quantrs2_device/

transpiler.rs

//! Circuit transpilation utilities for hardware device connectivity.
//!
//! Converts QuantRS2 circuits to QASM and transpiles them to match device
//! qubit connectivity graphs, basis gate sets, and physical qubit mappings.

use quantrs2_circuit::prelude::Circuit;
use quantrs2_core::gate::single::{RotationX, RotationY, RotationZ};
use quantrs2_core::prelude::QubitId;
use std::collections::{HashMap, VecDeque};

use crate::DeviceError;
use crate::DeviceResult;

/// QASM string representation of a quantum circuit
#[derive(Debug, Clone)]
pub struct QasmCircuit {
    /// QASM content
    pub content: String,
    /// Number of qubits in the circuit
    pub qubit_count: usize,
    /// Number of classical bits
    pub cbit_count: usize,
    /// Gate counts by type
    pub gate_counts: HashMap<String, usize>,
}

/// Circuit transpiler that converts between different quantum circuit representations.
///
/// Converts QuantRS2 [`Circuit`] instances to OpenQASM 2.0 text format and
/// performs physical qubit remapping for hardware connectivity constraints.
///
/// # Examples
///
/// ```rust
/// use quantrs2_device::transpiler::CircuitTranspiler;
/// use quantrs2_circuit::builder::Circuit;
///
/// let mut circ: Circuit<2> = Circuit::new();
/// circ.h(0).expect("h").cnot(0, 1).expect("cnot");
///
/// let qasm = CircuitTranspiler::circuit_to_qasm(&circ, None)
///     .expect("transpile failed");
/// assert!(qasm.content.contains("OPENQASM 2.0"));
/// assert_eq!(qasm.qubit_count, 2);
/// ```
pub struct CircuitTranspiler;

impl CircuitTranspiler {
    /// Convert a Quantrs circuit to OpenQASM 2.0 format
    pub fn circuit_to_qasm<const N: usize>(
        circuit: &Circuit<N>,
        qubit_mapping: Option<&HashMap<usize, usize>>,
    ) -> DeviceResult<QasmCircuit> {
        // Start QASM generation
        let mut qasm = String::from("OPENQASM 2.0;\ninclude \"qelib1.inc\";\n\n");

        // Define the quantum and classical registers
        use std::fmt::Write;
        let _ = writeln!(qasm, "qreg q[{N}];");
        let _ = writeln!(qasm, "creg c[{N}];");

        // Track gate counts
        let mut gate_counts = HashMap::new();

        // Process each gate in the circuit
        for gate in circuit.gates() {
            let gate_qasm = match gate.name() {
                "X" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("x".to_string()).or_insert(0) += 1;
                    format!("x q[{q}];")
                }
                "Y" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("y".to_string()).or_insert(0) += 1;
                    format!("y q[{q}];")
                }
                "Z" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("z".to_string()).or_insert(0) += 1;
                    format!("z q[{q}];")
                }
                "H" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("h".to_string()).or_insert(0) += 1;
                    format!("h q[{q}];")
                }
                "S" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("s".to_string()).or_insert(0) += 1;
                    format!("s q[{q}];")
                }
                "S†" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("sdg".to_string()).or_insert(0) += 1;
                    format!("sdg q[{q}];")
                }
                "T" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("t".to_string()).or_insert(0) += 1;
                    format!("t q[{q}];")
                }
                "T†" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    *gate_counts.entry("tdg".to_string()).or_insert(0) += 1;
                    format!("tdg q[{q}];")
                }
                "CNOT" => {
                    if gate.qubits().len() != 2 {
                        continue;
                    }
                    let control = gate.qubits()[0];
                    let target = gate.qubits()[1];
                    let c = map_qubit(control.id() as usize, &qubit_mapping);
                    let t = map_qubit(target.id() as usize, &qubit_mapping);
                    *gate_counts.entry("cx".to_string()).or_insert(0) += 1;
                    format!("cx q[{c}], q[{t}];")
                }
                "CZ" => {
                    if gate.qubits().len() != 2 {
                        continue;
                    }
                    let control = gate.qubits()[0];
                    let target = gate.qubits()[1];
                    let c = map_qubit(control.id() as usize, &qubit_mapping);
                    let t = map_qubit(target.id() as usize, &qubit_mapping);
                    *gate_counts.entry("cz".to_string()).or_insert(0) += 1;
                    format!("cz q[{c}], q[{t}];")
                }
                "SWAP" => {
                    if gate.qubits().len() != 2 {
                        continue;
                    }
                    let q1 = gate.qubits()[0];
                    let q2 = gate.qubits()[1];
                    let q1_mapped = map_qubit(q1.id() as usize, &qubit_mapping);
                    let q2_mapped = map_qubit(q2.id() as usize, &qubit_mapping);
                    *gate_counts.entry("swap".to_string()).or_insert(0) += 1;
                    format!("swap q[{q1_mapped}], q[{q2_mapped}];")
                }
                "RX" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    let theta = gate
                        .as_any()
                        .downcast_ref::<RotationX>()
                        .map(|g| g.theta)
                        .unwrap_or(0.0);
                    *gate_counts.entry("rx".to_string()).or_insert(0) += 1;
                    format!("rx({theta}) q[{q}];")
                }
                "RY" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    let theta = gate
                        .as_any()
                        .downcast_ref::<RotationY>()
                        .map(|g| g.theta)
                        .unwrap_or(0.0);
                    *gate_counts.entry("ry".to_string()).or_insert(0) += 1;
                    format!("ry({theta}) q[{q}];")
                }
                "RZ" => {
                    if gate.qubits().len() != 1 {
                        continue;
                    }
                    let qubit = gate.qubits()[0];
                    let q = map_qubit(qubit.id() as usize, &qubit_mapping);
                    let theta = gate
                        .as_any()
                        .downcast_ref::<RotationZ>()
                        .map(|g| g.theta)
                        .unwrap_or(0.0);
                    *gate_counts.entry("rz".to_string()).or_insert(0) += 1;
                    format!("rz({theta}) q[{q}];")
                }
                _ => {
                    // For now, return an error for unsupported gates
                    return Err(DeviceError::CircuitConversion(format!(
                        "Unsupported gate type for QASM conversion: {}",
                        gate.name()
                    )));
                }
            };

            let _ = writeln!(qasm, "{gate_qasm}");
        }

        Ok(QasmCircuit {
            content: qasm,
            qubit_count: N,
            cbit_count: N, // Assuming same number of classical bits as qubits
            gate_counts,
        })
    }

    /// Find the optimal qubit mapping for a circuit based on the device's coupling map
    pub fn optimize_qubit_mapping<const N: usize>(
        circuit: &Circuit<N>,
        coupling_map: &[(usize, usize)],
    ) -> HashMap<usize, usize> {
        // This is a simplified implementation of a qubit mapping algorithm
        // In a production system, this would use more sophisticated algorithms
        // like simulated annealing or subgraph isomorphism

        // Count the interactions between each pair of qubits
        let mut qubit_interactions = HashMap::new();

        for gate in circuit.gates() {
            let qubits = gate.qubits();
            if qubits.len() == 2 {
                let q1 = qubits[0].id() as usize;
                let q2 = qubits[1].id() as usize;
                *qubit_interactions.entry((q1, q2)).or_insert(0) += 1;
            }
        }

        // Sort qubit pairs by interaction frequency
        let mut pairs: Vec<_> = qubit_interactions.iter().collect();
        pairs.sort_by(|a, b| b.1.cmp(a.1));

        // Create a mapping from circuit qubits to device qubits
        let mut mapping = HashMap::new();
        let mut used_device_qubits = std::collections::HashSet::new();

        // First, map qubit pairs with the most interactions to connected device qubits
        for (&(q1, q2), _) in &pairs {
            if mapping.contains_key(&q1) && mapping.contains_key(&q2) {
                continue;
            }

            // Find an available connected pair on the device
            for &(dev_q1, dev_q2) in coupling_map {
                if (!mapping.contains_key(&q1) || mapping[&q1] == dev_q1)
                    && (!mapping.contains_key(&q2) || mapping[&q2] == dev_q2)
                    && !used_device_qubits.contains(&dev_q1)
                    && !used_device_qubits.contains(&dev_q2)
                {
                    // Map this pair
                    if let std::collections::hash_map::Entry::Vacant(e) = mapping.entry(q1) {
                        e.insert(dev_q1);
                        used_device_qubits.insert(dev_q1);
                    }
                    if let std::collections::hash_map::Entry::Vacant(e) = mapping.entry(q2) {
                        e.insert(dev_q2);
                        used_device_qubits.insert(dev_q2);
                    }
                    break;
                }

                if (!mapping.contains_key(&q1) || mapping[&q1] == dev_q2)
                    && (!mapping.contains_key(&q2) || mapping[&q2] == dev_q1)
                    && !used_device_qubits.contains(&dev_q1)
                    && !used_device_qubits.contains(&dev_q2)
                {
                    // Map this pair (reversed)
                    if let std::collections::hash_map::Entry::Vacant(e) = mapping.entry(q1) {
                        e.insert(dev_q2);
                        used_device_qubits.insert(dev_q2);
                    }
                    if let std::collections::hash_map::Entry::Vacant(e) = mapping.entry(q2) {
                        e.insert(dev_q1);
                        used_device_qubits.insert(dev_q1);
                    }
                    break;
                }
            }
        }

        // For any unmapped qubits, assign them to any unused device qubits
        for q in 0..N {
            if !mapping.contains_key(&q) {
                // Find any unused device qubit
                for dev_q in 0..N {
                    if used_device_qubits.insert(dev_q) {
                        mapping.insert(q, dev_q);
                        break;
                    }
                }
            }
        }

        // If there are still unmapped qubits, just use identity mapping for simplicity
        // In a real implementation, this would be more sophisticated
        for q in 0..N {
            mapping.entry(q).or_insert(q);
        }

        mapping
    }

    /// Transpile a circuit to adapt to device constraints
    pub fn transpile_circuit<const N: usize>(
        circuit: &Circuit<N>,
        coupling_map: &[(usize, usize)],
    ) -> DeviceResult<(Circuit<N>, HashMap<usize, usize>)> {
        // First, determine the optimal qubit mapping
        let mapping = Self::optimize_qubit_mapping(circuit, coupling_map);

        // Create a new circuit with the same number of qubits
        let mut new_circuit = Circuit::<N>::new();

        // Track which gates have been used for mapping
        let mut used_gates = vec![false; circuit.gates().len()];

        // Process each gate in the original circuit
        for (i, gate) in circuit.gates().iter().enumerate() {
            if used_gates[i] {
                continue;
            }

            // Process gate based on its name and properties
            let qubits = gate.qubits();
            match gate.name() {
                "CNOT" => {
                    if qubits.len() != 2 {
                        continue;
                    }
                    let control = qubits[0];
                    let target = qubits[1];
                    let c = control.id() as usize;
                    let t = target.id() as usize;
                    let mapped_c = mapping[&c];
                    let mapped_t = mapping[&t];

                    // Check if the mapped qubits are connected in the coupling map
                    let directly_connected = coupling_map.contains(&(mapped_c, mapped_t))
                        || coupling_map.contains(&(mapped_t, mapped_c));

                    if directly_connected {
                        // If directly connected, just add the gate with mapped qubits
                        if coupling_map.contains(&(mapped_c, mapped_t)) {
                            let _ = new_circuit
                                .cnot(QubitId::new(mapped_c as u32), QubitId::new(mapped_t as u32));
                        } else {
                            // If connected in reverse direction, we need to use SWAP tricks
                            // H - CNOT - H sequence to reverse the control and target
                            let _ = new_circuit.h(QubitId::new(mapped_c as u32));
                            let _ = new_circuit.h(QubitId::new(mapped_t as u32));
                            let _ = new_circuit
                                .cnot(QubitId::new(mapped_t as u32), QubitId::new(mapped_c as u32));
                            let _ = new_circuit.h(QubitId::new(mapped_c as u32));
                            let _ = new_circuit.h(QubitId::new(mapped_t as u32));
                        }
                    } else {
                        // If not directly connected, we need to find a path and insert SWAP gates
                        // This is a simplified version - in a real implementation this would
                        // be more sophisticated
                        let path = find_shortest_path(mapped_c, mapped_t, coupling_map);
                        if path.is_empty() {
                            return Err(DeviceError::CircuitConversion(format!(
                                "No path found between qubits {mapped_c} and {mapped_t}"
                            )));
                        }

                        // Apply SWAP gates along the path to bring qubits next to each other
                        for i in 0..path.len() - 2 {
                            let _ = new_circuit.swap(
                                QubitId::new(path[i] as u32),
                                QubitId::new(path[i + 1] as u32),
                            );
                        }

                        // Now the control and target are adjacent, so apply the CNOT
                        let final_c = path[path.len() - 2];
                        let final_t = path[path.len() - 1];
                        let _ = new_circuit
                            .cnot(QubitId::new(final_c as u32), QubitId::new(final_t as u32));

                        // Undo the SWAP gates in reverse order
                        for i in (0..path.len() - 2).rev() {
                            let _ = new_circuit.swap(
                                QubitId::new(path[i] as u32),
                                QubitId::new(path[i + 1] as u32),
                            );
                        }
                    }

                    used_gates[i] = true;
                }
                // Handle other gate types - most are straightforward as they're single-qubit
                "X" => {
                    if qubits.len() != 1 {
                        continue;
                    }
                    let qubit = qubits[0];
                    let q = qubit.id() as usize;
                    let mapped_q = mapping[&q];
                    let _ = new_circuit.x(QubitId::new(mapped_q as u32));
                    used_gates[i] = true;
                }
                "Y" => {
                    if qubits.len() != 1 {
                        continue;
                    }
                    let qubit = qubits[0];
                    let q = qubit.id() as usize;
                    let mapped_q = mapping[&q];
                    let _ = new_circuit.y(QubitId::new(mapped_q as u32));
                    used_gates[i] = true;
                }
                "Z" => {
                    if qubits.len() != 1 {
                        continue;
                    }
                    let qubit = qubits[0];
                    let q = qubit.id() as usize;
                    let mapped_q = mapping[&q];
                    let _ = new_circuit.z(QubitId::new(mapped_q as u32));
                    used_gates[i] = true;
                }
                "H" => {
                    if qubits.len() != 1 {
                        continue;
                    }
                    let qubit = qubits[0];
                    let q = qubit.id() as usize;
                    let mapped_q = mapping[&q];
                    let _ = new_circuit.h(QubitId::new(mapped_q as u32));
                    used_gates[i] = true;
                }
                // Add other gate types as needed
                _ => {
                    // For now, return an error for unsupported gates
                    return Err(DeviceError::CircuitConversion(format!(
                        "Unsupported gate type for transpilation: {}",
                        gate.name()
                    )));
                }
            }
        }

        Ok((new_circuit, mapping))
    }
}

// Helper function to map a qubit index based on a mapping
fn map_qubit(qubit: usize, mapping: &Option<&HashMap<usize, usize>>) -> usize {
    match mapping {
        Some(map) => *map.get(&qubit).unwrap_or(&qubit),
        None => qubit,
    }
}

// Helper function to find the shortest path between two qubits in the coupling map
fn find_shortest_path(start: usize, end: usize, coupling_map: &[(usize, usize)]) -> Vec<usize> {
    if start == end {
        return vec![start];
    }

    // Create an adjacency list from the coupling map
    let mut adj_list = HashMap::new();
    for &(a, b) in coupling_map {
        adj_list.entry(a).or_insert_with(Vec::new).push(b);
        adj_list.entry(b).or_insert_with(Vec::new).push(a);
    }

    // Perform BFS to find the shortest path
    let mut queue = VecDeque::new();
    let mut visited = std::collections::HashSet::new();
    let mut parent = HashMap::new();

    queue.push_back(start);
    visited.insert(start);

    while let Some(node) = queue.pop_front() {
        if node == end {
            // Reconstruct the path
            let mut path = Vec::new();
            let mut current = node;

            while let Some(&p) = parent.get(&current) {
                path.push(current);
                current = p;
                if current == start {
                    path.push(current);
                    path.reverse();
                    return path;
                }
            }
        }

        if let Some(neighbors) = adj_list.get(&node) {
            for &neighbor in neighbors {
                if visited.insert(neighbor) {
                    queue.push_back(neighbor);
                    parent.insert(neighbor, node);
                }
            }
        }
    }

    // No path found
    Vec::new()
}

#[cfg(test)]
mod tests {
    use super::*;
    use quantrs2_circuit::builder::Circuit;

    #[test]
    fn test_qasm_export_rotation_angles() {
        // Build a 3-qubit circuit with RX(0.5), RY(1.0), RZ(1.5)
        let mut circ: Circuit<3> = Circuit::new();
        circ.rx(0u32, 0.5).expect("rx gate");
        circ.ry(1u32, 1.0).expect("ry gate");
        circ.rz(2u32, 1.5).expect("rz gate");

        let qasm = CircuitTranspiler::circuit_to_qasm(&circ, None)
            .expect("circuit_to_qasm should succeed");

        assert!(
            qasm.content.contains("rx(0.5)"),
            "QASM should contain rx(0.5), got: {}",
            qasm.content
        );
        // Rust formats 1.0_f64 as "1" via Display
        assert!(
            qasm.content.contains("ry(1)") || qasm.content.contains("ry(1.0)"),
            "QASM should contain ry(1) or ry(1.0), got: {}",
            qasm.content
        );
        assert!(
            qasm.content.contains("rz(1.5)"),
            "QASM should contain rz(1.5), got: {}",
            qasm.content
        );

        // Ensure old placeholder values are absent — "rx(0) " (with trailing space) was the bug
        assert!(
            !qasm.content.contains("rx(0) "),
            "QASM must not contain hardcoded placeholder rx(0), got: {}",
            qasm.content
        );
        assert!(
            !qasm.content.contains("ry(0) "),
            "QASM must not contain hardcoded placeholder ry(0), got: {}",
            qasm.content
        );
        assert!(
            !qasm.content.contains("rz(0) "),
            "QASM must not contain hardcoded placeholder rz(0), got: {}",
            qasm.content
        );
    }
}