1 files changed, 74 insertions, 2 deletions

diff --git a/packages/base/src/Internal/Algorithms.hs b/packages/base/src/Internal/Algorithms.hs
index 23a5e13..99c9e34 100644
--- a/packages/base/src/Internal/Algorithms.hs
+++ b/packages/base/src/Internal/Algorithms.hs
@@ -62,6 +62,7 @@ class (Numeric t,
     linearSolve' :: Matrix t -> Matrix t -> Matrix t
     cholSolve'   :: Matrix t -> Matrix t -> Matrix t
     triSolve'   :: UpLo -> Matrix t -> Matrix t -> Matrix t
+    triDiagSolve' :: Vector t -> Vector t -> Vector t -> Matrix t -> Matrix t
     ldlPacked'   :: Matrix t -> (Matrix t, [Int])
     ldlSolve'    :: (Matrix t, [Int]) -> Matrix t -> Matrix t
     linearSolveSVD' :: Matrix t -> Matrix t -> Matrix t
@@ -88,6 +89,7 @@ instance Field Double where
     mbLinearSolve' = mbLinearSolveR
     cholSolve' = cholSolveR
     triSolve' = triSolveR
+    triDiagSolve' = triDiagSolveR
     linearSolveLS' = linearSolveLSR
     linearSolveSVD' = linearSolveSVDR Nothing
     eig' = eigR
@@ -118,6 +120,7 @@ instance Field (Complex Double) where
     mbLinearSolve' = mbLinearSolveC
     cholSolve' = cholSolveC
     triSolve' = triSolveC
+    triDiagSolve' = triDiagSolveC
     linearSolveLS' = linearSolveLSC
     linearSolveSVD' = linearSolveSVDC Nothing
     eig' = eigC
@@ -356,10 +359,79 @@ cholSolve
     -> Matrix t -- ^ solution
 cholSolve = {-# SCC "cholSolve" #-} cholSolve'
 
--- | Solve a triangular linear system.
-triSolve :: Field t => UpLo -> Matrix t -> Matrix t -> Matrix t
+-- | Solve a triangular linear system. If `Upper` is specified then
+-- all elements below the diagonal are ignored; if `Lower` is
+-- specified then all elements above the diagonal are ignored.
+triSolve
+  :: Field t
+  => UpLo     -- ^ `Lower` or `Upper`
+  -> Matrix t -- ^ coefficient matrix
+  -> Matrix t -- ^ right hand sides
+  -> Matrix t -- ^ solution
 triSolve = {-# SCC "triSolve" #-} triSolve'
 
+-- | Solve a tridiagonal linear system. Suppose you wish to solve \(Ax = b\) where
+--
+-- \[
+-- A =
+-- \begin{bmatrix}
+--    1.0 & 4.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0
+-- \\ 3.0 & 1.0 & 4.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0
+-- \\ 0.0 & 3.0 & 1.0 & 4.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0
+-- \\ 0.0 & 0.0 & 3.0 & 1.0 & 4.0 & 0.0 & 0.0 & 0.0 & 0.0
+-- \\ 0.0 & 0.0 & 0.0 & 3.0 & 1.0 & 4.0 & 0.0 & 0.0 & 0.0
+-- \\ 0.0 & 0.0 & 0.0 & 0.0 & 3.0 & 1.0 & 4.0 & 0.0 & 0.0
+-- \\ 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 3.0 & 1.0 & 4.0 & 0.0
+-- \\ 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 3.0 & 1.0 & 4.0
+-- \\ 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 3.0 & 1.0
+-- \end{bmatrix}
+-- \quad
+-- b =
+-- \begin{bmatrix}
+--    1.0 &  1.0 &  1.0
+-- \\ 1.0 & -1.0 &  2.0
+-- \\ 1.0 &  1.0 &  3.0
+-- \\ 1.0 & -1.0 &  4.0
+-- \\ 1.0 &  1.0 &  5.0
+-- \\ 1.0 & -1.0 &  6.0
+-- \\ 1.0 &  1.0 &  7.0
+-- \\ 1.0 & -1.0 &  8.0
+-- \\ 1.0 &  1.0 &  9.0
+-- \end{bmatrix}
+-- \]
+--
+-- then
+--
+-- @
+-- dL =  fromList [3.0, 3.0, 3.0, 3.0, 3.0, 3.0, 3.0, 3.0]
+-- d  =  fromList [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0]
+-- dU =  fromList [4.0, 4.0, 4.0, 4.0, 4.0, 4.0, 4.0, 4.0]
+--
+-- b = (9><3)
+--     [
+--       1.0,   1.0,   1.0,
+--       1.0,  -1.0,   2.0,
+--       1.0,   1.0,   3.0,
+--       1.0,  -1.0,   4.0,
+--       1.0,   1.0,   5.0,
+--       1.0,  -1.0,   6.0,
+--       1.0,   1.0,   7.0,
+--       1.0,  -1.0,   8.0,
+--       1.0,   1.0,   9.0
+--     ]
+--
+-- x = triDiagSolve dL d dU b
+-- @
+--
+triDiagSolve
+  :: Field t
+  => Vector t -- ^ lower diagonal: \(n - 1\) elements
+  -> Vector t -- ^ diagonal: \(n\) elements
+  -> Vector t -- ^ upper diagonal: \(n - 1\) elements
+  -> Matrix t -- ^ right hand sides
+  -> Matrix t -- ^ solution
+triDiagSolve = {-# SCC "triDiagSolve" #-} triDiagSolve'
+
 -- | Minimum norm solution of a general linear least squares problem Ax=B using the SVD. Admits rank-deficient systems but it is slower than 'linearSolveLS'. The effective rank of A is determined by treating as zero those singular valures which are less than 'eps' times the largest singular value.
 linearSolveSVD :: Field t => Matrix t -> Matrix t -> Matrix t
 linearSolveSVD = {-# SCC "linearSolveSVD" #-} linearSolveSVD'