Add Pop Count practice exercise

config.json

@@ -1582,6 +1582,18 @@
         ],
         "difficulty": 3
       },
+      {
+        "slug": "pop-count",
+        "name": "Pop Count",
+        "uuid": "8a9d6e1e-9188-44c9-8681-ade84d340dd0",
+        "practices": [
+          "bit-manipulation"
+        ],
+        "prerequisites": [
+          "bit-manipulation"
+        ],
+        "difficulty": 3
+      },
       {
         "slug": "prime-factors",
         "name": "Prime Factors",

exercises/practice/pop-count/.docs/instructions.md

@@ -0,0 +1,8 @@
+# Instructions
+
+Your task is to count the number of 1 bits in the binary representation of a number.
+
+## Restrictions
+
+Keep your hands off that bit-count functionality provided by your standard library!
+Solve this one yourself using other basic tools instead.

exercises/practice/pop-count/.docs/introduction.md

@@ -0,0 +1,47 @@
+# Introduction
+
+Your friend Eliud inherited a farm from her grandma Tigist.
+Her granny was an inventor and had a tendency to build things in an overly complicated manner.
+The chicken coop has a digital display showing an encoded number representing the positions of all eggs that could be picked up.
+
+Eliud is asking you to write a program that shows the actual number of eggs in the coop.
+
+The position information encoding is calculated as follows:
+
+1. Scan the potential egg-laying spots and mark down a `1` for an existing egg or a `0` for an empty spot.
+2. Convert the number from binary to decimal.
+3. Show the result on the display.
+
+Example 1:
+
+```text
+Chicken Coop:
+ _ _ _ _ _ _ _
+|E| |E|E| | |E|
+
+Resulting Binary:
+ 1 0 1 1 0 0 1
+
+Decimal number on the display:
+89
+
+Actual eggs in the coop:
+4
+```
+
+Example 2:
+
+```text
+Chicken Coop:
+ _ _ _ _ _ _ _ _
+| | | |E| | | | |
+
+Resulting Binary:
+ 0 0 0 1 0 0 0 0
+
+Decimal number on the display:
+16
+
+Actual eggs in the coop:
+1
+```

exercises/practice/pop-count/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

exercises/practice/pop-count/.gitignore

@@ -0,0 +1,24 @@
+# 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").
+isogram-*.tar
+

exercises/practice/pop-count/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "kahgoh"
+  ],
+  "files": {
+    "solution": [
+      "lib/pop_count.ex"
+    ],
+    "test": [
+      "test/pop_count_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "blurb": "Count the 1 bits in a number",
+  "source": "Christian Willner, Eric Willigers",
+  "source_url": "https://forum.exercism.org/t/new-exercise-suggestion-pop-count/7632/5"
+}

exercises/practice/pop-count/.meta/example.ex

@@ -0,0 +1,17 @@
+import Bitwise
+
+defmodule PopCount do
+  @doc """
+  Given the number, count the number of eggs.
+  """
+  @spec egg_count(number :: integer()) :: non_neg_integer()
+  def egg_count(number) do
+    do_count(number, 0)
+  end
+
+  defp do_count(0, acc), do: acc
+
+  defp do_count(number, acc) when number > 0 do
+    do_count(number >>> 1, acc + (number &&& 1))
+  end
+end

exercises/practice/pop-count/.meta/tests.toml

@@ -0,0 +1,23 @@
+# 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.
+
+[559e789d-07d1-4422-9004-3b699f83bca3]
+description = "0 eggs"
+include = false
+
+[97223282-f71e-490c-92f0-b3ec9e275aba]
+description = "1 egg"
+
+[1f8fd18f-26e9-4144-9a0e-57cdfc4f4ff5]
+description = "4 eggs"
+
+[0c18be92-a498-4ef2-bcbb-28ac4b06cb81]
+description = "13 eggs"

exercises/practice/pop-count/lib/pop_count.ex

@@ -0,0 +1,8 @@
+defmodule PopCount do
+  @doc """
+  Given the number, count the number of eggs.
+  """
+  @spec egg_count(number :: integer()) :: non_neg_integer()
+  def egg_count(number) do
+  end
+end

exercises/practice/pop-count/mix.exs

@@ -0,0 +1,28 @@
+defmodule PopCount.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :pop_count,
+      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

exercises/practice/pop-count/test/pop_count_test.exs

@@ -0,0 +1,24 @@
+defmodule PopCountTest do
+  use ExUnit.Case
+
+  describe "egg count" do
+    test "0 eggs" do
+      assert PopCount.egg_count(0) == 0
+    end
+
+    @tag :pending
+    test "1 egg" do
+      assert PopCount.egg_count(16) == 1
+    end
+
+    @tag :pending
+    test "4 eggs" do
+      assert PopCount.egg_count(89) == 4
+    end
+
+    @tag :pending
+    test "13 eggs" do
+      assert PopCount.egg_count(2_000_000_000) == 13
+    end
+  end
+end

exercises/practice/pop-count/test/test_helper.exs

@@ -0,0 +1,2 @@
+ExUnit.start()
+ExUnit.configure(exclude: :pending, trace: true)