From 3668532fabc4ba4eaf70cf0ad1a55522aa33cdb3 Mon Sep 17 00:00:00 2001
From: Jack Koenig
Date: Tue, 11 Aug 2020 10:28:15 -0700
Subject: 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.---
 src/main/scala/chisel3/util/Counter.scala | 26 ++++++++++++++++++++++++--
 src/test/scala/chiselTests/Counter.scala  |  8 ++++++--
 2 files changed, 30 insertions(+), 4 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
diff --git a/src/test/scala/chiselTests/Counter.scala b/src/test/scala/chiselTests/Counter.scala
index 31bfe7eb..985cf3fd 100644
--- a/src/test/scala/chiselTests/Counter.scala
+++ b/src/test/scala/chiselTests/Counter.scala
@@ -8,8 +8,10 @@ import chisel3.util._
 
 class CountTester(max: Int) extends BasicTester {
   val cnt = Counter(max)
+  assert(cnt.n == max)
   when(true.B) { cnt.inc() }
-  when(cnt.value === (max-1).asUInt) {
+  val expected = if (max == 0) 0.U else (max - 1).U
+  when(cnt.value === expected) {
     stop()
   }
 }
@@ -50,7 +52,9 @@ class RangeTester(r: Range) extends BasicTester {
 
 class CounterSpec extends ChiselPropSpec {
   property("Counter should count up") {
-    forAll(smallPosInts) { (max: Int) => assertTesterPasses{ new CountTester(max) } }
+    for (i <- 0 until 4) {
+      assertTesterPasses(new CountTester(i))
+    }
   }
 
   property("Counter can be en/disabled") {
-- 
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