.com/mvoidex/nurbs
 license:             BSD3
 license-file:        LICENSE
@@ -14,6 +78,7 @@
 library
   exposed-modules:
     Linear.NURBS
+    Linear.NURBS.Types
   build-depends:
     base >= 4.8 && < 5,
     base-unicode-symbols,
diff --git a/src/Linear/NURBS.hs b/src/Linear/NURBS.hs
--- a/src/Linear/NURBS.hs
+++ b/src/Linear/NURBS.hs
@@ -1,16 +1,19 @@
-{-# LANGUAGE TypeFamilies, TemplateHaskell, RankNTypes, MultiParamTypeClasses, FlexibleInstances, DefaultSignatures, FlexibleContexts, UndecidableInstances #-}
+{-# LANGUAGE TypeFamilies, RankNTypes, MultiParamTypeClasses, FlexibleInstances, DefaultSignatures, FlexibleContexts, UndecidableInstances #-}
 
 module Linear.NURBS (
 	binomial, size,
-	Weight(..), ofWeight, weight, wpoint,
-	Span(..), spanStart, spanEnd, spanId, grow, fall, coords, rangeSpan, mergeSpan,
+	spanId, spanEmpty, spanLength, inSpan, grow, fall, coords, rangeSpan, mergeSpan,
 	knotSpans, growSpans, fallSpans,
-	KnotData(..), knotData, makeData, iterData, evalData,
+	dataSpan, makeData, iterData, evalData,
 	basis, rbasis,
-	NURBS(..), eval, uniformKnot, degree, wpoints, knotVector, iknotVector, knotSpan, normalizeKnot, nurbs, wnurbs,
-	insertKnot, insertKnots, appendPoint, prependPoint, split, cut, removeKnot, removeKnot_, removeKnots, purgeKnots,
-	ndist, SimEq(..),
-	joint, (⊕)
+	eval, uniformKnot, cycleKnot, periodic, degree, wpoints, points, knotVector, iknotVector, knotSpan, normalizeKnot, nurbs, wnurbs,
+	insertKnot, insertKnots, appendPoint, prependPoint, split, cut, breakLoop, removeKnot, removeKnot_, removeKnots, purgeKnot, purgeKnots,
+	ndist, SimEq(..), simEq, simNeq,
+	joint, (⊕),
+
+	pline, circle,
+
+	module Linear.NURBS.Types
 	) where
 
 import Prelude.Unicode
@@ -20,76 +23,43 @@
 import Data.List
 import Data.Maybe (fromMaybe)
 import Linear.Vector hiding (basis)
-import Linear.Affine
 import Linear.Metric
+import Linear.V2
 
+import Linear.NURBS.Types
+
+-- | Binomial coefficients
 binomial ∷ Integral a ⇒ a → a → a
 binomial n k
 	| k > n ∨ k < 0 = 0
 	| otherwise = product [k + 1 .. n] `div` product [1 .. n - k]
 
+-- | Size of vector
 size ∷ (Additive f, Floating a, Foldable f) ⇒ f a → a
 size = sqrt ∘ sum ∘ fmap (^ (2 ∷ Integer))
 
-data Weight f a = Weight (f a) a deriving (Eq, Ord, Read, Show)
-
-ofWeight ∷ Additive f ⇒ f a → a → Weight f a
-pt `ofWeight` w = Weight pt w
-
-weight ∷ (Additive f, Fractional a) ⇒ Lens' (Weight f a) a
-weight = lens fromw tow where
-	fromw (Weight _ w) = w
-	tow (Weight pt w) w' = Weight ((w' / w) *^ pt) w'
-
-wpoint ∷ (Additive f, Additive g, Fractional a) ⇒ Lens (Weight f a) (Weight g a) (f a) (g a)
-wpoint = lens fromw tow where
-	fromw (Weight pt w) = (1.0 / w) *^ pt
-	tow (Weight _ w) pt' = Weight (w *^ pt') w
-
-instance Functor f ⇒ Functor (Weight f) where
-	fmap f (Weight pt w) = Weight (fmap f pt) (f w)	
-
-instance Additive f ⇒ Additive (Weight f) where
-	zero = Weight zero 0
-	Weight lx lw ^+^ Weight rx rw = Weight (lx ^+^ rx) (lw + rw)
-	Weight lx lw ^-^ Weight rx rw = Weight (lx ^-^ rx) (lw - rw)
-	lerp a (Weight lx lw) (Weight rx rw) = Weight (lerp a lx rx) (a * lw + (1 - a) * rw)
-	liftU2 f (Weight lx lw) (Weight rx rw) = Weight (liftU2 f lx rx) (f lw rw)
-	liftI2 f (Weight lx lw) (Weight rx rw) = Weight (liftI2 f lx rx) (f lw rw)
-
-instance Affine f ⇒ Affine (Weight f) where
-	type Diff (Weight f) = Weight (Diff f)
-	Weight lx lw .-. Weight rx rw = Weight (lx .-. rx) (lw - rw)
-	Weight lx lw .+^ Weight x w = Weight (lx .+^ x) (lw + w)
-	Weight lx lw .-^ Weight x w = Weight (lx .-^ x) (lw - w)
-
-instance Foldable f ⇒ Foldable (Weight f) where
-	foldMap f (Weight x w) = foldMap f x `mappend` f w
-
-instance Metric f ⇒ Metric (Weight f) where
-	dot (Weight lx lw) (Weight rx rw) = dot lx rx + lw * rw	
-
-data Span a = Span {
-	_spanStart ∷ a,
-	_spanEnd ∷ a }
-		deriving (Eq, Ord, Read)
-
-instance Show a ⇒ Show (Span a) where
-	show (Span s e) = show (s, e)
-
 -- | Piecewise constant function, returns 1 in span, 0 otherwise
 spanId ∷ (Ord a, Num a) ⇒ a → Span a → a
-spanId u (Span s e)
-	| s ≤ u ∧ u ≤ e = 1
+spanId u s
+	| u `inSpan` s = 1
 	| otherwise = 0
 
+-- | Is span empty
+spanEmpty ∷ Eq a ⇒ Span a → Bool
+spanEmpty (Span s e) = s ≡ e
+
+-- | Check whether value in span
+inSpan ∷ Ord a ⇒ a → Span a → Bool
+x `inSpan` Span s e = x ≥ s ∧ x ≤ e
+
+-- | Span length
+spanLength ∷ Num a ⇒ Span a → a
+spanLength (Span s e) = e - s
+
 safeDiv ∷ (Eq a, Fractional a) ⇒ a → a → a
 safeDiv _ 0.0 = 0.0
 safeDiv x y = x / y
 
-tailsOf ∷ Int → [a] → [[a]]
-tailsOf n = filter ((≡ n) ∘ length) ∘ map (take n) ∘ tails
-
 -- | Grow within span from 0 to 1
 grow ∷ (Ord a, Fractional a) ⇒ a → Span a → a
 grow u (Span l h)
@@ -102,14 +72,20 @@
 fall ∷ (Ord a, Fractional a) ⇒ a → Span a → a
 fall u s = 1 - grow u s
 
+-- | Grop within span from 0 to 1, periodic
+cycleGrow ∷ (Ord a, Fractional a) ⇒ a → a → Span a → a
+cycleGrow per u s = grow (until (≥ (s ^. spanStart)) (+ per) u) s
+
+-- | Fall within span from 1 to 0, periodic
+cycleFall ∷ (Ord a, Fractional a) ⇒ a → a → Span a → a
+cycleFall per u s = fall (until (≥ (s ^. spanStart)) (+ per) u) s
+
 -- | Map value to span coordinates, span start mapped to 0, end to 1
 coords ∷ (Eq a, Fractional a) ⇒ Span a → Iso' a a
 coords (Span s e) = iso fromc toc where
 	fromc u = (u - s) `safeDiv` (e - s)
 	toc u' = (u' * (e - s)) + s
 
-makeLenses ''Span
-
 -- | Make span from knot vector
 rangeSpan ∷ [a] → Span a
 rangeSpan = uncurry Span ∘ (head &&& last)
@@ -119,33 +95,35 @@
 mergeSpan l r = Span (min (_spanStart l) (_spanStart r)) (max (_spanEnd l) (_spanEnd r))
 
 -- | Generate knot spans of degree
-knotSpans ∷ Int → [a] → [Span a]
-knotSpans d = map rangeSpan ∘ tailsOf (d + 2)
+knotSpans ∷ Num a ⇒ Int → [a] → [Span a]
+knotSpans n k = foldr ($) (zipWith Span k (tail k)) (replicate n merge') where
+	merge' s = zipWith mergeSpan' s (tail s ++ [over traversed (+ _spanEnd sp) $ head s])
+	mergeSpan' l r = Span (_spanStart l) (_spanEnd r)
+	sp = rangeSpan k
 
 -- | Generate drow spans of degree
-growSpans ∷ Int → [a] → [Span a]
+growSpans ∷ Num a ⇒ Int → [a] → [Span a]
 growSpans d = knotSpans (pred d) ∘ init
 
 -- | Generate fall spans of degree
-fallSpans ∷ Int → [a] → [Span a]
+fallSpans ∷ Num a ⇒ Int → [a] → [Span a]
 fallSpans d = knotSpans (pred d) ∘ tail
 
--- | Knot evaluation data
-data KnotData a = KnotData a [(Span a, a)] deriving (Eq, Ord, Read, Show)
-
-knotData ∷ Lens' (KnotData a) [(Span a, a)]
-knotData = lens fromk tok where
-	fromk (KnotData _ d) = d
-	tok (KnotData u _) = KnotData u
+-- | Span of knot data
+dataSpan ∷ (Eq a, Fractional a) ⇒ Lens' (KnotData a) (Span a)
+dataSpan = lens fromk tok where
+	fromk (KnotData _ s _) = s
+	tok (KnotData u s d) s' = KnotData (scale u) s' (map (fmap scale *** scale) d) where
+		scale x = x ^. coords s . from (coords s')
 
 -- | Make initial knot data
 makeData ∷ (Ord a, Num a) ⇒ [a] → a → KnotData a
-makeData knot u = KnotData u $ map (id &&& spanId u) $ knotSpans 0 knot
+makeData knot u = KnotData u (rangeSpan knot) $ map (id &&& spanId u) $ knotSpans 0 knot
 
 -- | Eval basis function for next degree
 iterData ∷ (Ord a, Fractional a) ⇒ KnotData a → KnotData a
-iterData (KnotData u vs) = KnotData u $ zipWith mergeSpans vs (tail vs) where
-	mergeSpans (ls, l) (rs, r) = (mergeSpan ls rs, grow u ls * l + fall u rs * r)
+iterData (KnotData u s vs) = KnotData u s $ zipWith mergeSpans vs (tail vs ++ [over (_1 . traversed) (+ _spanEnd s) $ head vs]) where
+	mergeSpans (ls, l) (rs, r) = (mergeSpan ls rs, cycleGrow (spanLength s) u ls * l + cycleFall (spanLength s) u rs * r)
 
 -- | Eval for n degree
 evalData ∷ (Ord a, Fractional a) ⇒ Int → KnotData a → KnotData a
@@ -160,8 +138,6 @@
 rbasis ws knot i n u = (dat ^?! knotData . ix i . _2) * (ws ^?! ix i) / sum (zipWith (*) (dat ^.. knotData . each . _2) ws) where
 	dat = evalData n (makeData knot u)
 
-data NURBS f a = NURBS [Weight f a] [a] deriving (Eq, Ord, Read, Show)
-
 -- | Evaluate nurbs point
 eval ∷ (Additive f, Ord a, Fractional a) ⇒ NURBS f a → a → f a
 eval n t = foldr (^+^) zero [rbasis ws knot i deg t *^ pt | (i, pt) ← zip [0..] pts] where
@@ -170,34 +146,66 @@
 	pts = n ^.. wpoints . each . wpoint
 	ws = n ^.. wpoints . each . weight
 
+-- [0.0 .. 1.0] divided on n parts
+unitRange ∷ Fractional a ⇒ Int → [a]
+unitRange n = [fromIntegral i / fromIntegral n | i ← [0 .. n]]
+
 -- | Generate knot of degree for points
 uniformKnot ∷ Fractional a ⇒ Int → Int → [a]
 uniformKnot deg pts = concat [
-	replicate (succ deg) 0,
-	[1 / fromIntegral (pts - deg) * fromIntegral i | i ← [1 .. pts - succ deg]],
-	replicate (succ deg) 1]
+	replicate deg 0,
+	unitRange (fromIntegral (pts - deg)),
+	replicate deg 1]
 
+-- | Generate cycle knot
+cycleKnot ∷ Fractional a ⇒ Int → [a]
+cycleKnot = unitRange
+
+-- | Cut first and last same knots
+cutKnot ∷ Int → [a] → [a]
+cutKnot deg k = drop deg (take (length k - deg) k)
+
+-- | Add first and last same knots
+extendKnot ∷ Int → [a] → [a]
+extendKnot deg k = replicate deg (k ^?! _head) ++ k ++ replicate deg (k ^?! _last)
+
+-- | NURBS degree
 degree ∷ Fractional a ⇒ Lens' (NURBS f a) Int
 degree = lens fromn ton where
-	fromn (NURBS wpts k) = length k - length wpts - 1
-	ton n@(NURBS wpts _) d
-		| d ≥ length wpts = n
-		| d < 1 = n
-		| otherwise = NURBS wpts (uniformKnot d $ length wpts)
+	fromn (NURBS _ _ d) = d
+	ton n@(NURBS wpts k d) d'
+		| n ^. periodic = NURBS wpts k d'
+		| otherwise = NURBS wpts (extendKnot d' $ cutKnot d k) d'
 
+-- | Is NURBS periodic
+periodic ∷ Fractional a ⇒ Lens' (NURBS f a) Bool
+periodic = lens fromn ton where
+	fromn (NURBS wpts k _) = length k ≡ succ (length wpts)
+	ton n@(NURBS wpts k d) p
+		| (length k ≡ succ (length wpts)) ≡ p = n
+		| p = NURBS wpts (cycleKnot (length wpts)) d
+		| otherwise = NURBS wpts (uniformKnot d (length wpts)) d
+
+-- | NURBS points with weights
 wpoints ∷ Fractional a ⇒ Lens (NURBS f a) (NURBS g a) [Weight f a] [Weight g a]
 wpoints = lens fromn ton where
-	fromn (NURBS wpts _) = wpts
-	ton n@(NURBS wpts k) wpts'
-		| length wpts ≡ length wpts' = NURBS wpts' k
-		| otherwise = NURBS wpts' (uniformKnot (view degree n) (length wpts'))
+	fromn (NURBS wpts _ _) = wpts
+	ton (NURBS wpts k d) wpts'
+		| length wpts ≡ length wpts' = NURBS wpts' k d
+		| otherwise = NURBS wpts' (uniformKnot d (length wpts')) d
 
+-- | NURBS points without weights
+points ∷ (Additive f, Additive g, Fractional a) ⇒ Traversal (NURBS f a) (NURBS g a) (f a) (g a)
+points = wpoints . each . wpoint
+
+-- | NURBS knot vector
 knotVector ∷ Eq a ⇒ Lens' (NURBS f a) [a]
 knotVector = lens fromn ton where
-	fromn (NURBS _ k) = k
-	ton n@(NURBS wpts _) k'
+	fromn (NURBS _ k _) = k
+	ton n@(NURBS wpts _ d) k'
+		| length k' ≡ succ (length wpts) = NURBS wpts k' d
 		| length k' > length wpts * 2 ∨ length k' < length wpts + 2 = n
-		| allSame (take (succ deg') k') ∧ allSame (take (succ deg') $ reverse k') = NURBS wpts k'
+		| allSame (take (succ deg') k') ∧ allSame (take (succ deg') $ reverse k') = NURBS wpts k' deg'
 		| otherwise = n
 		where
 			deg' = length k' - length wpts - 1
@@ -207,17 +215,22 @@
 
 iknotVector ∷ (Eq a, Fractional a) ⇒ Lens' (NURBS f a) [a]
 iknotVector = lens fromn ton where
-	fromn n@(NURBS _ k) = drop (n ^. degree + 1) $ take (length k - n^. degree - 1) k
-	ton n@(NURBS wpts k) k'
-		| length k' + 2 * (n ^. degree + 1) > length wpts * 2 ∨ length k' + 2 * (n ^. degree + 1) < length wpts + 2 = n
-		| otherwise = NURBS wpts (take (n ^. degree + 1) k ++  k' ++ take (n ^. degree + 1) (reverse k))
+	fromn n@(NURBS _ k _)
+		| n ^. periodic = k
+		| otherwise = cutKnot (n ^. degree) k
+	ton n@(NURBS wpts _ d) k'
+		| (n ^. periodic) ∧ length k' ≡ succ (length wpts) = set knotVector k' n
+		| length k' ≡ succ (length wpts - d) = set knotVector (extendKnot (n ^. degree) k') n
+		| otherwise = n
 
+-- | NURBS knot span
 knotSpan ∷ (Eq a, Fractional a) ⇒ Lens' (NURBS f a) (Span a)
 knotSpan = lens fromn ton where
-	fromn (NURBS _ k) = rangeSpan k
-	ton (NURBS wpts k) s = NURBS wpts (map (view norm') k) where
+	fromn (NURBS _ k _) = rangeSpan k
+	ton (NURBS wpts k d) s = NURBS wpts (map (view norm') k) d where
 		norm' = coords (rangeSpan k) ∘ from (coords s)
 
+-- | Scale NURBS params to ∈ [0, 1]
 normalizeKnot ∷ (Eq a, Fractional a) ⇒ NURBS f a → NURBS f a
 normalizeKnot = set knotSpan (Span 0 1)
 
@@ -227,17 +240,24 @@
 
 -- | Make nurbs of degree from weighted points
 wnurbs ∷ (Additive f, Fractional a) ⇒ Int → [Weight f a] → NURBS f a
-wnurbs deg pts = NURBS pts (uniformKnot deg (length pts))
+wnurbs deg pts = NURBS pts (uniformKnot deg (length pts)) deg
 
 -- | Insert knot
+-- qᵢ₊₁ = fᵢ⋅pᵢ + (1-fᵢ)⋅pᵢ₊₁
+-- q₀ = p₀ (f₋₁ ≡ 0) (non periodic nurbs)
+-- qₙ₊₁ = pₙ (fₙ ≡ 1) (non periodic nurbs)
+-- fᵢ, pᵢ, pᵢ₊₁ ↦ qᵢ₊₁
 insertKnot ∷ (Additive f, Ord a, Fractional a) ⇒ a → NURBS f a → NURBS f a
 insertKnot u n
-	| u ≤ head (n ^. knotVector) ∨ u ≥ last (n ^. knotVector) = error "Invalid knot value"
-	| otherwise = NURBS qs (sort $ u : (n ^. knotVector))
+	| not (u `inSpan` (n ^. knotSpan)) = error "Invalid knot value"
+	| otherwise = NURBS qs (sort $ u : (n ^. knotVector)) (n ^. degree)
 	where
 		wpts = n ^. wpoints
-		fs = map (fall u) $ fallSpans (n ^. degree) (n ^. knotVector)
-		qs = [head wpts] ++ zipWith3 lerp fs wpts (tail wpts) ++ [last wpts]
+		fs = map (cycleFall (spanLength (n ^. knotSpan)) u) $ take (length (n ^. wpoints)) $ knotSpans (pred (n ^. degree)) (n ^. knotVector)
+		-- find place to insert additional point
+		-- after last non null fall param
+		(growth, restart) = over (both . mapped . _1) snd $ span (uncurry (≤) ∘ fst) $ zip (zip (0.0 : fs) fs) (zip (last wpts : wpts) wpts)
+		qs = map (\(f, (prev, cur)) → lerp f prev cur) $ growth ++ (set _1 0.0 (last growth)) : restart
 
 -- | Insert knots
 insertKnots ∷ (Additive f, Ord a, Fractional a) ⇒ [(Int, a)] → NURBS f a → NURBS f a
@@ -245,47 +265,68 @@
 
 -- | Append point
 appendPoint ∷ (Eq a, Fractional a) ⇒ a → Weight f a → NURBS f a → NURBS f a
-appendPoint knot_end pt n = NURBS
-	(view wpoints n ++ [pt])
-	(take (succ $ length $ view wpoints n) (view knotVector n) ++ replicate (view degree n + 1) knot_end)
+appendPoint knot_end pt =
+	over nurbsPoints (++ [pt]) ∘
+	over nurbsKnoti (++ [knot_end])
 
 -- | Prepend point
 prependPoint ∷ (Eq a, Fractional a) ⇒ a → Weight f a → NURBS f a → NURBS f a
-prependPoint knot_start pt n = NURBS
-	(pt : view wpoints n)
-	(replicate (view degree n + 1) knot_start ++ drop (view degree n) (view knotVector n))
+prependPoint knot_start pt =
+	over nurbsPoints (pt :) ∘
+	over nurbsKnoti (knot_start :)
 
 -- | Split NURBS
 split ∷ (Additive f, Ord a, Fractional a) ⇒ a → NURBS f a → (NURBS f a, NURBS f a)
-split u n = (before, after) where
-	n' = foldr ($) n $ replicate (view degree n - existed) (insertKnot u)
-	before = NURBS (take (length bknots) $ view wpoints n') (bknots ++ replicate (view degree n' + 1) u) where
-		bknots = takeWhile (< u) (view knotVector n')
-	after = NURBS (drop (length (view wpoints n') - length aknots) $ view wpoints n') (replicate (view degree n' + 1) u ++ aknots) where
-		aknots = dropWhile (≤ u) (view knotVector n')
-	existed = length $ filter (≡ u) $ view knotVector n
+split u n
+	| n ^. periodic = error "Can't split periodic nurbs"
+	| otherwise = (before, after)
+	where
+		n' = insertKnots [(n ^. degree - existed, u)] n
+		before = NURBS (take (length bknots) $ view wpoints n') (bknots ++ replicate (view degree n' + 1) u) (n ^. degree) where
+			bknots = takeWhile (< u) (view knotVector n')
+		after = NURBS (drop (length (view wpoints n') - length aknots) $ view wpoints n') (replicate (view degree n' + 1) u ++ aknots) (n ^. degree) where
+			aknots = dropWhile (≤ u) (view knotVector n')
+		existed = length $ filter (≡ u) $ view knotVector n
 
 -- | Cut NURBS
 cut ∷ (Additive f, Ord a, Fractional a) ⇒ Span a → NURBS f a → NURBS f a
-cut (Span l h) = snd ∘ split l ∘ fst ∘ split h
+cut (Span l h) n
+	| not (n ^. periodic) = snd ∘ split l ∘ fst ∘ split h $ n
+	| otherwise = fst ∘ split h ∘ breakLoop l $ n
 
+-- | Break periodic NURBS at param
+breakLoop ∷ (Additive f, Ord a, Fractional a) ⇒ a → NURBS f a → NURBS f a
+breakLoop u n
+	| not (n ^. periodic) = error "Can't break not periodic nurbs"
+	| otherwise = NURBS (wpts' ++ [head wpts']) knot' (n ^. degree)
+	where
+		n' = insertKnots [(n ^. degree - existed, u)] n
+		existed = length $ filter (≡ u) $ n ^. knotVector
+		(knot_tail, knot_init) = break (≡ u) $ n' ^. knotVector . _init
+		knot' =
+			extendKnot (n' ^. degree) $
+				drop (pred $ n' ^. degree) knot_init ++
+				over each (+ (n' ^. knotSpan . spanEnd)) (knot_tail ++ [u])
+		wpts' = rotate (pred $ length knot_tail) (n' ^. wpoints)
+
 -- | Remove knot
+-- pᵢ₊₁ = (qᵢ₊₁ - fᵢ⋅pᵢ)/(1-fᵢ) = hᵢ⋅qᵢ₊₁ + (1-hᵢ)⋅pᵢ, where hᵢ = 1/(1-fᵢ) ∧ fᵢ ≢ 1
+-- if fᵢ = 1 then pᵢ₊₁ = qᵢ₊₁
+-- p₀ = q₀ (h₋₁ ≡ 1)
+-- pₙ = qₙ₊₁ (hₙ ≡ ∞, fₙ ≡ 1)
+-- hᵢ, qᵢ₊₁, pᵢ ↦ pᵢ₊₁
 removeKnot ∷ (Foldable f, Additive f, Ord a, Floating a, SimEq (NURBS f a)) ⇒ a → NURBS f a → Maybe (NURBS f a)
 removeKnot u n
+	| n ^. periodic = Nothing
 	| n' ≃ n = Just n'
 	| otherwise = Nothing
 	where
-		n' = NURBS (pts ++ drop (succ $ length pts) wpts) knots'
+		n' = NURBS (pts ++ drop (succ $ length pts) wpts) knots' (n ^. degree)
 		knots' = delete u $ n ^. knotVector
 		fs = map (fall u) $ fallSpans (n ^. degree) knots'
-		hs = takeWhile (> 0.0) $ 1 : [inv (1 - f) | f ← fs]
+		hs = takeWhile (> 0.0) [inv (1 - f) | f ← fs]
 		wpts = n ^. wpoints
-		pts = zipWith eval' qs hs_ where
-			qs = tail (inits wpts)
-			hs_ = tail (inits hs)
-			eval' qs' hs' = foldr (^+^) zero $ zipWith (*^) (map h' (tails hs')) qs'
-			h' [] = error "Impossible"
-			h' (hi : his) = hi * product [1 - hk | hk ← his]
+		pts = head wpts : zipWith3 lerp hs (tail wpts) pts
 		inv 0.0 = 0.0
 		inv x = 1.0 / x
 
@@ -297,10 +338,18 @@
 removeKnots ∷ (Foldable f, Additive f, Ord a, Floating a, SimEq (NURBS f a)) ⇒ [(Int, a)] → NURBS f a → NURBS f a
 removeKnots iu n = foldr ($) n $ concat [replicate i (removeKnot_ u) | (i, u) ← iu]
 
+-- | Try remove knot as much times as possible
+purgeKnot ∷ (Foldable f, Additive f, Ord a, Floating a, SimEq (NURBS f a)) ⇒ a → NURBS f a → NURBS f a
+purgeKnot u n = fromMaybe n (purgeKnot u <$> removeKnot u n)
+
 -- | Try remove knots
 purgeKnots ∷ (Foldable f, Additive f, Ord a, Floating a, SimEq (NURBS f a)) ⇒ NURBS f a → NURBS f a
-purgeKnots n = foldr ($) n [removeKnot_ u | u ← n ^. iknotVector] where
+purgeKnots n = foldr ($) n [removeKnot_ u | u ← vs] where
+	vs
+		| n ^. periodic = n ^. knotVector
+		| otherwise = cutKnot (n ^. degree + 1) $ n ^. knotVector
 
+-- | Distance between points
 ndist ∷ (Metric f, Ord a, Floating a) ⇒ f a → f a → a
 ndist l r = distance l r / sqrt (max (norm l) (norm r))
 
@@ -310,6 +359,12 @@
 	(≄) ∷ a → a → Bool
 	x ≄ y = not (x ≃ y)
 
+simEq ∷ SimEq a ⇒ a → a → Bool
+simEq = (≃)
+
+simNeq ∷ SimEq a ⇒ a → a → Bool
+simNeq = (≄)
+
 instance SimEq a ⇒ SimEq (Maybe a) where
 	Just x ≃ Just y = x ≃ y
 	_ ≃ _ = False
@@ -332,14 +387,40 @@
 		i = max (length (x ^. wpoints)) (length (y ^. wpoints)) * 4
 		norm' n = maximum $ map norm (n ^. wpoints)
 
+-- | Try to joint two NURBS
 joint ∷ (Ord a, Num a, Floating a, Foldable f, Metric f, SimEq (Weight f a), SimEq (NURBS f a)) ⇒ NURBS f a → NURBS f a → Maybe (NURBS f a)
 joint l r
-	| (l ^?! wpoints . _last) ≃ (r ^?! wpoints . _head) ∧ (l ^. degree ≡ r ^. degree) = Just $ purgeKnots $ NURBS ((l ^. wpoints) ++ (r ^. wpoints . _tail)) knots'
+	| (l ^?! wpoints . _last) ≃ (r ^?! wpoints . _head) ∧ (l ^. degree ≡ r ^. degree) = Just $ purgeKnot (l ^?! knotVector . _last) $ NURBS ((l ^. wpoints) ++ (r ^. wpoints . _tail)) knots' (l ^. degree)
 	| otherwise = Nothing
 	where
 		knots' = (l ^. knotVector . _init) ++ over mapped moveKnot (r ^.. knotVector . _tail . dropping deg' each)
 		deg' = l ^. degree
 		moveKnot k = k - (r ^?! knotVector . _head) + (l ^?! knotVector . _last)
 
+-- | Joint
 (⊕) ∷ (Ord a, Num a, Floating a, Foldable f, Metric f, SimEq (Weight f a), SimEq (NURBS f a)) ⇒ NURBS f a → NURBS f a → Maybe (NURBS f a)
 l ⊕ r = joint l r
+
+-- | Make pline NURBS
+pline ∷ (Additive f, Fractional a) ⇒ [f a] → NURBS f a
+pline = nurbs 1
+
+-- | Make circle NURBS
+circle ∷ (Eq a, Floating a) ⇒ V2 a → a → NURBS V2 a
+circle c r = over points move' $ set nurbsKnot knot' $ wnurbs 2 [
+	V2 r r `ofWeight` sq,
+	V2 0 r `ofWeight` 1,
+	V2 (-r) r `ofWeight` sq,
+	V2 (-r) 0 `ofWeight` 1,
+	V2 (-r) (-r) `ofWeight` sq,
+	V2 0 (-r) `ofWeight` 1,
+	V2 r (-r) `ofWeight` sq,
+	V2 r 0 `ofWeight` 1]
+	where
+		sq = sqrt 2.0 / 2.0
+		knot' = [0, 0, 0.25, 0.25, 0.5, 0.5, 0.75, 0.75, 1.0]
+		move' pt = pt ^+^ c
+
+rotate ∷ Int → [a] → [a]
+rotate n l = uncurry (flip (++)) $ splitAt n' l where
+	n' = until (≥ 0) (+ length l) n
diff --git a/src/Linear/NURBS/Types.hs b/src/Linear/NURBS/Types.hs
new file mode 100644
--- /dev/null
+++ b/src/Linear/NURBS/Types.hs
@@ -0,0 +1,115 @@
+{-# LANGUAGE TemplateHaskell, TypeFamilies #-}
+
+module Linear.NURBS.Types (
+	Weight(..), weightPoint, weightValue, ofWeight, weight, wpoint,
+	Span(..), spanStart, spanEnd,
+	KnotData(..), knotData, knotDataAt, knotDataSpan,
+	NURBS(..), nurbsPoints, nurbsKnot, nurbsKnoti, nurbsDegree
+	) where
+
+import Prelude.Unicode
+
+import Control.Lens
+import Linear.Vector hiding (basis)
+import Linear.Affine
+import Linear.Metric
+
+-- | Point with weight
+data Weight f a = Weight { _weightPoint ∷ f a, _weightValue ∷ a } deriving (Eq, Ord, Read, Show)
+
+makeLenses ''Weight
+
+-- | Make point with weight
+ofWeight ∷ Additive f ⇒ f a → a → Weight f a
+pt `ofWeight` w = Weight pt w
+
+-- | Weight lens
+weight ∷ (Additive f, Fractional a) ⇒ Lens' (Weight f a) a
+weight = lens fromw tow where
+	fromw (Weight _ w) = w
+	tow (Weight pt w) w' = Weight ((w' / w) *^ pt) w'
+
+-- | Point lens
+wpoint ∷ (Additive f, Additive g, Fractional a) ⇒ Lens (Weight f a) (Weight g a) (f a) (g a)
+wpoint = lens fromw tow where
+	fromw (Weight pt w) = (1.0 / w) *^ pt
+	tow (Weight _ w) pt' = Weight (w *^ pt') w
+
+instance Functor f ⇒ Functor (Weight f) where
+	fmap f (Weight pt w) = Weight (fmap f pt) (f w)	
+
+instance Traversable f ⇒ Traversable (Weight f) where
+	traverse f (Weight pt w) = Weight <$> traverse f pt <*> f w
+
+instance Additive f ⇒ Additive (Weight f) where
+	zero = Weight zero 0
+	Weight lx lw ^+^ Weight rx rw = Weight (lx ^+^ rx) (lw + rw)
+	Weight lx lw ^-^ Weight rx rw = Weight (lx ^-^ rx) (lw - rw)
+	lerp a (Weight lx lw) (Weight rx rw) = Weight (lerp a lx rx) (a * lw + (1 - a) * rw)
+	liftU2 f (Weight lx lw) (Weight rx rw) = Weight (liftU2 f lx rx) (f lw rw)
+	liftI2 f (Weight lx lw) (Weight rx rw) = Weight (liftI2 f lx rx) (f lw rw)
+
+instance Affine f ⇒ Affine (Weight f) where
+	type Diff (Weight f) = Weight (Diff f)
+	Weight lx lw .-. Weight rx rw = Weight (lx .-. rx) (lw - rw)
+	Weight lx lw .+^ Weight x w = Weight (lx .+^ x) (lw + w)
+	Weight lx lw .-^ Weight x w = Weight (lx .-^ x) (lw - w)
+
+instance Foldable f ⇒ Foldable (Weight f) where
+	foldMap f (Weight x w) = foldMap f x `mappend` f w
+
+instance Metric f ⇒ Metric (Weight f) where
+	dot (Weight lx lw) (Weight rx rw) = dot lx rx + lw * rw	
+
+-- | Knot span
+data Span a = Span {
+	_spanStart ∷ a,
+	_spanEnd ∷ a }
+		deriving (Eq, Ord, Read)
+
+makeLenses ''Span
+
+instance Functor Span where
+	fmap f (Span s e) = Span (f s) (f e)
+
+instance Foldable Span where
+	foldMap f (Span s e) = f s `mappend` f e
+
+instance Traversable Span where
+	traverse f (Span s e) = Span <$> f s <*> f e
+
+instance Show a ⇒ Show (Span a) where
+	show (Span s e) = show (s, e)
+
+-- | Knot evaluation data, used to compute basis functions
+data KnotData a = KnotData {
+	_knotDataAt ∷ a,
+	_knotDataSpan ∷ Span a,
+	_knotData ∷ [(Span a, a)] }
+		deriving (Eq, Ord, Read, Show)
+
+makeLenses ''KnotData
+
+-- | NURBS
+data NURBS f a = NURBS {
+	_nurbsPoints ∷ [Weight f a],
+	_nurbsKnot ∷ [a],
+	_nurbsDegree ∷ Int }
+		deriving (Eq, Ord, Read, Show)
+
+makeLenses ''NURBS
+
+instance Functor f ⇒ Functor (NURBS f) where
+	fmap f (NURBS pts k d) = NURBS (map (fmap f) pts) (map f k) d
+
+instance Foldable f ⇒ Foldable (NURBS f) where
+	foldMap f (NURBS pts k _) = mconcat (map (foldMap f) pts) `mappend` mconcat (map f k)
+
+nurbsKnoti ∷ Lens' (NURBS f a) [a]
+nurbsKnoti = lens fromn ton where
+	fromn (NURBS wpts k d)
+		| length k ≡ succ (length wpts) = k
+		| otherwise = drop d $ reverse $ drop d $ reverse k
+	ton (NURBS wpts k d) k'
+		| length k ≡ succ (length wpts) = NURBS wpts k' d
+		| otherwise = NURBS wpts (replicate d (k' ^?! _head) ++ k' ++ replicate d (k' ^?! _last)) d
diff --git a/tests/Test.hs b/tests/Test.hs
--- a/tests/Test.hs
+++ b/tests/Test.hs
@@ -2,18 +2,14 @@
 	main
 	) where
 
-import Prelude.Unicode
-
-import Control.Applicative
 import Control.Lens
-import Data.Ratio
 import Linear.NURBS
-import Linear.Vector
 import Linear.V2
 import Test.Hspec
 
-test ∷ NURBS V2 Double
-test = nurbs 3 [
+-- | Simple NURBS of degree 3
+test₁ ∷ NURBS V2 Double
+test₁ = nurbs 3 [
 	V2 0.0 0.0,
 	V2 10.0 0.0,
 	V2 10.0 10.0,
@@ -21,34 +17,49 @@
 	V2 0.0 20.0,
 	V2 (-20.0) 0.0]
 
-test2 ∷ NURBS V2 Double
-test2 = nurbs 3 [
+-- | Another NURBS of degree 3
+test₂ ∷ NURBS V2 Double
+test₂ = nurbs 3 [
 	V2 (-20.0) 0.0,
 	V2 (-20.0) (-20.0),
 	V2 0.0 (-40.0),
 	V2 20.0 20.0]
 
+-- | Make test₁ periodic
+testₒ ∷ NURBS V2 Double
+testₒ = set periodic True test₁
+
 main ∷ IO ()
 main = hspec $ do
+	describe "evaluate point" $ do
+		it "should start from first control point" $
+			(test₁ `eval` 0.0) ≃ (test₁ ^?! wpoints . _head . wpoint)
+		it "should end at last control point" $
+			(test₁ `eval` 1.0) ≃ (test₁ ^?! wpoints . _last . wpoint)
 	describe "insert knot" $ do
 		it "should not change nurbs curve" $
-			insertKnots [(1, 0.1), (2, 0.3)] test ≃ test
+			insertKnots [(1, 0.1), (2, 0.3)] test₁ ≃ test₁
 	describe "remove knots" $ do
 		it "should not change nurbs curve" $
-			removeKnots [(1, 0.1), (2, 0.3)] (insertKnots [(1, 0.1), (2, 0.3)] test) ≃ test
+			removeKnots [(1, 0.1), (2, 0.3)] (insertKnots [(1, 0.1), (2, 0.3)] test₁) ≃ test₁
 	describe "purge knots" $ do
 		it "should not change nurbs curve" $
-			purgeKnots (insertKnots [(1, 0.1), (2, 0.6)] test) ≃ test
+			purgeKnots (insertKnots [(1, 0.1), (2, 0.6)] test₁) ≃ test₁
 	describe "split" $ do
 		it "should work as cut" $
-			snd (split 0.4 test) ≃ cut (Span 0.4 1.0) test
+			snd (split 0.4 test₁) ≃ cut (Span 0.4 1.0) test₁
 	describe "normalize" $ do
 		it "should not affect curve" $
-			cut (Span 0.2 0.8) test ≃ normalizeKnot (cut (Span 0.2 0.8) test)
+			cut (Span 0.2 0.8) test₁ ≃ normalizeKnot (cut (Span 0.2 0.8) test₁)
 	describe "joint" $ do
 		it "should joint cutted nurbs" $
-			uncurry joint (split 0.3 test) ≃ Just test
+			uncurry joint (split 0.3 test₁) ≃ Just test₁
 		it "should cut jointed nurbs" $
-			(cut (Span 0.0 1.0) <$> (test ⊕ test2)) ≃ Just test
+			(cut (Span 0.0 1.0) <$> (test₁ ⊕ test₂)) ≃ Just test₁
 		it "should cut jointed nurbs" $
-			(cut (Span 1.0 2.0) <$> (test ⊕ test2)) ≃ Just test2
+			(cut (Span 1.0 2.0) <$> (test₁ ⊕ test₂)) ≃ Just test₂
+	describe "periodic" $ do
+		it "can be broken into simple nurbs" $
+			breakLoop 0.0 testₒ ≃ testₒ
+		it "can be broken in any place" $
+			uncurry (flip (⊕)) (split 0.5 (breakLoop 0.0 testₒ)) ≃ Just (breakLoop 0.5 testₒ)