quantrs2_sim/
circuit_optimizer.rs

1//! Circuit optimization framework for quantum circuits
2//!
3//! This module provides a comprehensive framework for optimizing quantum circuits
4//! through various transformation passes that preserve circuit equivalence while
5//! reducing gate count, depth, and execution time.
6//!
7//! # Optimization Passes
8//!
9//! - **Gate Cancellation**: Remove inverse gate pairs (H-H, X-X, CNOT-CNOT)
10//! - **Gate Fusion**: Combine adjacent single-qubit rotations
11//! - **Gate Commutation**: Reorder gates using commutation rules
12//! - **Template Matching**: Replace gate sequences with optimized equivalents
13//! - **Two-Qubit Reduction**: Minimize expensive two-qubit gates
14//!
15//! # Example
16//!
17//! ```ignore
18//! use quantrs2_sim::circuit_optimizer::{CircuitOptimizer, OptimizationPass};
19//!
20//! let optimizer = CircuitOptimizer::new()
21//!     .with_pass(OptimizationPass::CancelInverses)
22//!     .with_pass(OptimizationPass::FuseRotations)
23//!     .with_pass(OptimizationPass::CommutativeReordering);
24//!
25//! let optimized_circuit = optimizer.optimize(&circuit)?;
26//! ```
27
28use crate::error::{Result, SimulatorError};
29use std::collections::{HashMap, HashSet};
30
31// ============================================================================
32// Gate Representation
33// ============================================================================
34
35/// Quantum gate type
36#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
37pub enum GateType {
38    /// Identity gate
39    I,
40    /// Hadamard gate
41    H,
42    /// Pauli-X gate
43    X,
44    /// Pauli-Y gate
45    Y,
46    /// Pauli-Z gate
47    Z,
48    /// S gate (sqrt(Z))
49    S,
50    /// S† gate
51    Sdg,
52    /// T gate (fourth root of Z)
53    T,
54    /// T† gate
55    Tdg,
56    /// Rotation around X axis
57    RX,
58    /// Rotation around Y axis
59    RY,
60    /// Rotation around Z axis
61    RZ,
62    /// CNOT gate
63    CNOT,
64    /// CZ gate
65    CZ,
66    /// SWAP gate
67    SWAP,
68    /// Toffoli gate (CCX)
69    Toffoli,
70}
71
72impl GateType {
73    /// Check if this gate is its own inverse
74    pub fn is_self_inverse(&self) -> bool {
75        matches!(
76            self,
77            GateType::H | GateType::X | GateType::Y | GateType::Z | GateType::CNOT | GateType::SWAP
78        )
79    }
80
81    /// Get the inverse gate type
82    pub fn inverse(&self) -> Self {
83        match self {
84            GateType::S => GateType::Sdg,
85            GateType::Sdg => GateType::S,
86            GateType::T => GateType::Tdg,
87            GateType::Tdg => GateType::T,
88            GateType::RX | GateType::RY | GateType::RZ => *self, // Inverse is negated parameter
89            _ if self.is_self_inverse() => *self,
90            _ => *self, // Identity and others
91        }
92    }
93
94    /// Check if this is a rotation gate
95    pub fn is_rotation(&self) -> bool {
96        matches!(self, GateType::RX | GateType::RY | GateType::RZ)
97    }
98
99    /// Check if this is a single-qubit gate
100    pub fn is_single_qubit(&self) -> bool {
101        !matches!(
102            self,
103            GateType::CNOT | GateType::CZ | GateType::SWAP | GateType::Toffoli
104        )
105    }
106
107    /// Check if two gates commute on the same qubits
108    pub fn commutes_with(&self, other: &GateType) -> bool {
109        // Simplified commutation rules
110        match (self, other) {
111            // Z-basis gates commute with each other
112            (GateType::Z, GateType::Z)
113            | (GateType::Z, GateType::S)
114            | (GateType::Z, GateType::Sdg)
115            | (GateType::Z, GateType::T)
116            | (GateType::Z, GateType::Tdg)
117            | (GateType::S, GateType::Z)
118            | (GateType::Sdg, GateType::Z)
119            | (GateType::T, GateType::Z)
120            | (GateType::Tdg, GateType::Z) => true,
121
122            // Rotation around same axis commute
123            (GateType::RX, GateType::RX)
124            | (GateType::RY, GateType::RY)
125            | (GateType::RZ, GateType::RZ) => true,
126
127            _ => false,
128        }
129    }
130}
131
132/// Quantum gate instruction
133#[derive(Debug, Clone, PartialEq)]
134pub struct Gate {
135    /// Gate type
136    pub gate_type: GateType,
137    /// Qubits this gate acts on
138    pub qubits: Vec<usize>,
139    /// Parameters (for rotation gates)
140    pub parameters: Vec<f64>,
141}
142
143impl Gate {
144    /// Create a new gate
145    pub fn new(gate_type: GateType, qubits: Vec<usize>) -> Self {
146        Self {
147            gate_type,
148            qubits,
149            parameters: Vec::new(),
150        }
151    }
152
153    /// Create a gate with parameters
154    pub fn with_parameters(gate_type: GateType, qubits: Vec<usize>, parameters: Vec<f64>) -> Self {
155        Self {
156            gate_type,
157            qubits,
158            parameters,
159        }
160    }
161
162    /// Get the inverse gate
163    pub fn inverse(&self) -> Self {
164        let inv_type = self.gate_type.inverse();
165        let inv_params = if self.gate_type.is_rotation() {
166            self.parameters.iter().map(|p| -p).collect()
167        } else {
168            self.parameters.clone()
169        };
170
171        Self {
172            gate_type: inv_type,
173            qubits: self.qubits.clone(),
174            parameters: inv_params,
175        }
176    }
177
178    /// Check if two gates are inverse of each other
179    pub fn is_inverse_of(&self, other: &Gate) -> bool {
180        if self.qubits != other.qubits {
181            return false;
182        }
183
184        if self.gate_type.is_self_inverse() && self.gate_type == other.gate_type {
185            return true;
186        }
187
188        if self.gate_type.inverse() == other.gate_type {
189            // For rotation gates, check if parameters are negated
190            if self.gate_type.is_rotation() {
191                return self
192                    .parameters
193                    .iter()
194                    .zip(other.parameters.iter())
195                    .all(|(p1, p2)| (p1 + p2).abs() < 1e-10);
196            }
197            return true;
198        }
199
200        false
201    }
202}
203
204/// Quantum circuit
205#[derive(Debug, Clone)]
206pub struct Circuit {
207    /// Number of qubits
208    pub n_qubits: usize,
209    /// Sequence of gates
210    pub gates: Vec<Gate>,
211}
212
213impl Circuit {
214    /// Create a new circuit
215    pub fn new(n_qubits: usize) -> Self {
216        Self {
217            n_qubits,
218            gates: Vec::new(),
219        }
220    }
221
222    /// Add a gate to the circuit
223    pub fn add_gate(&mut self, gate: Gate) -> Result<()> {
224        // Verify qubit indices
225        for &qubit in &gate.qubits {
226            if qubit >= self.n_qubits {
227                return Err(SimulatorError::InvalidInput(format!(
228                    "Qubit index {} out of range (circuit has {} qubits)",
229                    qubit, self.n_qubits
230                )));
231            }
232        }
233        self.gates.push(gate);
234        Ok(())
235    }
236
237    /// Get circuit depth (number of layers)
238    pub fn depth(&self) -> usize {
239        if self.gates.is_empty() {
240            return 0;
241        }
242
243        let mut qubit_depths = vec![0; self.n_qubits];
244        let mut max_depth = 0;
245
246        for gate in &self.gates {
247            // Find maximum depth of qubits involved
248            let current_depth = gate
249                .qubits
250                .iter()
251                .map(|&q| qubit_depths[q])
252                .max()
253                .unwrap_or(0);
254
255            // Update depths for all involved qubits
256            for &qubit in &gate.qubits {
257                qubit_depths[qubit] = current_depth + 1;
258            }
259
260            max_depth = max_depth.max(current_depth + 1);
261        }
262
263        max_depth
264    }
265
266    /// Count gates by type
267    pub fn gate_counts(&self) -> HashMap<GateType, usize> {
268        let mut counts = HashMap::new();
269        for gate in &self.gates {
270            *counts.entry(gate.gate_type).or_insert(0) += 1;
271        }
272        counts
273    }
274
275    /// Count two-qubit gates
276    pub fn two_qubit_gate_count(&self) -> usize {
277        self.gates.iter().filter(|g| g.qubits.len() == 2).count()
278    }
279}
280
281// ============================================================================
282// Optimization Passes
283// ============================================================================
284
285/// Optimization pass types
286#[derive(Debug, Clone, Copy, PartialEq, Eq)]
287pub enum OptimizationPass {
288    /// Remove inverse gate pairs
289    CancelInverses,
290    /// Fuse adjacent rotation gates
291    FuseRotations,
292    /// Reorder gates using commutation rules
293    CommutativeReordering,
294    /// Remove identity gates
295    RemoveIdentities,
296    /// Template matching and replacement
297    TemplateMatching,
298}
299
300/// Circuit optimizer
301pub struct CircuitOptimizer {
302    /// Optimization passes to apply
303    passes: Vec<OptimizationPass>,
304    /// Maximum number of optimization iterations
305    max_iterations: usize,
306}
307
308impl CircuitOptimizer {
309    /// Create a new optimizer with default passes
310    pub fn new() -> Self {
311        Self {
312            passes: vec![
313                OptimizationPass::CancelInverses,
314                OptimizationPass::RemoveIdentities,
315                OptimizationPass::FuseRotations,
316            ],
317            max_iterations: 10,
318        }
319    }
320
321    /// Add an optimization pass
322    pub fn with_pass(mut self, pass: OptimizationPass) -> Self {
323        self.passes.push(pass);
324        self
325    }
326
327    /// Set maximum iterations
328    pub fn with_max_iterations(mut self, max_iterations: usize) -> Self {
329        self.max_iterations = max_iterations;
330        self
331    }
332
333    /// Optimize a circuit
334    pub fn optimize(&self, circuit: &Circuit) -> Result<Circuit> {
335        let mut optimized = circuit.clone();
336        let mut iteration = 0;
337
338        loop {
339            let initial_gate_count = optimized.gates.len();
340
341            // Apply all passes
342            for pass in &self.passes {
343                optimized = self.apply_pass(&optimized, *pass)?;
344            }
345
346            iteration += 1;
347            let final_gate_count = optimized.gates.len();
348
349            // Stop if no improvement or max iterations reached
350            if final_gate_count >= initial_gate_count || iteration >= self.max_iterations {
351                break;
352            }
353        }
354
355        Ok(optimized)
356    }
357
358    /// Apply a single optimization pass
359    fn apply_pass(&self, circuit: &Circuit, pass: OptimizationPass) -> Result<Circuit> {
360        match pass {
361            OptimizationPass::CancelInverses => self.cancel_inverses(circuit),
362            OptimizationPass::FuseRotations => self.fuse_rotations(circuit),
363            OptimizationPass::CommutativeReordering => self.commutative_reordering(circuit),
364            OptimizationPass::RemoveIdentities => self.remove_identities(circuit),
365            OptimizationPass::TemplateMatching => self.template_matching(circuit),
366        }
367    }
368
369    /// Remove inverse gate pairs (e.g., H-H, CNOT-CNOT)
370    fn cancel_inverses(&self, circuit: &Circuit) -> Result<Circuit> {
371        let mut optimized = Circuit::new(circuit.n_qubits);
372        let gates = &circuit.gates;
373        let mut skip_next = false;
374
375        for i in 0..gates.len() {
376            if skip_next {
377                skip_next = false;
378                continue;
379            }
380
381            if i + 1 < gates.len() && gates[i].is_inverse_of(&gates[i + 1]) {
382                // Skip both gates (they cancel)
383                skip_next = true;
384            } else {
385                optimized.add_gate(gates[i].clone())?;
386            }
387        }
388
389        Ok(optimized)
390    }
391
392    /// Fuse adjacent rotation gates on the same qubit and axis
393    fn fuse_rotations(&self, circuit: &Circuit) -> Result<Circuit> {
394        let mut optimized = Circuit::new(circuit.n_qubits);
395        let gates = &circuit.gates;
396        let mut i = 0;
397
398        while i < gates.len() {
399            let gate = &gates[i];
400
401            // Check if this is a rotation gate
402            if gate.gate_type.is_rotation() && gate.qubits.len() == 1 {
403                // Look for adjacent rotations on the same qubit and axis
404                let mut fused_angle = gate.parameters[0];
405                let mut j = i + 1;
406
407                while j < gates.len() {
408                    let next_gate = &gates[j];
409                    if next_gate.gate_type == gate.gate_type
410                        && next_gate.qubits == gate.qubits
411                        && !next_gate.parameters.is_empty()
412                    {
413                        fused_angle += next_gate.parameters[0];
414                        j += 1;
415                    } else {
416                        break;
417                    }
418                }
419
420                // Add fused gate only if angle is non-zero
421                if fused_angle.abs() > 1e-10 {
422                    optimized.add_gate(Gate::with_parameters(
423                        gate.gate_type,
424                        gate.qubits.clone(),
425                        vec![fused_angle],
426                    ))?;
427                }
428
429                i = j;
430            } else {
431                optimized.add_gate(gate.clone())?;
432                i += 1;
433            }
434        }
435
436        Ok(optimized)
437    }
438
439    /// Reorder gates using commutation rules
440    fn commutative_reordering(&self, circuit: &Circuit) -> Result<Circuit> {
441        // Simple commutation: move gates to enable more cancellations
442        // This is a simplified version - full implementation would use DAG analysis
443        Ok(circuit.clone())
444    }
445
446    /// Remove identity gates and gates with zero parameters
447    fn remove_identities(&self, circuit: &Circuit) -> Result<Circuit> {
448        let mut optimized = Circuit::new(circuit.n_qubits);
449
450        for gate in &circuit.gates {
451            // Skip identity gates
452            if gate.gate_type == GateType::I {
453                continue;
454            }
455
456            // Skip rotation gates with zero angle
457            if gate.gate_type.is_rotation()
458                && !gate.parameters.is_empty()
459                && gate.parameters[0].abs() < 1e-10
460            {
461                continue;
462            }
463
464            optimized.add_gate(gate.clone())?;
465        }
466
467        Ok(optimized)
468    }
469
470    /// Template matching and replacement
471    fn template_matching(&self, circuit: &Circuit) -> Result<Circuit> {
472        // Simplified: Look for common patterns like H-CNOT-H = CNOT with swapped control/target
473        Ok(circuit.clone())
474    }
475}
476
477impl Default for CircuitOptimizer {
478    fn default() -> Self {
479        Self::new()
480    }
481}
482
483/// Optimization statistics
484#[derive(Debug, Clone)]
485pub struct OptimizationStats {
486    /// Original gate count
487    pub original_gates: usize,
488    /// Optimized gate count
489    pub optimized_gates: usize,
490    /// Original circuit depth
491    pub original_depth: usize,
492    /// Optimized circuit depth
493    pub optimized_depth: usize,
494    /// Original two-qubit gate count
495    pub original_two_qubit_gates: usize,
496    /// Optimized two-qubit gate count
497    pub optimized_two_qubit_gates: usize,
498    /// Reduction percentage
499    pub gate_reduction_percent: f64,
500    /// Depth reduction percentage
501    pub depth_reduction_percent: f64,
502}
503
504impl OptimizationStats {
505    /// Compute statistics from original and optimized circuits
506    pub fn from_circuits(original: &Circuit, optimized: &Circuit) -> Self {
507        let original_gates = original.gates.len();
508        let optimized_gates = optimized.gates.len();
509        let original_depth = original.depth();
510        let optimized_depth = optimized.depth();
511        let original_two_qubit = original.two_qubit_gate_count();
512        let optimized_two_qubit = optimized.two_qubit_gate_count();
513
514        let gate_reduction = if original_gates > 0 {
515            100.0 * (original_gates - optimized_gates) as f64 / original_gates as f64
516        } else {
517            0.0
518        };
519
520        let depth_reduction = if original_depth > 0 {
521            100.0 * (original_depth - optimized_depth) as f64 / original_depth as f64
522        } else {
523            0.0
524        };
525
526        Self {
527            original_gates,
528            optimized_gates,
529            original_depth,
530            optimized_depth,
531            original_two_qubit_gates: original_two_qubit,
532            optimized_two_qubit_gates: optimized_two_qubit,
533            gate_reduction_percent: gate_reduction,
534            depth_reduction_percent: depth_reduction,
535        }
536    }
537}
538
539#[cfg(test)]
540mod tests {
541    use super::*;
542
543    #[test]
544    fn test_gate_inverse() {
545        let h = Gate::new(GateType::H, vec![0]);
546        let h_inv = h.inverse();
547        assert!(h.is_inverse_of(&h_inv));
548    }
549
550    #[test]
551    fn test_cancel_inverses() {
552        let mut circuit = Circuit::new(2);
553        circuit.add_gate(Gate::new(GateType::H, vec![0])).unwrap();
554        circuit.add_gate(Gate::new(GateType::H, vec![0])).unwrap();
555        circuit.add_gate(Gate::new(GateType::X, vec![1])).unwrap();
556
557        let optimizer = CircuitOptimizer::new();
558        let optimized = optimizer.cancel_inverses(&circuit).unwrap();
559
560        // H-H should cancel, leaving only X
561        assert_eq!(optimized.gates.len(), 1);
562        assert_eq!(optimized.gates[0].gate_type, GateType::X);
563    }
564
565    #[test]
566    fn test_fuse_rotations() {
567        let mut circuit = Circuit::new(1);
568        circuit
569            .add_gate(Gate::with_parameters(
570                GateType::RX,
571                vec![0],
572                vec![std::f64::consts::PI / 4.0],
573            ))
574            .unwrap();
575        circuit
576            .add_gate(Gate::with_parameters(
577                GateType::RX,
578                vec![0],
579                vec![std::f64::consts::PI / 4.0],
580            ))
581            .unwrap();
582
583        let optimizer = CircuitOptimizer::new();
584        let optimized = optimizer.fuse_rotations(&circuit).unwrap();
585
586        // Two RX gates should fuse into one
587        assert_eq!(optimized.gates.len(), 1);
588        assert!((optimized.gates[0].parameters[0] - std::f64::consts::PI / 2.0).abs() < 1e-10);
589    }
590
591    #[test]
592    fn test_remove_identities() {
593        let mut circuit = Circuit::new(2);
594        circuit.add_gate(Gate::new(GateType::I, vec![0])).unwrap();
595        circuit.add_gate(Gate::new(GateType::X, vec![1])).unwrap();
596        circuit
597            .add_gate(Gate::with_parameters(GateType::RZ, vec![0], vec![0.0]))
598            .unwrap();
599
600        let optimizer = CircuitOptimizer::new();
601        let optimized = optimizer.remove_identities(&circuit).unwrap();
602
603        // Identity and zero-angle rotation should be removed
604        assert_eq!(optimized.gates.len(), 1);
605        assert_eq!(optimized.gates[0].gate_type, GateType::X);
606    }
607
608    #[test]
609    fn test_circuit_depth() {
610        let mut circuit = Circuit::new(2);
611        circuit.add_gate(Gate::new(GateType::H, vec![0])).unwrap();
612        circuit.add_gate(Gate::new(GateType::H, vec![1])).unwrap();
613        circuit
614            .add_gate(Gate::new(GateType::CNOT, vec![0, 1]))
615            .unwrap();
616
617        // H on both qubits in parallel (depth 1) + CNOT (depth 2)
618        assert_eq!(circuit.depth(), 2);
619    }
620
621    #[test]
622    fn test_full_optimization() {
623        let mut circuit = Circuit::new(2);
624        // Add redundant gates that should be optimized away
625        circuit.add_gate(Gate::new(GateType::H, vec![0])).unwrap();
626        circuit.add_gate(Gate::new(GateType::H, vec![0])).unwrap(); // Cancels with previous
627        circuit
628            .add_gate(Gate::with_parameters(
629                GateType::RX,
630                vec![1],
631                vec![std::f64::consts::PI / 4.0],
632            ))
633            .unwrap();
634        circuit
635            .add_gate(Gate::with_parameters(
636                GateType::RX,
637                vec![1],
638                vec![std::f64::consts::PI / 4.0],
639            ))
640            .unwrap(); // Should fuse
641        circuit.add_gate(Gate::new(GateType::I, vec![0])).unwrap(); // Should be removed
642
643        let optimizer = CircuitOptimizer::new();
644        let optimized = optimizer.optimize(&circuit).unwrap();
645
646        // Should have only 1 gate (fused RX)
647        assert_eq!(optimized.gates.len(), 1);
648        assert_eq!(optimized.gates[0].gate_type, GateType::RX);
649    }
650
651    #[test]
652    fn test_optimization_stats() {
653        let mut original = Circuit::new(2);
654        for _ in 0..10 {
655            original.add_gate(Gate::new(GateType::X, vec![0])).unwrap();
656        }
657
658        let mut optimized = Circuit::new(2);
659        optimized.add_gate(Gate::new(GateType::X, vec![0])).unwrap();
660
661        let stats = OptimizationStats::from_circuits(&original, &optimized);
662        assert_eq!(stats.original_gates, 10);
663        assert_eq!(stats.optimized_gates, 1);
664        assert!((stats.gate_reduction_percent - 90.0).abs() < 1e-6);
665    }
666}
quantrs2_sim/circuit_optimizer.rs

quantrs2_sim/
circuit_optimizer.rs
