Add Expression Fuzzer (#1741)

Includes:
* Random generator of FIRRTL Expressions (UInt and SInt types)
* JQF SBT plugin and CLI
* Documentation in README.md

Co-authored-by: Jack Koenig <koenig@sifive.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/fuzzer/src/main/scala/firrtl/ExprState.scala b/fuzzer/src/main/scala/firrtl/ExprState.scala
new file mode 100644
index 00000000..f1a2f0d1
--- /dev/null
+++ b/fuzzer/src/main/scala/firrtl/ExprState.scala
@@ -0,0 +1,30 @@
+package firrtl.fuzzer
+
+import firrtl.ir.{Expression, Reference, Type}
+
+import scala.language.higherKinds
+
+/** A typeclass for types that represent the state of a random expression generator
+  */
+trait ExprState[State] {
+
+  /** Creates a [[StateGen]] that adds a reference to the input state and returns that reference
+    */
+  def withRef[Gen[_]: GenMonad](ref: Reference): StateGen[State, Gen, Reference]
+
+  /** Creates a [[StateGen]] that returns an [[firrtl.ir.Expression Expression]] with the specified type
+    */
+  def exprGen[Gen[_]: GenMonad](tpe: Type): StateGen[State, Gen, Expression]
+
+  /** Gets the set of unbound references
+    */
+  def unboundRefs(s: State): Set[Reference]
+
+  /** Gets the maximum allowed width of any Expression
+    */
+  def maxWidth(s: State): Int
+}
+
+object ExprState {
+  def apply[S: ExprState] = implicitly[ExprState[S]]
+}