From f82638cd89296af146dd53cacc3e3670e95ef041 Mon Sep 17 00:00:00 2001
From: Michael Pilquist <mpilquist@gmail.com>
Date: Sun, 18 Aug 2024 13:14:56 -0400
Subject: [PATCH] Add ByteVector#equalsConstantTime

---
 .../src/main/scala/scodec/bits/ByteVector.scala   | 15 +++++++++++++++
 .../test/scala/scodec/bits/ByteVectorTest.scala   |  7 +++++++
 2 files changed, 22 insertions(+)

diff --git a/core/shared/src/main/scala/scodec/bits/ByteVector.scala b/core/shared/src/main/scala/scodec/bits/ByteVector.scala
index df8d139e..59741225 100644
--- a/core/shared/src/main/scala/scodec/bits/ByteVector.scala
+++ b/core/shared/src/main/scala/scodec/bits/ByteVector.scala
@@ -1293,6 +1293,21 @@ sealed abstract class ByteVector
       case _                => false
     }
 
+  /** Like [[equals]] but compares all bytes instead of returning after first non-matching byte.
+    * @group collection
+    */
+  def equalsConstantTime(other: ByteVector): Boolean =
+    if (this.size != other.size) false
+    else {
+      var result, idx = 0
+      val s = this.size
+      while (idx < s) {
+        result = result | (this(idx) ^ other(idx))
+        idx += 1
+      }
+      result == 0
+    }
+
   /** Display the size and bytes of this `ByteVector`. For bit vectors beyond a certain size, only a
     * hash of the contents are shown.
     * @group collection
diff --git a/core/shared/src/test/scala/scodec/bits/ByteVectorTest.scala b/core/shared/src/test/scala/scodec/bits/ByteVectorTest.scala
index 798458e9..2d44b8db 100644
--- a/core/shared/src/test/scala/scodec/bits/ByteVectorTest.scala
+++ b/core/shared/src/test/scala/scodec/bits/ByteVectorTest.scala
@@ -59,6 +59,13 @@ class ByteVectorTest extends BitsSuite {
     forAll((b: ByteVector, b2: ByteVector) => assert((b == b2) == (b === b2)))
   }
 
+  property("equalsConstantTime") {
+    forAll(bytesWithIndex) { case (b, m) =>
+      assert((b.take(m) ++ b.drop(m)).equalsConstantTime(b))
+    } &&
+    forAll((b: ByteVector, b2: ByteVector) => assert((b === b2) == (b.equalsConstantTime(b2))))
+  }
+
   test("compact is a no-op for already compact byte vectors") {
     val b = ByteVector(0x80)
     assert((b.compact eq b.compact) == true)