\subsection{ONNX-to-IN Translation}
\label{sec:translation}

An ONNX neural network is organized as a \textit{Directed Acyclic Graph} (DAG), where nodes represent
a specific mathematical operation. While in a standard DAG, a node can be connected to many nodes to
share its value or result, in an IN, an agent has a fixed number of ports, with each port supporting
at most one connection. This limitation is solved by utilizing the \textit{Dup} agent to create the
necessary copies of a value required by the next operations. Since nodes do not know how successor
nodes will utilize their outputs, the translation layer traverses the DAG in reverse order
to be able to instantiate the correct number of \textit{Dup} agents. The main algorithm, illustrated
in \textbf{\Cref{alg:onnx-to-in}}, maintains an \textit{interactions} dictionary data-structure, that maps each tensor
name to a list of ports, to keep track of the graph traversal. The ONNX operators supported are Gemm
(General matrix multiplication) and ReLU.

\begin{algorithm}[H]
	\caption{Backwards ONNX-to-IN Translation}
	\label{alg:onnx-to-in}
	\SetKwInOut{Input}{Input}\SetKwInOut{Output}{Output}
	\Input{ONNX Graph $G$}
	\Output{Inpla script $S$}

	$interactions \leftarrow \emptyset$ \tcp*{Map: Tensor Name $\rightarrow$ List of ports}
	$S \leftarrow \emptyset$
	
	\ForEach{neuron $y$ in $G.output$}{
		$interactions[G.output][y] \leftarrow [\text{Materialize}(result_y)]$
	}

	\ForEach{node $N$ in \textbf{reverse}($G.nodes$)}{
		\Switch{$N.type$}{
			\Case{ReLU}{
				\ForEach{neuron $i$ in $N.output$}{
					$sink \leftarrow \text{BalancedFanOut}(interactions[N.output][i], \text{Dup}, S)$

					$v \leftarrow \text{generate\_wire}()$

					$interactions[N.input][i].\text{append}(\text{ReLU}(v))$

					$S.\text{append}(v \sim sink)$
				}
			}
			\Case{Gemm}{
				\ForEach{neuron $j$ in $N.output$}{
					$sink \leftarrow \text{BalancedFanOut}(interactions[N.output][j], \text{Dup}, S)$

					$neuron\_terms \leftarrow \emptyset$

					\ForEach{neuron $i$ in $N.input$}{
						$v \leftarrow \text{generate\_wire}()$

						$interactions[N.input][i].\text{append}(\text{Mul}(v, \text{Concrete}(N.alpha * N.weight[j, i])))$

						$neuron\_terms.\text{append}(v)$
					}

					$neuron\_terms.\text{append}(\text{Concrete}(N.beta * N.bias[j]))$

					$root \leftarrow \text{BalancedFanIn}(neuron\_terms, \text{Add}, S)$

					$S.\text{append}(root \sim sink)$
				}
			}
		}
	}

	\ForEach{neuron $x$ in $G.input$}{
		$sink \leftarrow \text{BalancedFanOut}(interactions[G.input][x], \text{Dup}, S)$

		$S.\text{append}(sink \sim \text{Linear}(\text{TermSymbolic}(x), 1.0, 0.0))$
	}

	\ForEach{$y$ in $\text{len}(interactions[G.output])$}{
		$S.\text{append}(result_y)$
	}

	\Return{S}
\end{algorithm}

To maximize the concurrency of the INPLA engine, the translation layer avoids generating linear
chains of agents, opting instead for balanced binary trees for signal distribution (single input to
multiple output) and signal reduction (multiple input to single output). As \textbf{\Cref{alg:balanced-fan-in}}
and \textbf{\Cref{alg:balanced-fan-out}} illustrate, the depth of agent chains (especially \textit{Dup} chains)
is limited to $O(\log N)$. The two algorithms are very similar; the difference is in how they wire the
agents together: in the \textit{Fan-In} the principal port of the agents are facing the leaves, while in the
\textit{Fan-Out} they face the root.

\begin{algorithm}[H]
	\caption{Balanced Fan-In}
	\label{alg:balanced-fan-in}
	\SetKwInOut{Input}{Input}\SetKwInOut{Output}{Output}
	\Input{A list of input signal wires $T$, Agent type $A$, Script $S$}
	\Output{A single wire connected to the output of the balanced tree}

	\If{$T = \emptyset$}{\Return{\text{Eraser}}}
	\If{$|T| = 1$}{\Return{$T[0]$}}

	\While{$|T| > 1$}{
		$T' \leftarrow \emptyset$

		\For{$i \leftarrow 0$ \KwTo $|T|-1$ \textbf{by} 2}{
			\eIf{$i+1 < |T|$}{
				$w_{out} \leftarrow \text{generate\_wire}()$

				$S.\text{append}(T[i] \sim A(w_{out}, T[i+1]))$

				$T'.\text{append}(w_{out})$
			}{
				$T'.\text{append}(T[i])$
			}
		}
		$T \leftarrow T'$
	}
	\Return{$T[0]$}
\end{algorithm}

\begin{algorithm}[H]
	\caption{Balanced Fan-Out}
	\label{alg:balanced-fan-out}
	\SetKwInOut{Input}{Input}\SetKwInOut{Output}{Output}
	\Input{A list of output signal wires $T$, Agent type $A$, Script $S$}
	\Output{A single wire connected to the input of the balanced tree}

	\If{$T = \emptyset$}{\Return{\text{Eraser}}}
	\If{$|T| = 1$}{\Return{$T[0]$}}

	\While{$|T| > 1$}{
		$T' \leftarrow \emptyset$

		\For{$i \leftarrow 0$ \KwTo $|T|-1$ \textbf{by} 2}{
			\eIf{$i+1 < |T|$}{
				$w_{out} \leftarrow \text{generate\_wire}()$

				$S.\text{append}(w_{out} \sim A(T[i], T[i+1]))$

				$T'.\text{append}(w_{out})$
			}{
				$T'.\text{append}(T[i])$
			}
		}
		$T \leftarrow T'$
	}
	\Return{$T[0]$}
\end{algorithm}
