Restore Counter.n API (#1546)

Includes special case support for Counter(0) which has identical
behavior to Counter(1) except for the value of n.

1 files changed, 24 insertions, 2 deletions

diff --git a/src/main/scala/chisel3/util/Counter.scala b/src/main/scala/chisel3/util/Counter.scala
index 11daa515..fa1bb80c 100644
--- a/src/main/scala/chisel3/util/Counter.scala
+++ b/src/main/scala/chisel3/util/Counter.scala
@@ -19,20 +19,42 @@ import chisel3.internal.naming.chiselName  // can't use chisel3_ version because
   *     ...
   *   }
   * }}}
+  * @example {{{
+  *   // Using Scala Range API
+  *   val (counterValue, counterWrap) = Counter(0 until 10 by 2)
+  *   when (counterValue === 4.U) {
+  *     ...
+  *   }
+  * }}}
   */
 @chiselName
-class Counter private (r: Range) {
+class Counter private (r: Range, oldN: Option[Int] = None) {
   require(r.length > 0, s"Counter range cannot be empty, got: $r")
   require(r.start >= 0 && r.end >= 0, s"Counter range must be positive, got: $r")
 
   private lazy val delta = math.abs(r.step)
   private lazy val width = math.max(log2Up(r.last + 1), log2Up(r.head + 1))
 
+  /** Number of counts before the counter resets
+    *
+    * @note Only defined for ranges starting at zero with steps of size 1. Use [[range]] for other
+    * use cases.
+    */
+  def n: Int = oldN match {
+    case Some(x) => x
+    case None =>
+      // Reasonable for typical ranges
+      require(r.start == 0 && r.step == 1,
+        s"Counter.n only defined on ranges starting at 0 with step == 1, got $r. " +
+        "Use underlying range.")
+      r.last + 1
+  }
+
   /** Creates a counter with the specified number of steps.
     *
     * @param n number of steps before the counter resets
     */
-  def this(n: Int) { this(0 until math.max(1, n)) }
+  def this(n: Int) { this(0 until math.max(1, n), Some(n)) }
 
   /** The current value of the counter. */
   val value = if (r.length > 1) RegInit(r.head.U(width.W)) else r.head.U