cargo-coupling 0.2.8

A coupling analysis tool for Rust projects - measuring the 'right distance' in your code
Documentation
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
# cargo-coupling

[![Crates.io](https://img.shields.io/crates/v/cargo-coupling.svg)](https://crates.io/crates/cargo-coupling)
[![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
[![Rust](https://img.shields.io/badge/rust-2024-orange.svg)](https://www.rust-lang.org)

**Measure the "right distance" in your Rust code.**

`cargo-coupling` analyzes coupling in Rust projects based on Vlad Khononov's "Balancing Coupling in Software Design" framework. It calculates a **Balance Score** from three core dimensions: **Integration Strength**, **Distance**, and **Volatility**.

![CLI Output](docs/images/cli-output.png)

> ⚠️ **Experimental Project**
>
> This tool is currently experimental. The scoring algorithms, thresholds, and detected patterns are subject to change based on real-world feedback.
>
> **We want your input!** If you try this tool on your project, please share your experience:
> - Are the grades and scores meaningful for your codebase?
> - Are there false positives or patterns that shouldn't be flagged?
> - What additional metrics would be useful?
>
> Please open an issue at [GitHub Issues]https://github.com/nwiizo/cargo-coupling/issues to discuss. Your feedback helps improve the tool for everyone.

## Quick Start

### 1. Install

```bash
cargo install cargo-coupling
```

Or use Docker:

```bash
docker pull ghcr.io/nwiizo/cargo-coupling
```

### 2. Analyze

```bash
# Analyze current project (default: shows only important issues)
cargo coupling ./src

# Show summary only
cargo coupling --summary ./src

# Japanese output with explanations (日本語出力)
cargo coupling --summary --japanese ./src
cargo coupling --summary --jp ./src

# Show all issues including Low severity
cargo coupling --summary --all ./src
```

### 3. Refactor with AI

```bash
# Generate AI-friendly output
cargo coupling --ai ./src
```

Copy the output and use this prompt with Claude, Copilot, or any AI coding assistant:

```
The following is the output of `cargo coupling --ai`, which analyzes coupling issues in a Rust project.
For each issue, suggest specific code changes to reduce coupling.
Focus on introducing traits, moving code closer, or breaking circular dependencies.
```

Example output:

```
Coupling Issues in my-project:
────────────────────────────────────────────────────────────

Grade: B (Good) | Score: 0.88 | Issues: 0 High, 5 Medium

Issues:

1. 🟡 api::handler → db::internal::Query
   Type: Global Complexity
   Problem: Intrusive coupling to db::internal::Query across module boundary
   Fix: Introduce trait `QueryTrait` with methods: // Extract required methods

2. 🟡 25 dependents → core::types
   Type: High Afferent Coupling
   Problem: Module core::types is depended on by 25 other components
   Fix: Introduce trait `TypesInterface` with methods: // Define stable public API
```

The AI will analyze patterns and suggest specific refactoring strategies.

### 4. Interactive Web Visualization (Experimental)

> ⚠️ **Experimental Feature**: The Web UI is currently in an experimental state. The interface, features, and behavior may change significantly in future versions.

![Web UI](docs/images/web-ui.png)

```bash
# Start interactive web UI
cargo coupling --web ./src

# Custom port
cargo coupling --web --port 8080 ./src
```

The web UI provides:
- Interactive graph visualization with Cytoscape.js
- **Hotspots panel**: Top refactoring targets ranked by severity
- **Blast Radius**: Impact analysis with risk score
- **Clusters**: Architecture grouping detection
- Filtering by strength, distance, volatility, balance score
- Source code viewing with syntax highlighting

### 5. Job-Focused CLI Commands

For quick, focused analysis without opening the web UI:

```bash
# Find top refactoring targets
cargo coupling --hotspots ./src
cargo coupling --hotspots=10 ./src

# With beginner-friendly explanations
cargo coupling --hotspots --verbose ./src

# Analyze change impact for a specific module
cargo coupling --impact main ./src
cargo coupling --impact analyzer ./src

# Trace dependencies for a specific function or type
cargo coupling --trace analyze_file ./src
cargo coupling --trace BalanceScore ./src

# CI/CD quality gate (exits with code 1 on failure)
cargo coupling --check ./src
cargo coupling --check --min-grade=B ./src
cargo coupling --check --max-critical=0 --max-circular=0 ./src

# Machine-readable JSON output
cargo coupling --json ./src
cargo coupling --json ./src | jq '.hotspots[0]'
```

Example `--hotspots --verbose` output:

```
#1 my-project::main (Score: 55)
   🟡 Medium: High Efferent Coupling

   💡 What it means:
      This module depends on too many other modules

   ⚠️  Why it's a problem:
      • Changes elsewhere may break this module
      • Testing requires many mocks/stubs
      • Hard to understand in isolation

   🔧 How to fix:
      Split into smaller modules with clear responsibilities
      e.g., Split main.rs into cli.rs, config.rs, runner.rs
```

### More Options

```bash
# Generate detailed report to file
cargo coupling -o report.md ./src

# Show timing information
cargo coupling --summary --timing ./src

# Use 4 threads for parallel processing
cargo coupling -j 4 ./src

# Skip Git history analysis for faster results
cargo coupling --no-git ./src
```

## Features

- **3-Dimensional Balance Score**: Calculates coupling balance based on **Integration Strength**, **Distance**, and **Volatility** (0.0 - 1.0)
- **Khononov Balance Formula**: `BALANCE = (STRENGTH XOR DISTANCE) OR NOT VOLATILITY`
- **Interactive Web UI**: `--web` flag starts a browser-based visualization with graph, hotspots, and blast radius analysis
- **Job-Focused CLI**: Quick commands for common tasks (`--hotspots`, `--impact`, `--check`, `--json`)
- **Japanese Support**: `--japanese` / `--jp` flag for Japanese output with explanations and design decision matrix
- **Noise Reduction**: Default strict mode hides Low severity issues (`--all` to show all)
- **Beginner-Friendly**: `--verbose` flag explains issues in plain language with fix examples
- **CI/CD Quality Gate**: `--check` command with configurable thresholds and exit codes
- **AI-Friendly Output**: `--ai` flag generates output optimized for coding agents (Claude, Copilot, etc.)
- **Rust Pattern Detection**: Detects newtype usage, serde derives, public fields, primitive obsession
- **Issue Detection**: Automatically identifies problematic coupling patterns (God Module, etc.)
- **Circular Dependency Detection**: Detects and reports dependency cycles
- **Visibility Tracking**: Analyzes Rust visibility modifiers (pub, pub(crate), etc.)
- **Git Integration**: Analyzes change frequency from Git history for volatility scoring
- **Configuration File**: Supports `.coupling.toml` for volatility overrides
- **Parallel Processing**: Uses Rayon for fast analysis of large codebases
- **Configurable Thresholds**: Customize dependency limits via CLI or config
- **Markdown Reports**: Generates detailed analysis reports
- **Cargo Integration**: Works as a cargo subcommand

## Khononov's Coupling Balance

**Coupling Balance** is a framework proposed by Vlad Khononov that evaluates coupling between modules across three dimensions to guide design decisions.

### Core Principle

Coupling is not inherently bad. What matters is **the balance between coupling strength, distance, and volatility**.

## The Three Dimensions

### 1. Strength (Integration Strength)

Represents how tightly components depend on each other.

| Level | Description | Rust Example | Score |
|-------|-------------|--------------|-------|
| **Intrusive** | Direct dependency on internal implementation | Direct access to `struct.field` | 1.00 (strong) |
| **Functional** | Dependency on behavior | Method calls on concrete types | 0.75 |
| **Model** | Dependency on data structures | Sharing type definitions | 0.50 |
| **Contract** | Dependency on interfaces only | Access via `trait` | 0.25 (weak) |

→ Lower in the table = **weaker** coupling (preferred)

### 2. Distance

The physical or logical distance between dependent components.

| Level | Description | Score |
|-------|-------------|-------|
| **Same Module** | Within the same module | 0.25 (close) |
| **Different Module** | Different module in the same crate | 0.50 |
| **External Crate** | Dependency on external crate | 1.00 (far) |

→ Lower in the table = **farther** distance

### 3. Volatility

How frequently a component changes (automatically calculated from Git history).

| Level | Description | Changes (6 months) | Score |
|-------|-------------|-------------------|-------|
| **Low** | Stable, rarely changes | 0-2 times | 0.00 |
| **Medium** | Occasionally changes | 3-10 times | 0.50 |
| **High** | Frequently changes | 11+ times | 1.00 |

> **Note**: Volatility requires Git history. Use `cargo coupling ./src` (not `--no-git`) to enable volatility analysis.

## The Balance Law

Good design follows this principle:

```
Strong coupling is only acceptable when distance is close OR volatility is low
```

Expressed as a logical formula:

```
BALANCED = (STRENGTH ≤ threshold) OR (DISTANCE = near) OR (VOLATILITY = low)
```

Or Khononov's formula:

```
BALANCE = (STRENGTH XOR DISTANCE) OR NOT VOLATILITY
```

- **STRENGTH XOR DISTANCE**: Strong coupling × close distance OR weak coupling × far distance = Good
- **OR NOT VOLATILITY**: Even if the above isn't satisfied, low volatility makes it acceptable

## Design Decision Matrix

| Strength | Distance | Volatility | Decision | Reason |
|----------|----------|------------|----------|--------|
| Strong | Close | Low-Medium | ✅ OK | High cohesion, changes are localized |
| Weak | Far | Any | ✅ OK | Loose coupling with healthy dependencies |
| Strong | Far | Any | ⚠️ Needs improvement | Change impact spreads widely (global complexity) |
| Strong | Any | High | ⚠️ Needs improvement | Changes cascade through the system |
| Weak | Close | Low | 🤔 Consider | Opportunity for integration (possibly over-modularized) |

## Improvement Patterns

### Pattern 1: Reducing Coupling Strength via Abstraction

**Problem**: Strong coupling + far distance

```
┌─────────────┐         ┌─────────────┐
│  Module A   │ ──────▶ │  Module B   │
│             │  strong  │  (impl)     │
└─────────────┘         └─────────────┘
       far distance (different module)
```

**Solution**: Introduce a Contract (trait)

```
┌─────────────┐         ┌─────────────┐
│  Module A   │ ──────▶ │   trait T   │
│             │   weak   │ (contract)  │
└─────────────┘         └─────────────┘
                              │ implements
                        ┌─────────────┐
                        │  Module B   │
                        │   (impl)    │
                        └─────────────┘
```

### Pattern 2: Isolating Volatility

**Problem**: Strong coupling + high volatility

**Solution**: Insert a stable interface layer

## Concrete Example (Rust)

### Before: Problematic Code

```rust
// module_a.rs
fn process_user(user: &User) {
    // Direct access to struct internal fields (Intrusive)
    let name = &user.name;           // ← strong coupling
    let age = user.age;              // ← strong coupling
    let email = &user.email_address; // ← breaks if field name changes
    // ...
}
```

```rust
// module_b.rs (frequently modified)
pub struct User {
    pub name: String,
    pub age: u32,
    pub email_address: String,  // ← renamed from email
}
```

**Issues**:
- Coupling strength: Intrusive (direct field access)
- Distance: Different Module
- Volatility: High (User struct changes frequently)

### After: Improved Code

```rust
// contracts.rs (stable layer)
pub trait UserInfo {
    fn display_name(&self) -> &str;
    fn age(&self) -> u32;
    fn contact_email(&self) -> &str;
}
```

```rust
// module_b.rs (implementation details hidden)
pub struct User {
    name: String,        // changed to private
    age: u32,
    email_address: String,
}

impl UserInfo for User {
    fn display_name(&self) -> &str { &self.name }
    fn age(&self) -> u32 { self.age }
    fn contact_email(&self) -> &str { &self.email_address }
}
```

```rust
// module_a.rs (access via trait)
fn process_user(user: &impl UserInfo) {
    let name = user.display_name();    // ← Contract coupling
    let age = user.age();              // ← Contract coupling
    let email = user.contact_email();  // ← unaffected by internal changes
    // ...
}
```

**Improvements**:
- Coupling strength: Reduced to Contract (via trait)
- Changes are contained within the `User` struct
- `module_a` no longer needs to know `User`'s internal structure

## Coupling Balance Summary

| Perspective | Guideline |
|-------------|-----------|
| Strong coupling... | Keep components close, or reduce volatility |
| Far dependencies... | Use weak coupling (Contract) |
| Highly volatile components... | Isolate with stable abstraction layers |

Coupling balance is not about "eliminating coupling" but about "placing the right strength of coupling in the right place."

## Numeric Implementation

In the actual implementation:

```rust
let alignment = 1.0 - (strength - (1.0 - distance)).abs();
let volatility_impact = 1.0 - (volatility * strength);
let score = alignment * volatility_impact;
```

## CLI Options

```
cargo coupling [OPTIONS] [PATH]

Arguments:
  [PATH]  Path to analyze [default: ./src]

Options:
  -o, --output <FILE>           Output report to file
  -s, --summary                 Show summary only
      --ai                      AI-friendly output for coding agents
      --all                     Show all issues (default: hide Low severity)
      --japanese, --jp          Japanese output with explanations (日本語)
      --git-months <MONTHS>     Git history period [default: 6]
      --no-git                  Skip Git analysis
  -c, --config <CONFIG>         Config file path (default: .coupling.toml)
  -v, --verbose                 Verbose output with explanations
      --timing                  Show timing information
  -j, --jobs <N>                Number of threads (default: auto)
      --max-deps <N>            Max outgoing dependencies [default: 20]
      --max-dependents <N>      Max incoming dependencies [default: 30]

Web Visualization:
      --web                     Start interactive web UI
      --port <PORT>             Web server port [default: 3000]
      --no-open                 Don't auto-open browser
      --api-endpoint <URL>      API endpoint URL (for separate deployments)

Job-Focused Commands:
      --hotspots[=<N>]          Show top N refactoring targets [default: 5]
      --impact <MODULE>         Analyze change impact for a module
      --trace <ITEM>            Trace dependencies for a function/type
      --check                   CI/CD quality gate (exit code 1 on failure)
      --min-grade <GRADE>       Minimum grade for --check (A/B/C/D/F)
      --max-critical <N>        Max critical issues for --check
      --max-circular <N>        Max circular dependencies for --check
      --fail-on <SEVERITY>      Fail --check on severity (critical/high/medium/low)
      --json                    Output in JSON format

  -h, --help                    Print help
  -V, --version                 Print version
```

## Thresholds

### Issue Detection Thresholds

The tool uses the following default thresholds for detecting coupling issues:

| Threshold | Default | CLI Flag | Description |
|-----------|---------|----------|-------------|
| Strong Coupling | 0.75 | - | Minimum strength value considered "strong" (Intrusive level) |
| Far Distance | 0.50 | - | Minimum distance value considered "far" (DifferentModule+) |
| High Volatility | 0.75 | - | Minimum volatility value considered "high" |
| Max Dependencies | 20 | `--max-deps` | Outgoing dependencies before flagging High Efferent Coupling |
| Max Dependents | 30 | `--max-dependents` | Incoming dependencies before flagging High Afferent Coupling |

### Health Grade Calculation

Health grades are calculated based on internal couplings only (external crate dependencies are excluded):

| Grade | Description | Criteria |
|-------|-------------|----------|
| **S (Over-optimized!)** | Stop refactoring! | Medium density <= 5% with >= 20 couplings |
| **A (Well-balanced)** | Coupling is appropriate | Medium density 5-10%, no high issues |
| **B (Healthy)** | Minor issues, manageable | Medium density > 10%, no critical issues |
| **C (Room for improvement)** | Some structural issues | Any high issues OR medium density > 25% |
| **D (Attention needed)** | Significant issues | Any critical issues OR high density > 5% |
| **F (Immediate action required)** | Critical issues | More than 3 critical issues |

**Note**: S is a WARNING, not a reward. It means you might be over-engineering. Aim for A.

### Severity Classification

Issues are classified by severity based on:

| Severity | Criteria |
|----------|----------|
| **Critical** | Multiple critical issues detected (circular dependencies, etc.) |
| **High** | Count > threshold × 2 (e.g., > 40 dependencies when threshold is 20) |
| **Medium** | Count > threshold but <= threshold × 2 |
| **Low** | Minor issues, generally informational |

## Output Example

### Summary Mode (English)

```
$ cargo coupling --summary ./src

Balanced Coupling Analysis: my-project
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Grade: B (Good) | Score: 0.67/1.00 | Modules: 14

3-Dimensional Analysis:
  Strength:   Contract 1% / Model 24% / Functional 66% / Intrusive 8%
  Distance:   Same 6% / Different 2% / External 91%
  Volatility: Low 2% / Medium 98% / High 0%

Balance State:
  ✅ High Cohesion (strong+close): 24 (6%)
  ✅ Loose Coupling (weak+far): 5 (1%)
  🤔 Acceptable (strong+far+stable): 352 (92%)

Detected Issues:
  🟡 Medium: 3

Top Priorities:
  - [Medium] metrics → 17 functions, 17 types, 11 impls
  - [Medium] main → 21 dependencies
```

### Summary Mode (Japanese)

```
$ cargo coupling --summary --jp ./src

カップリング分析: my-project
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

評価: B (Good) | スコア: 0.67/1.00 | モジュール数: 14

3次元分析:
  結合強度: Contract 1% / Model 24% / Functional 66% / Intrusive 8%
           (トレイト)   (型)      (関数)        (内部アクセス)
  距離:     同一モジュール 6% / 別モジュール 2% / 外部 91%
  変更頻度: 低 2% / 中 98% / 高 0%

バランス状態:
  ✅ 高凝集 (強い結合 + 近い距離): 24 (6%) ← 理想的
  ✅ 疎結合 (弱い結合 + 遠い距離): 5 (1%) ← 理想的
  🤔 許容可能 (強い結合 + 遠い距離 + 安定): 352 (92%)

優先的に対処すべき問題:
  - 神モジュール (責務が多すぎる) | metrics
    → モジュールを分割: metrics_core, metrics_helpers

設計判断ガイド (Khononov):
  ✅ 強い結合 + 近い距離 → 高凝集 (理想的)
  ✅ 弱い結合 + 遠い距離 → 疎結合 (理想的)
  🤔 強い結合 + 遠い距離 + 安定 → 許容可能
  ❌ 強い結合 + 遠い距離 + 頻繁に変更 → 要リファクタリング
```

### Coupling Distribution

The tool shows how couplings are distributed by Integration Strength:

```
By Integration Strength:
| Strength   | Count | %   | Description                    |
|------------|-------|-----|--------------------------------|
| Contract   | 23    | 4%  | Depends on traits/interfaces   |
| Model      | 199   | 31% | Uses data types/structs        |
| Functional | 382   | 59% | Calls specific functions       |
| Intrusive  | 46    | 7%  | Accesses internal details      |
```

## Detected Issues

### Critical Severity
- **Circular Dependencies**: Modules that depend on each other in a cycle

### High Severity
- **Global Complexity**: Strong coupling spanning long distances
- **Cascading Change Risk**: Strong coupling with frequently changing components

### Medium Severity
- **God Module**: Module with too many functions, types, or implementations
- **High Efferent Coupling**: Module depends on too many other modules
- **High Afferent Coupling**: Too many modules depend on this module
- **Inappropriate Intimacy**: Intrusive coupling across module boundaries

### Low Severity (hidden by default, use `--all` to show)
- **Public Field Exposure**: Public fields that could use getter methods
- **Primitive Obsession**: Functions with many primitive parameters (suggest newtype)

## Performance

`cargo-coupling` is optimized for large codebases with parallel AST analysis and streaming Git processing.

### Benchmark Results (Large OSS Projects)

| Project | Files | With Git | Without Git | Speed |
|---------|-------|----------|-------------|-------|
| tokio | 488 | 655ms | 234ms | 745 files/sec |
| alacritty | 83 | 298ms | 161ms | 514 files/sec |
| ripgrep | 59 | 181ms | - | 326 files/sec |
| bat | 40 | 318ms | - | 126 files/sec |

### Performance Features

1. **Parallel AST Analysis**: Uses Rayon for multi-threaded file processing
2. **Optimized Git Analysis**: Streaming processing with path filtering
3. **Configurable Thread Count**: Use `-j N` to control parallelism

```bash
# Show timing information
cargo coupling --timing ./src

# Use 4 threads
cargo coupling -j 4 ./src

# Skip Git analysis for faster results
cargo coupling --no-git ./src
```

### Git Analysis Optimization

The Git volatility analysis is optimized with:

- **Path filtering**: `-- "*.rs"` filters at Git level (reduces data transfer)
- **Diff filtering**: `--diff-filter=AMRC` skips deleted files
- **Streaming**: `BufReader` processes output without loading all into memory
- **Async spawn**: Starts processing before Git completes

These optimizations provide **5x-47x speedup** compared to naive implementation on large repositories.

## Library Usage

```rust
use cargo_coupling::{
    analyze_workspace,
    generate_report_with_thresholds,
    IssueThresholds,
    VolatilityAnalyzer
};
use std::path::Path;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Analyze project with workspace support
    let mut metrics = analyze_workspace(Path::new("./src"))?;

    // Add volatility from Git history
    let mut volatility = VolatilityAnalyzer::new(6);
    if let Ok(()) = volatility.analyze(Path::new("./src")) {
        metrics.file_changes = volatility.file_changes;
        metrics.update_volatility_from_git();
    }

    // Detect circular dependencies
    let circular = metrics.circular_dependency_summary();
    if circular.total_cycles > 0 {
        println!("Found {} cycles!", circular.total_cycles);
    }

    // Generate report with custom thresholds
    let thresholds = IssueThresholds {
        max_dependencies: 20,
        max_dependents: 25,
        ..Default::default()
    };
    generate_report_with_thresholds(&metrics, &thresholds, &mut std::io::stdout())?;

    Ok(())
}
```

## Docker

Run cargo-coupling without installing Rust:

```bash
# Basic analysis
docker run --rm -v $(pwd):/workspace ghcr.io/nwiizo/cargo-coupling coupling /workspace/src

# Summary mode
docker run --rm -v $(pwd):/workspace ghcr.io/nwiizo/cargo-coupling coupling --summary /workspace/src

# Web UI (access at http://localhost:3000)
docker run --rm -p 3000:3000 -v $(pwd):/workspace ghcr.io/nwiizo/cargo-coupling coupling --web --no-open /workspace/src

# Japanese output
docker run --rm -v $(pwd):/workspace ghcr.io/nwiizo/cargo-coupling coupling --summary --jp /workspace/src
```

### Docker Compose

```bash
# Run analysis
docker compose run --rm analyze

# Start Web UI
docker compose up web
```

### Available Tags

| Tag | Description |
|-----|-------------|
| `latest` | Latest release |
| `main` | Latest main branch build |
| `vX.Y.Z` | Specific version |

## CI/CD Integration

```yaml
# .github/workflows/coupling.yml
name: Coupling Analysis

on: [push, pull_request]

jobs:
  analyze:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # Full history for volatility analysis

      - name: Install cargo-coupling
        run: cargo install cargo-coupling

      - name: Run coupling analysis
        run: cargo coupling --summary --timing ./src

      - name: Quality gate check
        run: cargo coupling --check --min-grade=C --max-circular=0 ./src

      - name: Generate report
        run: cargo coupling -o coupling-report.md ./src

      - name: Upload report
        uses: actions/upload-artifact@v4
        with:
          name: coupling-report
          path: coupling-report.md
```

### Quality Gate Options

The `--check` command provides flexible quality gate configuration:

```bash
# Fail if grade is below C
cargo coupling --check --min-grade=C ./src

# Fail if there are any circular dependencies
cargo coupling --check --max-circular=0 ./src

# Fail if there are any critical issues
cargo coupling --check --max-critical=0 ./src

# Fail on any high severity or above
cargo coupling --check --fail-on=high ./src

# Combine multiple conditions
cargo coupling --check --min-grade=B --max-circular=0 --max-critical=0 ./src
```

Exit codes:
- `0`: All checks passed
- `1`: One or more checks failed

## Best Practices

### ✅ Good: Strong Coupling at Close Distance

```rust
mod user_profile {
    pub struct User { /* ... */ }
    pub struct UserProfile { /* ... */ }

    impl User {
        pub fn get_profile(&self) -> &UserProfile { /* ... */ }
    }
}
```

### ✅ Good: Weak Coupling at Far Distance

```rust
// core/src/lib.rs
pub trait NotificationService {
    fn send(&self, message: &str) -> Result<()>;
}

// adapters/email/src/lib.rs
impl NotificationService for EmailService { /* ... */ }
```

### ❌ Bad: Strong Coupling at Far Distance

```rust
// src/api/handlers.rs
impl Handler {
    fn handle(&self) {
        // Direct dependency on internal implementation ❌
        let result = database::internal::execute_raw_sql(...);
    }
}
```

### ❌ Bad: Circular Dependencies

```rust
// module_a.rs
use crate::module_b::TypeB;  // ❌ Creates cycle

// module_b.rs
use crate::module_a::TypeA;  // ❌ Creates cycle
```

## Limitations

**This tool is a measurement aid, not an absolute authority on code quality.**

Please keep the following limitations in mind:

### What This Tool Cannot Do

- **Understand Business Context**: The tool analyzes structural patterns but cannot understand why certain couplings exist. Some "problematic" patterns may be intentional design decisions.
- **Replace Human Judgment**: Coupling metrics are heuristics. A high coupling score doesn't always mean bad code, and a low score doesn't guarantee good design.
- **Detect All Issues**: Static analysis has inherent limitations. Runtime behavior, dynamic dispatch, and macro-generated code may not be fully analyzed.
- **Provide Perfect Thresholds**: The default thresholds are calibrated for typical Rust projects but may not fit every codebase. Adjust them based on your project's needs.

### Important Considerations

- **External Dependencies Are Excluded**: The health grade only considers internal couplings. Dependencies on external crates (serde, tokio, etc.) are not penalized since you cannot control their design.
- **Git History Affects Volatility**: If Git history is unavailable or limited, volatility analysis will be incomplete.
- **Small Projects May Score Differently**: Projects with very few internal couplings (< 10) may receive a Grade B by default, as there's insufficient data for accurate assessment.

### Recommended Usage

1. **Use as a Starting Point**: The tool highlights areas worth investigating, not definitive problems.
2. **Combine with Code Review**: Human review should validate any suggested refactoring.
3. **Track Trends Over Time**: Use the tool regularly to track coupling trends rather than focusing on absolute scores.
4. **Customize Thresholds**: Adjust `--max-deps` and `--max-dependents` to match your project's architecture.

**The goal is to provide visibility into coupling patterns, empowering developers to make informed decisions.**

## References

- [Vlad Khononov - "Balancing Coupling in Software Design"]https://www.amazon.com/dp/B0FVDYKJYQ

## Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

## License

This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.