defmodule Machete.StructLikeMatcher do
  @moduledoc """
  Defines a matcher that matches structs only on a specified list of fields
  """

  import Machete.AllMatcher
  import Machete.IsAMatcher
  import Machete.Operators
  import Machete.SupersetMatcher

  defstruct type: nil, fields: nil

  @typedoc """
  Describes an instance of this matcher
  """
  @opaque t :: %__MODULE__{}

  @doc """
  Matches structs only on a specified list of fields

  Structs can be tricky to match because they have default values for fields which are not
  specified at the point of creation. After creation these default values are otherwise
  indistinguishable from user-specified ones, and often get in the way of effective matching. This
  matcher provides a convenient way around this by allowing the user to describe both the type of
  struct expected as well as a set of matchers for fields within the struct; only the fields
  explicitly listed in the matcher are examined.

  Fields can be specified via a map or keyword list in a manner identical to that of
  `Kernel.struct/2`. Fields may be matched using any matcher, not just literal values.

  Examples:
      iex> assert %URI{host: "example.com", path: "/abc"} ~> struct_like(URI, %{host: "example.com"})
      true

      iex> assert %URI{host: "example.com", path: "/abc"} ~> struct_like(URI, host: "example.com")
      true

      iex> assert %URI{host: "example.com", path: "/abc"} ~> struct_like(URI, host: string())
      true

      iex> refute %URI{host: "example.com"} ~> struct_like(URI, host: "example.com", path: "/abc")
      false
  """
  @spec struct_like(struct(), Enum.t()) :: t()
  def struct_like(type, fields),
    do: struct!(__MODULE__, type: type, fields: Enum.into(fields, %{}))

  defimpl Machete.Matchable do
    def mismatches(%@for{} = a, b) do
      b ~>> all([is_a(a.type), superset(a.fields)])
    end
  end
end