Fix parsing of huge OpenQASM 2 conditionals (#12774)

* Fix parsing of huge OpenQASM 2 conditionals

We fixed handling of giant integers in gate expression positions
gh-12140, and this commit fixes the handling in conditionals.
Unfortunately, this means pulling in big-int handling properly; the
integers simply _are_ bigints, and we're not immediately converting them
into something else.

The need to support this may influence how the Rust-space data models of
`QuantumCircuit` evolve.

* Move `num-bigint` dependency to workspace

(cherry picked from commit 85f9860)

Cargo.toml

@@ -17,6 +17,7 @@ license = "Apache-2.0"
 bytemuck = "1.16"
 indexmap.version = "2.2.6"
 hashbrown.version = "0.14.0"
+num-bigint = "0.4"
 num-complex = "0.4"
 ndarray = "^0.15.6"
 numpy = "0.21.0"

crates/accelerate/Cargo.toml

@@ -18,7 +18,7 @@ rand_distr = "0.4.3"
 ahash = "0.8.11"
 num-traits = "0.2"
 num-complex.workspace = true
-num-bigint = "0.4"
+num-bigint.workspace = true
 rustworkx-core = "0.15"
 faer = "0.19.1"
 itertools = "0.13.0"

crates/qasm2/Cargo.toml

@@ -10,6 +10,7 @@ name = "qiskit_qasm2"
 doctest = false
 
 [dependencies]
+num-bigint.workspace = true
 hashbrown.workspace = true
 pyo3.workspace = true
 qiskit-circuit.workspace = true

crates/qasm2/src/bytecode.rs

@@ -10,6 +10,7 @@
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
 
+use num_bigint::BigUint;
 use pyo3::prelude::*;
 
 use crate::error::QASM2ParseError;
@@ -160,7 +161,7 @@ pub enum InternalBytecode {
         arguments: Vec<f64>,
         qubits: Vec<QubitId>,
         creg: CregId,
-        value: usize,
+        value: BigUint,
     },
     Measure {
         qubit: QubitId,
@@ -170,15 +171,15 @@ pub enum InternalBytecode {
         qubit: QubitId,
         clbit: ClbitId,
         creg: CregId,
-        value: usize,
+        value: BigUint,
     },
     Reset {
         qubit: QubitId,
     },
     ConditionedReset {
         qubit: QubitId,
         creg: CregId,
-        value: usize,
+        value: BigUint,
     },
     Barrier {
         qubits: Vec<QubitId>,

crates/qasm2/src/lex.rs

@@ -24,6 +24,7 @@
 //! real-number tokenization.
 
 use hashbrown::HashMap;
+use num_bigint::BigUint;
 use pyo3::prelude::PyResult;
 
 use std::path::Path;
@@ -279,6 +280,15 @@ impl Token {
         context.text[self.index].parse().unwrap()
     }
 
+    /// If the token is an integer (by type, not just by value), this method can be called to
+    /// evaluate its value as a big integer.  Panics if the token is not an integer type.
+    pub fn bigint(&self, context: &TokenContext) -> BigUint {
+        if self.ttype != TokenType::Integer {
+            panic!()
+        }
+        context.text[self.index].parse().unwrap()
+    }
+
     /// If the token is a filename path, this method can be called to get a (regular) string
     /// representing it.  Panics if the token type was not a filename.
     pub fn filename(&self, context: &TokenContext) -> String {

crates/qasm2/src/parse.rs

@@ -16,6 +16,7 @@
 //! operator-precedence parser.
 
 use hashbrown::{HashMap, HashSet};
+use num_bigint::BigUint;
 use pyo3::prelude::{PyObject, PyResult, Python};
 
 use crate::bytecode::InternalBytecode;
@@ -188,9 +189,10 @@ enum GateParameters {
 
 /// An equality condition from an `if` statement.  These can condition gate applications, measures
 /// and resets, although in practice they're basically only ever used on gates.
+#[derive(Clone)]
 struct Condition {
     creg: CregId,
-    value: usize,
+    value: BigUint,
 }
 
 /// Find the first match for the partial [filename] in the directories along [path].  Returns
@@ -1105,7 +1107,7 @@ impl State {
         } {
             return match parameters {
                 GateParameters::Constant(parameters) => {
-                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, &condition)
+                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, condition)
                 }
                 GateParameters::Expression(parameters) => {
                     self.emit_single_gate_gate(bc, gate_id, parameters, qubits)
@@ -1174,7 +1176,7 @@ impl State {
             }
             return match parameters {
                 GateParameters::Constant(parameters) => {
-                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, &condition)
+                    self.emit_single_global_gate(bc, gate_id, parameters, qubits, condition)
                 }
                 GateParameters::Expression(parameters) => {
                     self.emit_single_gate_gate(bc, gate_id, parameters, qubits)
@@ -1196,7 +1198,7 @@ impl State {
                         gate_id,
                         parameters.clone(),
                         qubits,
-                        &condition,
+                        condition.clone(),
                     )?;
                 }
                 // Gates used in gate-body definitions can't ever broadcast, because their only
@@ -1215,7 +1217,7 @@ impl State {
         gate_id: GateId,
         arguments: Vec<f64>,
         qubits: Vec<QubitId>,
-        condition: &Option<Condition>,
+        condition: Option<Condition>,
     ) -> PyResult<usize> {
         if let Some(condition) = condition {
             bc.push(Some(InternalBytecode::ConditionedGate {
@@ -1262,7 +1264,7 @@ impl State {
         self.expect(TokenType::Equals, "'=='", &if_token)?;
         let value = self
             .expect(TokenType::Integer, "an integer", &if_token)?
-            .int(&self.context);
+            .bigint(&self.context);
         self.expect(TokenType::RParen, "')'", &lparen_token)?;
         let name = name_token.id(&self.context);
         let creg = match self.symbols.get(&name) {
@@ -1408,7 +1410,7 @@ impl State {
                             qubit: q_start + i,
                             clbit: c_start + i,
                             creg,
-                            value,
+                            value: value.clone(),
                         })
                     }));
                     Ok(q_size)
@@ -1477,7 +1479,7 @@ impl State {
                         Some(InternalBytecode::ConditionedReset {
                             qubit: start + offset,
                             creg,
-                            value,
+                            value: value.clone(),
                         })
                     }));
                     Ok(size)

releasenotes/notes/qasm2-big-condition-cfd203d53540d4ca.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The OpenQASM 2 parser (:mod:`qiskit.qasm2`) can now handle conditionals
+    with integers that do not fit within a 64-bit integer.  Fixed
+    `#12773 <https://github.com/Qiskit/qiskit/issues/12773>`__.

test/python/qasm2/test_structure.py

@@ -324,6 +324,35 @@ def test_parameterless_gates_accept_parentheses(self):
         qc.cx(1, 0)
         self.assertEqual(parsed, qc)
 
+    def test_huge_conditions(self):
+        # Something way bigger than any native integer.
+        bigint = (1 << 300) + 123456789
+        program = f"""
+            qreg qr[2];
+            creg cr[2];
+            creg cond[500];
+            if (cond=={bigint}) U(0, 0, 0) qr[0];
+            if (cond=={bigint}) U(0, 0, 0) qr;
+            if (cond=={bigint}) reset qr[0];
+            if (cond=={bigint}) reset qr;
+            if (cond=={bigint}) measure qr[0] -> cr[0];
+            if (cond=={bigint}) measure qr -> cr;
+        """
+        parsed = qiskit.qasm2.loads(program)
+        qr, cr = QuantumRegister(2, "qr"), ClassicalRegister(2, "cr")
+        cond = ClassicalRegister(500, "cond")
+        qc = QuantumCircuit(qr, cr, cond)
+        qc.u(0, 0, 0, qr[0]).c_if(cond, bigint)
+        qc.u(0, 0, 0, qr[0]).c_if(cond, bigint)
+        qc.u(0, 0, 0, qr[1]).c_if(cond, bigint)
+        qc.reset(qr[0]).c_if(cond, bigint)
+        qc.reset(qr[0]).c_if(cond, bigint)
+        qc.reset(qr[1]).c_if(cond, bigint)
+        qc.measure(qr[0], cr[0]).c_if(cond, bigint)
+        qc.measure(qr[0], cr[0]).c_if(cond, bigint)
+        qc.measure(qr[1], cr[1]).c_if(cond, bigint)
+        self.assertEqual(parsed, qc)
+
 
 class TestGateDefinition(QiskitTestCase):
     def test_simple_definition(self):