From f1451b0759c8f0055aca9afb387f9847d86ece4c Mon Sep 17 00:00:00 2001
From: Olivier Giniaux <oginiaux@smartadserver.com>
Date: Fri, 13 Oct 2023 23:58:28 +0200
Subject: [PATCH] Prevent right padded zeros from triggering collisions

---
 src/gxhash.rs | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/gxhash.rs b/src/gxhash.rs
index 9456b9d..9ecd46e 100644
--- a/src/gxhash.rs
+++ b/src/gxhash.rs
@@ -37,7 +37,12 @@ mod platform_defs {
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ];
 
         let mask = vld1q_s8((MASK.as_ptr() as *const i8).offset(size_of::<state>() as isize - len));
-        vandq_s8(load_unaligned(p), mask)
+        let mut vec = vandq_s8(load_unaligned(p), mask);
+
+        // To avoid collisions for zero right padded inputs, we mutate this vector using its used length
+        vec = vaddq_s8(vec, vdupq_n_s8(len as i8));
+
+        return vec;
     }
 
     #[inline]
@@ -256,6 +261,7 @@ pub fn gxhash(input: &[u8]) -> u32 {
 mod tests {
 
     use super::*;
+    use rand::Rng;
 
     #[test]
     fn all_blocks_are_consumed() {
@@ -273,6 +279,22 @@ mod tests {
         }
     }
 
+    #[test]
+    fn add_zeroes_mutates_hash() {
+        let mut bytes = [0u8; 1200];
+
+        let mut rng = rand::thread_rng();
+        rng.fill(&mut bytes[..32]);
+
+        let mut ref_hash = 0;
+
+        for i in 32..100 {
+            let new_hash = gxhash(&mut bytes[..i]);
+            assert_ne!(ref_hash, new_hash, "Same hash at size {i} ({new_hash})");
+            ref_hash = new_hash;
+        }
+    }
+
     #[test]
     fn hash_of_zero_is_not_zero() {
         assert_ne!(0, gxhash(&[0u8; 0]));