fix custom hasher to also use a salt

Author: arvidn

Cargo.lock

@@ -77,6 +77,7 @@ k256 = { version = "0.13.4", features = ["ecdsa"] }
 p256 = { version = "0.13.2", features = ["ecdsa"] }
 # for keccak256
 sha3 = "0.10.8"
+rand = { workspace = true }
 
 [dev-dependencies]
 rstest = { workspace = true }

Cargo.toml

@@ -1,5 +1,6 @@
 use bitvec::prelude::*;
 use bitvec::vec::BitVec;
+use rand::Rng;
 /// When deserializing a clvm object, a stack of deserialized child objects
 /// is created, which can be used with back-references. A `ReadCacheLookup` keeps
 /// track of the state of this stack and all child objects under each root
@@ -23,27 +24,43 @@ use std::hash::{BuildHasher, Hasher};
 use super::bytes32::{hash_blob, hash_blobs, Bytes32};
 
 #[derive(Default, Clone, Copy)]
-pub struct IdentityHash(u64);
+pub struct IdentityHash(u64, u64);
+
+impl IdentityHash {
+    fn new(salt: u64) -> Self {
+        Self(0, salt)
+    }
+}
 
 impl Hasher for IdentityHash {
     fn finish(&self) -> u64 {
         self.0
     }
 
     fn write(&mut self, bytes: &[u8]) {
-        self.0 = u64::from_le_bytes(bytes[0..8].try_into().expect("expected 32 byte hashes"));
+        self.0 =
+            u64::from_le_bytes(bytes[0..8].try_into().expect("expected 32 byte hashes")) ^ self.1;
     }
 
     fn write_u64(&mut self, _i: u64) {
         panic!("This hasher only takes bytes");
     }
 }
 
-impl BuildHasher for IdentityHash {
-    type Hasher = Self;
+pub struct RandomState(u64);
+
+impl RandomState {
+    fn new() -> Self {
+        let mut rng = rand::thread_rng();
+        Self(rng.gen())
+    }
+}
+
+impl BuildHasher for RandomState {
+    type Hasher = IdentityHash;
 
     fn build_hasher(&self) -> Self::Hasher {
-        *self
+        IdentityHash::new(self.0)
     }
 }
 
@@ -56,10 +73,10 @@ pub struct ReadCacheLookup {
     /// the tree hashes of the contents on the left and right
     read_stack: Vec<(Bytes32, Bytes32)>,
 
-    count: HashMap<Bytes32, u32, IdentityHash>,
+    count: HashMap<Bytes32, u32, RandomState>,
 
     /// a mapping of tree hashes to `(parent, is_right)` tuples
-    parent_lookup: HashMap<Bytes32, Vec<(Bytes32, bool)>, IdentityHash>,
+    parent_lookup: HashMap<Bytes32, Vec<(Bytes32, bool)>, RandomState>,
 }
 
 impl Default for ReadCacheLookup {
@@ -74,9 +91,9 @@ impl ReadCacheLookup {
         let read_stack = Vec::with_capacity(1000);
         // all keys in count and parent_lookup are tree-hashes. There's no need
         // to hash them again for the hash map
-        let mut count = HashMap::with_hasher(IdentityHash::default());
+        let mut count = HashMap::with_hasher(RandomState::new());
         count.insert(root_hash, 1);
-        let parent_lookup = HashMap::with_hasher(IdentityHash::default());
+        let parent_lookup = HashMap::with_hasher(RandomState::new());
         Self {
             root_hash,
             read_stack,
@@ -161,10 +178,8 @@ impl ReadCacheLookup {
 
         // all the values we put in this hash set are themselves sha256 hashes.
         // There's no point in hashing the hashes
-        let mut seen_ids = HashSet::<&Bytes32, IdentityHash>::with_capacity_and_hasher(
-            1000,
-            IdentityHash::default(),
-        );
+        let mut seen_ids =
+            HashSet::<&Bytes32, RandomState>::with_capacity_and_hasher(1000, RandomState::new());
 
         let max_bytes_for_path_encoding = serialized_length - 2; // 1 byte for 0xfe, 1 min byte for savings
         let max_path_length: usize = (max_bytes_for_path_encoding.saturating_mul(8) - 1)