September 2017 Update

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-4.js

@@ -0,0 +1,52 @@
+(() => {
+    'use strict';
+
+    // QUICKSELECT ------------------------------------------------------------
+
+    // quickselect :: Ord a => Int -> [a] -> a
+    const quickSelect = (k, xxs) => {
+        const
+            [x, xs] = uncons(xxs),
+            [ys, zs] = partition(v => v < x, xs),
+            l = length(ys);
+
+        return (k < l) ? (
+            quickSelect(k, ys)
+        ) : (k > l) ? (
+            quickSelect(k - l - 1, zs)
+        ) : x;
+    };
+
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
+
+    // length :: [a] -> Int
+    const length = xs => xs.length;
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // partition :: Predicate -> List -> (Matches, nonMatches)
+    // partition :: (a -> Bool) -> [a] -> ([a], [a])
+    const partition = (p, xs) =>
+        xs.reduce((a, x) =>
+            p(x) ? [a[0].concat(x), a[1]] : [a[0], a[1].concat(x)], [
+                [],
+                []
+            ]);
+
+    // uncons :: [a] -> Maybe (a, [a])
+    const uncons = xs => xs.length ? [xs[0], xs.slice(1)] : undefined;
+
+
+    // TEST -------------------------------------------------------------------
+    const v = [9, 8, 7, 6, 5, 0, 1, 2, 3, 4];
+
+    return map(i => quickSelect(i, v), enumFromTo(0, length(v) - 1));
+})();