2016 Update

Task/Matrix-multiplication/JavaScript/matrix-multiplication-2.js

@@ -0,0 +1,67 @@
+(function () {
+    'use strict';
+
+    // matrixMultiply:: [[n]] -> [[n]] -> [[n]]
+    function matrixMultiply(a, b) {
+        var bCols = transpose(b);
+
+        return a.map(function (aRow) {
+            return bCols.map(function (bCol) {
+                return dotProduct(aRow, bCol);
+            });
+        });
+    }
+
+    // [[n]] -> [[n]] -> [[n]]
+    function dotProduct(xs, ys) {
+        return sum(zipWith(product, xs, ys));
+    }
+
+    return matrixMultiply(
+        [[-1,  1,  4],
+         [ 6, -4,  2],
+         [-3,  5,  0],
+         [ 3,  7, -2]],
+
+        [[-1,  1,  4,  8],
+         [ 6,  9, 10,  2],
+         [11, -4,  5, -3]]
+    );
+
+    // --> [[51, -8, 26, -18], [-8, -38, -6, 34],
+    //        [33, 42, 38, -14], [17, 74, 72, 44]]
+
+
+    // GENERIC LIBRARY FUNCTIONS
+
+    // (a -> b -> c) -> [a] -> [b] -> [c]
+    function zipWith(f, xs, ys) {
+        return xs.length === ys.length ? (
+            xs.map(function (x, i) {
+                return f(x, ys[i]);
+            })
+        ) : undefined;
+    }
+
+    // [[a]] -> [[a]]
+    function transpose(lst) {
+        return lst[0].map(function (_, iCol) {
+            return lst.map(function (row) {
+                return row[iCol];
+            });
+        });
+    }
+
+    // sum :: (Num a) => [a] -> a
+    function sum(xs) {
+        return xs.reduce(function (a, x) {
+            return a + x;
+        }, 0);
+    }
+
+    // product :: n -> n -> n
+    function product(a, b) {
+        return a * b;
+    }
+
+})();