# A `Proc` represents a function pointer with an optional context (the closure data).
# It is typically created with a proc literal:
#
# ```
# # A proc without arguments
# ->{ 1 } # Proc(Int32)
#
# # A proc with one argument
# ->(x : Int32) { x.to_s } # Proc(Int32, String)
#
# # A proc with two arguments:
# ->(x : Int32, y : Int32) { x + y } # Proc(Int32, Int32, Int32)
# ```
#
# The types of the arguments (`T`) are mandatory, except when directly
# sending a proc literal to a lib fun in C bindings.
#
# The return type (`R`) is inferred from the proc's body.
#
# A special new method is provided too:
#
# ```
# Proc(Int32, String).new { |x| x.to_s } # Proc(Int32, String)
# ```
#
# This form allows you to specify the return type and to check it
# against the proc's body.
#
# Another way to create a `Proc` is by capturing a block:
#
# ```
# def capture(&block : Int32 -> Int32)
#   # block argument is used, so block is turned into a Proc
#   block
# end
#
# proc = capture { |x| x + 1 } # Proc(Int32, Int32)
# proc.call(1)                 # => 2
# ```
#
# When capturing blocks, the type of the arguments and return type
# must be specified in the capturing method block signature.
#
# ### Passing a Proc to a C function
#
# Passing a `Proc` to a C function, for example as a callback, is possible
# as long as the `Proc` isn't a closure. If it is, either a compile-time or
# runtime error will happen depending on whether the compiler can check this.
# The reason is that a `Proc` is internally represented as two void pointers,
# one having the function pointer and another the closure data. If just
# the function pointer is passed, the closure data will be missing at invocation time.
#
# Most of the time a C function that allows setting a callback also provide
# an argument for custom data. This custom data is then sent as an argument
# to the callback. For example, suppose a C function that invokes a callback
# at every tick, passing that tick:
#
# ```
# lib LibTicker
#   fun on_tick(callback : (Int32, Void* ->), data : Void*)
# end
# ```
#
# To properly define a wrapper for this function we must send the `Proc` as the
# callback data, and then convert that callback data to the `Proc` and finally invoke it.
#
# ```
# module Ticker
#   # The callback for the user doesn't have a Void*
#   @@box : Pointer(Void)?
#
#   def self.on_tick(&callback : Int32 ->)
#     # Since Proc is a {Void*, Void*}, we can't turn that into a Void*, so we
#     # "box" it: we allocate memory and store the Proc there
#     boxed_data = Box.box(callback)
#
#     # We must save this in Crystal-land so the GC doesn't collect it (*)
#     @@box = boxed_data
#
#     # We pass a callback that doesn't form a closure, and pass the boxed_data as
#     # the callback data
#     LibTicker.on_tick(->(tick, data) {
#       # Now we turn data back into the Proc, using Box.unbox
#       data_as_callback = Box(typeof(callback)).unbox(data)
#       # And finally invoke the user's callback
#       data_as_callback.call(tick)
#     }, boxed_data)
#   end
# end
#
# Ticker.on_tick do |tick|
#   puts tick
# end
# ```
#
# Note that we save the box in `@@box`. The reason is that if we don't do it,
# and our code doesn't reference it anymore, the GC will collect it.
# The C library will of course store the callback, but Crystal's GC has
# no way of knowing that.
struct Proc
  def self.new(pointer : Void*, closure_data : Void*)
    func = {pointer, closure_data}
    ptr = pointerof(func).as(self*)
    ptr.value
  end

  # Returns a new `Proc` that has its first arguments fixed to
  # the values given by *args*.
  #
  # See [Wikipedia, Partial application](https://en.wikipedia.org/wiki/Partial_application)
  #
  # ```
  # add = ->(x : Int32, y : Int32) { x + y }
  # add.call(1, 2) # => 3
  #
  # add_one = add.partial(1)
  # add_one.call(2)  # => 3
  # add_one.call(10) # => 11
  #
  # add_one_and_two = add_one.partial(2)
  # add_one_and_two.call # => 3
  # ```
  def partial(*args : *U) forall U
    {% begin %}
      {% remaining = (T.size - U.size) %}
      ->(
          {% for i in 0...remaining %}
            arg{{i}} : {{T[i + U.size]}},
          {% end %}
        ) {
        call(
          *args,
          {% for i in 0...remaining %}
            arg{{i}},
          {% end %}
        )
      }
    {% end %}
  end

  # Returns the number of arguments of this `Proc`.
  #
  # ```
  # add = ->(x : Int32, y : Int32) { x + y }
  # add.arity # => 2
  #
  # neg = ->(x : Int32) { -x }
  # neg.arity # => 1
  # ```
  def arity
    {{T.size}}
  end

  def pointer
    internal_representation[0]
  end

  def closure_data
    internal_representation[1]
  end

  def closure?
    !closure_data.null?
  end

  private def internal_representation
    func = self
    ptr = pointerof(func).as({Void*, Void*}*)
    ptr.value
  end

  def ==(other : self)
    pointer == other.pointer && closure_data == other.closure_data
  end

  def ===(other : self)
    self == other
  end

  def ===(other)
    call(other)
  end

  # See `Object#hash(hasher)`
  def hash(hasher)
    internal_representation.hash(hasher)
  end

  def clone
    self
  end

  def to_s(io : IO) : Nil
    io << "#<"
    io << {{@type.name.stringify}}
    io << ":0x"
    pointer.address.to_s(16, io)
    if closure?
      io << ":closure"
    end
    io << '>'
    nil
  end
end
