September 2017 Update

Task/Five-weekends/JavaScript/five-weekends-3.js

@@ -1,7 +1,6 @@
 (function () {
     'use strict';
 
-
     // longMonthsStartingFriday :: Int -> Int
     function longMonthsStartingFriday(y) {
         return [0, 2, 4, 6, 7, 9, 11]
@@ -11,7 +10,6 @@
             });
     }
 
-
     // range :: Int -> Int -> [Int]
     function range(m, n) {
         return Array.apply(null, Array(n - m + 1))
@@ -53,5 +51,4 @@
         },
         null, 2
     );
-
 })();

Task/Five-weekends/JavaScript/five-weekends-4.js

@@ -0,0 +1,60 @@
+(() => {
+    // longMonthsStartingFriday :: Int -> [Int]
+    const longMonthsStartingFriday = y =>
+        filter(m => (new Date(Date.UTC(y, m, 1)))
+            .getDay() === 5, [0, 2, 4, 6, 7, 9, 11]);
+
+    // Years -> YearMonths
+    // fullMonths :: [Int] -> [String]
+    const fullMonths = xs =>
+        foldl((a, y) => a.concat(
+            map(m => `${y.toString()} ${[
+                            'January', '', 'March', '', 'May', '',
+                            'July', 'August', '', 'October', '', 'December'
+                        ][m]}`, longMonthsStartingFriday(y))
+        ), [], xs);
+
+    // leanYears :: [Int] -> [Int]
+    const leanYears = years =>
+        filter(y => longMonthsStartingFriday(y)
+            .length === 0, years);
+
+    // GENERIC ----------------------------------------------------------------
+
+    // A list of functions applied to a list of arguments
+    // <*> :: [(a -> b)] -> [a] -> [b]
+    const ap = (fs, xs) => //
+        [].concat.apply([], fs.map(f => //
+            [].concat.apply([], xs.map(x => [f(x)]))));
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // filter :: (a -> Bool) -> [a] -> [a]
+    const filter = (f, xs) => xs.filter(f);
+
+    // foldl :: (b -> a -> b) -> b -> [a] -> b
+    const foldl = (f, a, xs) => xs.reduce(f, a);
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // show :: a -> String
+    const show = x => JSON.stringify(x, null, 2);
+
+
+    // TEST -------------------------------------------------------------------
+    const [lstFullMonths, lstLeanYears] = ap(
+        [fullMonths, leanYears], [enumFromTo(1900, 2100)]
+    );
+
+    return show({
+        number: lstFullMonths.length,
+        firstFive: lstFullMonths.slice(0, 5),
+        lastFive: lstFullMonths.slice(-5),
+        leanYearCount: lstLeanYears.length
+    });
+})();