1 files changed, 59 insertions, 0 deletions
diff --git a/chiselFrontend/src/main/scala/chisel3/core/When.scala b/chiselFrontend/src/main/scala/chisel3/core/When.scala
new file mode 100644
index 00000000..5d484313
--- /dev/null
+++ b/chiselFrontend/src/main/scala/chisel3/core/When.scala
@@ -0,0 +1,59 @@
+// See LICENSE for license details.
+
+package chisel.core
+
+import scala.language.experimental.macros
+
+import chisel.internal._
+import chisel.internal.Builder.pushCommand
+import chisel.internal.firrtl._
+import chisel.internal.sourceinfo.{SourceInfo}
+
+object when {  // scalastyle:ignore object.name
+  /** Create a `when` condition block, where whether a block of logic is
+    * executed or not depends on the conditional.
+    *
+    * @param cond condition to execute upon
+    * @param block logic that runs only if `cond` is true
+    *
+    * @example
+    * {{{
+    * when ( myData === UInt(3) ) {
+    *   // Some logic to run when myData equals 3.
+    * } .elsewhen ( myData === UInt(1) ) {
+    *   // Some logic to run when myData equals 1.
+    * } .otherwise {
+    *   // Some logic to run when myData is neither 3 nor 1.
+    * }
+    * }}}
+    */
+  def apply(cond: Bool)(block: => Unit)(implicit sourceInfo: SourceInfo): WhenContext = {
+    new WhenContext(sourceInfo, cond, !cond, block)
+  }
+}
+
+/** Internal mechanism for generating a when. Because of the way FIRRTL
+  * commands are emitted, generating a FIRRTL elsewhen or nested whens inside
+  * elses would be difficult. Instead, this keeps track of the negative of the
+  * previous conditions, so when an elsewhen or otherwise is used, it checks
+  * that both the condition is true and all the previous conditions have been
+  * false.
+  */
+final class WhenContext(sourceInfo: SourceInfo, cond: Bool, prevCond: => Bool, block: => Unit) {
+  /** This block of logic gets executed if above conditions have been false
+    * and this condition is true.
+    */
+  def elsewhen (elseCond: Bool)(block: => Unit)(implicit sourceInfo: SourceInfo): WhenContext = {
+    new WhenContext(sourceInfo, prevCond && elseCond, prevCond && !elseCond, block)
+  }
+
+  /** This block of logic gets executed only if the above conditions were all
+    * false. No additional logic blocks may be appended past the `otherwise`.
+    */
+  def otherwise(block: => Unit)(implicit sourceInfo: SourceInfo): Unit =
+    new WhenContext(sourceInfo, prevCond, null, block)
+
+  pushCommand(WhenBegin(sourceInfo, cond.ref))
+  block
+  pushCommand(WhenEnd(sourceInfo))
+}