default hasher is removed, need to be explicit (#145)

follow up to arkworks-rs/algebra#825

crypto-primitives/Cargo.toml

@@ -33,7 +33,7 @@ ark-snark = { version = "^0.4.0", default-features = false }
 rayon = { version = "1.0", optional = true }
 derivative = { version = "2.0", features = ["use_core"] }
 tracing = { version = "0.1", default-features = false, features = [ "attributes" ], optional = true }
-hashbrown = { version = "^0.14", default-features = false, optional = true }
+hashbrown = { version = "0.14", default-features = false, features = ["inline-more", "allocator-api2"], optional = true  }
 
 [features]
 default = ["std"]
@@ -50,6 +50,12 @@ prf = []
 snark = []
 signature = []
 
+[target.'cfg(all(target_has_atomic = "8", target_has_atomic = "16", target_has_atomic = "32", target_has_atomic = "64", target_has_atomic = "ptr"))'.dependencies]
+ahash = { version = "0.8", default-features = false}
+
+[target.'cfg(not(all(target_has_atomic = "8", target_has_atomic = "16", target_has_atomic = "32", target_has_atomic = "64", target_has_atomic = "ptr")))'.dependencies]
+fnv = { version = "1.0", default-features = false }
+
 [dev-dependencies]
 ark-ed-on-bls12-377 = { version = "^0.4.0", default-features = false }
 ark-ed-on-bls12-381 = { version = "^0.4.0", default-features = false, features = [ "r1cs" ] }

crypto-primitives/src/merkle_tree/mod.rs

@@ -1,5 +1,7 @@
 #![allow(clippy::needless_range_loop)]
 
+use core::hash::BuildHasherDefault;
+
 /// Defines a trait to chain two types of CRHs.
 use crate::crh::TwoToOneCRHScheme;
 use crate::sponge::Absorb;
@@ -21,6 +23,24 @@ pub mod constraints;
 #[cfg(feature = "parallel")]
 use rayon::prelude::*;
 
+#[cfg(all(
+    target_has_atomic = "8",
+    target_has_atomic = "16",
+    target_has_atomic = "32",
+    target_has_atomic = "64",
+    target_has_atomic = "ptr"
+))]
+type DefaultHasher = ahash::AHasher;
+
+#[cfg(not(all(
+    target_has_atomic = "8",
+    target_has_atomic = "16",
+    target_has_atomic = "32",
+    target_has_atomic = "64",
+    target_has_atomic = "ptr"
+)))]
+type DefaultHasher = fnv::FnvHasher;
+
 /// Convert the hash digest in different layers by converting previous layer's output to
 /// `TargetType`, which is a `Borrow` to next layer's input.
 pub trait DigestConverter<From, To: ?Sized> {
@@ -248,7 +268,8 @@ impl<P: Config> MultiPath<P> {
         let mut leaves = leaves.into_iter();
 
         // LookUp table to speedup computation avoid redundant hash computations
-        let mut hash_lut: HashMap<usize, P::InnerDigest> = HashMap::new();
+        let mut hash_lut: HashMap<usize, P::InnerDigest, _> =
+            HashMap::with_hasher(BuildHasherDefault::<DefaultHasher>::default());
 
         // init prev path for decoding
         let mut prev_path: Vec<_> = self.auth_paths_suffixes[0].clone();