Poseidon instructions

src/circuit/gadget.rs

@@ -1 +1,2 @@
 pub(crate) mod ecc;
+pub(crate) mod poseidon;

src/circuit/gadget/poseidon.rs

@@ -0,0 +1,22 @@
+//! Gadget and chips for the Poseidon algebraic hash function.
+
+use std::fmt;
+
+use halo2::{
+    arithmetic::FieldExt,
+    circuit::{Chip, Layouter},
+    plonk::Error,
+};
+
+/// The set of circuit instructions required to use the [`Poseidon`] gadget.
+pub trait PoseidonInstructions<F: FieldExt>: Chip<F> {
+    /// Variable representing the state over which the Poseidon permutation operates.
+    type State: fmt::Debug;
+
+    /// Applies the Poseidon permutation to the given state.
+    fn permute(
+        &self,
+        layouter: &mut impl Layouter<F>,
+        initial_state: &Self::State,
+    ) -> Result<Self::State, Error>;
+}