Fix SVGCircuit() for cirq.Circuit with empty moments (#4535)

* Modified SVG Representation to include circuits with empty moments

* TODOs

* Reverted the Casting of Ints to Floats

* Consistent TODOs

* removed redundancies, added comments and improved tests

* Formatted Code

* Coverage Error

* Added tests to increase coverage

* Simplified, removed try except

Co-authored-by: Gaurav Singh <gauravsinghk03@gmail.com>
Co-authored-by: Gaurav Singh <71246220+vanhalen42@users.noreply.github.com>
Co-authored-by: Cirq Bot <craiggidney+github+cirqbot@google.com>

cirq-core/cirq/contrib/svg/svg.py

@@ -8,10 +8,13 @@
 
 QBLUE = '#1967d2'
 FONT = "Arial"
+EMPTY_MOMENT_COLWIDTH = float(21)  # assumed default column width
 
 
 def fixup_text(text: str):
     if '\n' in text:
+        # https://github.com/quantumlib/Cirq/issues/4499
+        # TODO: Visualize Custom MatrixGate
         return '?'
     if '[<virtual>]' in text:
         # https://github.com/quantumlib/Cirq/issues/2905
@@ -24,6 +27,8 @@ def fixup_text(text: str):
 
 
 def _get_text_width(t: str) -> float:
+    if t == '':  # in case of an empty moment
+        return EMPTY_MOMENT_COLWIDTH
     t = fixup_text(t)
     tp = matplotlib.textpath.TextPath((0, 0), t, size=14, prop=FONT)
     bb = tp.get_extents()
@@ -234,6 +239,8 @@ def tdd_to_svg(
         if v.text == '×':
             t += _text(x, y + 3, '×', fontsize=40)
             continue
+        if v.text == '':
+            continue
 
         v_text = fixup_text(v.text)
         t += _rect(boxx, boxy, boxwidth, boxheight)
@@ -247,12 +254,6 @@ def _validate_circuit(circuit: 'cirq.Circuit'):
     if len(circuit) == 0:
         raise ValueError("Can't draw SVG diagram for empty circuits")
 
-    if any(len(mom) == 0 for mom in circuit.moments):
-        raise ValueError(
-            "Can't draw SVG diagram for circuits with empty "
-            "moments. Run it through cirq.DropEmptyMoments()"
-        )
-
 
 class SVGCircuit:
     """A wrapper around cirq.Circuit to enable rich display in a Jupyter
@@ -269,13 +270,13 @@ def __init__(self, circuit: 'cirq.Circuit'):
 
     def _repr_svg_(self) -> str:
         # coverage: ignore
-        _validate_circuit(self.circuit)
-        tdd = self.circuit.to_text_diagram_drawer(transpose=False)
-        return tdd_to_svg(tdd)
+        return circuit_to_svg(self.circuit)
 
 
 def circuit_to_svg(circuit: 'cirq.Circuit') -> str:
     """Render a circuit as SVG."""
     _validate_circuit(circuit)
     tdd = circuit.to_text_diagram_drawer(transpose=False)
+    if len(tdd.horizontal_lines) == 0:  # in circuits with no non-empty moments,return a blank SVG
+        return '<svg></svg>'
     return tdd_to_svg(tdd)

cirq-core/cirq/contrib/svg/svg_test.py

@@ -37,6 +37,23 @@ def test_validation():
     with pytest.raises(ValueError):
         circuit_to_svg(cirq.Circuit())
 
-    q0 = cirq.LineQubit(0)
-    with pytest.raises(ValueError):
-        circuit_to_svg(cirq.Circuit([cirq.Moment([cirq.X(q0)]), cirq.Moment([])]))
+
+def test_empty_moments():
+    a, b = cirq.LineQubit.range(2)
+    svg_1 = circuit_to_svg(
+        cirq.Circuit(
+            cirq.Moment(),
+            cirq.Moment(cirq.CNOT(a, b)),
+            cirq.Moment(),
+            cirq.Moment(cirq.SWAP(a, b)),
+            cirq.Moment(cirq.Z(a)),
+            cirq.Moment(cirq.measure(a, b, key='z')),
+            cirq.Moment(),
+        )
+    )
+    assert '<svg' in svg_1
+    assert '</svg>' in svg_1
+
+    svg_2 = circuit_to_svg(cirq.Circuit(cirq.Moment()))
+    assert '<svg' in svg_2
+    assert '</svg>' in svg_2