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davearonson's solution

to Secret Handshake in the Elixir Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

There are 10 types of people in the world: Those who understand binary, and those who don't.

You and your fellow cohort of those in the "know" when it comes to binary decide to come up with a secret "handshake".

1 = wink
10 = double blink
100 = close your eyes
1000 = jump


10000 = Reverse the order of the operations in the secret handshake.

Given a decimal number, convert it to the appropriate sequence of events for a secret handshake.

Here's a couple of examples:

Given the input 3, the function would return the array ["wink", "double blink"] because 3 is 11 in binary.

Given the input 19, the function would return the array ["double blink", "wink"] because 19 is 10011 in binary. Notice that the addition of 16 (10000 in binary) has caused the array to be reversed.

use Bitwise (or div/rem)

If you use Bitwise, an easy way to see if a particular bit is set is to compare the binary AND (&&&) of a set of bits with the particular bit pattern you want to check, and determine if the result is the same as the pattern you're checking.

Example:

Flags: 0b11011 Check: 0b11010

Flags &&& Check: 0b11010 (All checked bits are set)

Another:

Flags: 0b11011 Check: 0b10110

Flags &&& Check: 0b10010 (Third bit not set)

Running tests

Execute the tests with:

$ elixir secret_handshake_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

For more detailed information about the Elixir track, please see the help page.

Source

Bert, in Mary Poppins http://www.imdb.com/title/tt0058331/quotes/qt0437047

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

secret_handshake_test.exs

if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("secret_handshake.exs", __DIR__)
end

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

defmodule SecretHandshakeTest do
  use ExUnit.Case

  describe "Create a handshake for a number" do
    # @tag :pending
    test "wink for 1" do
      assert SecretHandshake.commands(1) == ["wink"]
    end

    @tag :pending
    test "double blink for 10" do
      assert SecretHandshake.commands(2) == ["double blink"]
    end

    @tag :pending
    test "close your eyes for 100" do
      assert SecretHandshake.commands(4) == ["close your eyes"]
    end

    @tag :pending
    test "jump for 1000" do
      assert SecretHandshake.commands(8) == ["jump"]
    end

    @tag :pending
    test "combine two actions" do
      assert SecretHandshake.commands(3) == ["wink", "double blink"]
    end

    @tag :pending
    test "reverse two actions" do
      assert SecretHandshake.commands(19) == ["double blink", "wink"]
    end

    @tag :pending
    test "reversing one action gives the same action" do
      assert SecretHandshake.commands(24) == ["jump"]
    end

    @tag :pending
    test "reversing no actions still gives no actions" do
      assert SecretHandshake.commands(16) == []
    end

    @tag :pending
    test "all possible actions" do
      assert SecretHandshake.commands(15) == ["wink", "double blink", "close your eyes", "jump"]
    end

    @tag :pending
    test "reverse all possible actions" do
      assert SecretHandshake.commands(31) == ["jump", "close your eyes", "double blink", "wink"]
    end

    @tag :pending
    test "do nothing for zero" do
      assert SecretHandshake.commands(0) == []
    end

    @tag :pending
    test "do nothing if lower 5 bits not set" do
      assert SecretHandshake.commands(32) == []
    end
  end
end
use Bitwise
defmodule SecretHandshake do
  @doc """
  Determine the actions of a secret handshake based on the binary
  representation of the given `code`.

  If the following bits are set, include the corresponding action in your list
  of commands, in order from lowest to highest.

  1 = wink
  10 = double blink
  100 = close your eyes
  1000 = jump

  10000 = Reverse the order of the operations in the secret handshake
  """
  @spec commands(code :: integer) :: list(String.t())
  def commands(code) do
    code
    |> Integer.to_string(2)
    |> String.graphemes
    |> Enum.reverse
    |> Enum.with_index
    |> Enum.map(&{elem(&1,0), 1 <<< elem(&1,1)})
    |> do_commands([])
  end

  @actions %{
    0b1 => "wink",
    0b10 => "double blink",
    0b100 => "close your eyes",
    0b1000 => "jump"
  }

  @action_nums @actions |> Map.keys

  defp do_commands([], acc), do: acc |> Enum.reverse

  defp do_commands([{"1",num}|tail], acc) when num in @action_nums do
    do_commands(tail, [@actions[num]|acc])
  end

  # note: depends on assumption that 16 is LAST!
  defp do_commands([{"1",16}|tail], acc), do: acc

  defp do_commands([_|tail], acc), do: do_commands(tail, acc)

end

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