From 74dea4ff6470cf5ce23db73040451aab1e540d2e Mon Sep 17 00:00:00 2001
From: Tom Grosso <tgrosso@eryx.co>
Date: Mon, 14 Oct 2024 16:28:19 -0300
Subject: [PATCH] Implement trailing zeros

---
 stwo_cairo_verifier/src/channel.cairo      | 48 ++++++++++++++++++++++
 stwo_cairo_verifier/src/pcs/verifier.cairo |  6 +++
 2 files changed, 54 insertions(+)

diff --git a/stwo_cairo_verifier/src/channel.cairo b/stwo_cairo_verifier/src/channel.cairo
index 408359b5..26840172 100644
--- a/stwo_cairo_verifier/src/channel.cairo
+++ b/stwo_cairo_verifier/src/channel.cairo
@@ -137,6 +137,27 @@ pub impl ChannelImpl of ChannelTrait {
             };
         bytes
     }
+
+    fn trailing_zeros(self: Channel) -> u32 {
+        let digest: felt252 = self.digest;
+        let mut digest_as_u256: u256 = digest.try_into().unwrap();
+
+        return Self::count_trailing_zeros(@digest_as_u256);
+    }
+
+    fn count_trailing_zeros(n: @u256) -> u32 {
+        let mut count = 0;
+        let mut value = n.clone();
+        while value > 0 {
+            if value & 1_u256 == 1 {
+                break;
+            }
+            count += 1;
+            value = value / 2;
+        };
+
+        return count;
+    }
 }
 
 #[inline]
@@ -332,4 +353,31 @@ mod tests {
         let second_result = channel.draw_random_bytes();
         assert_ne!(first_result, second_result);
     }
+
+    #[test]
+    pub fn test_can_return_trailing_zeros_of_digest() {
+        let initial_digest = 0xcafecafe;
+        let mut channel = ChannelTrait::new(initial_digest);
+        let trailing_zeros: u32 = channel.trailing_zeros();
+        assert_eq!(trailing_zeros, 1);
+    }
+
+    #[test]
+    pub fn test_can_return_trailing_zeros_of_integer() {
+        let mut value = 1024_u256;
+        let mut tz = ChannelTrait::count_trailing_zeros(@value);
+        assert_eq!(tz, 10);
+
+        value = 16;
+        let mut tz = ChannelTrait::count_trailing_zeros(@value);
+        assert_eq!(tz, 4);
+
+        value = 3405695742;
+        let mut tz = ChannelTrait::count_trailing_zeros(@value);
+        assert_eq!(tz, 1);
+
+        value = 11111;
+        let mut tz = ChannelTrait::count_trailing_zeros(@value);
+        assert_eq!(tz, 0);
+    }
 }
diff --git a/stwo_cairo_verifier/src/pcs/verifier.cairo b/stwo_cairo_verifier/src/pcs/verifier.cairo
index fac7ac77..7e314ac8 100644
--- a/stwo_cairo_verifier/src/pcs/verifier.cairo
+++ b/stwo_cairo_verifier/src/pcs/verifier.cairo
@@ -98,6 +98,12 @@ impl CommitmentSchemeVerifierImpl of CommitmentSchemeVerifierTrait {
 
         channel.mix_nonce(proof.proof_of_work);
 
+        // TODO: implement
+        // let proof_of_work_bits: u32 = *self.config.pow_bits;
+        // if channel.trailing_zeros() < proof_of_work_bits {
+        //     return Result::Err(VerificationError::ProofOfWork);
+        // }
+
         // Verify merkle decommitments.
         assert_eq!(self.trees.len(), proof.queried_values.len());
         assert_eq!(self.trees.len(), proof.decommitments.len());