Merge pull request #4 from platogo/feature/add-pncounter

Implement PNCounter

lib/crdt/pn_counter.ex

@@ -0,0 +1,129 @@
+defmodule CRDT.PNCounter do
+  @moduledoc """
+  A PN-Counter is a CRDT that can be used to count events that can be both incremented and decremented.
+
+  They are no delta CRDTs, so they are not space efficient. They are however commutative and associative,
+  so they can be merged without any conflicts.
+  """
+
+  defstruct pos: %{}, neg: %{}
+
+  @type t :: %__MODULE__{
+          pos: %{CRDT.actor() => non_neg_integer()},
+          neg: %{CRDT.actor() => non_neg_integer()}
+        }
+
+  @doc """
+  Creates a new PN-Counter.
+
+  Starts with a counter of 0.
+  """
+  @spec new() :: t
+  def new(), do: %__MODULE__{}
+
+  @doc """
+  Creates a new PN-Counter with given values.
+
+  ### Examples
+
+      iex> CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"c" => 3})
+      %CRDT.PNCounter{neg: %{"c" => 3}, pos: %{"a" => 1, "b" => 2}}
+  """
+  @spec new(
+          pos: %{CRDT.actor() => non_neg_integer()},
+          neg: %{CRDT.actor() => non_neg_integer()}
+        ) :: t
+  def new(pos: pos, neg: neg), do: %__MODULE__{pos: pos, neg: neg}
+
+  @doc """
+  Increments the counter on behalf of the actor by the given value, default 1.
+
+  ### Examples
+
+      iex> CRDT.PNCounter.new |> CRDT.PNCounter.increment("a") |> CRDT.value
+      1
+      iex> CRDT.PNCounter.new |> CRDT.PNCounter.increment("a", 2) |> CRDT.value
+      2
+  """
+  @spec increment(t(), CRDT.actor(), non_neg_integer()) :: t()
+  def increment(%__MODULE__{} = counter, actor, value \\ 1) do
+    pos = Map.update(counter.pos, actor, value, &(&1 + value))
+    %__MODULE__{counter | pos: pos}
+  end
+
+  @doc """
+  Decrements the counter on behalf of the actor by the given value, default 1.
+
+  ### Examples
+
+      iex> CRDT.PNCounter.new |> CRDT.PNCounter.decrement("a") |> CRDT.value
+      -1
+      iex> CRDT.PNCounter.new |> CRDT.PNCounter.decrement("a", 2) |> CRDT.value
+      -2
+  """
+  @spec decrement(t(), CRDT.actor(), non_neg_integer()) :: t()
+  def decrement(%__MODULE__{} = counter, actor, value \\ 1) do
+    neg = Map.update(counter.neg, actor, value, &(&1 + value))
+    %__MODULE__{counter | neg: neg}
+  end
+
+  @doc """
+  Merges two PN-Counters.
+
+  ### Examples
+
+      iex> pncounter1 = CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"c" => 3})
+      iex> pncounter2 = CRDT.PNCounter.new(pos: %{"a" => 2, "b" => 1}, neg: %{"d" => 4})
+      iex> CRDT.PNCounter.merge(pncounter1, pncounter2)
+      %CRDT.PNCounter{neg: %{"c" => 3, "d" => 4}, pos: %{"a" => 2, "b" => 2}}
+  """
+  @spec merge(t(), t()) :: t()
+  def merge(%__MODULE__{} = counter1, %__MODULE__{} = counter2) do
+    pos = Map.merge(counter1.pos, counter2.pos, fn _, v1, v2 -> max(v1, v2) end)
+    neg = Map.merge(counter1.neg, counter2.neg, fn _, v1, v2 -> max(v1, v2) end)
+    %__MODULE__{pos: pos, neg: neg}
+  end
+
+  @doc """
+  Returns the actual value of the PN-Counter.
+
+  ### Examples
+
+      iex> CRDT.PNCounter.new |> CRDT.PNCounter.value
+      0
+      iex> CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"c" => 3}) |> CRDT.PNCounter.value
+      0
+      iex> CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"b" => 4}) |> CRDT.PNCounter.value
+      -1
+  """
+  @spec value(t()) :: integer()
+  def value(%__MODULE__{} = counter) do
+    (Map.values(counter.pos) |> Enum.sum()) - (Map.values(counter.neg) |> Enum.sum())
+  end
+end
+
+defimpl CRDT, for: CRDT.PNCounter do
+  @doc """
+  ### Examples
+
+      iex> CRDT.PNCounter.new |> CRDT.value
+      0
+      iex> CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"c" => 3}) |> CRDT.value
+      0
+      iex> CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"b" => 4}) |> CRDT.value
+      -1
+  """
+
+  def value(%CRDT.PNCounter{} = counter), do: CRDT.PNCounter.value(counter)
+
+  @doc """
+  ### Examples
+
+      iex> pncounter1 = CRDT.PNCounter.new(pos: %{"a" => 1, "b" => 2}, neg: %{"c" => 3})
+      iex> pncounter2 = CRDT.PNCounter.new(pos: %{"a" => 2, "b" => 1}, neg: %{"d" => 4})
+      iex> CRDT.merge(pncounter1, pncounter2)
+      %CRDT.PNCounter{neg: %{"c" => 3, "d" => 4}, pos: %{"a" => 2, "b" => 2}}
+  """
+  def merge(%CRDT.PNCounter{} = counter1, %CRDT.PNCounter{} = counter2),
+    do: CRDT.PNCounter.merge(counter1, counter2)
+end

test/crdt/pn_counter_test.exs

@@ -0,0 +1,4 @@
+defmodule CRDT.PNCounterTest do
+  use ExUnit.Case
+  doctest CRDT.PNCounter
+end