From 3ddbb329ceda631449b1e5c4a79cefccd8a12be4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20Gillet?= <jie.gillet@gmail.com>
Date: Sat, 12 Jun 2021 13:56:27 +0900
Subject: [PATCH 1/4] [#726] New practice exercise

---
 config.json                                   | 15 ++++++
 .../practice/satellite/.docs/instructions.md  | 29 +++++++++++
 exercises/practice/satellite/.formatter.exs   |  4 ++
 .../practice/satellite/.meta/config.json      | 18 +++++++
 exercises/practice/satellite/.meta/example.ex | 41 +++++++++++++++
 exercises/practice/satellite/.meta/tests.toml | 22 ++++++++
 exercises/practice/satellite/lib/satellite.ex | 14 +++++
 exercises/practice/satellite/mix.exs          | 28 ++++++++++
 .../satellite/test/satellite_test.exs         | 52 +++++++++++++++++++
 .../practice/satellite/test/test_helper.exs   |  2 +
 10 files changed, 225 insertions(+)
 create mode 100644 exercises/practice/satellite/.docs/instructions.md
 create mode 100644 exercises/practice/satellite/.formatter.exs
 create mode 100644 exercises/practice/satellite/.meta/config.json
 create mode 100644 exercises/practice/satellite/.meta/example.ex
 create mode 100644 exercises/practice/satellite/.meta/tests.toml
 create mode 100644 exercises/practice/satellite/lib/satellite.ex
 create mode 100644 exercises/practice/satellite/mix.exs
 create mode 100644 exercises/practice/satellite/test/satellite_test.exs
 create mode 100644 exercises/practice/satellite/test/test_helper.exs

diff --git a/config.json b/config.json
index e910031b30..a5b7808e0a 100644
--- a/config.json
+++ b/config.json
@@ -2380,6 +2380,21 @@
         ],
         "difficulty": 9
       },
+      {
+        "slug": "satellite",
+        "name": "Satellite",
+        "uuid": "a07e01e6-7cea-4db9-a514-7c95788e90a9",
+        "prerequisites": [
+          "pattern-matching",
+          "tuple",
+          "cond",
+          "recursion"
+        ],
+        "practices": [
+          "recusrion"
+        ],
+        "difficulty": 6
+      },
       {
         "slug": "zebra-puzzle",
         "name": "Zebra Puzzle",
diff --git a/exercises/practice/satellite/.docs/instructions.md b/exercises/practice/satellite/.docs/instructions.md
new file mode 100644
index 0000000000..3a8077960f
--- /dev/null
+++ b/exercises/practice/satellite/.docs/instructions.md
@@ -0,0 +1,29 @@
+# Description
+
+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]
+
+```
+  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
diff --git a/exercises/practice/satellite/.formatter.exs b/exercises/practice/satellite/.formatter.exs
new file mode 100644
index 0000000000..d2cda26edd
--- /dev/null
+++ b/exercises/practice/satellite/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/exercises/practice/satellite/.meta/config.json b/exercises/practice/satellite/.meta/config.json
new file mode 100644
index 0000000000..ca834e65a6
--- /dev/null
+++ b/exercises/practice/satellite/.meta/config.json
@@ -0,0 +1,18 @@
+{
+  "blurb": "Rebuild binary trees from pre-order and in-order traversals.",
+  "authors": [
+    "jiegillet"
+  ],
+  "contributors": [],
+  "files": {
+    "solution": [
+      "lib/satellite.ex"
+    ],
+    "test": [
+      "test/satellite_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  }
+}
diff --git a/exercises/practice/satellite/.meta/example.ex b/exercises/practice/satellite/.meta/example.ex
new file mode 100644
index 0000000000..663a27566b
--- /dev/null
+++ b/exercises/practice/satellite/.meta/example.ex
@@ -0,0 +1,41 @@
+defmodule Satellite do
+  @typedoc """
+  A tree, which can be empty, or made from a left branch, a node and a right branch
+  """
+  @type tree :: {} | {tree, any, tree}
+
+  @doc """
+  Build a tree from the elements given in a pre-order and in-order style
+  """
+  @spec build_tree(preorder :: [any], inorder :: [any]) :: tree | {:error, String.t()}
+
+  def build_tree(preorder, inorder) do
+    p_length = length(preorder)
+    i_length = length(inorder)
+    p_set = MapSet.new(preorder)
+    i_set = MapSet.new(inorder)
+
+    cond do
+      p_length != i_length -> {:error, "traversals must have the same length"}
+      p_set != i_set -> {:error, "traversals must have the same elements"}
+      p_length != MapSet.size(p_set) -> {:error, "traversals must contain unique items"}
+      true -> build_tree(preorder, inorder, :safe)
+    end
+  end
+
+  def build_tree([], [], :safe), do: {}
+
+  def build_tree([root | preorder], inorder, :safe) do
+    {in_left, in_right, pre_left, pre_right} = split(root, inorder, preorder)
+
+    {build_tree(pre_left, in_left, :safe), root, build_tree(pre_right, in_right, :safe)}
+  end
+
+  defp split(root, [root | inorder], preorder), do: {[], inorder, [], preorder}
+
+  defp split(root, [i | inorder], [p | preorder]) do
+    {in_left, in_right, pre_left, pre_right} = split(root, inorder, preorder)
+
+    {[i | in_left], in_right, [p | pre_left], pre_right}
+  end
+end
diff --git a/exercises/practice/satellite/.meta/tests.toml b/exercises/practice/satellite/.meta/tests.toml
new file mode 100644
index 0000000000..2d5ff6015d
--- /dev/null
+++ b/exercises/practice/satellite/.meta/tests.toml
@@ -0,0 +1,22 @@
+# This is an auto-generated file. Regular comments will be removed when this
+# file is regenerated. Regenerating will not touch any manually added keys,
+# so comments can be added in 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"
+
diff --git a/exercises/practice/satellite/lib/satellite.ex b/exercises/practice/satellite/lib/satellite.ex
new file mode 100644
index 0000000000..12ff2bcf99
--- /dev/null
+++ b/exercises/practice/satellite/lib/satellite.ex
@@ -0,0 +1,14 @@
+defmodule Satellite do
+  @typedoc """
+  A tree, which can be empty, or made from a left branch, a node and a right branch
+  """
+  @type tree :: {} | {tree, any, tree}
+
+  @doc """
+  Build a tree from the elements given in a pre-order and in-order style
+  """
+  @spec build_tree(preorder :: [any], inorder :: [any]) :: tree | {:error, String.t()}
+
+  def build_tree(preorder, inorder) do
+  end
+end
diff --git a/exercises/practice/satellite/mix.exs b/exercises/practice/satellite/mix.exs
new file mode 100644
index 0000000000..370bdd443c
--- /dev/null
+++ b/exercises/practice/satellite/mix.exs
@@ -0,0 +1,28 @@
+defmodule Satellite.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :satellite,
+      version: "0.1.0",
+      # elixir: "~> 1.8",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end
diff --git a/exercises/practice/satellite/test/satellite_test.exs b/exercises/practice/satellite/test/satellite_test.exs
new file mode 100644
index 0000000000..33cee4cfa0
--- /dev/null
+++ b/exercises/practice/satellite/test/satellite_test.exs
@@ -0,0 +1,52 @@
+defmodule SatelliteTest do
+  use ExUnit.Case
+
+  # @tag :pending
+  test "Empty tree" do
+    preorder = []
+    inorder = []
+    tree = {}
+    assert Satellite.build_tree(preorder, inorder) == tree
+  end
+
+  @tag :pending
+  test "Tree with one item" do
+    preorder = [:a]
+    inorder = [:a]
+    tree = {{}, :a, {}}
+    assert Satellite.build_tree(preorder, inorder) == tree
+  end
+
+  @tag :pending
+  test "Tree with many items" do
+    preorder = ~w(a i x f r)a
+    inorder = ~w(i a f x r)a
+    tree = {{{}, :i, {}}, :a, {{{}, :f, {}}, :x, {{}, :r, {}}}}
+
+    assert Satellite.build_tree(preorder, inorder) == tree
+  end
+
+  @tag :pending
+  test "Reject traversals of different length" do
+    preorder = [:a, :b]
+    inorder = [:b, :a, :r]
+    error = {:error, "traversals must have the same length"}
+    assert Satellite.build_tree(preorder, inorder) == error
+  end
+
+  @tag :pending
+  test "Reject inconsistent traversals of same length" do
+    preorder = [:x, :y, :z]
+    inorder = [:a, :b, :c]
+    error = {:error, "traversals must have the same elements"}
+    assert Satellite.build_tree(preorder, inorder) == error
+  end
+
+  @tag :pending
+  test "Reject traversals with repeated items" do
+    preorder = [:a, :b, :a]
+    inorder = [:b, :a, :a]
+    error = {:error, "traversals must contain unique items"}
+    assert Satellite.build_tree(preorder, inorder) == error
+  end
+end
diff --git a/exercises/practice/satellite/test/test_helper.exs b/exercises/practice/satellite/test/test_helper.exs
new file mode 100644
index 0000000000..35fc5bff82
--- /dev/null
+++ b/exercises/practice/satellite/test/test_helper.exs
@@ -0,0 +1,2 @@
+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true)

From c432839213fffbd52d5a6f7b6d3571e04d7271c8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20Gillet?= <jie.gillet@gmail.com>
Date: Sat, 12 Jun 2021 22:41:02 +0900
Subject: [PATCH 2/4] Change type to {:ok, tree}

---
 exercises/practice/satellite/.meta/example.ex        | 4 ++--
 exercises/practice/satellite/lib/satellite.ex        | 2 +-
 exercises/practice/satellite/test/satellite_test.exs | 7 +++----
 3 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/exercises/practice/satellite/.meta/example.ex b/exercises/practice/satellite/.meta/example.ex
index 663a27566b..778a30e40c 100644
--- a/exercises/practice/satellite/.meta/example.ex
+++ b/exercises/practice/satellite/.meta/example.ex
@@ -7,7 +7,7 @@ defmodule Satellite do
   @doc """
   Build a tree from the elements given in a pre-order and in-order style
   """
-  @spec build_tree(preorder :: [any], inorder :: [any]) :: tree | {:error, String.t()}
+  @spec build_tree(preorder :: [any], inorder :: [any]) :: {:ok, tree} | {:error, String.t()}
 
   def build_tree(preorder, inorder) do
     p_length = length(preorder)
@@ -19,7 +19,7 @@ defmodule Satellite do
       p_length != i_length -> {:error, "traversals must have the same length"}
       p_set != i_set -> {:error, "traversals must have the same elements"}
       p_length != MapSet.size(p_set) -> {:error, "traversals must contain unique items"}
-      true -> build_tree(preorder, inorder, :safe)
+      true -> {:ok, build_tree(preorder, inorder, :safe)}
     end
   end
 
diff --git a/exercises/practice/satellite/lib/satellite.ex b/exercises/practice/satellite/lib/satellite.ex
index 12ff2bcf99..7768a85e9a 100644
--- a/exercises/practice/satellite/lib/satellite.ex
+++ b/exercises/practice/satellite/lib/satellite.ex
@@ -7,7 +7,7 @@ defmodule Satellite do
   @doc """
   Build a tree from the elements given in a pre-order and in-order style
   """
-  @spec build_tree(preorder :: [any], inorder :: [any]) :: tree | {:error, String.t()}
+  @spec build_tree(preorder :: [any], inorder :: [any]) :: {:ok, tree} | {:error, String.t()}
 
   def build_tree(preorder, inorder) do
   end
diff --git a/exercises/practice/satellite/test/satellite_test.exs b/exercises/practice/satellite/test/satellite_test.exs
index 33cee4cfa0..f4b393a84a 100644
--- a/exercises/practice/satellite/test/satellite_test.exs
+++ b/exercises/practice/satellite/test/satellite_test.exs
@@ -6,7 +6,7 @@ defmodule SatelliteTest do
     preorder = []
     inorder = []
     tree = {}
-    assert Satellite.build_tree(preorder, inorder) == tree
+    assert Satellite.build_tree(preorder, inorder) == {:ok, tree}
   end
 
   @tag :pending
@@ -14,7 +14,7 @@ defmodule SatelliteTest do
     preorder = [:a]
     inorder = [:a]
     tree = {{}, :a, {}}
-    assert Satellite.build_tree(preorder, inorder) == tree
+    assert Satellite.build_tree(preorder, inorder) == {:ok, tree}
   end
 
   @tag :pending
@@ -22,8 +22,7 @@ defmodule SatelliteTest do
     preorder = ~w(a i x f r)a
     inorder = ~w(i a f x r)a
     tree = {{{}, :i, {}}, :a, {{{}, :f, {}}, :x, {{}, :r, {}}}}
-
-    assert Satellite.build_tree(preorder, inorder) == tree
+    assert Satellite.build_tree(preorder, inorder) == {:ok, tree}
   end
 
   @tag :pending

From ee04547ca6fdb155098d986bdde3c145f8037cc9 Mon Sep 17 00:00:00 2001
From: Jie <jie.gillet@gmail.com>
Date: Sat, 12 Jun 2021 22:42:52 +0900
Subject: [PATCH 3/4] Typos and prerequisites

Co-authored-by: Angelika Tyborska <angelikatyborska@fastmail.com>
---
 config.json | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/config.json b/config.json
index a5b7808e0a..7e211a7840 100644
--- a/config.json
+++ b/config.json
@@ -2386,12 +2386,15 @@
         "uuid": "a07e01e6-7cea-4db9-a514-7c95788e90a9",
         "prerequisites": [
           "pattern-matching",
-          "tuple",
+          "tuples",
           "cond",
-          "recursion"
+          "recursion",
+          "atoms",
+          "lists",
+          "enum"
         ],
         "practices": [
-          "recusrion"
+          "recursion"
         ],
         "difficulty": 6
       },

From 178499b270422a39320d3d1520659b68924f3192 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=A9mie=20Gillet?= <jie.gillet@gmail.com>
Date: Sat, 12 Jun 2021 22:43:47 +0900
Subject: [PATCH 4/4] Update contributions

---
 exercises/practice/satellite/.meta/config.json | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/exercises/practice/satellite/.meta/config.json b/exercises/practice/satellite/.meta/config.json
index ca834e65a6..3cd9373242 100644
--- a/exercises/practice/satellite/.meta/config.json
+++ b/exercises/practice/satellite/.meta/config.json
@@ -3,7 +3,9 @@
   "authors": [
     "jiegillet"
   ],
-  "contributors": [],
+  "contributors": [
+    "angelikatyborska"
+  ],
   "files": {
     "solution": [
       "lib/satellite.ex"