quantrs2_core/optimizations_stable/
gate_fusion.rs

1//! Quantum Gate Fusion Engine
2//!
3//! Optimizes quantum circuits by fusing adjacent gates into efficient sequences,
4//! reducing the total number of matrix multiplications and improving performance.
5
6use crate::error::{QuantRS2Error, QuantRS2Result};
7use scirs2_core::Complex64;
8use std::collections::HashMap;
9use std::sync::{Arc, OnceLock, RwLock};
10
11/// Types of gates that can be fused
12#[derive(Debug, Clone, PartialEq, Eq, Hash)]
13pub enum GateType {
14    // Single-qubit gates
15    PauliX,
16    PauliY,
17    PauliZ,
18    Hadamard,
19    Phase(u64), // Quantized angle
20    RX(u64),    // Quantized angle
21    RY(u64),    // Quantized angle
22    RZ(u64),    // Quantized angle
23    S,
24    T,
25
26    // Two-qubit gates
27    CNOT,
28    CZ,
29    SWAP,
30    CRZ(u64), // Controlled RZ with quantized angle
31
32    // Multi-qubit gates
33    Toffoli,
34    Fredkin,
35}
36
37/// A quantum gate with target qubits
38#[derive(Debug, Clone)]
39pub struct QuantumGate {
40    pub gate_type: GateType,
41    pub qubits: Vec<usize>,
42    pub matrix: Vec<Complex64>,
43}
44
45impl QuantumGate {
46    /// Create a new quantum gate
47    pub fn new(gate_type: GateType, qubits: Vec<usize>) -> QuantRS2Result<Self> {
48        let matrix = Self::compute_matrix(&gate_type)?;
49        Ok(Self {
50            gate_type,
51            qubits,
52            matrix,
53        })
54    }
55
56    /// Compute the unitary matrix for a gate type
57    fn compute_matrix(gate_type: &GateType) -> QuantRS2Result<Vec<Complex64>> {
58        use std::f64::consts::{FRAC_1_SQRT_2, PI};
59
60        let matrix = match gate_type {
61            GateType::PauliX => vec![
62                Complex64::new(0.0, 0.0),
63                Complex64::new(1.0, 0.0),
64                Complex64::new(1.0, 0.0),
65                Complex64::new(0.0, 0.0),
66            ],
67            GateType::PauliY => vec![
68                Complex64::new(0.0, 0.0),
69                Complex64::new(0.0, -1.0),
70                Complex64::new(0.0, 1.0),
71                Complex64::new(0.0, 0.0),
72            ],
73            GateType::PauliZ => vec![
74                Complex64::new(1.0, 0.0),
75                Complex64::new(0.0, 0.0),
76                Complex64::new(0.0, 0.0),
77                Complex64::new(-1.0, 0.0),
78            ],
79            GateType::Hadamard => vec![
80                Complex64::new(FRAC_1_SQRT_2, 0.0),
81                Complex64::new(FRAC_1_SQRT_2, 0.0),
82                Complex64::new(FRAC_1_SQRT_2, 0.0),
83                Complex64::new(-FRAC_1_SQRT_2, 0.0),
84            ],
85            GateType::S => vec![
86                Complex64::new(1.0, 0.0),
87                Complex64::new(0.0, 0.0),
88                Complex64::new(0.0, 0.0),
89                Complex64::new(0.0, 1.0),
90            ],
91            GateType::T => vec![
92                Complex64::new(1.0, 0.0),
93                Complex64::new(0.0, 0.0),
94                Complex64::new(0.0, 0.0),
95                Complex64::new(FRAC_1_SQRT_2, FRAC_1_SQRT_2),
96            ],
97            GateType::RX(quantized_angle) => {
98                let angle = (*quantized_angle as f64) / 1_000_000.0;
99                let cos_half = (angle / 2.0).cos();
100                let sin_half = (angle / 2.0).sin();
101                vec![
102                    Complex64::new(cos_half, 0.0),
103                    Complex64::new(0.0, -sin_half),
104                    Complex64::new(0.0, -sin_half),
105                    Complex64::new(cos_half, 0.0),
106                ]
107            }
108            GateType::RY(quantized_angle) => {
109                let angle = (*quantized_angle as f64) / 1_000_000.0;
110                let cos_half = (angle / 2.0).cos();
111                let sin_half = (angle / 2.0).sin();
112                vec![
113                    Complex64::new(cos_half, 0.0),
114                    Complex64::new(-sin_half, 0.0),
115                    Complex64::new(sin_half, 0.0),
116                    Complex64::new(cos_half, 0.0),
117                ]
118            }
119            GateType::RZ(quantized_angle) => {
120                let angle = (*quantized_angle as f64) / 1_000_000.0;
121                let cos_half = (angle / 2.0).cos();
122                let sin_half = (angle / 2.0).sin();
123                vec![
124                    Complex64::new(cos_half, -sin_half),
125                    Complex64::new(0.0, 0.0),
126                    Complex64::new(0.0, 0.0),
127                    Complex64::new(cos_half, sin_half),
128                ]
129            }
130            GateType::Phase(quantized_angle) => {
131                let angle = (*quantized_angle as f64) / 1_000_000.0;
132                vec![
133                    Complex64::new(1.0, 0.0),
134                    Complex64::new(0.0, 0.0),
135                    Complex64::new(0.0, 0.0),
136                    Complex64::new(angle.cos(), angle.sin()),
137                ]
138            }
139            GateType::CNOT => vec![
140                Complex64::new(1.0, 0.0),
141                Complex64::new(0.0, 0.0),
142                Complex64::new(0.0, 0.0),
143                Complex64::new(0.0, 0.0),
144                Complex64::new(0.0, 0.0),
145                Complex64::new(1.0, 0.0),
146                Complex64::new(0.0, 0.0),
147                Complex64::new(0.0, 0.0),
148                Complex64::new(0.0, 0.0),
149                Complex64::new(0.0, 0.0),
150                Complex64::new(0.0, 0.0),
151                Complex64::new(1.0, 0.0),
152                Complex64::new(0.0, 0.0),
153                Complex64::new(0.0, 0.0),
154                Complex64::new(1.0, 0.0),
155                Complex64::new(0.0, 0.0),
156            ],
157            _ => return Err(QuantRS2Error::UnsupportedGate(format!("{gate_type:?}"))),
158        };
159
160        Ok(matrix)
161    }
162
163    /// Get the number of qubits this gate acts on
164    pub const fn num_qubits(&self) -> usize {
165        match self.gate_type {
166            GateType::PauliX
167            | GateType::PauliY
168            | GateType::PauliZ
169            | GateType::Hadamard
170            | GateType::Phase(_)
171            | GateType::RX(_)
172            | GateType::RY(_)
173            | GateType::RZ(_)
174            | GateType::S
175            | GateType::T => 1,
176
177            GateType::CNOT | GateType::CZ | GateType::SWAP | GateType::CRZ(_) => 2,
178
179            GateType::Toffoli | GateType::Fredkin => 3,
180        }
181    }
182}
183
184/// Rule for fusing gates
185#[derive(Debug, Clone)]
186pub struct FusionRule {
187    pub pattern: Vec<GateType>,
188    pub replacement: Vec<GateType>,
189    pub efficiency_gain: f64, // Expected speedup
190}
191
192impl FusionRule {
193    /// Create common fusion rules
194    pub fn common_rules() -> Vec<Self> {
195        vec![
196            // X * X = I (eliminate double X)
197            Self {
198                pattern: vec![GateType::PauliX, GateType::PauliX],
199                replacement: vec![], // Identity = no gates
200                efficiency_gain: 2.0,
201            },
202            // Y * Y = I
203            Self {
204                pattern: vec![GateType::PauliY, GateType::PauliY],
205                replacement: vec![],
206                efficiency_gain: 2.0,
207            },
208            // Z * Z = I
209            Self {
210                pattern: vec![GateType::PauliZ, GateType::PauliZ],
211                replacement: vec![],
212                efficiency_gain: 2.0,
213            },
214            // H * H = I
215            Self {
216                pattern: vec![GateType::Hadamard, GateType::Hadamard],
217                replacement: vec![],
218                efficiency_gain: 2.0,
219            },
220            // S * S = Z
221            Self {
222                pattern: vec![GateType::S, GateType::S],
223                replacement: vec![GateType::PauliZ],
224                efficiency_gain: 2.0,
225            },
226            // T * T * T * T = I
227            Self {
228                pattern: vec![GateType::T, GateType::T, GateType::T, GateType::T],
229                replacement: vec![],
230                efficiency_gain: 4.0,
231            },
232            // Commute Z and RZ (can be parallelized)
233            // This would be handled by specialized logic
234        ]
235    }
236}
237
238/// A sequence of fused gates
239#[derive(Debug, Clone)]
240pub struct FusedGateSequence {
241    pub gates: Vec<QuantumGate>,
242    pub fused_matrix: Vec<Complex64>,
243    pub target_qubits: Vec<usize>,
244    pub efficiency_gain: f64,
245}
246
247impl FusedGateSequence {
248    /// Create a fused sequence from individual gates
249    pub fn from_gates(gates: Vec<QuantumGate>) -> QuantRS2Result<Self> {
250        if gates.is_empty() {
251            return Err(QuantRS2Error::InvalidInput(
252                "Empty gate sequence".to_string(),
253            ));
254        }
255
256        // All gates must act on the same qubits for fusion
257        let target_qubits = gates[0].qubits.clone();
258        for gate in &gates {
259            if gate.qubits != target_qubits {
260                return Err(QuantRS2Error::InvalidInput(
261                    "All gates must act on the same qubits for fusion".to_string(),
262                ));
263            }
264        }
265
266        // Compute fused matrix by multiplying individual matrices
267        let matrix_size = gates[0].matrix.len();
268        let sqrt_size = (matrix_size as f64).sqrt() as usize;
269
270        let mut fused_matrix = Self::identity_matrix(sqrt_size);
271
272        // Multiply matrices in reverse order (gates are applied left to right)
273        for gate in gates.iter().rev() {
274            fused_matrix = Self::matrix_multiply(&fused_matrix, &gate.matrix, sqrt_size)?;
275        }
276
277        let efficiency_gain = gates.len() as f64; // Each gate fusion saves one matrix multiplication
278
279        Ok(Self {
280            gates,
281            fused_matrix,
282            target_qubits,
283            efficiency_gain,
284        })
285    }
286
287    /// Create identity matrix
288    fn identity_matrix(size: usize) -> Vec<Complex64> {
289        let mut matrix = vec![Complex64::new(0.0, 0.0); size * size];
290        for i in 0..size {
291            matrix[i * size + i] = Complex64::new(1.0, 0.0);
292        }
293        matrix
294    }
295
296    /// Check if matrix is approximately identity
297    fn is_identity_matrix(&self) -> bool {
298        let size = (self.fused_matrix.len() as f64).sqrt() as usize;
299        let identity = Self::identity_matrix(size);
300
301        for (a, b) in self.fused_matrix.iter().zip(identity.iter()) {
302            if (a - b).norm() > 1e-10 {
303                return false;
304            }
305        }
306        true
307    }
308
309    /// Multiply two matrices
310    fn matrix_multiply(
311        a: &[Complex64],
312        b: &[Complex64],
313        size: usize,
314    ) -> QuantRS2Result<Vec<Complex64>> {
315        if a.len() != size * size || b.len() != size * size {
316            return Err(QuantRS2Error::InvalidInput(
317                "Matrix size mismatch".to_string(),
318            ));
319        }
320
321        let mut result = vec![Complex64::new(0.0, 0.0); size * size];
322
323        for i in 0..size {
324            for j in 0..size {
325                for k in 0..size {
326                    result[i * size + j] += a[i * size + k] * b[k * size + j];
327                }
328            }
329        }
330
331        Ok(result)
332    }
333}
334
335/// Gate fusion engine
336pub struct GateFusionEngine {
337    rules: Vec<FusionRule>,
338    statistics: Arc<RwLock<FusionStatistics>>,
339}
340
341/// Fusion performance statistics
342#[derive(Debug, Clone, Default)]
343pub struct FusionStatistics {
344    pub total_fusions: u64,
345    pub gates_eliminated: u64,
346    pub total_efficiency_gain: f64,
347    pub fusion_types: HashMap<String, u64>,
348}
349
350impl GateFusionEngine {
351    /// Create a new fusion engine
352    pub fn new() -> Self {
353        Self {
354            rules: FusionRule::common_rules(),
355            statistics: Arc::new(RwLock::new(FusionStatistics::default())),
356        }
357    }
358
359    /// Add a custom fusion rule
360    pub fn add_rule(&mut self, rule: FusionRule) {
361        self.rules.push(rule);
362    }
363
364    /// Fuse a sequence of gates
365    pub fn fuse_gates(&self, gates: Vec<QuantumGate>) -> QuantRS2Result<Vec<FusedGateSequence>> {
366        if gates.is_empty() {
367            return Ok(vec![]);
368        }
369
370        let mut fused_sequences = Vec::new();
371        let mut i = 0;
372
373        while i < gates.len() {
374            let gate = &gates[i];
375
376            // Try to find fusable patterns
377            if let Some(fusion_length) = self.find_fusion_pattern(&gates[i..]) {
378                // Found a fusable pattern
379                let fusion_gates = gates[i..i + fusion_length].to_vec();
380                let fused_sequence = FusedGateSequence::from_gates(fusion_gates)?;
381
382                // Only add non-identity sequences
383                if fused_sequence.is_identity_matrix() {
384                    // Identity matrix - gates cancelled out, count them as eliminated
385                    if let Ok(mut stats) = self.statistics.write() {
386                        stats.total_fusions += 1;
387                        stats.gates_eliminated += fusion_length as u64; // All gates eliminated
388                    }
389                } else {
390                    // Update statistics
391                    if let Ok(mut stats) = self.statistics.write() {
392                        stats.total_fusions += 1;
393                        stats.gates_eliminated += (fusion_length - 1) as u64;
394                        stats.total_efficiency_gain += fused_sequence.efficiency_gain;
395
396                        let fusion_type = format!("{:?}_fusion", gate.gate_type);
397                        *stats.fusion_types.entry(fusion_type).or_insert(0) += 1;
398                    }
399
400                    fused_sequences.push(fused_sequence);
401                }
402                i += fusion_length;
403            } else {
404                // No fusion pattern found, group consecutive gates on the same qubit
405                let mut gate_group = vec![gate.clone()];
406                let mut j = i + 1;
407
408                // Collect consecutive gates on the same qubit
409                while j < gates.len() && gates[j].qubits == gate.qubits {
410                    gate_group.push(gates[j].clone());
411                    j += 1;
412                }
413
414                // Create a fused sequence for the group
415                let fused_sequence = FusedGateSequence::from_gates(gate_group)?;
416                fused_sequences.push(fused_sequence);
417                i = j;
418            }
419        }
420
421        Ok(fused_sequences)
422    }
423
424    /// Find fusion patterns in gate sequence
425    fn find_fusion_pattern(&self, gates: &[QuantumGate]) -> Option<usize> {
426        for rule in &self.rules {
427            if gates.len() >= rule.pattern.len() {
428                let matches = gates[..rule.pattern.len()]
429                    .iter()
430                    .zip(&rule.pattern)
431                    .all(|(gate, pattern_gate)| gate.gate_type == *pattern_gate);
432
433                // Also check that all gates in the pattern act on the same qubits
434                let same_qubits = if rule.pattern.len() > 1 {
435                    let first_qubits = &gates[0].qubits;
436                    gates[1..rule.pattern.len()]
437                        .iter()
438                        .all(|gate| gate.qubits == *first_qubits)
439                } else {
440                    true // Single gate patterns always match
441                };
442
443                if matches && same_qubits {
444                    return Some(rule.pattern.len());
445                }
446            }
447        }
448
449        // Check for consecutive identical single-qubit gates on the same qubits (can be optimized)
450        if gates.len() >= 2 {
451            let first = &gates[0];
452            if first.num_qubits() == 1 {
453                let mut count = 1;
454                for gate in gates.iter().skip(1) {
455                    if gate.gate_type == first.gate_type && gate.qubits == first.qubits {
456                        count += 1;
457                    } else {
458                        break;
459                    }
460                }
461                if count > 1 {
462                    return Some(count); // Found consecutive identical gates on same qubits
463                }
464            }
465        }
466
467        None
468    }
469
470    /// Get fusion statistics
471    pub fn get_statistics(&self) -> FusionStatistics {
472        self.statistics
473            .read()
474            .map(|guard| guard.clone())
475            .unwrap_or_default()
476    }
477
478    /// Get global fusion statistics
479    pub fn get_global_statistics() -> FusionStatistics {
480        if let Some(engine) = GLOBAL_FUSION_ENGINE.get() {
481            engine.get_statistics()
482        } else {
483            FusionStatistics::default()
484        }
485    }
486
487    /// Reset statistics
488    pub fn reset_statistics(&self) {
489        if let Ok(mut stats) = self.statistics.write() {
490            *stats = FusionStatistics::default();
491        }
492    }
493}
494
495impl Default for GateFusionEngine {
496    fn default() -> Self {
497        Self::new()
498    }
499}
500
501/// Global gate fusion engine
502static GLOBAL_FUSION_ENGINE: OnceLock<GateFusionEngine> = OnceLock::new();
503
504/// Get the global gate fusion engine
505pub fn get_global_fusion_engine() -> &'static GateFusionEngine {
506    GLOBAL_FUSION_ENGINE.get_or_init(GateFusionEngine::new)
507}
508
509/// Apply gate fusion to a circuit
510pub fn apply_gate_fusion(gates: Vec<QuantumGate>) -> QuantRS2Result<Vec<FusedGateSequence>> {
511    let engine = get_global_fusion_engine();
512    engine.fuse_gates(gates)
513}
514
515#[cfg(test)]
516mod tests {
517    use super::*;
518
519    #[test]
520    fn test_pauli_x_fusion() {
521        let gates = vec![
522            QuantumGate::new(GateType::PauliX, vec![0]).expect("failed to create PauliX gate"),
523            QuantumGate::new(GateType::PauliX, vec![0]).expect("failed to create PauliX gate"),
524        ];
525
526        let engine = GateFusionEngine::new();
527        let fused = engine.fuse_gates(gates).expect("failed to fuse gates");
528
529        // Should eliminate both X gates (X*X = I)
530        assert_eq!(fused.len(), 0);
531
532        let stats = engine.get_statistics();
533        assert_eq!(stats.gates_eliminated, 2);
534    }
535
536    #[test]
537    fn test_hadamard_fusion() {
538        let gates = vec![
539            QuantumGate::new(GateType::Hadamard, vec![0]).expect("failed to create Hadamard gate"),
540            QuantumGate::new(GateType::Hadamard, vec![0]).expect("failed to create Hadamard gate"),
541        ];
542
543        let engine = GateFusionEngine::new();
544        let fused = engine.fuse_gates(gates).expect("failed to fuse gates");
545
546        // Should eliminate both H gates (H*H = I)
547        assert_eq!(fused.len(), 0);
548    }
549
550    #[test]
551    fn test_mixed_gate_fusion() {
552        let gates = vec![
553            QuantumGate::new(GateType::PauliX, vec![0]).expect("failed to create PauliX gate"),
554            QuantumGate::new(GateType::PauliY, vec![0]).expect("failed to create PauliY gate"),
555            QuantumGate::new(GateType::PauliZ, vec![0]).expect("failed to create PauliZ gate"),
556        ];
557
558        let engine = GateFusionEngine::new();
559        let fused = engine.fuse_gates(gates).expect("failed to fuse gates");
560
561        // Should create one fused sequence with all three gates
562        assert_eq!(fused.len(), 1);
563        assert_eq!(fused[0].gates.len(), 3);
564    }
565
566    #[test]
567    fn test_no_fusion_different_qubits() {
568        let gates = vec![
569            QuantumGate::new(GateType::PauliX, vec![0]).expect("failed to create PauliX gate"),
570            QuantumGate::new(GateType::PauliX, vec![1]).expect("failed to create PauliX gate"), // Different qubit
571        ];
572
573        let engine = GateFusionEngine::new();
574        let fused = engine.fuse_gates(gates).expect("failed to fuse gates");
575
576        // Should create two separate sequences
577        assert_eq!(fused.len(), 2);
578    }
579
580    #[test]
581    fn test_matrix_multiplication() {
582        // Test identity multiplication
583        let identity = vec![
584            Complex64::new(1.0, 0.0),
585            Complex64::new(0.0, 0.0),
586            Complex64::new(0.0, 0.0),
587            Complex64::new(1.0, 0.0),
588        ];
589        let pauli_x = vec![
590            Complex64::new(0.0, 0.0),
591            Complex64::new(1.0, 0.0),
592            Complex64::new(1.0, 0.0),
593            Complex64::new(0.0, 0.0),
594        ];
595
596        let result = FusedGateSequence::matrix_multiply(&identity, &pauli_x, 2)
597            .expect("matrix multiplication failed");
598
599        // I * X should equal X
600        for (a, b) in result.iter().zip(pauli_x.iter()) {
601            assert!((a - b).norm() < 1e-10);
602        }
603    }
604
605    #[test]
606    fn test_efficiency_gain_calculation() {
607        let gates = vec![
608            QuantumGate::new(GateType::S, vec![0]).expect("failed to create S gate"),
609            QuantumGate::new(GateType::T, vec![0]).expect("failed to create T gate"),
610            QuantumGate::new(GateType::Hadamard, vec![0]).expect("failed to create Hadamard gate"),
611        ];
612
613        let fused = FusedGateSequence::from_gates(gates).expect("failed to create fused sequence");
614        assert_eq!(fused.efficiency_gain, 3.0); // Three gates fused into one
615    }
616}
quantrs2_core/optimizations_stable/gate_fusion.rs

quantrs2_core/optimizations_stable/
gate_fusion.rs
