Poseidon merkle hasher (#2)

.github/workflows/cairo-ci.yaml

@@ -25,6 +25,6 @@ jobs:
       - uses: actions/checkout@v3
       - uses: software-mansion/setup-scarb@v1
         with:
-          scarb-version: "2.6.4"
+          scarb-version: "nightly-2024-04-24"
       - run: scarb fmt --check
       - run: scarb test

stwo_cairo_verifier/.tool-versions

@@ -0,0 +1 @@
+scarb nightly-2024-04-24

stwo_cairo_verifier/src/fields.cairo

@@ -1,2 +1,4 @@
 pub mod m31;
 pub mod cm31;
+
+pub type BaseField = m31::M31;

stwo_cairo_verifier/src/fields/m31.cairo

@@ -8,7 +8,7 @@ const P64NZ: NonZero<u64> = 0x7fffffff;
 
 #[derive(Copy, Drop, Debug, PartialEq, Eq)]
 pub struct M31 {
-    inner: u32
+    pub inner: u32
 }
 
 #[generate_trait]

stwo_cairo_verifier/src/lib.cairo

@@ -1,3 +1,6 @@
 mod fields;
+mod vcs;
+
+pub use fields::BaseField;
 
 fn main() {}

stwo_cairo_verifier/src/vcs.cairo

@@ -0,0 +1,74 @@
+use core::array::ArrayTrait;
+use core::option::OptionTrait;
+use core::poseidon::poseidon_hash_span;
+use stwo_cairo_verifier::BaseField;
+
+// A Merkle node hash is a hash of:
+//   [left_child_hash, right_child_hash], column0_value, column1_value, ...
+// "[]" denotes optional values.
+// The largest Merkle layer has no left and right child hashes. The rest of the layers have
+// children hashes.
+// At each layer, the tree may have multiple columns of the same length as the layer.
+// Each node in that layer contains one value from each column.
+pub trait MerkleHasher {
+    type Hash;
+    // Hashes a single Merkle node.
+    fn hash_node(
+        children_hashes: Option<(Self::Hash, Self::Hash)>, column_values: Array<BaseField>,
+    ) -> Self::Hash;
+}
+
+// 8 M31 elements fit in a hash, since 31*8 = 242 < 252.
+const M31_ELS_IN_HASH: usize = 8;
+const M31_ELS_IN_HASH_MINUS1: usize = M31_ELS_IN_HASH - 1;
+const M31_IN_HASH_SHIFT: felt252 = 0x80000000; // 2**31.
+pub impl PoseidonMerkleHasher of MerkleHasher {
+    type Hash = felt252;
+
+    fn hash_node(
+        children_hashes: Option<(Self::Hash, Self::Hash)>, mut column_values: Array<BaseField>,
+    ) -> Self::Hash {
+        let mut hash_array: Array<felt252> = Default::default();
+        if let Option::Some((x, y)) = children_hashes {
+            hash_array.append(x);
+            hash_array.append(y);
+        }
+
+        // Pad column_values to a multiple of 8.
+        let mut pad_len = M31_ELS_IN_HASH_MINUS1
+            - ((column_values.len() + M31_ELS_IN_HASH_MINUS1) % M31_ELS_IN_HASH);
+        while pad_len > 0 {
+            column_values.append(core::num::traits::Zero::zero());
+            pad_len = M31_ELS_IN_HASH_MINUS1
+                - ((column_values.len() + M31_ELS_IN_HASH_MINUS1) % M31_ELS_IN_HASH);
+        };
+
+        while !column_values.is_empty() {
+            let mut word = 0;
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+            word = word * M31_IN_HASH_SHIFT + column_values.pop_front().unwrap().inner.into();
+        };
+
+        poseidon_hash_span(hash_array.span())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::PoseidonMerkleHasher;
+    use stwo_cairo_verifier::fields::m31::{m31};
+
+    #[test]
+    fn test_m31() {
+        assert_eq!(
+            PoseidonMerkleHasher::hash_node(Option::None, array![m31(0), m31(1)]),
+            973835572668429495915136902981656666590582180872133591629269551720657739196
+        );
+    }
+}