Data commit

Task/Sum-of-elements-below-main-diagonal-of-matrix/JavaScript/sum-of-elements-below-main-diagonal-of-matrix.js

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+(() => {
+    "use strict";
+
+    // -------- LOWER TRIANGLE OF A SQUARE MATRIX --------
+
+    // lowerTriangle :: [[a]] -> Either String [[a]]
+    const lowerTriangle = matrix =>
+        // Either a message, if the matrix is not square,
+        // or the lower triangle of the matrix.
+        isSquare(matrix) ? (
+            Right(
+                matrix.reduce(
+                    ([n, rows], xs) => [
+                        1 + n,
+                        rows.concat([xs.slice(0, n)])
+                    ],
+                    [0, []]
+                )[1]
+            )
+        ) : Left("Not a square matrix");
+
+
+    // isSquare :: [[a]] -> Bool
+    const isSquare = rows => {
+        // True if the length of every row in the matrix
+        // matches the number of rows in the matrix.
+        const n = rows.length;
+
+        return rows.every(x => n === x.length);
+    };
+
+    // ---------------------- TEST -----------------------
+    const main = () =>
+        either(
+            msg => `Lower triangle undefined :: ${msg}`
+        )(
+            rows => sum([].concat(...rows))
+        )(
+            lowerTriangle([
+                [1, 3, 7, 8, 10],
+                [2, 4, 16, 14, 4],
+                [3, 1, 9, 18, 11],
+                [12, 14, 17, 18, 20],
+                [7, 1, 3, 9, 5]
+            ])
+        );
+
+    // --------------------- GENERIC ---------------------
+
+    // Left :: a -> Either a b
+    const Left = x => ({
+        type: "Either",
+        Left: x
+    });
+
+
+    // Right :: b -> Either a b
+    const Right = x => ({
+        type: "Either",
+        Right: x
+    });
+
+
+    // either :: (a -> c) -> (b -> c) -> Either a b -> c
+    const either = fl =>
+        // Application of the function fl to the
+        // contents of any Left value in e, or
+        // the application of fr to its Right value.
+        fr => e => e.Left ? (
+            fl(e.Left)
+        ) : fr(e.Right);
+
+
+    // sum :: [Num] -> Num
+    const sum = xs =>
+        // The numeric sum of all values in xs.
+        xs.reduce((a, x) => a + x, 0);
+
+    // MAIN ---
+    return main();
+})();