2 files changed, 90 insertions, 29 deletions
diff --git a/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala b/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
index 86973e5b..8c85b6d1 100644
--- a/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
+++ b/src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
@@ -87,6 +87,6 @@ object EspressoMinimizer extends Minimizer with LazyLogging {
     logger.trace(s"""espresso output table:
                     |$output
                     |""".stripMargin)
-    TruthTable(readTable(output), table.default)
+    TruthTable.fromEspressoOutput(readTable(output), table.default)
   }
 }
diff --git a/src/main/scala/chisel3/util/experimental/decode/TruthTable.scala b/src/main/scala/chisel3/util/experimental/decode/TruthTable.scala
index 00fa0f9c..2720e690 100644
--- a/src/main/scala/chisel3/util/experimental/decode/TruthTable.scala
+++ b/src/main/scala/chisel3/util/experimental/decode/TruthTable.scala
@@ -3,6 +3,7 @@
 package chisel3.util.experimental.decode
 
 import chisel3.util.BitPat
+import scala.collection.mutable
 
 sealed class TruthTable private (val table: Seq[(BitPat, BitPat)], val default: BitPat, val sort: Boolean) {
   def inputWidth = table.head._1.getWidth
@@ -29,40 +30,89 @@ sealed class TruthTable private (val table: Seq[(BitPat, BitPat)], val default:
 
 object TruthTable {
 
-  /** Convert a table and default output into a [[TruthTable]]. */
-  def apply(table: Iterable[(BitPat, BitPat)], default: BitPat, sort: Boolean = true): TruthTable = {
+  /** Pad the input signals to equalize all input widths. Pads input signals
+    *  to the maximum width found in the table.
+    *
+    * @param table the truth table whose rows will be padded
+    * @return the same truth table but with inputs padded
+    */
+  private def padInputs(table: Iterable[(BitPat, BitPat)]): Iterable[(BitPat, BitPat)] = {
     val inputWidth = table.map(_._1.getWidth).max
-    require(table.map(_._2.getWidth).toSet.size == 1, "output width not equal.")
-    val outputWidth = table.map(_._2.getWidth).head
-    val mergedTable = table.map {
-      // pad input signals if necessary
+    table.map {
       case (in, out) if inputWidth > in.width =>
         (BitPat.N(inputWidth - in.width) ## in, out)
       case (in, out) => (in, out)
     }
-      .groupBy(_._1.toString)
-      .map {
-        case (key, values) =>
-          // merge same input inputs.
-          values.head._1 -> BitPat(s"b${Seq
-            .tabulate(outputWidth) { i =>
-              val outputSet = values
-                .map(_._2)
-                .map(_.rawString)
-                .map(_(i))
-                .toSet
-                .filterNot(_ == '?')
-              require(
-                outputSet.size != 2,
-                s"TruthTable conflict in :\n${values.map { case (i, o) => s"${i.rawString}->${o.rawString}" }.mkString("\n")}"
-              )
-              outputSet.headOption.getOrElse('?')
-            }
-            .mkString}")
-      }
-      .toSeq
+  }
+
+  /** For each duplicated input, collect the outputs into a single Seq.
+    *
+    * @param table the truth table
+    * @return a Seq of tuple of length 2, where the first element is the
+    *         input and the second element is a Seq of OR-ed outputs
+    *         for the input
+    */
+  private def mergeTableOnInputs(table: Iterable[(BitPat, BitPat)]): Seq[(BitPat, Seq[BitPat])] = {
+    groupByIntoSeq(table)(_._1).map {
+      case (input, mappings) =>
+        input -> mappings.map(_._2)
+    }
+  }
+
+  /** Merge two BitPats by OR-ing the values and masks, and setting the
+    *  width to the max width among the two
+    */
+  private def merge(a: BitPat, b: BitPat): BitPat = {
+    new BitPat(a.value | b.value, a.mask | b.mask, a.width.max(b.width))
+  }
+
+  /** Public method for calling with the Espresso decoder format fd
+    *
+    * For Espresso, for each output, a 1 means this product term belongs to the ON-set,
+    *  a 0 means this product term has no meaning for the value of this function.
+    * This is the same as the fd (or f) type in espresso.
+    *
+    * @param table the truth table
+    * @param default the default BitPat is made up of a single bit type, either "?", "0" or "1".
+    *                A default of "?" sets Espresso to fr-format, while a "0" or "1" sets it to the
+    *                fd-format.
+    * @param sort whether to sort the final truth table using BitPat.bitPatOrder
+    * @return a fully built TruthTable
+    */
+  def fromEspressoOutput(table: Iterable[(BitPat, BitPat)], default: BitPat, sort: Boolean = true): TruthTable = {
+    apply_impl(table, default, sort, false)
+  }
+
+  /** Public apply method to TruthTable. Calls apply_impl with the default value true of checkCollisions */
+  def apply(table: Iterable[(BitPat, BitPat)], default: BitPat, sort: Boolean = true): TruthTable = {
+    apply_impl(table, default, sort, true)
+  }
+
+  /** Convert a table and default output into a [[TruthTable]]. */
+  private def apply_impl(
+    table:           Iterable[(BitPat, BitPat)],
+    default:         BitPat,
+    sort:            Boolean,
+    checkCollisions: Boolean
+  ): TruthTable = {
+    val paddedTable = padInputs(table)
+
+    require(table.map(_._2.getWidth).toSet.size == 1, "output width not equal.")
+
+    val mergedTable = mergeTableOnInputs(paddedTable)
+
+    val finalTable: Seq[(BitPat, BitPat)] = mergedTable.map {
+      case (input, outputs) =>
+        val (result, noCollisions) = outputs.tail.foldLeft((outputs.head, checkCollisions)) {
+          case ((acc, ok), o) => (merge(acc, o), ok && acc.overlap(o))
+        }
+        // Throw an error if checkCollisions is true but there are bits with a non-zero overlap.
+        require(!checkCollisions || noCollisions, s"TruthTable conflict on merged row:\n  $input -> $outputs")
+        (input, result)
+    }
+
     import BitPat.bitPatOrder
-    new TruthTable(if (sort) mergedTable.sorted else mergedTable, default, sort)
+    new TruthTable(if (sort) finalTable.sorted else finalTable, default, sort)
   }
 
   /** Parse TruthTable from its string representation. */
@@ -140,4 +190,15 @@ object TruthTable {
       bitPat(tables.flatMap { case (table, indexes) => table.default.rawString.zip(indexes) })
     )
   }
+
+  /** Similar to Seq.groupBy except that it preserves ordering of elements within each group */
+  private def groupByIntoSeq[A, K](xs: Iterable[A])(f: A => K): Seq[(K, Seq[A])] = {
+    val map = mutable.LinkedHashMap.empty[K, mutable.ListBuffer[A]]
+    for (x <- xs) {
+      val key = f(x)
+      val l = map.getOrElseUpdate(key, mutable.ListBuffer.empty[A])
+      l += x
+    }
+    map.view.map({ case (k, vs) => k -> vs.toList }).toList
+  }
 }