Diffusers.jl/examples/ConditionalChain.jl

using Flux
import Flux._show_children
import Flux._big_show


abstract type AbstractParallel end

_maybe_forward(layer::AbstractParallel, x::AbstractArray, ys::AbstractArray...) = layer(x, ys...)
_maybe_forward(layer::Parallel, x::AbstractArray, ys::AbstractArray...) = layer(x, ys...)
_maybe_forward(layer, x::AbstractArray, ys::AbstractArray...) = layer(x)

"""
    ConditionalChain(layers...)

Based off `Flux.Chain` except takes in multiple inputs.
If a layer is of type `AbstractParallel` it uses all inputs else it uses only the first one.
The first input can therefore be conditioned on the other inputs.
"""
struct ConditionalChain{T<:Union{Tuple,NamedTuple}} <: AbstractParallel
  layers::T
end
Flux.@functor ConditionalChain

ConditionalChain(xs...) = ConditionalChain(xs)
function ConditionalChain(; kw...)
  :layers in keys(kw) && throw(ArgumentError("a Chain cannot have a named layer called `layers`"))
  isempty(kw) && return ConditionalChain(())
  ConditionalChain(values(kw))
end

Flux.@forward ConditionalChain.layers Base.getindex, Base.length, Base.first, Base.last,
Base.iterate, Base.lastindex, Base.keys, Base.firstindex

Base.getindex(c::ConditionalChain, i::AbstractArray) = ConditionalChain(c.layers[i]...)

function (c::ConditionalChain)(x, ys...)
  for layer in c.layers
    x = _maybe_forward(layer, x, ys...)
  end
  x
end

function Base.show(io::IO, c::ConditionalChain)
  print(io, "ConditionalChain(")
  Flux._show_layers(io, c.layers)
  print(io, ")")
end

function _big_show(io::IO, m::ConditionalChain{T}, indent::Int=0, name=nothing) where {T<:NamedTuple}
  println(io, " "^indent, isnothing(name) ? "" : "$name = ", "ConditionalChain(")
  for k in Base.keys(m.layers)
    _big_show(io, m.layers[k], indent + 2, k)
  end
  if indent == 0
    print(io, ") ")
    _big_finale(io, m)
  else
    println(io, " "^indent, ")", ",")
  end
end

"""
    ConditionalSkipConnection(layers, connection)

The output is equivalent to `connection(layers(x, ys...), x)`.
Based off Flux.SkipConnection except it passes multiple arguments to layers.
"""
struct ConditionalSkipConnection{T,F} <: AbstractParallel
  layers::T
  connection::F
end

Flux.@functor ConditionalSkipConnection

function (skip::ConditionalSkipConnection)(x, ys...)
  skip.connection(skip.layers(x, ys...), x)
end

function Base.show(io::IO, b::ConditionalSkipConnection)
  print(io, "ConditionalSkipConnection(", b.layers, ", ", b.connection, ")")
end

### Show. Copied from Flux.jl/src/layers/show.jl

for T in [
  :ConditionalChain, ConditionalSkipConnection
]
  @eval function Base.show(io::IO, m::MIME"text/plain", x::$T)
    if get(io, :typeinfo, nothing) === nothing  # e.g. top level in REPL
      Flux._big_show(io, x)
    elseif !get(io, :compact, false)  # e.g. printed inside a Vector, but not a Matrix
      Flux._layer_show(io, x)
    else
      show(io, x)
    end
  end
end

_show_children(c::ConditionalChain) = c.layers