defmodule Riptide.Mutation do
  @typedoc "A map containing a path to be added (merge) and a path to be removed (delete)."
  @type t :: %{merge: map, delete: map}

  @typedoc "A key-value pair representing a layer of the mutation. The key
  is a list of strings representing the path to the current layer. The value is a
  mutation, representing any deeper sub-mutations."
  @type layer :: {list(String.t()), t}

  @doc ~S"""
  Creates a new mutation with empty delete and merge maps.

  ## Example

    iex> Riptide.Mutation.new
    %{delete: %{}, merge: %{}}

  """
  @spec new(map, map) :: t
  def new(merge \\ %{}, delete \\ %{}) do
    %{
      merge: merge || %{},
      delete: delete || %{}
    }
  end

  @doc false
  @spec merge(list(String.t()), any) :: t
  def merge(path, value), do: new() |> merge(path, value)

  @doc ~S"""
  Places the value at the given path in the merge.

  ## Example

    iex> mutation = %{delete: %{}, merge: %{"a" => %{"b" => 1}}}
    iex> Riptide.Mutation.merge(mutation, ["a","c"], 2)
    %{delete: %{}, merge: %{"a" => %{"b" => 1, "c" => 2}}}

  """
  @spec merge(t, list(String.t()), any) :: t
  def merge(input, _path, value) when value == %{}, do: input
  def merge(input, path, value), do: Dynamic.put(input, [:merge | path], value)

  @doc false
  @spec delete(list(String.t())) :: t
  def delete(path), do: new() |> delete(path)

  @doc ~S"""
  Adds a path to be deleted to the input mutation.

  ## Example

    iex> Riptide.Mutation.delete(
    ...>	%{
    ...>		delete: %{},
    ...>		merge: %{
    ...>			"a" => %{
    ...>				"b" => %{
    ...>					"c" => true
    ...>				}
    ...>			}
    ...>		}
    ...>	},
    ...>	["c"]
    ...> )
    %{delete: %{"c" => 1}, merge: %{"a" => %{"b" => %{"c" => true}}}}
  """
  @spec delete(t, list(String.t())) :: t
  def delete(input, path), do: Dynamic.put(input, [:delete | path], 1)

  @doc ~S"""
  Returns a map of levels for the given mutation. Each level is a key-value
  pair, where the key is a list of keys representing the current path, and the
  value is the remaining part of the mutation structure.

  ## Example

    iex> %{delete: %{}, merge: %{"a" => %{"b" => true}}} |> Riptide.Mutation.layers
    %{
      [] => %{
        delete: %{},
        merge: %{
          "a" => %{
            "b" => true
          }
        }
      },
      ["a"] => %{
        delete: %{},
        merge: %{
          "b" => true
        }
      }
    }
  """
  @spec layers(t) :: %{required(list(String.t())) => layer}
  def layers(%{merge: merge, delete: delete}) do
    merge
    |> layers(:merge)
    |> Dynamic.combine(layers(delete, :delete))
    |> Stream.map(fn {path, value} ->
      merge = Map.get(value, :merge, %{})
      delete = Map.get(value, :delete, %{})

      {path,
       %{
         merge: merge,
         delete: delete
       }}
    end)
    |> Enum.into(%{})
  end

  @doc false
  @spec layers(t, :merge | :delete) :: %{required(list(String.t())) => layer}
  def layers(input, type) do
    input
    |> Dynamic.layers()
    |> Enum.reduce(%{}, fn {path, value}, collect ->
      Dynamic.put(collect, [path, type], value)
    end)
  end

  @doc ~S"""
  Combines two mutations into one.

  ## Example

    iex> Riptide.Mutation.combine(
    ...>	%{delete: %{}, merge: %{"a" => true}},
    ...>	%{delete: %{}, merge: %{"b" => false}}
    ...> )
    %{delete: %{}, merge: %{"a" => true, "b" => false}}
  """
  @spec combine(t, t) :: t
  def combine(left, right) do
    %{
      merge:
        left.merge
        |> Riptide.Mutation.apply(%{delete: right.delete, merge: %{}})
        |> Riptide.Mutation.apply(%{delete: %{}, merge: right.merge}),
      delete:
        Dynamic.combine(
          left.delete,
          right.delete
        )
    }
  end

  @spec combine(Enum.t()) :: t
  def combine(input) do
    input
    |> Stream.filter(fn item -> item != nil end)
    |> Enum.reduce(new(), &combine(&2, &1))
  end

  @doc ~S"""
  Applies the entire mutation to the input map.

  ## Example

    iex> Riptide.Mutation.apply(
    ...> 	%{"b" => false},
    ...> 	%{delete: %{}, merge: %{"a" => true}}
    ...> )
    %{"a" => true, "b" => false}
  """
  @spec apply(map, t) :: map
  def apply(input, mutation) do
    deleted =
      mutation.delete
      |> Dynamic.flatten()
      |> Enum.reduce(input, fn {path, _value}, collect ->
        Dynamic.delete(collect, path)
      end)

    mutation.merge
    |> Dynamic.flatten()
    |> Enum.reduce(deleted, fn {path, value}, collect ->
      Dynamic.put(collect, path, value)
    end)
  end

  @doc ~S"""
  Accepts a list and mutation, and returns a new mutation with the given
  mutation nested at the given path.

  ## Example

    iex> Riptide.Mutation.inflate(
    ...>	["a", "b"],
    ...>	%{
    ...>		delete: %{},
    ...>		merge: %{
    ...>			"a" => 1
    ...>		}
    ...>	}
    ...>)
    %{
      delete: %{
        "a" => %{
          "b" => %{}
        }
      },
      merge: %{
        "a" => %{
          "b" => %{
            "a" => 1
          }
        }
      }
    }
  """
  @spec inflate(list(String.t()), t) :: t
  def inflate(path, mut) do
    new()
    |> Dynamic.put([:merge | path], mut.merge)
    |> Dynamic.put([:delete | path], mut.delete)
  end

  @doc ~S"""
  Takes two maps and returns a mutation that could be applied to turn the
  the first map into the second.

  ## Example

    iex> Riptide.Mutation.from_diff(
    ...>	%{"a" => 1},
    ...>	%{"b" => 2}
    ...>)
    %{delete: %{"a" => 1}, merge: %{"b" => 2}}
  """
  def from_diff(old, new) do
    old
    |> Dynamic.flatten()
    |> Enum.reduce(new(new), fn {path, value}, collect ->
      case Dynamic.get(new, path) do
        ^value -> Dynamic.delete(collect, [:merge | path])
        nil -> delete(collect, path)
        next -> merge(collect, path, next)
      end
    end)
  end

  def chunk(stream, count) do
    stream
    |> Stream.chunk_every(count)
    |> Stream.map(&Riptide.Mutation.combine/1)
  end
end