Initial data commit

Task/Tree-traversal/JavaScript/tree-traversal-6.js

@@ -0,0 +1,128 @@
+(() => {
+    "use strict";
+
+    // preorder :: a -> [[a]] -> [a]
+    const preorder = x =>
+        xs => [x, ...xs.flat()];
+
+
+    // inorder :: a -> [[a]] -> [a]
+    const inorder = x =>
+        xs => Boolean(xs.length) ? (
+            [...xs[0], x, ...xs.slice(1).flat()]
+        ) : [x];
+
+
+    // postorder :: a -> [[a]] -> [a]
+    const postorder = x =>
+        xs => [...xs.flat(), x];
+
+
+    // levelOrder :: Tree a -> [a]
+    const levelOrder = tree =>
+        levels(tree).flat();
+
+
+    // ------------------------TEST------------------------
+    // main :: IO ()
+    const main = () => {
+        const tree = Node(1)([
+            Node(2)([
+                Node(4)([
+                    Node(7)([])
+                ]),
+                Node(5)([])
+            ]),
+            Node(3)([
+                Node(6)([
+                    Node(8)([]),
+                    Node(9)([])
+                ])
+            ])
+        ]);
+
+        // Generated by code in Rosetta Code
+        // task: 'Visualize a tree'
+        console.log([
+            "       + 4 - 7",
+            "   + 2 ¦",
+            "   ¦   + 5",
+            " 1 ¦",
+            "   ¦       + 8",
+            "   + 3 - 6 ¦",
+            "           + 9"
+        ].join("\n"));
+
+        [preorder, inorder, postorder]
+        .forEach(f => console.log(
+            justifyRight(11)(" ")(`${f.name}:`),
+            foldTree(f)(
+                tree
+            )
+        ));
+
+        console.log(
+            `levelOrder: ${levelOrder(tree)}`
+        );
+    };
+
+    // ---------------------- TREES ----------------------
+
+    // Node :: a -> [Tree a] -> Tree a
+    const Node = v =>
+        // Constructor for a Tree node which connects a
+        // value of some kind to a list of zero or
+        // more child trees.
+        xs => ({
+            type: "Node",
+            root: v,
+            nest: xs || []
+        });
+
+
+    // foldTree :: (a -> [b] -> b) -> Tree a -> b
+    const foldTree = f => {
+        // The catamorphism on trees. A summary
+        // value obtained by a depth-first fold.
+        const go = tree => f(
+            tree.root
+        )(
+            tree.nest.map(go)
+        );
+
+        return go;
+    };
+
+
+    // levels :: Tree a -> [[a]]
+    const levels = tree => {
+        // A list of lists, grouping the root
+        // values of each level of the tree.
+        const go = (a, node) => {
+            const [h, ...t] = 0 < a.length ? a : [
+                []
+            ];
+
+            return [
+                [node.root, ...h],
+                ...node.nest.reduceRight(go, t)
+            ];
+        };
+
+        return go([], tree);
+    };
+
+
+    // --------------------- GENERIC ---------------------
+
+    // justifyRight :: Int -> Char -> String -> String
+    const justifyRight = n =>
+        // The string s, preceded by enough padding (with
+        // the character c) to reach the string length n.
+        c => s => Boolean(s) ? (
+            s.padStart(n, c)
+        ) : "";
+
+    // MAIN ---
+    return main();
+})();