quantrs2_circuit/

graph_optimizer.rs

//! Graph-based circuit optimizer using SciRS2 algorithms
//!
//! This module implements advanced circuit optimization using graph representations
//! and algorithms from SciRS2 for optimal gate scheduling and optimization.

use crate::builder::Circuit;
use scirs2_core::Complex64;
use quantrs2_core::error::QuantRS2Error;
use quantrs2_core::qubit::QubitId;
use std::collections::{HashMap, HashSet, VecDeque};

// SciRS2 integration for graph algorithms when available
#[cfg(feature = "scirs")]
use scirs2_core::sparse::{CsrMatrix, SparseMatrix};
#[cfg(feature = "scirs")]
use scirs2_optimize::graph::{
    find_critical_path, graph_coloring, minimum_feedback_arc_set, topological_sort_weighted,
};

/// Helper function to multiply two 2x2 matrices
fn matrix_multiply_2x2(a: &[Vec<Complex64>], b: &[Vec<Complex64>]) -> Vec<Vec<Complex64>> {
    vec![
        vec![
            a[0][0] * b[0][0] + a[0][1] * b[1][0],
            a[0][0] * b[0][1] + a[0][1] * b[1][1],
        ],
        vec![
            a[1][0] * b[0][0] + a[1][1] * b[1][0],
            a[1][0] * b[0][1] + a[1][1] * b[1][1],
        ],
    ]
}

/// Represents a gate in the circuit graph
#[derive(Debug, Clone, PartialEq)]
pub struct GraphGate {
    pub id: usize,
    pub gate_type: String,
    pub qubits: Vec<QubitId>,
    pub params: Vec<f64>,
    pub matrix: Option<Vec<Vec<Complex64>>>,
}

/// Edge types in the circuit graph
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum EdgeType {
    /// Data dependency (same qubit)
    DataDependency,
    /// Commutation constraint
    NonCommuting,
    /// Can be parallelized
    Parallelizable,
}

/// Circuit DAG (Directed Acyclic Graph) representation
pub struct CircuitDAG {
    nodes: Vec<GraphGate>,
    edges: HashMap<(usize, usize), EdgeType>,
    qubit_chains: HashMap<u32, Vec<usize>>, // qubit_id -> [gate_ids in order]
}

impl CircuitDAG {
    /// Create a new empty DAG
    pub fn new() -> Self {
        Self {
            nodes: Vec::new(),
            edges: HashMap::new(),
            qubit_chains: HashMap::new(),
        }
    }

    /// Add a gate to the DAG
    pub fn add_gate(&mut self, gate: GraphGate) -> usize {
        let gate_id = self.nodes.len();

        // Update qubit chains
        for qubit in &gate.qubits {
            self.qubit_chains
                .entry(qubit.id())
                .or_default()
                .push(gate_id);
        }

        // Add edges for data dependencies
        for qubit in &gate.qubits {
            if let Some(chain) = self.qubit_chains.get(&qubit.id()) {
                if chain.len() > 1 {
                    let prev_gate = chain[chain.len() - 2];
                    self.edges
                        .insert((prev_gate, gate_id), EdgeType::DataDependency);
                }
            }
        }

        self.nodes.push(gate);
        gate_id
    }

    /// Check if two gates commute
    fn gates_commute(&self, g1: &GraphGate, g2: &GraphGate) -> bool {
        // Gates on different qubits always commute
        let qubits1: HashSet<_> = g1.qubits.iter().map(|q| q.id()).collect();
        let qubits2: HashSet<_> = g2.qubits.iter().map(|q| q.id()).collect();

        if qubits1.is_disjoint(&qubits2) {
            return true;
        }

        // Special cases for common gates
        match (g1.gate_type.as_str(), g2.gate_type.as_str()) {
            // Z gates always commute with each other
            ("z", "z") | ("rz", "rz") | ("z", "rz") | ("rz", "z") => true,
            // CNOT gates commute if they share only control or only target
            ("cnot", "cnot") => {
                if g1.qubits.len() == 2 && g2.qubits.len() == 2 {
                    let same_control = g1.qubits[0] == g2.qubits[0];
                    let same_target = g1.qubits[1] == g2.qubits[1];
                    same_control && same_target // Only if identical
                } else {
                    false
                }
            }
            _ => false,
        }
    }

    /// Compute commutation graph
    pub fn compute_commutation_edges(&mut self) {
        #[cfg(feature = "scirs")]
        {
            if self.scirs_compute_commutation_edges() {
                return;
            }
        }

        self.standard_compute_commutation_edges();
    }

    #[cfg(feature = "scirs")]
    /// SciRS2-powered commutation edge computation
    fn scirs_compute_commutation_edges(&mut self) -> bool {
        let n = self.nodes.len();
        if n == 0 {
            return true;
        }

        // Build interference graph for gates
        let mut interference = vec![vec![false; n]; n];

        for i in 0..n {
            for j in i + 1..n {
                let g1 = &self.nodes[i];
                let g2 = &self.nodes[j];

                // Check if gates interfere (can't be scheduled in parallel)
                if !self.gates_commute(g1, g2) && !self.has_path(i, j) && !self.has_path(j, i) {
                    interference[i][j] = true;
                    interference[j][i] = true;
                }
            }
        }

        // Use graph coloring to find maximum parallelism
        if let Ok(coloring) = graph_coloring(&interference) {
            // Use coloring information to add parallelization hints
            for i in 0..n {
                for j in i + 1..n {
                    if coloring[i] == coloring[j] && !interference[i][j] {
                        // Same color = can be scheduled in parallel
                        self.edges.insert((i, j), EdgeType::Parallelizable);
                    }
                }
            }
            return true;
        }

        false
    }

    /// Standard commutation edge computation
    fn standard_compute_commutation_edges(&mut self) {
        let n = self.nodes.len();

        for i in 0..n {
            for j in i + 1..n {
                let g1 = &self.nodes[i];
                let g2 = &self.nodes[j];

                // Skip if already connected
                if self.edges.contains_key(&(i, j)) || self.edges.contains_key(&(j, i)) {
                    continue;
                }

                // Check commutation
                if !self.gates_commute(g1, g2) {
                    // Check if they could be reordered (no data dependency path)
                    if !self.has_path(i, j) && !self.has_path(j, i) {
                        self.edges.insert((i, j), EdgeType::NonCommuting);
                    }
                } else if g1.qubits.iter().any(|q| g2.qubits.contains(q)) {
                    // Gates commute but share qubits
                    self.edges.insert((i, j), EdgeType::Parallelizable);
                }
            }
        }
    }

    #[cfg(feature = "scirs")]
    /// Find optimal gate reordering using minimum feedback arc set
    pub fn optimize_with_feedback_arc_set(&self) -> Option<Vec<usize>> {
        let n = self.nodes.len();
        if n == 0 {
            return Some(vec![]);
        }

        // Build directed graph of non-commuting edges
        let mut edges = Vec::new();
        let mut weights = Vec::new();

        for ((u, v), edge_type) in &self.edges {
            if *edge_type == EdgeType::NonCommuting {
                edges.push((*u, *v));
                // Weight by impact on parallelism
                let weight = self.gate_weight(*u) * self.gate_weight(*v);
                weights.push(weight);
            }
        }

        // Find minimum feedback arc set to break cycles
        if let Ok(feedback_arcs) = minimum_feedback_arc_set(&edges, &weights) {
            // Remove feedback arcs and compute new ordering
            let mut filtered_edges = edges.clone();
            for &arc_idx in &feedback_arcs {
                filtered_edges[arc_idx] = (n, n); // Mark for removal
            }
            filtered_edges.retain(|&(u, v)| u < n && v < n);

            // Build new DAG and compute topological sort
            let mut new_dag = CircuitDAG::new();
            new_dag.nodes = self.nodes.clone();
            for (u, v) in filtered_edges {
                new_dag.edges.insert((u, v), EdgeType::DataDependency);
            }

            return Some(new_dag.optimized_topological_sort());
        }

        None
    }

    /// Check if there's a path from src to dst
    fn has_path(&self, src: usize, dst: usize) -> bool {
        let mut visited = vec![false; self.nodes.len()];
        let mut queue = VecDeque::new();

        queue.push_back(src);
        visited[src] = true;

        while let Some(node) = queue.pop_front() {
            if node == dst {
                return true;
            }

            for ((u, v), edge_type) in &self.edges {
                if *u == node && !visited[*v] && *edge_type == EdgeType::DataDependency {
                    visited[*v] = true;
                    queue.push_back(*v);
                }
            }
        }

        false
    }

    /// Topological sort with optimization
    pub fn optimized_topological_sort(&self) -> Vec<usize> {
        #[cfg(feature = "scirs")]
        {
            // Use SciRS2's optimized topological sort when available
            if let Some(order) = self.scirs_topological_sort() {
                return order;
            }
        }

        // Fallback to standard implementation
        self.standard_topological_sort()
    }

    #[cfg(feature = "scirs")]
    /// SciRS2-powered topological sort with advanced optimization
    fn scirs_topological_sort(&self) -> Option<Vec<usize>> {
        let n = self.nodes.len();
        if n == 0 {
            return Some(vec![]);
        }

        // Convert to sparse adjacency matrix for SciRS2
        let mut row_indices = Vec::new();
        let mut col_indices = Vec::new();
        let mut values = Vec::new();

        for ((u, v), edge_type) in &self.edges {
            if *edge_type == EdgeType::DataDependency {
                row_indices.push(*u);
                col_indices.push(*v);
                // Weight edges by gate complexity
                let weight = self.gate_weight(*u) + self.gate_weight(*v);
                values.push(weight);
            }
        }

        // Create sparse matrix
        let matrix = CsrMatrix::from_triplets(n, n, &row_indices, &col_indices, &values);

        // Use weighted topological sort for optimal scheduling
        if let Ok(order) = topological_sort_weighted(&matrix, |i| self.gate_priority(i)) {
            // Find critical path for depth optimization
            if let Ok(critical) = find_critical_path(&matrix, &order) {
                // Reorder based on critical path analysis
                return Some(self.optimize_order_by_critical_path(order, critical));
            }
            return Some(order);
        }

        None
    }

    /// Calculate gate weight for scheduling priority
    fn gate_weight(&self, gate_id: usize) -> f64 {
        let gate = &self.nodes[gate_id];
        match gate.gate_type.as_str() {
            // Two-qubit gates are more expensive
            "cnot" | "cz" | "swap" => 10.0,
            "rzz" | "rxx" | "ryy" => 15.0,
            // Rotation gates
            "rx" | "ry" | "rz" => 2.0,
            // Clifford gates are cheap
            "h" | "s" | "t" | "x" | "y" | "z" => 1.0,
            // Unknown gates
            _ => 5.0,
        }
    }

    /// Calculate gate priority for scheduling
    fn gate_priority(&self, gate_id: usize) -> f64 {
        // Prioritize gates that enable more parallelism
        let parallelism_score = self.count_parallel_successors(gate_id) as f64;
        // Factor in gate weight
        let weight = self.gate_weight(gate_id);
        // Higher priority for gates that unlock more parallelism
        parallelism_score / weight
    }

    /// Count how many gates can be executed in parallel after this gate
    fn count_parallel_successors(&self, gate_id: usize) -> usize {
        let mut count = 0;
        for ((u, _v), edge_type) in &self.edges {
            if *u == gate_id && *edge_type == EdgeType::Parallelizable {
                count += 1;
            }
        }
        count
    }

    #[cfg(feature = "scirs")]
    /// Optimize gate order based on critical path analysis
    fn optimize_order_by_critical_path(
        &self,
        order: Vec<usize>,
        critical: Vec<usize>,
    ) -> Vec<usize> {
        let mut optimized = Vec::new();
        let mut scheduled = HashSet::new();
        let critical_set: HashSet<_> = critical.into_iter().collect();

        // Schedule critical path gates first
        for &gate_id in &order {
            if critical_set.contains(&gate_id) && !scheduled.contains(&gate_id) {
                optimized.push(gate_id);
                scheduled.insert(gate_id);
            }
        }

        // Then schedule remaining gates
        for &gate_id in &order {
            if !scheduled.contains(&gate_id) {
                optimized.push(gate_id);
                scheduled.insert(gate_id);
            }
        }

        optimized
    }

    /// Standard topological sort implementation (fallback)
    fn standard_topological_sort(&self) -> Vec<usize> {
        let n = self.nodes.len();
        let mut in_degree = vec![0; n];
        let mut adj_list: HashMap<usize, Vec<usize>> = HashMap::new();

        // Build adjacency list and compute in-degrees
        for ((u, v), edge_type) in &self.edges {
            if *edge_type == EdgeType::DataDependency {
                adj_list.entry(*u).or_default().push(*v);
                in_degree[*v] += 1;
            }
        }

        // Priority queue for selecting next gate (minimize circuit depth)
        let mut ready: Vec<usize> = Vec::new();
        for (i, &degree) in in_degree.iter().enumerate() {
            if degree == 0 {
                ready.push(i);
            }
        }

        let mut result = Vec::new();
        let mut layer_qubits: HashSet<u32> = HashSet::new();

        while !ready.is_empty() {
            // Sort ready gates by number of qubits (prefer single-qubit gates)
            ready.sort_by_key(|&i| self.nodes[i].qubits.len());

            // Try to pack gates that don't conflict
            let mut next_layer = Vec::new();
            let mut used = vec![false; ready.len()];

            for (idx, &gate_id) in ready.iter().enumerate() {
                if used[idx] {
                    continue;
                }

                let gate = &self.nodes[gate_id];
                let gate_qubits: HashSet<_> = gate.qubits.iter().map(|q| q.id()).collect();

                // Check if this gate conflicts with current layer
                if gate_qubits.is_disjoint(&layer_qubits) {
                    next_layer.push(gate_id);
                    layer_qubits.extend(&gate_qubits);
                    used[idx] = true;
                }
            }

            // If no gates selected, pick the first one
            if next_layer.is_empty() && !ready.is_empty() {
                next_layer.push(ready[0]);
                used[0] = true;
            }

            // Remove selected gates from ready
            ready.retain(|&g| !next_layer.contains(&g));

            // Add to result and update graph
            for &gate_id in &next_layer {
                result.push(gate_id);

                if let Some(neighbors) = adj_list.get(&gate_id) {
                    for &neighbor in neighbors {
                        in_degree[neighbor] -= 1;
                        if in_degree[neighbor] == 0 {
                            ready.push(neighbor);
                        }
                    }
                }
            }

            layer_qubits.clear();
        }

        result
    }
}

impl Default for CircuitDAG {
    fn default() -> Self {
        Self::new()
    }
}

/// Graph-based circuit optimizer
pub struct GraphOptimizer {
    merge_threshold: f64,
    #[allow(dead_code)]
    max_lookahead: usize,
}

impl GraphOptimizer {
    /// Create a new graph optimizer
    pub fn new() -> Self {
        Self {
            merge_threshold: 1e-6,
            max_lookahead: 10,
        }
    }

    /// Convert circuit to DAG representation
    pub fn circuit_to_dag<const N: usize>(
        &self,
        circuit: &Circuit<N>,
    ) -> Result<CircuitDAG, QuantRS2Error> {
        let mut dag = CircuitDAG::new();

        // Extract gates from circuit and convert to GraphGates
        for (gate_id, gate) in circuit.gates().iter().enumerate() {
            let graph_gate = GraphGate {
                id: gate_id,
                gate_type: gate.name().to_string(),
                qubits: gate.qubits(),
                params: if gate.is_parameterized() {
                    // Extract parameters from specific gate types
                    match gate.name() {
                        "RX" | "RY" | "RZ" => {
                            // Try to extract rotation parameters using downcast
                            if let Some(rx_gate) =
                                gate.as_any()
                                    .downcast_ref::<quantrs2_core::gate::single::RotationX>()
                            {
                                vec![rx_gate.theta]
                            } else if let Some(ry_gate) =
                                gate.as_any()
                                    .downcast_ref::<quantrs2_core::gate::single::RotationY>()
                            {
                                vec![ry_gate.theta]
                            } else if let Some(rz_gate) =
                                gate.as_any()
                                    .downcast_ref::<quantrs2_core::gate::single::RotationZ>()
                            {
                                vec![rz_gate.theta]
                            } else {
                                vec![] // Default for unknown parameterized gates
                            }
                        }
                        "CRX" | "CRY" | "CRZ" => {
                            // Try to extract controlled rotation parameters
                            if let Some(crx_gate) = gate
                                .as_any()
                                .downcast_ref::<quantrs2_core::gate::multi::CRX>()
                            {
                                vec![crx_gate.theta]
                            } else if let Some(cry_gate) =
                                gate.as_any()
                                    .downcast_ref::<quantrs2_core::gate::multi::CRY>()
                            {
                                vec![cry_gate.theta]
                            } else if let Some(crz_gate) =
                                gate.as_any()
                                    .downcast_ref::<quantrs2_core::gate::multi::CRZ>()
                            {
                                vec![crz_gate.theta]
                            } else {
                                vec![]
                            }
                        }
                        _ => vec![], // Other parameterized gates
                    }
                } else {
                    vec![] // Non-parameterized gates have no parameters
                },
                matrix: None, // Compute on demand if needed
            };

            dag.add_gate(graph_gate);
        }

        Ok(dag)
    }

    /// Optimize gate sequence using commutation rules
    pub fn optimize_gate_sequence(&self, gates: Vec<GraphGate>) -> Vec<GraphGate> {
        let mut dag = CircuitDAG::new();

        // Add gates to DAG
        for gate in gates {
            dag.add_gate(gate);
        }

        // Compute commutation relationships
        dag.compute_commutation_edges();

        // Try advanced optimization methods if available
        #[cfg(feature = "scirs")]
        {
            // Try feedback arc set optimization for complex circuits
            if dag.nodes.len() > 10 {
                if let Some(optimized_order) = dag.optimize_with_feedback_arc_set() {
                    return optimized_order
                        .iter()
                        .map(|&i| dag.nodes[i].clone())
                        .collect();
                }
            }
        }

        // Get optimized ordering
        let order = dag.optimized_topological_sort();

        // Apply gate merging optimization
        self.merge_gates_in_sequence(order.iter().map(|&i| dag.nodes[i].clone()).collect())
    }

    /// Apply gate merging to an ordered sequence
    fn merge_gates_in_sequence(&self, gates: Vec<GraphGate>) -> Vec<GraphGate> {
        if gates.is_empty() {
            return gates;
        }

        let mut merged = Vec::new();
        let mut i = 0;

        while i < gates.len() {
            if i + 1 < gates.len() {
                // Try to merge consecutive gates
                if let Some(merged_gate) = self.try_merge_gates(&gates[i], &gates[i + 1]) {
                    merged.push(merged_gate);
                    i += 2; // Skip both gates
                    continue;
                }
            }
            merged.push(gates[i].clone());
            i += 1;
        }

        merged
    }

    /// Try to merge two gates if possible
    fn try_merge_gates(&self, g1: &GraphGate, g2: &GraphGate) -> Option<GraphGate> {
        // Only merge single-qubit gates on the same qubit
        if g1.qubits.len() == 1 && g2.qubits.len() == 1 && g1.qubits[0] == g2.qubits[0] {
            self.merge_single_qubit_gates(g1, g2)
        } else {
            None
        }
    }

    /// Merge consecutive single-qubit gates
    pub fn merge_single_qubit_gates(&self, g1: &GraphGate, g2: &GraphGate) -> Option<GraphGate> {
        // Check if both are single-qubit gates on the same qubit
        if g1.qubits.len() != 1 || g2.qubits.len() != 1 || g1.qubits[0] != g2.qubits[0] {
            return None;
        }

        // Get matrices
        let m1 = g1.matrix.as_ref()?;
        let m2 = g2.matrix.as_ref()?;

        // Multiply matrices (2x2)
        let combined = matrix_multiply_2x2(m2, m1);

        // Check if it's close to a known gate
        if let Some((gate_type, params)) = self.identify_gate(&combined) {
            Some(GraphGate {
                id: g1.id, // Use first gate's ID
                gate_type,
                qubits: g1.qubits.clone(),
                params,
                matrix: Some(combined),
            })
        } else {
            // Return as generic unitary
            Some(GraphGate {
                id: g1.id,
                gate_type: "u".to_string(),
                qubits: g1.qubits.clone(),
                params: vec![],
                matrix: Some(combined),
            })
        }
    }

    /// Identify a gate from its matrix
    fn identify_gate(&self, matrix: &[Vec<Complex64>]) -> Option<(String, Vec<f64>)> {
        let tolerance = self.merge_threshold;

        // Check for Pauli gates
        if self.is_pauli_x(matrix, tolerance) {
            return Some(("x".to_string(), vec![]));
        }
        if self.is_pauli_y(matrix, tolerance) {
            return Some(("y".to_string(), vec![]));
        }
        if self.is_pauli_z(matrix, tolerance) {
            return Some(("z".to_string(), vec![]));
        }

        // Check for Hadamard
        if self.is_hadamard(matrix, tolerance) {
            return Some(("h".to_string(), vec![]));
        }

        // Check for rotation gates
        if let Some(angle) = self.is_rz(matrix, tolerance) {
            return Some(("rz".to_string(), vec![angle]));
        }

        None
    }

    fn is_pauli_x(&self, matrix: &[Vec<Complex64>], tol: f64) -> bool {
        matrix.len() == 2
            && matrix[0].len() == 2
            && (matrix[0][0].norm() < tol)
            && (matrix[0][1] - Complex64::new(1.0, 0.0)).norm() < tol
            && (matrix[1][0] - Complex64::new(1.0, 0.0)).norm() < tol
            && (matrix[1][1].norm() < tol)
    }

    fn is_pauli_y(&self, matrix: &[Vec<Complex64>], tol: f64) -> bool {
        matrix.len() == 2
            && matrix[0].len() == 2
            && (matrix[0][0].norm() < tol)
            && (matrix[0][1] - Complex64::new(0.0, -1.0)).norm() < tol
            && (matrix[1][0] - Complex64::new(0.0, 1.0)).norm() < tol
            && (matrix[1][1].norm() < tol)
    }

    fn is_pauli_z(&self, matrix: &[Vec<Complex64>], tol: f64) -> bool {
        matrix.len() == 2
            && matrix[0].len() == 2
            && (matrix[0][0] - Complex64::new(1.0, 0.0)).norm() < tol
            && (matrix[0][1].norm() < tol)
            && (matrix[1][0].norm() < tol)
            && (matrix[1][1] - Complex64::new(-1.0, 0.0)).norm() < tol
    }

    fn is_hadamard(&self, matrix: &[Vec<Complex64>], tol: f64) -> bool {
        let h_val = 1.0 / 2.0_f64.sqrt();
        matrix.len() == 2
            && matrix[0].len() == 2
            && (matrix[0][0] - Complex64::new(h_val, 0.0)).norm() < tol
            && (matrix[0][1] - Complex64::new(h_val, 0.0)).norm() < tol
            && (matrix[1][0] - Complex64::new(h_val, 0.0)).norm() < tol
            && (matrix[1][1] - Complex64::new(-h_val, 0.0)).norm() < tol
    }

    fn is_rz(&self, matrix: &[Vec<Complex64>], tol: f64) -> Option<f64> {
        if matrix.len() != 2
            || matrix[0].len() != 2
            || matrix[0][1].norm() > tol
            || matrix[1][0].norm() > tol
        {
            return None;
        }

        let phase1 = matrix[0][0].arg();
        let phase2 = matrix[1][1].arg();

        if (matrix[0][0].norm() - 1.0).abs() < tol && (matrix[1][1].norm() - 1.0).abs() < tol {
            let angle = phase2 - phase1;
            Some(angle)
        } else {
            None
        }
    }
}

impl Default for GraphOptimizer {
    fn default() -> Self {
        Self::new()
    }
}

/// Optimization statistics
#[derive(Debug, Clone)]
pub struct OptimizationStats {
    pub original_gate_count: usize,
    pub optimized_gate_count: usize,
    pub original_depth: usize,
    pub optimized_depth: usize,
    pub gates_removed: usize,
    pub gates_merged: usize,
}

impl OptimizationStats {
    pub fn improvement_percentage(&self) -> f64 {
        if self.original_gate_count == 0 {
            0.0
        } else {
            100.0 * (self.original_gate_count - self.optimized_gate_count) as f64
                / self.original_gate_count as f64
        }
    }
}

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

    #[test]
    fn test_dag_construction() {
        let mut dag = CircuitDAG::new();

        let g1 = GraphGate {
            id: 0,
            gate_type: "h".to_string(),
            qubits: vec![QubitId::new(0)],
            params: vec![],
            matrix: None,
        };

        let g2 = GraphGate {
            id: 1,
            gate_type: "cnot".to_string(),
            qubits: vec![QubitId::new(0), QubitId::new(1)],
            params: vec![],
            matrix: None,
        };

        dag.add_gate(g1);
        dag.add_gate(g2);

        assert_eq!(dag.nodes.len(), 2);
        assert!(dag.edges.contains_key(&(0, 1)));
    }

    #[test]
    fn test_commutation_detection() {
        let _optimizer = GraphOptimizer::new();

        let g1 = GraphGate {
            id: 0,
            gate_type: "z".to_string(),
            qubits: vec![QubitId::new(0)],
            params: vec![],
            matrix: None,
        };

        let g2 = GraphGate {
            id: 1,
            gate_type: "z".to_string(),
            qubits: vec![QubitId::new(0)],
            params: vec![],
            matrix: None,
        };

        let dag = CircuitDAG::new();
        assert!(dag.gates_commute(&g1, &g2));
    }

    #[test]
    fn test_gate_identification() {
        let optimizer = GraphOptimizer::new();

        // Pauli X matrix
        let x_matrix = vec![
            vec![Complex64::new(0.0, 0.0), Complex64::new(1.0, 0.0)],
            vec![Complex64::new(1.0, 0.0), Complex64::new(0.0, 0.0)],
        ];

        if let Some((gate_type, _)) = optimizer.identify_gate(&x_matrix) {
            assert_eq!(gate_type, "x");
        } else {
            panic!("Failed to identify Pauli X gate");
        }
    }
}