From dc2daed0ebca05c3331d98311eed15465e1fbf33 Mon Sep 17 00:00:00 2001
From: Craig Gidney <craig.gidney@gmail.com>
Date: Sun, 24 Mar 2019 12:13:19 -0700
Subject: [PATCH] Fix detect-control-reset gates not being able to toggle
 classical wires

---
 src/circuit/Gate.js                    | 21 ++++++++++++++++++++-
 src/circuit/GateColumn.js              |  3 ++-
 src/gates/Detector.js                  |  2 +-
 test/circuit/CircuitDefinition.test.js |  6 ++++++
 4 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/src/circuit/Gate.js b/src/circuit/Gate.js
index 9db006b3..9e50e93c 100644
--- a/src/circuit/Gate.js
+++ b/src/circuit/Gate.js
@@ -158,6 +158,14 @@ class Gate {
          * @private
          */
         this._controlBit = undefined;
+        /**
+         * Determines if a controlled gate's control is guaranteed to be known classically. This is used by the
+         * combination detect-control-reset gates as a way to communicate that they can control permutation operations
+         * on classical wires.
+         * @type {boolean}
+         * @private
+         */
+        this._isClassicalControl = false;
         /**
          * Indicates that this gate is guaranteed to preserve probability (as opposed to e.g. post-selection).
          * When gates with this property not set to true are present in a column, the simulator computes losses/gains.
@@ -284,6 +292,7 @@ class Gate {
         g._effectCreatesSuperpositions = this._effectCreatesSuperpositions;
         g._affectsOtherWires = this._affectsOtherWires;
         g._controlBit = this._controlBit;
+        g._isClassicalControl = this._isClassicalControl;
         g.isControlWireSource = this.isControlWireSource;
         g._isDefinitelyUnitary = this._isDefinitelyUnitary;
         g.knownPhaseTurnsFunc = this.knownPhaseTurnsFunc;
@@ -393,6 +402,13 @@ class Gate {
         return this._controlBit !== undefined;
     }
 
+    /**
+     * @returns {!boolean}
+     */
+    isClassicalControl() {
+        return this._isClassicalControl;
+    }
+
     /**
      * @returns {undefined|!boolean}
      */
@@ -809,12 +825,15 @@ class GateBuilder {
     /**
      * Sets meta-properties to indicate a gate is a control.
      * @param {!boolean} bit: Whether gate is a control or anti-control. Use before/after operations for flexibility.
+     * @param {!boolean} guaranteedClassical Whether or not the control can be used to control permutations of classical
+     *     wires, even if placed on a coherent wire.
      * @returns {!GateBuilder}
      */
-    markAsControlExpecting(bit) {
+    markAsControlExpecting(bit, guaranteedClassical=false) {
         this.gate._controlBit = bit;
         this.gate.isControlWireSource = true;
         this.gate.interestedInControls = false;
+        this.gate._isClassicalControl = guaranteedClassical;
         return this;
     }
 
diff --git a/src/circuit/GateColumn.js b/src/circuit/GateColumn.js
index f809ebb8..d7ecf18e 100644
--- a/src/circuit/GateColumn.js
+++ b/src/circuit/GateColumn.js
@@ -87,7 +87,8 @@ class GateColumn {
         for (let i = 0; i < this.gates.length; i++) {
             if ((inputMeasureMask & (1 << i)) === 0 &&
                     this.gates[i] !== undefined &&
-                    this.gates[i].isControl()) {
+                    this.gates[i].isControl() &&
+                    !this.gates[i].isClassicalControl()) {
                 return true;
             }
         }
diff --git a/src/gates/Detector.js b/src/gates/Detector.js
index 1f9bcf78..857a7926 100644
--- a/src/gates/Detector.js
+++ b/src/gates/Detector.js
@@ -370,7 +370,7 @@ function makeDetectControlClearGate(axis) {
         setTitle(`${axis} Detect-Control-Reset`).
         setBlurb(`Does a collapsing ${axis}-axis measurement.\nControls operations with the result.\nResets the target to |0⟩.`).
         setDrawer(args => drawDetectClearReset(args, axis)).
-        markAsControlExpecting(true).
+        markAsControlExpecting(true, true).
         markAsReachingOtherWires().
         setActualEffectToUpdateFunc(() => {}).
         setStatTexturesMaker(withClearedControls(detectorStatTexture)).
diff --git a/test/circuit/CircuitDefinition.test.js b/test/circuit/CircuitDefinition.test.js
index 36d076c9..0b7d5cb6 100644
--- a/test/circuit/CircuitDefinition.test.js
+++ b/test/circuit/CircuitDefinition.test.js
@@ -901,6 +901,12 @@ suite.test("gateAtLocIsDisabledReason", () => {
     bad(1, 0, `MR`, ['R', Gates.Detectors.XDetectControlClear]);
     bad(1, 0, `MR`, ['R', Gates.Detectors.YDetectControlClear]);
     good(1, 0, `MR`, ['R', Gates.Detectors.ZDetectControlClear]);
+    good(3, 1, `---]-
+                -M-X-`, [']', Gates.Detectors.XDetectControlClear]);
+    good(3, 1, `---]-
+                -M-X-`, [']', Gates.Detectors.YDetectControlClear]);
+    good(3, 1, `---]-
+                -M-X-`, [']', Gates.Detectors.ZDetectControlClear]);
 
     // Permutation sub-groups.
     good(2, 0, `--P-