sinsemilla::message.rs: Add message module.

This defines a Sinsemilla message in terms of pieces and subpieces.
This is useful when decomposing field elements and packing them
into K-bit messages.

src/circuit/gadget/sinsemilla.rs

@@ -7,7 +7,7 @@ use halo2::{arithmetic::CurveAffine, circuit::Layouter, plonk::Error};
 use std::fmt::Debug;
 
 // pub mod chip;
-// mod message;
+mod message;
 
 // pub use chip::{SinsemillaChip, SinsemillaConfig};
 

src/circuit/gadget/sinsemilla/message.rs

@@ -0,0 +1,71 @@
+//! Gadget and chips for the Sinsemilla hash function.
+use crate::circuit::gadget::utilities::{CellValue, Var};
+use ff::PrimeFieldBits;
+use halo2::{arithmetic::FieldExt, circuit::Cell};
+use std::fmt::Debug;
+
+/// A [`Message`] composed of several [`MessagePiece`]s.
+#[derive(Clone, Debug)]
+pub struct Message<F: FieldExt, const K: usize, const MAX_WORDS: usize>(Vec<MessagePiece<F, K>>);
+
+impl<F: FieldExt + PrimeFieldBits, const K: usize, const MAX_WORDS: usize>
+    From<Vec<MessagePiece<F, K>>> for Message<F, K, MAX_WORDS>
+{
+    fn from(pieces: Vec<MessagePiece<F, K>>) -> Self {
+        // A message cannot contain more than `MAX_WORDS` words.
+        assert!(pieces.iter().map(|piece| piece.num_words()).sum::<usize>() < MAX_WORDS);
+        Message(pieces)
+    }
+}
+
+impl<F: FieldExt + PrimeFieldBits, const K: usize, const MAX_WORDS: usize> std::ops::Deref
+    for Message<F, K, MAX_WORDS>
+{
+    type Target = [MessagePiece<F, K>];
+
+    fn deref(&self) -> &[MessagePiece<F, K>] {
+        &self.0
+    }
+}
+
+/// A [`MessagePiece`] of some bitlength.
+///
+/// The piece must fit within a base field element, which means its length
+/// cannot exceed the base field's `NUM_BITS`.
+#[derive(Copy, Clone, Debug)]
+pub struct MessagePiece<F: FieldExt, const K: usize> {
+    cell: Cell,
+    field_elem: Option<F>,
+    // The number of K-bit words in this message piece.
+    num_words: usize,
+}
+
+#[allow(clippy::from_over_into)]
+impl<F: FieldExt + PrimeFieldBits, const K: usize> Into<CellValue<F>> for MessagePiece<F, K> {
+    fn into(self) -> CellValue<F> {
+        CellValue::new(self.cell(), self.field_elem())
+    }
+}
+
+impl<F: FieldExt + PrimeFieldBits, const K: usize> MessagePiece<F, K> {
+    pub fn new(cell: Cell, field_elem: Option<F>, num_words: usize) -> Self {
+        assert!(num_words * K < F::NUM_BITS as usize);
+        Self {
+            cell,
+            field_elem,
+            num_words,
+        }
+    }
+
+    pub fn num_words(&self) -> usize {
+        self.num_words
+    }
+
+    pub fn cell(&self) -> Cell {
+        self.cell
+    }
+
+    pub fn field_elem(&self) -> Option<F> {
+        self.field_elem
+    }
+}