fix incorrect deflation code

src/lib/Math/Bezier/Cubic.hs

@@ -182,7 +182,7 @@ closestPoint
 closestPoint pts c = pickClosest ( 0 :| 1 : roots ) -- todo: also include the self-intersection point if one exists
   where
     roots :: [ r ]
-    roots = filter ( \ r -> r > 0 && r < 1 ) ( realRoots 2000 $ ddist @v pts c )
+    roots = filter ( \ r -> r > 0 && r < 1 ) ( realRoots 500 $ ddist @v pts c )
 
     pickClosest :: NonEmpty r -> ArgMin r ( r, p )
     pickClosest ( s :| ss ) = go s q nm0 ss

src/lib/Math/Bezier/Cubic/Fit.hs

@@ -262,7 +262,7 @@ fitPiece dist_tol t_tol maxIters p tp qs r tr =
         let
           laguerreStepResult :: Complex Double
           laguerreStepResult = runST do
-            coeffs <- unsafeThawPrimArray . primArrayFromListN 6 . map (:+ 0)
+            coeffs <- unsafeThawPrimArray . primArrayFromListN 6
                     $ Cubic.ddist @( Vector2D Double ) bez q
             laguerre epsilon 1 coeffs ( ti :+ 0 )
         ti' <- case laguerreStepResult of

src/lib/Math/Roots.hs

@@ -10,11 +10,11 @@ module Math.Roots where
 
 -- base
 import Control.Monad
-  ( unless )
+  ( unless, when )
 import Control.Monad.ST
   ( ST, runST )
 import Data.Complex
-  ( Complex(..), magnitude )
+  ( Complex(..), conjugate, magnitude, realPart, imagPart )
 import Data.Maybe
   ( mapMaybe )
 
@@ -68,71 +68,139 @@ solveQuadratic a0 a1 a2
     disc :: a  
     disc = a1 * a1 - 4 * a0 * a2
 
--- | Find real roots of a polynomial.
+-- | Find real roots of a polynomial with real coefficients.
 --
 -- Coefficients are given in order of decreasing degree, e.g.:
 --  x² + 7 is given by [ 1, 0, 7 ].
 realRoots :: forall a. ( RealFloat a, Prim a, NFData a ) => Int -> [ a ] -> [ a ]
-realRoots maxIters coeffs = mapMaybe isReal ( roots epsilon maxIters ( map (:+ 0) coeffs ) )
+realRoots maxIters coeffs = mapMaybe isReal ( roots epsilon maxIters coeffs )
   where
     isReal :: Complex a -> Maybe a
     isReal ( a :+ b )
       | abs b < epsilon = Just a
       | otherwise       = Nothing
 
--- | Compute all roots of a polynomial using Laguerre's method and (forward) deflation.
+-- | Compute all roots of a polynomial with real coefficients using Laguerre's method and (forward) deflation.
 --
 -- Polynomial coefficients are given in order of descending degree (e.g. constant coefficient last).
 --
 -- N.B. The forward deflation process is only guaranteed to be numerically stable
 -- if Laguerre's method finds roots in increasing order of magnitude.
-roots :: forall a. ( RealFloat a, Prim a, NFData a ) => a -> Int -> [ Complex a ] -> [ Complex a ]
+roots :: forall a. ( RealFloat a, Prim a, NFData a ) => a -> Int -> [ a ] -> [ Complex a ]
 roots eps maxIters coeffs = runST do
   let
-    coeffPrimArray :: PrimArray ( Complex a )
+    coeffPrimArray :: PrimArray a
     coeffPrimArray = primArrayFromList coeffs
     sz :: Int
     sz = sizeofPrimArray coeffPrimArray
-  ( p :: MutablePrimArray s ( Complex a ) ) <- unsafeThawPrimArray coeffPrimArray
+  ( p :: MutablePrimArray s a ) <- unsafeThawPrimArray coeffPrimArray
   let
     go :: Int -> [ Complex a ] -> ST s [ Complex a ]
     go !i rs = do
-      !r <- force <$> laguerre eps maxIters p 0 -- Start Laguerre's method at 0 for best chance of numerical stability.
+      -- Estimate the root with minimum absolute value in order to
+      -- improve numerical stability of Laguerre's method with forward deflation.
+      !z0 <- minAbsStartPoint p
+      !r  <- force <$> laguerre eps maxIters p z0
       if i <= 2
       then pure ( r : rs )
-      else do
-        deflate r p
-        go ( i - 1 ) ( r : rs )
+      else
+        -- real root
+        if abs ( imagPart r ) < epsilon
+        then do
+          deflate ( realPart r ) p
+          go ( i - 1 ) ( r : rs )
+        else do
+          deflateConjugatePair r p
+          go ( i - 2 ) ( r : conjugate r : rs )
   go sz []
 
--- | Deflate a polynomial: factor out a root of the polynomial.
--- 
--- The polynomial must have degree at least 2.
+-- | Estimate the root with smallest absolute value.
+--
+-- Polynomial coefficients are given in order of descending degree (e.g. constant coefficient last).
+minAbsStartPoint :: forall a m s. ( RealFloat a, Prim a, PrimMonad m, s ~ PrimState m ) => MutablePrimArray s a -> m ( Complex a )
+minAbsStartPoint p = do
+  n <- subtract 1 <$> getSizeofMutablePrimArray p
+  an <- readPrimArray p n
+  if abs an < epsilon
+  then pure 0
+  else do
+    a0 <- readPrimArray p 0
+    let
+      r, m0 :: Complex a
+      r  = log ( abs an :+ 0 )
+      m0 = exp $ ( r - log ( abs a0 :+ 0 ) ) / fromIntegral n
+      go :: Int -> Complex a -> m ( Complex a )
+      go i m
+        | i >= n
+        = pure m
+        | otherwise
+        = do
+          ai <- readPrimArray p i
+          if abs ai < epsilon
+          then go ( i + 1 ) m
+          else do
+            let
+              mi :: Complex a
+              mi = exp $ ( r - log ( abs ai :+ 0 ) ) / fromIntegral ( n - i )
+            if magnitude mi < magnitude m
+            then go ( i + 1 ) mi
+            else go ( i + 1 ) m
+    m <- go 1 m0
+    pure ( 0.5 * m )
+
+-- | Forward deflation of a polynomial by a root: factors out the root.
 deflate :: forall a m s. ( Num a, Prim a, PrimMonad m, s ~ PrimState m ) => a -> MutablePrimArray s a -> m ()
 deflate r p = do
   deg <- subtract 1 <$> getSizeofMutablePrimArray p
-  case compare deg 2 of
-    LT -> pure ()
-    EQ -> shrinkMutablePrimArray p deg
-    GT -> do
-      shrinkMutablePrimArray p deg
-      let
-        go :: a -> Int -> m ()
-        go !b !i = unless ( i >= deg ) do
-          ai <- readPrimArray p i
-          writePrimArray p i ( ai + r * b )
-          go ai ( i + 1 )
-      a0 <- readPrimArray p 0
-      go a0 1
+  when ( deg >= 2 ) do
+    shrinkMutablePrimArray p deg
+    let
+      go :: a -> Int -> m ()
+      go !b !i = unless ( i >= deg ) do
+        ai <- readPrimArray p i
+        let
+          bi :: a
+          !bi = ai + r * b
+        writePrimArray p i bi
+        go bi ( i + 1 )
+    a0 <- readPrimArray p 0
+    go a0 1
+
+-- | Forward deflation of a polynomial with real coefficients by a pair of complex-conjugate roots.
+deflateConjugatePair :: forall a m s. ( Num a, Prim a, PrimMonad m, s ~ PrimState m ) => Complex a -> MutablePrimArray s a -> m ()
+deflateConjugatePair ( x :+ y ) p = do
+  deg <- subtract 1 <$> getSizeofMutablePrimArray p
+  when ( deg >= 3 ) do
+    shrinkMutablePrimArray p ( deg - 1 )
+    let
+      c1, c2 :: a
+      !c1 = 2 * x
+      !c2 = x * x + y * y
+    a0 <- readPrimArray p 0
+    a1 <- readPrimArray p 1
+    let
+      b1 :: a
+      !b1 = a1 + c1 * a0
+    writePrimArray p 1 b1
+    let
+      go :: a -> a -> Int -> m ()
+      go !b !b' !i = unless ( i >= deg - 1 ) do
+        ai <- readPrimArray p i
+        let
+          bi :: a
+          !bi = ai + c1 * b - c2 * b'
+        writePrimArray p i bi
+        go bi b ( i + 1 )
+    go b1 a0 2
 
 -- | Laguerre's method.
 laguerre
   :: forall a m s
   .  ( RealFloat a, Prim a, PrimMonad m, s ~ PrimState m )
-  => a                                -- ^ error tolerance
-  -> Int                              -- ^ max number of iterations
-  -> MutablePrimArray s ( Complex a ) -- ^ polynomial
-  -> Complex a                        -- ^ initial point
+  => a                    -- ^ error tolerance
+  -> Int                  -- ^ max number of iterations
+  -> MutablePrimArray s a -- ^ polynomial
+  -> Complex a            -- ^ initial point
   -> m ( Complex a )
 laguerre eps maxIters p x0 = do
   p'  <- derivative p
@@ -150,11 +218,11 @@ laguerre eps maxIters p x0 = do
 laguerreStep
   :: forall a m s
   .  ( RealFloat a, Prim a, PrimMonad m, s ~ PrimState m )
-  => a                                -- ^ error tolerance
-  -> MutablePrimArray s ( Complex a ) -- ^ polynomial
-  -> MutablePrimArray s ( Complex a ) -- ^ first derivative of polynomial
-  -> MutablePrimArray s ( Complex a ) -- ^ second derivative of polynomial
-  -> Complex a                        -- ^ initial point
+  => a                    -- ^ error tolerance
+  -> MutablePrimArray s a -- ^ polynomial
+  -> MutablePrimArray s a -- ^ first derivative of polynomial
+  -> MutablePrimArray s a -- ^ second derivative of polynomial
+  -> Complex a            -- ^ initial point
   -> m ( Complex a )
 laguerreStep eps p p' p'' x = do
   n  <- fromIntegral @_ @a <$> getSizeofMutablePrimArray p
@@ -168,7 +236,8 @@ laguerreStep eps p p' p'' x = do
       g     = p'x / px
       g²    = g * g
       h     = g² - p''x / px
-      delta = sqrt $ ( n - 1 ) *: ( n *: h - g² )
+      mult  = 1 --max 1 . min n . fromIntegral . round @_ @Integer . realPart $ log p'x / log ( px / p'x )
+      delta = sqrt $ ( n - 1 ) *: ( ( n / mult ) *: h - g² )
       gp    = g + delta
       gm    = g - delta
       denom
@@ -182,24 +251,24 @@ laguerreStep eps p p' p'' x = do
     (*:) :: a -> Complex a -> Complex a
     r *: (u :+ v) = ( r * u ) :+ ( r * v )
 
--- | Evaluate a polynomial.
+-- | Evaluate a polynomial with real coefficients at a complex number.
 eval
   :: forall a m s
-  . ( Num a, Prim a, PrimMonad m, s ~ PrimState m )
-  => MutablePrimArray s a -> a -> m a
+  . ( RealFloat a, Prim a, PrimMonad m, s ~ PrimState m )
+  => MutablePrimArray s a -> Complex a -> m ( Complex a )
 eval p x = do
   n <- getSizeofMutablePrimArray p
   let
-    go :: a -> Int -> m a
+    go :: Complex a -> Int -> m ( Complex a )
     go !a !i
       | i >= n
       = pure a
       | otherwise
       = do
         b <- readPrimArray p i
-        go ( b + x * a ) ( i + 1 )
+        go ( ( b :+ 0 ) + x * a ) ( i + 1 )
   an <- readPrimArray p 0
-  go an 1
+  go ( an :+ 0 ) 1
 
 -- | Derivative of a polynomial.
 derivative