From 20d4b715912a8e73c82c1b0bcbb5ecf09ff47016 Mon Sep 17 00:00:00 2001
From: Stephen Chung <schungx@live.com>
Date: Mon, 26 Sep 2022 23:45:50 +0800
Subject: [PATCH] Add stable_hash.

---
 CHANGELOG.md        | 5 +++++
 Cargo.toml          | 1 +
 src/func/hashing.rs | 6 +++++-
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d95bc3d0..9340630c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,11 @@ Rhai Release Notes
 Version 1.11.0
 ==============
 
+New features
+------------
+
+* A new feature flag, `stable_hash`, is added that forces hashing to be consistent using a fixed seed.
+
 Enhancements
 ------------
 
diff --git a/Cargo.toml b/Cargo.toml
index 1034f923..6451af51 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -62,6 +62,7 @@ metadata = ["serde", "serde_json", "rhai_codegen/metadata", "smartstring/serde"]
 internals = []                  # expose internal data structures
 debugging = ["internals"]       # enable debugging
 serde = ["dep:serde", "smartstring/serde", "smallvec/serde"] # implement serde for rhai types
+stable_hash = []                # perform all hashing with fixed seed value
 
 # compiling for no-std
 no_std = ["no-std-compat", "num-traits/libm", "core-error", "libm", "ahash/compile-time-rng", "hashbrown/ahash-compile-time-rng"]
diff --git a/src/func/hashing.rs b/src/func/hashing.rs
index bc29f67b..ae0a57bc 100644
--- a/src/func/hashing.rs
+++ b/src/func/hashing.rs
@@ -74,7 +74,11 @@ impl BuildHasher for StraightHasherBuilder {
 #[inline(always)]
 #[must_use]
 pub fn get_hasher() -> ahash::AHasher {
-    ahash::AHasher::default()
+    if cfg!(feature = "stable_hash") {
+        ahash::RandomState::with_seeds(42, 999, 123, 0).build_hasher()
+    } else {
+        ahash::AHasher::default()
+    }
 }
 
 /// Calculate a non-zero [`u64`] hash key from a namespace-qualified variable name.