Use bezier curves for long links in timeslice diagrams (#737)

![image](https://github.com/quantumlib/Stim/assets/79941/c01d90d9-5723-4676-a523-e76ac5f6229d)

src/stim/diagram/detector_slice/detector_slice_set.cc

@@ -409,6 +409,7 @@ FlattenedCoords FlattenedCoords::from(const DetectorSliceSet &set, float desired
     }
 
     float characteristic_distance = pick_characteristic_distance(used, result.qubit_coords);
+    result.unit_distance = desired_unit_distance;
     float scale = desired_unit_distance / characteristic_distance;
     for (auto &c : result.qubit_coords) {
         c *= scale;

src/stim/diagram/detector_slice/detector_slice_set.h

@@ -81,6 +81,7 @@ struct FlattenedCoords {
     std::vector<Coord<2>> qubit_coords;
     std::map<uint64_t, Coord<2>> det_coords;
     Coord<2> size;
+    float unit_distance;
 
     static FlattenedCoords from(const DetectorSliceSet &set, float desired_unit_distance);
 };

src/stim/diagram/timeline/timeline_svg_drawer.cc

@@ -483,9 +483,27 @@ void DiagramTimelineSvgDrawer::reserve_drawing_room_for_targets(SpanRef<const Ga
             svg_out << "<path d=\"";
             svg_out << "M" << coords[0].xyz[0] << "," << coords[0].xyz[1] << " ";
             for (size_t k = 1; k < coords.size(); k++) {
-                svg_out << "L" << coords[k].xyz[0] << "," << coords[k].xyz[1] << " ";
+                auto p1 = coords[k - 1];
+                auto p2 = coords[k];
+                auto dp = p2 - p1;
+                if (dp.norm() < coord_sys.unit_distance * 1.1) {
+                    svg_out << "L";
+                    svg_out << p2.xyz[0] << "," << p2.xyz[1] << " ";
+                } else {
+                    auto dp2 = Coord<2>{-dp.xyz[1], dp.xyz[0]};
+                    if (2 * dp2.xyz[0] + 3 * dp2.xyz[1] < 0) {
+                        dp2 *= -1;
+                    }
+                    auto p3 = p1 + dp * 0.2 + dp2 * 0.2;
+                    auto p4 = p2 + dp * -0.2 + dp2 * 0.2;
+                    svg_out << "C";
+                    svg_out << p3.xyz[0] << " " << p3.xyz[1] << ",";
+                    svg_out << p4.xyz[0] << " " << p4.xyz[1] << ",";
+                    svg_out << p2.xyz[0] << " " << p2.xyz[1] << " ";
+                }
             }
             svg_out << "\"";
+            write_key_val(svg_out, "fill", "none");
             write_key_val(svg_out, "stroke", "black");
             write_key_val(svg_out, "stroke-width", "5");
             svg_out << "/>\n";

src/stim/diagram/timeline/timeline_svg_drawer.test.cc

@@ -381,3 +381,22 @@ TEST(diagram_timeline_svg_drawer, anticommuting_detector_circuit) {
         circuit, ss, 0, UINT64_MAX, DiagramTimelineSvgDrawerMode::SVG_MODE_TIME_DETECTOR_SLICE, {&empty_filter});
     expect_string_is_identical_to_saved_file(ss.str(), "anticommuting_detslice.svg");
 }
+
+TEST(diagram_timeline_svg_drawer, bezier_curves) {
+    CoordFilter empty_filter;
+    std::stringstream ss;
+    auto circuit = Circuit(R"CIRCUIT(
+        QUBIT_COORDS(0, 0) 0
+        QUBIT_COORDS(1, 0) 1
+        QUBIT_COORDS(2, 0) 2
+        QUBIT_COORDS(3, 0) 3
+        CX 0 1
+        CX 2 3
+        TICK
+        CX 0 2
+        CX 1 3
+    )CIRCUIT");
+    DiagramTimelineSvgDrawer::make_diagram_write_to(
+        circuit, ss, 0, UINT64_MAX, DiagramTimelineSvgDrawerMode::SVG_MODE_TIME_SLICE, {&empty_filter});
+    expect_string_is_identical_to_saved_file(ss.str(), "bezier_time_slice.svg");
+}

src/stim/stabilizers/tableau_iter.test.cc

@@ -116,29 +116,30 @@ TEST_EACH_WORD_SIZE_W(tableau_iter, iter_tableau, {
     TableauIterator<W> iter1_signs(1, true);
     TableauIterator<W> iter2(2, false);
     TableauIterator<W> iter3(3, false);
-    int s1 = 0;
     int n1 = 0;
-    int n2 = 0;
-    int n3 = 0;
     while (iter1.iter_next()) {
         n1++;
     }
     ASSERT_EQ(n1, 6);
 
+    int s1 = 0;
     while (iter1_signs.iter_next()) {
         s1++;
     }
     ASSERT_EQ(s1, 24);
 
+    int n2 = 0;
     while (iter2.iter_next()) {
         n2++;
     }
     ASSERT_EQ(n2, 720);
 
+    // Note: disabled because it takes 2-3 seconds.
+    // int n3 = 0;
     // while (iter3.iter_next()) {
     //     n3++;
     // }
-    // ASSERT_EQ(n3, 1451520);  // Note: disabled because it takes 2-3 seconds.
+    // ASSERT_EQ(n3, 1451520);
 })
 
 TEST_EACH_WORD_SIZE_W(tableau_iter, iter_tableau_distinct, {