From 5d74c5e01d8032bc499f96f0019e55f03401df9d Mon Sep 17 00:00:00 2001
From: Daniel Bevenius <daniel.bevenius@gmail.com>
Date: Sat, 1 Jun 2019 15:08:17 +0200
Subject: [PATCH] allow custom gates to be overwritten

This commit adds the ability to overwrite/redefine the a custom gate.
The motivation for this came from wanting to have multiple circuits in a
program and allow them to have the same name for a custom gate, but the
matrix for the gate would be different for each circuit.
---
 CHANGELOG.md                                 |  1 +
 packages/qiskit-sim/lib/gates.js             |  4 +++-
 packages/qiskit-sim/test/functional/gates.js | 10 ++++++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3d47496..149086b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - `@qiskit/qiskit-sim`: add prettyMatrix method to Gate class
 - `@qiskit/qiskit-sim`: add gates to gates map in constructor
 - `@qiskit/qiskit-sim`: remove unused addGate function in grammar.jison
+- `@qiskit/qiskit-sim`: allow custom gates to be overwritten
 
 ## [0.9.0] - 2019-05-13
 
diff --git a/packages/qiskit-sim/lib/gates.js b/packages/qiskit-sim/lib/gates.js
index 76fd859..dae0fca 100644
--- a/packages/qiskit-sim/lib/gates.js
+++ b/packages/qiskit-sim/lib/gates.js
@@ -43,12 +43,13 @@ const phaseShift = shift =>
   [[1, 0], [0, math.pow(math.e, math.multiply(math.i, math.PI / shift))]];
 
 const gates = new Map();
+let builtinGateNames;
 
 class Gate {
   constructor(name, matrix) {
     this.name = name;
     this.matrix = matrix;
-    if (!gates.has(name)) {
+    if (!builtinGateNames || !builtinGateNames.has(name)) {
       gates.set(name, this);
     }
   }
@@ -103,6 +104,7 @@ Gate.cs = new Gate('cs', buildControlled(Gate.s));
 Gate.cr2 = new Gate('cr2', buildControlled(Gate.r2));
 Gate.cr4 = new Gate('cr4', buildControlled(Gate.r4));
 Gate.cr8 = new Gate('cr8', buildControlled(Gate.r8));
+builtinGateNames = new Set(gates.keys());
 
 module.exports = {
   Gate,
diff --git a/packages/qiskit-sim/test/functional/gates.js b/packages/qiskit-sim/test/functional/gates.js
index 5d85a5f..5697d6d 100644
--- a/packages/qiskit-sim/test/functional/gates.js
+++ b/packages/qiskit-sim/test/functional/gates.js
@@ -55,4 +55,14 @@ describe('sim:gates', () => {
     const custom = new Gate('custom', [[1, 0], [0, 1]]);
     assert.ok(gates.has(custom.name));
   });
+
+  it('custom gate should be allowed to be overwritten', () => {
+    const custom = new Gate('custom', [[1, 0], [0, 1]]);
+    const newMatrix = [[1, 0],
+                      [1, 1]];
+    const overwrite = new Gate(custom.name, newMatrix);
+    assert.ok(gates.has(overwrite.name));
+    assert.strictEqual(gates.get(custom.name).matrix, newMatrix);
+  });
+
 });