util: add GrayCode (#2353) (#2354)

(cherry picked from commit 91d7baa8e7162d736f46b6d4964b09016d2dd172)

Co-authored-by: Kevin Laeufer <laeufer@cs.berkeley.edu>

2 files changed, 66 insertions, 0 deletions

diff --git a/integration-tests/src/test/scala/chiselTests/util/GrayCodeTests.scala b/integration-tests/src/test/scala/chiselTests/util/GrayCodeTests.scala
new file mode 100644
index 00000000..9562abb4
--- /dev/null
+++ b/integration-tests/src/test/scala/chiselTests/util/GrayCodeTests.scala
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiselTests.util
+
+import chisel3._
+import chisel3.util._
+import chiseltest._
+import chiseltest.formal._
+import org.scalatest.flatspec.AnyFlatSpec
+
+class GrayCodeTests extends AnyFlatSpec with ChiselScalatestTester with Formal {
+  behavior.of("GrayCode")
+
+  val Widths = Seq(1, 2, 3, 5, 8, 17, 65)
+  Widths.foreach { w =>
+    it should s"maintain identity (width=$w)" in {
+      verify(new GrayCodeIdentityCheck(w), Seq(BoundedCheck(1)))
+    }
+
+    it should s"ensure hamming distance of one (width=$w)" in {
+      verify(new GrayCodeHammingCheck(w), Seq(BoundedCheck(1)))
+    }
+  }
+}
+
+/** Checks that when we go from binary -> gray -> binary the result is always the same as the input. */
+private class GrayCodeIdentityCheck(width: Int) extends Module {
+  val in = IO(Input(UInt(width.W)))
+  val gray = BinaryToGray(in)
+  val out = GrayToBinary(gray)
+  assert(in === out, "%b -> %b -> %b", in, gray, out)
+}
+
+/** Checks that if we increment the binary number, the gray code equivalent only changes by one bit. */
+private class GrayCodeHammingCheck(width: Int) extends Module {
+  val a = IO(Input(UInt(width.W)))
+  val b = a + 1.U
+  val aGray = BinaryToGray(a)
+  val bGray = BinaryToGray(b)
+  val hamming = PopCount(aGray ^ bGray)
+  assert(hamming === 1.U, "%b ^ %b = %b", aGray, bGray, hamming)
+}
diff --git a/src/main/scala/chisel3/util/GrayCode.scala b/src/main/scala/chisel3/util/GrayCode.scala
new file mode 100644
index 00000000..ef310ee9
--- /dev/null
+++ b/src/main/scala/chisel3/util/GrayCode.scala
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chisel3.util
+
+import chisel3._
+
+object BinaryToGray {
+
+  /** Turns a binary number into gray code. */
+  def apply(in: UInt): UInt = in ^ (in >> 1)
+}
+
+object GrayToBinary {
+
+  /** Inverts the [[BinaryToGray]] operation. */
+  def apply(in: UInt, width: Int): UInt = apply(in(width - 1, 0))
+
+  /** Inverts the [[BinaryToGray]] operation. */
+  def apply(in: UInt): UInt = if (in.getWidth < 2) { in }
+  else {
+    val bits = in.getWidth - 2 to 0 by -1
+    Cat(bits.scanLeft(in.head(1)) { case (prev, ii) => prev ^ in(ii) })
+  }
+}