September 2017 Update

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-1.js

@@ -0,0 +1,71 @@
+// this just helps make partition read better
+function swap(items, firstIndex, secondIndex) {
+  var temp = items[firstIndex];
+  items[firstIndex] = items[secondIndex];
+  items[secondIndex] = temp;
+};
+
+// many algorithms on this page violate
+// the constraint that partition operates in place
+function partition(array, from, to) {
+  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random
+  var pivotIndex = getRandomInt(from, to),
+      pivot = array[pivotIndex];
+  swap(array, pivotIndex, to);
+  pivotIndex = from;
+
+  for(var i = from; i <= to; i++) {
+    if(array[i] < pivot) {
+      swap(array, pivotIndex, i);
+      pivotIndex++;
+    }
+  };
+  swap(array, pivotIndex, to);
+
+  return pivotIndex;
+};
+
+// later versions of JS have TCO so this is safe
+function quickselectRecursive(array, from, to, statistic) {
+  if(array.length === 0 || statistic > array.length - 1) {
+    return undefined;
+  };
+
+  var pivotIndex = partition(array, from, to);
+  if(pivotIndex === statistic) {
+    return array[pivotIndex];
+  } else if(pivotIndex < statistic) {
+    return quickselectRecursive(array, pivotIndex, to, statistic);
+  } else if(pivotIndex > statistic) {
+    return quickselectRecursive(array, from, pivotIndex, statistic);
+  }
+};
+
+function quickselectIterative(array, k) {
+  if(array.length === 0 || k > array.length - 1) {
+    return undefined;
+  };
+
+  var from = 0, to = array.length,
+      pivotIndex = partition(array, from, to);
+
+  while(pivotIndex !== k) {
+    pivotIndex = partition(array, from, to);
+    if(pivotIndex < k) {
+      from = pivotIndex;
+    } else if(pivotIndex > k) {
+      to = pivotIndex;
+    }
+  };
+
+  return array[pivotIndex];
+};
+
+KthElement = {
+  find: function(array, element) {
+    var k = element - 1;
+    return quickselectRecursive(array, 0, array.length, k);
+    // you can also try out the Iterative version
+    // return quickselectIterative(array, k);
+  }
+}

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-2.js

@@ -0,0 +1,3 @@
+var array = [9, 8, 7, 6, 5, 0, 1, 2, 3, 4],
+    ks = Array.apply(null, {length: 10}).map(Number.call, Number);
+ks.map(k => { KthElement.find(array, k) });

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-3.js

@@ -0,0 +1 @@
+[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

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));
+})();

Task/Quickselect-algorithm/JavaScript/quickselect-algorithm-5.js

@@ -0,0 +1 @@
+[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]