Don't assume expressions being substituted are mod 4

pytket/tests/circuit_test.py

@@ -223,6 +223,18 @@ def test_symbolic_ops() -> None:
     assert np.allclose(commands[1].op.params, [2.4], atol=1e-10)
 
 
+def test_subst_4() -> None:
+    # https://github.com/CQCL/tket/issues/219
+    m = fresh_symbol("m")
+    c = Circuit(1)
+    a = m / 4
+    c.add_gate(OpType.Rx, a, [0])
+    c.symbol_substitution({m: 4})
+    angle = c.get_commands()[0].op.params[0]
+    print(angle)
+    assert np.isclose(angle, 1.0)
+
+
 def test_sympy_conversion() -> None:
     def get_type_tree(expr: sympy.Expr) -> str:
         # Format e.g. "<class 'sympy.core.numbers.Pi'>" to "Pi"

tket/src/Circuit/Circuit.cpp

@@ -142,7 +142,7 @@ void Circuit::symbol_substitution(const symbol_map_t &symbol_map) {
     // This is a workaround for a symengine issue: symengine currently has poor
     // handling of symbolic evaluations for atan2. However, this may not catch
     // every such issue, so we should revisit it.
-    if (equiv_0(e, 4)) {
+    if (approx_0(e)) {
       sub_map[s] = SymEngine::zero;
     } else {
       sub_map[s] = e;

tket/src/Gate/Rotation.cpp

@@ -21,11 +21,6 @@
 
 namespace tket {
 
-static bool approx_0(const Expr &e) {
-  std::optional<double> v = eval_expr(e);
-  return v && (std::abs(v.value()) < EPS);
-}
-
 static Expr atan2_bypi(const Expr &a, const Expr &b) {
   std::optional<double> va = eval_expr(a);
   std::optional<double> vb = eval_expr(b);

tket/src/Utils/Expression.cpp

@@ -19,6 +19,11 @@
 
 namespace tket {
 
+bool approx_0(const Expr& e, double tol) {
+  std::optional<double> v = eval_expr(e);
+  return v && (std::abs(v.value()) < tol);
+}
+
 double fmodn(double x, unsigned n) {
   x /= n;
   x -= floor(x);

tket/src/Utils/include/Utils/Expression.hpp

@@ -122,6 +122,16 @@ Expr cos_halfpi_times(const Expr& e);
  */
 Expr sin_halfpi_times(const Expr& e);
 
+/**
+ * Test if an expression is approximately zero
+ *
+ * @param e expression
+ * @param tol tolerance
+ *
+ * @return whether \p e is within \p tol of zero
+ */
+bool approx_0(const Expr& e, double tol = EPS);
+
 /**
  * Evaluate modulo n in the range [0,n)
  *