September 2017 Update

Task/Equilibrium-index/JavaScript/equilibrium-index-2.js

@@ -0,0 +1,93 @@
+(() => {
+    // EQUILIBRIUM INDICES ----------------------------------------------------
+
+    // equilibriumIndices :: [Int] -> [Int]
+    const equilibriumIndices = xs =>
+        foldr((a, [x, y], i) =>
+            x === y ? cons(i, a) : a,
+            [],
+            zip(
+                scanl1(plus, xs), // Sums from the left
+                scanr1(plus, xs)  // Sums from the right
+            )
+        );
+
+    // GENERIC FUNCTIONS ------------------------------------------------------
+
+    // cons :: a -> [a] -> [a]
+    const cons = (x, xs) => [x].concat(xs);
+
+    // foldr (a -> b -> a) -> a -> [b] -> a
+    const foldr = (f, a, xs) => xs.reduceRight(f, a);
+
+    // map :: (a -> b) -> [a] -> [b]
+    const map = (f, xs) => xs.map(f);
+
+    // plus :: Num a => a -> a -> a
+    const plus = (a, b) => a + b;
+
+    // scanl :: (b -> a -> b) -> b -> [a] -> [b]
+    const scanl = (f, startValue, xs) =>
+        xs.reduce((a, x) => {
+            const v = f(a.acc, x);
+            return {
+                acc: v,
+                scan: a.scan.concat(v)
+            };
+        }, {
+            acc: startValue,
+            scan: [startValue]
+        })
+        .scan;
+
+    // scanl1 :: (a -> a -> a) -> [a] -> [a]
+    const scanl1 = (f, xs) =>
+        xs.length > 0 ? scanl(f, xs[0], xs.slice(1)) : [];
+
+    // scanr :: (b -> a -> b) -> b -> [a] -> [b]
+    const scanr = (f, startValue, xs) =>
+        xs.reduceRight((a, x) => {
+            const v = f(a.acc, x);
+            return {
+                acc: v,
+                scan: [v].concat(a.scan)
+            };
+        }, {
+            acc: startValue,
+            scan: [startValue]
+        })
+        .scan;
+
+    // scanr1 :: (a -> a -> a) -> [a] -> [a]
+    const scanr1 = (f, xs) =>
+        xs.length > 0 ? scanr(f, xs.slice(-1)[0], xs.slice(0, -1)) : [];
+
+    // Any value -> optional number of indents -> String
+    // show :: a -> String
+    // show :: a -> Int -> String
+    const show = (...x) =>
+        JSON.stringify.apply(
+            null, x.length > 1 ? [x[0], null, x[1]] : x
+        );
+
+    // tail :: [a] -> [a]
+    const tail = xs => xs.length ? xs.slice(1) : undefined;
+
+    // zip :: [a] -> [b] -> [(a,b)]
+    const zip = (xs, ys) =>
+        xs.slice(0, Math.min(xs.length, ys.length))
+        .map((x, i) => [x, ys[i]]);
+
+    // TEST -------------------------------------------------------------------
+    return show(
+        map(equilibriumIndices, [
+            [-7, 1, 5, 2, -4, 3, 0],
+            [2, 4, 6],
+            [2, 9, 2],
+            [1, -1, 1, -1, 1, -1, 1],
+            [1],
+            []
+        ])
+    );
+    // -> [[3, 6], [], [1], [0, 1, 2, 3, 4, 5, 6], [0], []]
+})();

Task/Equilibrium-index/JavaScript/equilibrium-index-3.js

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