2016 Update

Task/Resistor-mesh/Haskell/resistor-mesh.hs

@@ -0,0 +1,31 @@
+{-# LANGUAGE ParallelListComp #-}
+import Numeric.LinearAlgebra (linearSolve, toDense, (!), flatten)
+import Data.Monoid ((<>), Sum(..))
+
+rMesh n (ar, ac) (br, bc)
+  | n < 2 = Nothing
+  | any (\x -> x < 1 || x > n) [ar, ac, br, bc] = Nothing
+  | otherwise = between a b <$> voltage
+  where
+    a = (ac - 1) + n*(ar - 1)
+    b = (bc - 1) + n*(br - 1)
+
+    between x y v = abs (v ! a - v ! b)
+
+    voltage = flatten <$> linearSolve matrixG current
+
+    matrixG = toDense $ concat [ element row col node
+                               | row <- [1..n], col <- [1..n]
+                               | node <- [0..] ]
+
+    element row col node =
+      let (Sum c, elements) =
+            (Sum 1, [((node, node-n), -1)]) `when` (row > 1) <>
+            (Sum 1, [((node, node+n), -1)]) `when` (row < n) <>
+            (Sum 1, [((node, node-1), -1)]) `when` (col > 1) <>
+            (Sum 1, [((node, node+1), -1)]) `when` (col < n)
+      in [((node, node), c)] <> elements
+
+    x `when` p = if p then x else mempty
+
+    current  = toDense [ ((a, 0), -1) , ((b, 0),  1) , ((n^2-1, 0), 0) ]