New concept exercise top-secret (concept: ast)

exercises/concept/top-secret/.docs/hints.md

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+# Hints
+
+## General

exercises/concept/top-secret/.docs/instructions.md

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+# Instructions
+
+You're part of a task force fighting against corporate espionage. You have a secret informer at Shady Company X, which you suspect of stealing secrets from its competitors.
+
+Your informer, Agent Ex, is an Elixir developer. She is encoding secret messages in her code.
+
+To decode the secret message:
+
+- Take all functions (public and private) in the order they're defined in.
+- For each function, take the first `n` characters from its name, where `n` is the function's arity.
+
+## 1. Turn code into data
+
+Implement the `TopSecret.to_ast/1` function. It should take a string with Elixir code and return its AST.
+
+```elixir
+TopSecret.to_ast("div(4, 3)")
+# => {:div, [line: 1], [4, 3]}
+```
+
+## 2. Parse a single AST node
+
+Implement the `TopSecret.decode_secret_message_part/2` function. It should take an AST and an accumulator for the secret message (a list). It should return a tuple with the AST unchanged as the first element, and the accumulator as the second element.
+
+If the top-most operation in the AST is defining a function (`def` or `defp`), prepend the function name (changed to a string) to the accumulator. If the top-most operation is something else, return the accumulator unchanged.
+
+```elixir
+ast = TopSecret.to_ast("defp cat(a, b, c), do: nil")
+TopSecret.decode_secret_message_part(ast, ["day"])
+# => {ast, ["cat", "day"]}
+
+ast = TopSecret.to_ast("10 + 3")
+TopSecret.decode_secret_message_part(ast, ["day"])
+# => {ast, ["day"]}
+```
+
+This function doesn't need to do any recursive calls to check the whole AST, only the top-most operation. We will traverse the whole AST with built-in tools in the last step.
+
+## 3. Decode the secret message part from function definition
+
+Extend the `TopSecret.decode_secret_message_part/2` function. If the top-most operation in the AST is defining a function, don't return the whole function name. Instead, check the function's arity. Then, return only first `n` character from the name, where `n` is the arity.
+
+```elixir
+ast = TopSecret.to_ast("defp cat(a, b), do: nil")
+TopSecret.decode_secret_message_part(ast, ["day"])
+# => {ast, ["ca", "day"]}
+
+ast = TopSecret.to_ast("defp cat(), do: nil")
+TopSecret.decode_secret_message_part(ast, ["day"])
+# => {ast, ["", "day"]}
+```
+
+## 4. Decode the full secret message
+
+Implement the `TopSecret.decode_secret_message/1` function. It should take a string with Elixir code and return the secret message as a string decoded from all function definitions found in the code. Make sure to reuse functions defined in previous steps. 
+
+```elixir
+code = """
+defmodule MyCalendar do
+  def busy?(date, time) do
+    Date.day_of_week(date) != 7 and
+      time.hour in 10..16
+  end
+
+  def yesterday?(date) do
+    Date.diff(Date.utc_today, date)
+  end
+end
+"""
+
+TopSecret.decode_secret_message(code)
+# => "buy"
+```

exercises/concept/top-secret/.docs/introduction.md

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+# Introduction
+
+## AST
+
+to write:
+
+AST, also called a quoted expression in Elixir
+
+necessary for metaprogramming
+
+what is arity

exercises/concept/top-secret/.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

exercises/concept/top-secret/.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+maps-*.tar
+

exercises/concept/top-secret/.meta/config.json

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+{
+  "blurb": "TODO",
+  "icon": "TODO",
+  "authors": [
+    "jiegillet",
+    "angelikatyborska"
+  ],
+  "files": {
+    "solution": [
+      "lib/top_secret.ex"
+    ],
+    "test": [
+      "test/top_secret_test.exs"
+    ],
+    "exemplar": [
+      ".meta/exemplar.ex"
+    ]
+  },
+  "language_versions": ">=1.10"
+}

exercises/concept/top-secret/.meta/design.md

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+# Design
+
+## Learning objectives
+
+- How to parse a string to AST
+- Understanding ASTs
+- Traversing ASTs
+
+## Out of scope
+
+- `quote`/`unquote`
+- macros
+
+## Concepts
+
+- `ast`
+
+## Prerequisites
+
+- `enum`
+- `strings`
+- `tuples`
+- `pattern-matching`
+
+## Analyzer
+
+- function reuse: `decode_secret_message` should use `decode_secret_message_part` and `to_ast`.

exercises/concept/top-secret/.meta/exemplar.ex

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+defmodule TopSecret do
+  def to_ast(string) do
+    Code.string_to_quoted!(string)
+  end
+
+  def decode_secret_message_part({keyword, _, children} = ast, acc)
+      when keyword in [:def, :defp] do
+    [{function_name, _, arguments} | _] = children
+
+    arity = length(arguments)
+
+    message_part =
+      function_name
+      |> to_string()
+      |> String.slice(0, arity)
+
+    {ast, [message_part | acc]}
+  end
+
+  def decode_secret_message_part(ast, acc) do
+    {ast, acc}
+  end
+
+  def decode_secret_message(string) do
+    ast = to_ast(string)
+    {_, acc} = Macro.prewalk(ast, [], &decode_secret_message_part/2)
+
+    acc
+    |> Enum.reverse()
+    |> Enum.join("")
+  end
+end

exercises/concept/top-secret/lib/top_secret.ex

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+defmodule TopSecret do
+  def to_ast(string) do
+    # Please implement the to_ast/1 function
+  end
+
+  def decode_secret_message_part(ast, acc) do
+    # Please implement the decode_secret_message_part/2 function
+  end
+
+  def decode_secret_message(string) do
+    # Please implement the decode_secret_message/1 function
+  end
+end

exercises/concept/top-secret/mix.exs

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+defmodule TopSecret.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :top_secret,
+      version: "0.1.0",
+      # elixir: "~> 1.10",
+      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

exercises/concept/top-secret/test/test_helper.exs

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+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true, seed: 0)