September 2017 Update

Task/Floyds-triangle/JavaScript/floyds-triangle-2.js

@@ -1,36 +1,97 @@
-#!/usr/bin/env js
+(() => {
+    'use strict';
 
-function main() {
-    print('Floyd 5:');
-    floyd(5);
-    print('\nFloyd 14:');
-    floyd(14);
-}
+    // FLOYD's TRIANGLE -------------------------------------------------------
+
+    // floyd :: Int -> [[Int]]
+    const floyd = n => snd(mapAccumL(
+        (start, row) => [start + row + 1, enumFromTo(start, start + row)],
+        1, enumFromTo(0, n - 1)
+    ));
+
+    // showFloyd :: [[Int]] -> String
+    const showFloyd = xss => {
+        const ws = map(compose([succ, length, show]), last(xss));
+        return unlines(
+            map(xs => concat(zipWith(
+                    (w, x) => justifyRight(w, ' ', show(x)), ws, xs
+                )),
+                xss
+            )
+        );
+    };
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // compose :: [(a -> a)] -> (a -> a)
+    const compose = fs => x => fs.reduceRight((a, f) => f(a), x);
+
+    // concat :: [[a]] -> [a] | [String] -> String
+    const concat = xs => {
+        if (xs.length > 0) {
+            const unit = typeof xs[0] === 'string' ? '' : [];
+            return unit.concat.apply(unit, xs);
+        } else return [];
+    };
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // justifyRight :: Int -> Char -> Text -> Text
+    const justifyRight = (n, cFiller, strText) =>
+        n > strText.length ? (
+            (cFiller.repeat(n) + strText)
+            .slice(-n)
+        ) : strText;
+
+    // last :: [a] -> a
+    const last = xs => xs.length ? xs.slice(-1)[0] : undefined;
+
+    // length :: [a] -> Int
+    const length = xs => xs.length;
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f)
+
+    // 'The mapAccumL function behaves like a combination of map and foldl;
+    // it applies a function to each element of a list, passing an accumulating
+    // parameter from left to right, and returning a final value of this
+    // accumulator together with the new list.' (See hoogle )
+
+    // mapAccumL :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
+    const mapAccumL = (f, acc, xs) =>
+        xs.reduce((a, x) => {
+            const pair = f(a[0], x);
+
+            return [pair[0], a[1].concat([pair[1]])];
+        }, [acc, []]);
+
+    // show ::
+    // (a -> String) f,  Num n =>
+    // a -> maybe f -> maybe n -> String
+    const show = JSON.stringify;
+
+    // snd :: (a, b) -> b
+    const snd = tpl => Array.isArray(tpl) ? tpl[1] : undefined;
+
+    // succ :: Int -> Int
+    const succ = x => x + 1
+
+    // unlines :: [String] -> String
+    const unlines = xs => xs.join('\n');
 
 
-function padLeft(s, w) {
-    for (s = String(s); s.length < w; s = ' ' + s);
-    return s;
-}
+    // zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
+    const zipWith = (f, xs, ys) => {
+        const ny = ys.length;
+        return (xs.length <= ny ? xs : xs.slice(0, ny))
+            .map((x, i) => f(x, ys[i]));
+    };
 
+    // TEST ( n=5 and n=14 rows ) ---------------------------------------------
 
-function floyd(nRows) {
-    var lowerLeft = nRows * (nRows - 1) / 2 + 1;
-    var lowerRight = nRows * (nRows + 1) / 2;
-
-    var colWidths = [];
-    for (var col = lowerLeft; col <= lowerRight; col++) {
-        colWidths.push(String(col).length);
-    }
-
-    var  num = 1;
-    for (var row = 0; row < nRows; row++) {
-        var line = [];
-        for (var col = 0; col <= row; col++, num++) {
-            line.push(padLeft(num, colWidths[col]));
-        }
-        print(line.join(' '));
-    }
-}
-
-main();
+    return unlines(map(n => showFloyd(floyd(n)) + '\n', [5, 14]))
+})();