Add satellite

Author: BNAndras

config.json

@@ -926,6 +926,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 6
+      },
+      {
+        "slug": "satellite",
+        "name": "Satellite",
+        "uuid": "b09b1106-6ca1-49e2-8cb6-8dcf86f37675",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },
@@ -984,4 +992,4 @@
     "used_for/scripts",
     "used_for/web_development"
   ]
-}
+}

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 +1,20 @@
+{
+  "authors": [
+    "lpizzinidev"
+  ],
+  "contributors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "satellite.coffee"
+    ],
+    "test": [
+      "satellite.spec.coffee"
+    ],
+    "example": [
+      ".meta/example.coffee"
+    ]
+  },
+  "blurb": "Rebuild binary trees from pre-order and in-order traversals."
+}

exercises/practice/satellite/.meta/example.coffee

@@ -0,0 +1,37 @@
+class Satellite
+  @treeFromTraversals: (preorder, inorder) ->
+    if preorder.length != inorder.length
+        throw new Error 'traversals must have the same length'
+    
+    uniquePreorder = new Set preorder
+    uniqueInorder = new Set inorder
+
+    if uniquePreorder.size != preorder.length or uniqueInorder.size != inorder.length
+        throw new Error 'traversals must contain unique items'
+    
+    preorderSorted = preorder.slice().sort().join(',')
+    inorderSorted = inorder.slice().sort().join(',')
+
+    if preorderSorted != inorderSorted
+        throw new Error 'traversals must have the same elements'
+    
+    if preorder.length == 0
+        return {}
+
+    head = preorder[0]
+    preorderRest = preorder.slice 1
+    inorderIndex = inorder.indexOf head
+
+    leftInorder = inorder.slice 0, inorderIndex
+    rightInorder = inorder.slice inorderIndex + 1
+
+    leftPreorder = preorderRest.filter node -> leftInorder.includes node
+    rightPreorder = preorderRest.filter node -> rightInorder.includes node
+
+    {
+        value: head
+        left: @treeFromTraversals leftPreorder, leftInorder
+        right: @treeFromTraversals rightPreorder, rightInorder
+    }
+
+module.exports = Satellite

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

@@ -0,0 +1,37 @@
+# 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"
+
+[af31ae02-7e5b-4452-a990-bccb3fca9148]
+description = "A degenerate binary tree"
+
+[ee54463d-a719-4aae-ade4-190d30ce7320]
+description = "Another degenerate binary tree"
+
+[87123c08-c155-4486-90a4-e2f75b0f3e8f]
+description = "Tree with many more items"

exercises/practice/satellite/satellite.coffee

@@ -0,0 +1,4 @@
+class Satellite
+  @treeFromTraversals: (preorder, inorder) ->
+
+module.exports = Satellite

exercises/practice/satellite/satellite.spec.coffee

@@ -0,0 +1,121 @@
+Satellite = require './satellite'
+
+describe 'Satellite', ->
+  it 'Empty tree', ->
+    preorder = []
+    inorder = []
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual {}
+
+  xit 'Tree with one item', ->
+    preorder = ['a']
+    inorder = ['a']
+    expected =
+      value: 'a'
+      left: {}
+      right: {}
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual expected
+
+  xit 'Tree with many items', ->
+    preorder = ['a', 'i', 'x', 'f', 'r']
+    inorder = ['i', 'a', 'f', 'x', 'r']
+    expected =
+      value: 'a'
+      left:
+        value: 'i'
+        left: {}
+        right: {}
+      right:
+        value: 'x'
+        left:
+          value: 'f'
+          left: {}
+          right: {}
+        right:
+          value: 'r'
+          left: {}
+          right: {}
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual expected
+
+  xit 'Reject traversals of different length', ->
+    preorder = ['a', 'b']
+    inorder = ['b', 'a', 'r']
+    expect(-> Satellite.treeFromTraversals(preorder, inorder)).toThrow new Error 'traversals must have the same length'
+
+  xit 'Reject inconsistent traversals of same length', ->
+    preorder = ['x', 'y', 'z']
+    inorder = ['a', 'b', 'c']
+    expect(-> Satellite.treeFromTraversals(preorder, inorder)).toThrow new Error 'traversals must have the same elements'
+
+  xit 'Reject traversals with repeated items', ->
+    preorder = ['a', 'b', 'a']
+    inorder = ['b', 'a', 'a']
+    expect(-> Satellite.treeFromTraversals(preorder, inorder)).toThrow new Error 'traversals must contain unique items'
+
+  xit 'A degenerate binary tree', ->
+    preorder = ['a', 'b', 'c', 'd']
+    inorder = ['d', 'c', 'b', 'a']
+    expected =
+      value: 'a'
+      left:
+        value: 'b'
+        left:
+          value: 'c'
+          left:
+            value: 'd'
+            left: {}
+            right: {}
+          right: {}
+        right: {}
+      right: {}
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual expected
+
+  xit 'Another degenerate binary tree', ->
+    preorder = ['a', 'b', 'c', 'd']
+    inorder = ['a', 'b', 'c', 'd']
+    expected =
+      value: 'a'
+      left: {}
+      right:
+        value: 'b'
+        left: {}
+        right:
+          value: 'c'
+          left: {}
+          right:
+            value: 'd'
+            left: {}
+            right: {}
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual expected
+
+  xit 'Tree with many more items', ->
+    preorder = ['a', 'b', 'd', 'g', 'h', 'c', 'e', 'f', 'i']
+    inorder = ['g', 'd', 'h', 'b', 'a', 'e', 'c', 'i', 'f']
+    expected =
+      value: 'a'
+      left:
+        value: 'b'
+        left:
+          value: 'd'
+          left:
+            value: 'g'
+            left: {}
+            right: {}
+          right:
+            value: 'h'
+            left: {}
+            right: {}
+        right: {}
+      right:
+        value: 'c'
+        left:
+          value: 'e'
+          left: {}
+          right: {}
+        right:
+          value: 'f'
+          left:
+            value: 'i'
+            left: {}
+            right: {}
+          right: {}
+    expect(Satellite.treeFromTraversals(preorder, inorder)).toEqual expected