From 0d521bb9da4fcf3779e9e3e3f62ef65147c63599 Mon Sep 17 00:00:00 2001
From: Hyunjin Song <tteu.ingog@gmail.com>
Date: Sun, 21 Jun 2020 11:45:03 +0900
Subject: [PATCH] Fix overestimation in MachineLearning.NQubitsRequired

---
 MachineLearning/src/Structure.qs        | 15 ++++++---------
 MachineLearning/tests/StructureTests.qs |  3 ++-
 2 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/MachineLearning/src/Structure.qs b/MachineLearning/src/Structure.qs
index a3a80c5abd1..1f80528c3e9 100644
--- a/MachineLearning/src/Structure.qs
+++ b/MachineLearning/src/Structure.qs
@@ -20,17 +20,14 @@ namespace Microsoft.Quantum.MachineLearning {
     /// may be applied.
     function NQubitsRequired(model : SequentialModel)
     : Int {
-        mutable nQubitsRequired = 0;
+        mutable lastQubitIndex = -1; // Need to return 0 if there are no gates
         for (gate in model::Structure) {
-            set nQubitsRequired = 1 + Fold(
-                MaxI, 0,
-                gate::ControlIndices + [
-                    gate::TargetIndex,
-                    nQubitsRequired
-                ]
+            set lastQubitIndex = Fold(
+                MaxI, lastQubitIndex,
+                gate::ControlIndices + [gate::TargetIndex]
             );
         }
-        return nQubitsRequired;
+        return lastQubitIndex + 1;
     }
 
     /// # Summary
@@ -210,4 +207,4 @@ namespace Microsoft.Quantum.MachineLearning {
         return combined;
     }
 
-}
\ No newline at end of file
+}
diff --git a/MachineLearning/tests/StructureTests.qs b/MachineLearning/tests/StructureTests.qs
index e9ee25a29ae..e9ea8618386 100644
--- a/MachineLearning/tests/StructureTests.qs
+++ b/MachineLearning/tests/StructureTests.qs
@@ -12,7 +12,8 @@ namespace Microsoft.Quantum.MachineLearning.Tests {
     function NQubitsRequiredFact() : Unit {
         let model = Default<ML.SequentialModel>()
             w/ Structure <- [
-                ML.ControlledRotation((3, [7, 9]), PauliX, 0)
+                ML.ControlledRotation((3, [7, 9]), PauliX, 0),
+                ML.ControlledRotation((8, new Int[0]), PauliY, 1)
             ];
         let actual = ML.NQubitsRequired(model);
         EqualityFactI(actual, 10, "Wrong output from NQubitsRequired.");