Add recursive prime number generation

Signed-off-by: trivernis <trivernis@protonmail.com>

.formatter.exs

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

.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").
+hello_world-*.tar
+
+
+# Temporary files for e.g. tests
+/tmp
+
+.idea

README.md

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+# Elixir Primes
+
+Multithreaded calculation of prime numbers
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `primes` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:primes, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at [https://hexdocs.pm/hello_world](https://hexdocs.pm/hello_world).
+

elixir_primes.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="ELIXIR_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <output url="file://$MODULE_DIR$/_build/dev/lib/hello_world/ebin" />
+    <output-test url="file://$MODULE_DIR$/_build/test/lib/hello_world/ebin" />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/lib" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/spec" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/test" isTestSource="true" />
+      <excludeFolder url="file://$MODULE_DIR$/assets/node_modules/phoenix" />
+      <excludeFolder url="file://$MODULE_DIR$/assets/node_modules/phoenix_html" />
+      <excludeFolder url="file://$MODULE_DIR$/cover" />
+      <excludeFolder url="file://$MODULE_DIR$/deps" />
+      <excludeFolder url="file://$MODULE_DIR$/doc" />
+      <excludeFolder url="file://$MODULE_DIR$/logs" />
+    </content>
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

lib/primes.ex

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+defmodule Primes do
+  def start(_type, _args) do
+    Enum.each(0..6, fn (n) -> Task.start(fn -> primes(2*n + 1, 16) end); end)
+
+    primes(15, 16)
+  end
+
+  defp primes(value, step) do
+    if rem(value, 2) != 0 && check_prime(value, 3) do
+      IO.puts(value)
+    end
+    primes(value + step, step)
+  end
+
+  defp check_prime(value, num) do
+    cond do
+      num > :math.sqrt(value) -> true
+      rem(value, num) != 0 -> check_prime(value, num + 2)
+      true -> false
+    end
+  end
+end

mix.exs

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+defmodule HelloWorld.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :primes,
+      version: "0.1.0",
+      elixir: "~> 1.11",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      mod: {Primes, []},
+      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

test/hello_world_test.exs

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+defmodule HelloWorldTest do
+  use ExUnit.Case
+  doctest HelloWorld
+
+  test "greets the world" do
+    assert HelloWorld.hello() == :world
+  end
+end

test/test_helper.exs

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+ExUnit.start()