Add satellite exercise

config.json

@@ -780,6 +780,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "satellite",
+        "name": "Satellite",
+        "uuid": "02d23d01-c4c6-4fd1-9e0b-f6a457c402ed",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
+      },
       {
         "slug": "knapsack",
         "name": "Knapsack",

exercises/practice/satellite/.docs/instructions.md

@@ -0,0 +1,27 @@
+# Instructions
+
+Imagine you need to transmit a binary tree to a satellite approaching Alpha Centauri and you have limited bandwidth.
+Since the tree has no repeating items it can be uniquely represented by its [pre-order and in-order traversals][wiki].
+
+Write the software for the satellite to rebuild the tree from the traversals.
+
+A pre-order traversal reads the value of the current node before (hence "pre") reading the left subtree in pre-order.
+Afterwards the right subtree is read in pre-order.
+
+An in-order traversal reads the left subtree in-order then the current node and finally the right subtree in-order.
+So in order from left to right.
+
+For example the pre-order traversal of this tree is [a, i, x, f, r].
+The in-order traversal of this tree is [i, a, f, x, r]
+
+```text
+  a
+ / \
+i   x
+   / \
+  f   r
+```
+
+Note: the first item in the pre-order traversal is always the root.
+
+[wiki]: https://en.wikipedia.org/wiki/Tree_traversal

exercises/practice/satellite/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "satellite.el"
+    ],
+    "test": [
+      "satellite-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Rebuild binary trees from pre-order and in-order traversals."
+}

exercises/practice/satellite/.meta/example.el

@@ -0,0 +1,31 @@
+;;; satellite.el --- Satellite (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+(require 'seq)
+
+(defun tree-from-traversals (preorder inorder)
+  (when (/= (length preorder) (length inorder))
+    (error "traversals must have the same length"))
+  (when (or (/= (length preorder) (length (seq-uniq preorder)))
+            (/= (length inorder) (length (seq-uniq inorder))))
+    (error "traversals must contain unique items"))
+  (cl-labels
+    ((traverse (successor)
+       (cond
+         ((and successor (equal (car inorder) successor)) nil)
+         ((null preorder) (error "traversals must have the same elements"))
+         (t (let ((v (car preorder)))
+              (setq preorder (cdr preorder))
+              (let ((l (funcall #'traverse v)))
+                (setq inorder (cdr inorder))
+                (let ((r (and inorder (funcall #'traverse successor))))
+                  (list (cons :v v) (cons :l l) (cons :r r)))))))))
+    (and preorder (funcall #'traverse nil))))
+
+(provide 'satellite)
+;;; satellite.el ends here
+

exercises/practice/satellite/.meta/tests.toml

@@ -0,0 +1,28 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[8df3fa26-811a-4165-9286-ff9ac0850d19]
+description = "Empty tree"
+
+[f945ccfc-05e3-47d7-825b-0270559d43ad]
+description = "Tree with one item"
+
+[a0121d5f-37b0-48dd-9c64-cba4c4464135]
+description = "Tree with many items"
+
+[6074041f-4891-4d81-a128-401050c2a3b0]
+description = "Reject traversals of different length"
+
+[27916ce4-45f3-4d8b-8528-496fedc157ca]
+description = "Reject inconsistent traversals of same length"
+
+[d86a3d72-76a9-43b5-9d3a-e64cb1216035]
+description = "Reject traversals with repeated items"

exercises/practice/satellite/satellite-test.el

@@ -0,0 +1,52 @@
+;;; satellite-test.el --- Tests for Satellite (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(load-file "satellite.el")
+(declare-function tree-from-traversals "satellite.el" (preorder inorder))
+
+
+(ert-deftest empty-tree ()
+  (should (equal nil
+            (tree-from-traversals '() '()))))
+
+
+(ert-deftest tree-with-one-item ()
+  (should (equal '((:v . "a")
+                   (:l . nil)
+                   (:r . nil))
+            (tree-from-traversals '("a") '("a")))))
+
+
+(ert-deftest tree-with-many-items ()
+  (should (equal '((:v . "a")
+                   (:l . ((:v . "i")
+                          (:l . nil)
+                          (:r . nil)))
+                   (:r . ((:v . "x")
+                          (:l . ((:v . "f")
+                                 (:l . nil)
+                                 (:r . nil)))
+                          (:r . ((:v . "r")
+                                 (:l . nil)
+                                 (:r . nil))))))
+            (tree-from-traversals '("a" "i" "x" "f" "r") '("i" "a" "f" "x" "r")))))
+
+
+(ert-deftest reject-traversals-of-different-length ()
+  (should-error (tree-from-traversals '("a" "b") '("b" "a" "r"))))
+
+
+(ert-deftest reject-inconsistent-traversals-of-same-length ()
+  (should-error (tree-from-traversals '("x" "y" "z") '("a" "b" "c"))))
+
+
+(ert-deftest reject-traversals-with-repeated-items ()
+  (should-error (tree-from-traversals '("a" "b" "a") '("b" "a" "a"))))
+
+
+(provide 'satellite-test)
+;;; satellite-test.el ends here

exercises/practice/satellite/satellite.el

@@ -0,0 +1,14 @@
+;;; satellite.el --- Satellite (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun tree-from-traversals (preorder inorder)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'satellite)
+;;; satellite.el ends here
+