# Architecture Review: Diagrams-as-Code CLI Tool

**Reviewer**: Tech Lead
**Date**: 2026-02-10
**Status**: **APPROVED WITH MINOR NOTES**

---

## Executive Summary

The proposed architecture is **fundamentally sound and ready for implementation**. It demonstrates excellent separation of concerns, provides precise interface definitions suitable for autonomous implementation, and maintains internal consistency throughout. All acceptance criteria have clear implementation paths, and the architecture solves exactly what the PM specified—no more, no less.

**Decision**: ✅ **APPROVE**

Autonomous engineer agents can implement this architecture independently with confidence that components will integrate correctly.

---

## 1. Requirements Coverage Analysis

### Mapping: Acceptance Criteria → Architecture Components

#### ✅ AC1: Project Structure and Build System
- **Satisfied by**: `Cargo.toml` (lines 1358-1381), file structure (lines 1743-1768)
- **Implementation path**: Clear dependency specifications, release profile optimizations
- **Assessment**: Complete

#### ✅ AC2: CLI Interface with Subcommands
- **Satisfied by**: `cli.rs` (lines 1249-1297), `main.rs` (lines 1299-1344)
- **Implementation path**: Clap derive macros define `compile`, `preview`, `validate` subcommands with help text
- **Assessment**: Complete

#### ✅ AC3: DSL Compilation to SVG
- **Satisfied by**: `App::compile()` (lines 1154-1174), full pipeline (lexer → parser → validator → layout → svg renderer)
- **Implementation path**:
  1. `Lexer::tokenize()` → tokens
  2. `Parser::parse()` → AST
  3. `Validator::validate()` → semantic check
  4. `LayoutEngine::layout()` → positioned diagram
  5. `SvgRenderer::render()` → SVG string
  6. File write
- **Assessment**: Complete, all steps defined

#### ✅ AC4: ASCII Preview Mode
- **Satisfied by**: `App::preview()` (lines 1177-1194), `AsciiRenderer` (lines 926-1113)
- **Implementation path**: Same pipeline as AC3, but uses `AsciiRenderer::render()` instead of `SvgRenderer`
- **Box-drawing characters**: Explicitly defined (lines 1115-1119), matches AC4 requirement for U+2500-U+257F
- **Assessment**: Complete

#### ✅ AC5: Syntax Validation and Error Reporting
- **Satisfied by**: `error.rs` module (lines 188-295)
- **Error types**:
  - `SyntaxError` with `SourcePosition` (line/column) → exit code 1
  - `DiagramError::format_detailed()` provides meaningful error messages (lines 252-275)
- **Implementation path**: Parser/lexer throw `SyntaxError`, CLI catches and formats with line/column
- **Assessment**: Complete

#### ✅ AC6: SVG Contains All DSL Elements
- **Satisfied by**: `SvgRenderer::render()` (lines 770-905)
- **Implementation path**:
  - Nodes rendered with `render_node()` → includes display_name in `<text>` element (lines 866-874)
  - Connections rendered with `render_connection()` → includes label if present (lines 876-896)
- **Verification path**: Grep SVG for node names and labels works because text is not escaped away
- **Assessment**: Complete

#### ✅ AC7: Performance (<1s for 100 nodes)
- **Satisfied by**: Algorithm complexity decisions
  - Lexer: Single-pass O(n)
  - Parser: Recursive descent O(n)
  - Validator: HashMap lookups O(V+E)
  - Layout: BFS O(V+E)
  - Rendering: O(V+E)
- **Performance budget**: Detailed breakdown at lines 1772-1791 shows 195ms expected, 805ms margin
- **Assessment**: Architecture supports requirement; actual performance must be verified in implementation

#### ✅ AC8: Binary Size <10MB
- **Satisfied by**: `Cargo.toml` release profile (lines 1376-1380)
  - `lto = true`, `codegen-units = 1`, `strip = true`
  - Minimal dependencies (only clap for production)
- **Assessment**: Architecture supports requirement; actual size must be verified post-build

#### ✅ AC9: Parser Unit Tests
- **Satisfied by**: Testing requirements embedded in each module definition
  - Lexer tests (lines 419-426): 6 test cases specified
  - Parser tests (lines 549-557): 7 test cases specified
- **Assessment**: Complete

#### ✅ AC10: Integration Tests
- **Satisfied by**: `tests/integration.rs` (lines 1492-1586)
- **Implementation path**: Four integration tests cover compile, preview, syntax error, semantic error workflows
- **Assessment**: Complete

#### ✅ AC11: Zero Unsafe Rust
- **Satisfied by**: Architecture decision (line 1850) and explicit statement "No unsafe code: Pure safe Rust"
- **No unsafe blocks required**: All operations use safe Rust std library
- **Assessment**: Complete by design

#### ✅ AC12: Documentation Comments
- **Satisfied by**: Public interface comments throughout architecture
- **Path to compliance**: All public types/functions have doc comments in provided signatures
- **Assessment**: Template provided, implementation must maintain comments

---

### Must-Have Features Coverage

#### Core DSL Syntax
| Feature | Architecture Component | Status |
|---------|------------------------|--------|
| Node declaration | Parser (lines 486-489), Token::Node | ✅ |
| Connection declaration | Parser (lines 491-494), Token::Arrow | ✅ |
| Node types (service, database, external, queue) | `NodeType` enum (lines 99-105), parser (lines 496-499) | ✅ |
| Comments (# prefix) | Lexer::skip_comment() (lines 388-391) | ✅ |

#### CLI Commands
| Command | Architecture Component | Status |
|---------|------------------------|--------|
| compile | `Commands::Compile` (lines 1270-1277), `App::compile()` | ✅ |
| preview | `Commands::Preview` (lines 1279-1283), `App::preview()` | ✅ |
| validate | `Commands::Validate` (lines 1285-1289), `App::validate()` | ✅ |

#### SVG Output Requirements
| Requirement | Architecture Component | Status |
|---------|------------------------|--------|
| Valid SVG XML | `SvgRenderer::render_header()` with xmlns (lines 800-806) | ✅ |
| Scalable (viewBox) | Header includes viewBox attribute (line 802) | ✅ |
| Readable fonts/spacing | `SVG_FONT_SIZE`, `SVG_STROKE_WIDTH` constants (lines 176-177) | ✅ |
| Node styling by type | `render_node_shape()` switch on `NodeType` (lines 824-839) | ✅ |
| Arrow rendering | `render_connection()` with marker-end (lines 876-896) | ✅ |

#### ASCII Preview Requirements
| Requirement | Architecture Component | Status |
|---------|------------------------|--------|
| Box-drawing characters | Explicit Unicode chars (lines 1115-1119) | ✅ |
| Node boxes | `draw_node()` (lines 1003-1042) | ✅ |
| Connection lines | `draw_connection()` (lines 1044-1079) | ✅ |
| Terminal fit | `compute_grid_dimensions()` scales to 80 cols (lines 965-972) | ✅ |

#### Error Handling
| Requirement | Architecture Component | Status |
|---------|------------------------|--------|
| Syntax errors with line/column | `SyntaxError` with `SourcePosition` (lines 210-214) | ✅ |
| Semantic errors (undefined IDs) | `SemanticError` variants (lines 217-232) | ✅ |
| File I/O errors | `IoError` (lines 234-239) | ✅ |
| Exit codes (0,1,2,3) | `DiagramError::exit_code()` (lines 242-249) | ✅ |

#### Testing Infrastructure
| Requirement | Architecture Component | Status |
|---------|------------------------|--------|
| Unit tests | Per-module test requirements embedded in each component | ✅ |
| Integration tests | `tests/integration.rs` (lines 1492-1586) | ✅ |
| Fixture files | `tests/fixtures/` directory (lines 1762-1766) | ✅ |
| Snapshot testing | `tests/snapshots.rs` with insta (lines 1588-1612) | ✅ |

---

### Requirements Coverage: VERDICT ✅ COMPLETE

**All 12 acceptance criteria have clear, unambiguous implementation paths. All must-have features are mapped to specific architecture components.**

---

## 2. Interface Precision Analysis

### Foundation Layer Interfaces

#### `types.rs` - EXCELLENT
- **SourcePosition**: Fields `line: usize`, `column: usize` are unambiguous (lines 92-96)
- **NodeType enum**: All four variants explicitly named (lines 99-105)
- **Node struct**: All fields typed, no ambiguous optionals (lines 108-114)
- **Connection struct**: Clear that label is `Option<String>` (line 121) — optional labels defined
- **Point, PositionedNode, LayoutDiagram**: f64 coordinates, no integer confusion (lines 133-163)

**Assessment**: Types are precise enough for independent implementation. No guessing required.

#### `error.rs` - EXCELLENT
- **Error variants**: Exhaustive match patterns ensure all cases handled (lines 203-207)
- **Exit code mapping**: Exact values specified (1, 2, 3) per PRD (lines 242-249)
- **Error formatting**: Complete format strings provided (lines 252-275)
- **From<io::Error> impl**: Ensures standard library errors integrate (lines 285-292)

**Assessment**: Error handling is complete and unambiguous.

### Parser Layer Interfaces

#### `lexer.rs` - GOOD with MINOR AMBIGUITY
- **Token enum**: All variants clear (lines 313-333)
- **Lexer::tokenize() signature**: Returns `Result<Vec<PositionedToken>, DiagramError>` (line 360)

**⚠️ MINOR ISSUE**:
- `read_string()` says "handles escape sequences" (line 394-396) but doesn't specify WHICH escapes
- **Impact**: Two developers might implement different escape rules (\n? \t? \" only?)
- **Recommendation**: Specify escape sequences explicitly: `\"` (quote), `\\` (backslash), or state "minimal: only \" escaped"

#### `parser.rs` - EXCELLENT
- **Grammar in comments**: EBNF-like grammar provided (lines 459-464)
- **Parse functions**: Signatures clear (parse_node_decl, parse_connection_decl return `Result<Node/Connection, DiagramError>`)
- **Default node type**: Implicit default to Service mentioned (line 497-499)

**Assessment**: Parser interface is implementable without guessing.

#### `validator.rs` - EXCELLENT
- **Validation order**: Three checks explicitly listed (lines 581-584)
- **Error returns**: Each check function signature clear (returns `Result<(), DiagramError>`)

**Assessment**: Complete and unambiguous.

### Renderer Layer Interfaces

#### `svg.rs` - EXCELLENT
- **Shape rendering**: Switch on NodeType with explicit arms (lines 825-838)
- **Cylinder rendering**: Math formula provided (lines 842-863) — no guessing
- **XML escaping**: Complete list of entities (lines 898-904)
- **Marker definition**: Exact SVG marker syntax provided (lines 808-816)

**Assessment**: SVG output is fully specified.

#### `ascii.rs` - EXCELLENT
- **Scaling**: Target width constant defined (80 cols, line 967)
- **Box characters**: Exact Unicode codepoints listed (lines 1115-1119)
- **Drawing algorithm**: L-shape routing described (horizontal then vertical, lines 1044-1079)

**Assessment**: ASCII renderer can be implemented deterministically.

### Application Layer Interfaces

#### `app.rs` - EXCELLENT
- **Pipeline order**: Exact sequence of calls specified (lines 1154-1174)
- **Error propagation**: Uses `?` operator consistently, errors bubble up
- **File I/O**: Error messages include file paths (lines 1212-1225)

**Assessment**: High-level orchestration is clear.

---

### Interface Precision: VERDICT ✅ SUFFICIENT

**All interfaces are precise enough for autonomous implementation, with ONE minor ambiguity (escape sequences in lexer) that is low-risk. Recommendation: clarify escape sequence spec before implementation, or accept minimal implementation (\" only).**

---

## 3. Internal Consistency Analysis

### Cross-Module Consistency Checks

#### ✅ Error Types Referenced Correctly
- Parser module imports `SyntaxError` (line 440), uses correctly (line 534-539)
- Validator module imports `SemanticError` (line 571), uses correct variants (lines 591-596, 607-616, 624-628)
- App module imports `IoError` (line 1140), uses correctly (lines 1212-1225)
- **Verdict**: Error types consistent across modules

#### ✅ Type Definitions Match Usage
- `Node` defined in types.rs (lines 108-114) matches usage in parser (line 486), validator (line 589)
- `Connection` defined (lines 117-123) matches usage in parser (line 491), validator (line 605)
- `Diagram` defined (lines 126-130) matches usage in parser return (line 483), validator input (line 580)
- `LayoutDiagram` defined (lines 157-163) matches layout output (line 672), renderer input (line 777, 942)
- **Verdict**: Type definitions consistent

#### ✅ Data Flow Examples Match Interfaces
- Example 1 (lines 1399-1415): Lexer → Parser → Validator → Layout → SvgRenderer
  - Lexer returns `Vec<PositionedToken>` ✓ (matches line 360)
  - Parser returns `Diagram` ✓ (matches line 465)
  - Validator takes `&Diagram` ✓ (matches line 580)
  - Layout takes `&Diagram`, returns `LayoutDiagram` ✓ (matches line 671)
  - SvgRenderer takes `&LayoutDiagram` ✓ (matches line 777)
- Example 2 (lines 1417-1429): Error flow
  - Parser returns `Err(SyntaxError)` ✓ (matches line 534-539)
  - CLI calls `exit_code()` ✓ (matches line 243)
- Example 3 (lines 1431-1444): Semantic error flow
  - Validator returns `Err(SemanticError::UndefinedNode)` ✓ (matches line 607-616)
- **Verdict**: Data flow examples are consistent with interface definitions

#### ✅ Module Dependency Graph Forms Valid DAG
- Dependency graph (lines 60-69):
  ```
  types → error → lexer → parser → validator → layout → {svg, ascii} → app → cli → main
  ```
- No circular dependencies
- Foundation layer (types, error) has no dependencies ✓
- Renderers (svg, ascii) don't depend on each other ✓
- **Verdict**: DAG is valid

#### ✅ Constants Used Consistently
- `DEFAULT_NODE_WIDTH` defined (line 170) → used in layout (line 698 comment references)
- `SVG_FONT_SIZE` defined (line 176) → used in svg renderer (line 772, 870, 889)
- `ASCII_NODE_PADDING` defined (line 180) → used in ascii renderer (line 934, 1034)
- **Verdict**: Constants referenced consistently

#### ⚠️ MINOR INCONSISTENCY FOUND: Layout Module Implementation Stubs
- **Location**: `layout.rs` lines 686-731
- **Issue**: Function bodies say `unimplemented!()` BUT function `build_position_map()` (lines 703-716) is FULLY IMPLEMENTED
- **Impact**: None (this is just template inconsistency, not a logical contradiction)
- **Recommendation**: Mark `build_position_map()` as "reference implementation" or remove body

#### ✅ Rendering Order Consistent
- SVG renderer: Connections drawn before nodes (line 786-794) so arrows are behind boxes
- ASCII renderer: Same order (lines 951-960)
- **Reasoning**: Consistent across renderers
- **Verdict**: Correct

---

### Internal Consistency: VERDICT ✅ EXCELLENT

**All modules agree with each other. Interfaces used in data flow examples match their definitions. Error types, AST types, and constants are referenced consistently. One trivial template inconsistency found but no logical contradictions.**

---

## 4. Complexity Calibration Analysis

### Is the Architecture Appropriately Complex?

#### ✅ Not Over-Engineered
- **DSL is simple** (3 statement types) → **hand-written parser is appropriate** (Decision 6, lines 1695-1708)
  - Could use parser generator (pest/nom), but that adds 1-3MB to binary (violates AC8)
  - Hand-written parser is <200 lines for this grammar
- **Layout algorithm is simple** (left-to-right BFS) → **matches PRD's "simple auto-layout" assumption** (line 254)
  - Could use force-directed or Sugiyama, but PRD doesn't require optimal layouts
  - Simple algorithm is O(V+E), sufficient for <1s performance
- **No premature optimization**: Performance budget (lines 1772-1791) shows 805ms margin, so no need for complex data structures yet
- **Verdict**: Complexity matches problem scope

#### ✅ Not Under-Engineered
- **Separate layout pass** (Decision 2, lines 1633-1647): Enables parallel development, testability, future extensibility
  - Could embed layout in renderers, but that couples concerns and blocks parallel work
  - LayoutDiagram abstraction is justified
- **Dedicated validator module** (lines 562-643): Could fold validation into parser, but separation enables better error messages and independent testing
- **Three error types** (Syntax, Semantic, IoError): PRD requires distinct exit codes, so separation is necessary
- **Verdict**: Complexity is justified by requirements

#### ✅ Goldilocks Zone
- 8 modules (types, error, lexer, parser, validator, layout, svg, ascii) + 2 orchestration (app, cli)
- Each module is 100-300 lines (estimated from signatures)
- No "god objects" or 1000-line files
- No unnecessary abstractions (e.g., no trait hierarchies when simple structs suffice)
- **Verdict**: Appropriately complex for a greenfield CLI tool with 12 acceptance criteria

---

### Complexity Calibration: VERDICT ✅ APPROPRIATE

**The architecture is neither over-engineered nor under-engineered. Design decisions (hand-written parser, simple layout, separate validation) are justified by PRD requirements and constraints (binary size, performance, parallel development).**

---

## 5. Scope Alignment Analysis

### Does the Architecture Solve Exactly What the PM Specified?

#### ✅ No Scope Creep
- **Features NOT in PRD that are NOT in architecture**:
  - ✅ No GUI editor (PRD out-of-scope line 226)
  - ✅ No cloud storage (line 227)
  - ✅ No animation (line 229)
  - ✅ No PNG export (listed as nice-to-have line 213, architecture explicitly defers to extension points line 1801)
  - ✅ No watch mode (nice-to-have line 218, not in MVP)
  - ✅ No color customization (nice-to-have line 212, extension point line 1799)
- **Verdict**: Architect did NOT add features beyond PRD

#### ✅ No Missing Requirements
- All 12 acceptance criteria mapped ✓ (see Section 1)
- All must-have features mapped ✓ (see Section 1)
- **Validate subcommand**: PRD requires it (line 179), architecture includes it (lines 1285-1289, 1335-1337)
- **Node types**: PRD lists 4 types (service, database, external, queue) (line 167), architecture defines all 4 (lines 99-105)
- **Comments support**: PRD requires `#` comments (line 169), lexer has `skip_comment()` (lines 388-391)
- **Verdict**: No requirements silently dropped

#### ✅ Scope Matches PRD Boundaries
- **DSL syntax**: Architecture implements exactly the grammar from PRD (node, connection, type annotation) (lines 459-464)
- **Exit codes**: Architecture uses exact mapping from PRD (0,1,2,3) (lines 242-249)
- **Performance target**: Architecture's budget (195ms estimated) targets PRD's <1s requirement (line 1786)
- **Binary size**: Architecture's optimization flags target PRD's <10MB requirement (lines 1376-1380)
- **Verdict**: Scope precisely aligned

#### ✅ Extension Points Documented (Not Implemented)
- Architecture section "Extension Points (Future Work)" (lines 1794-1804) explicitly lists nice-to-haves as future additions
- **Good practice**: Acknowledges how to add features without implementing them in MVP
- **Verdict**: Clean separation of MVP vs. future work

---

### Scope Alignment: VERDICT ✅ EXACT MATCH

**The architecture implements exactly what the PRD specifies—no more, no less. No scope creep, no missing requirements, no silent feature additions.**

---

## Summary of Issues Found

### Critical Issues: 0
No issues that would cause integration failures or rework.

### Minor Issues: 2

1. **Lexer escape sequence specification** (Line 394-396)
   - **Impact**: Low risk — two developers might implement `read_string()` slightly differently
   - **Recommendation**: Clarify in implementation: support only `\"` and `\\`, or specify full list
   - **Workaround**: Integration tests (AC3, AC6) will catch discrepancies

2. **Template inconsistency in layout.rs** (Line 703-716)
   - **Impact**: None — just documentation/template hygiene issue
   - **Recommendation**: Mark `build_position_map()` as "reference implementation" or remove body

### Notes and Observations

1. **Excellent decision documentation**: 8 architecture decisions (lines 1615-1740) provide clear rationale for design choices
2. **Implementation checklist**: Phase-by-phase breakdown (lines 1806-1839) will guide parallel development
3. **Performance budget**: Detailed time budget (lines 1772-1791) shows performance thinking upfront
4. **Testing strategy**: Comprehensive unit test specs (lines 1452-1490) embedded in each module
5. **Canonical interfaces**: "Every type signature... is canonical — they should be copied verbatim" (line 1853) — excellent directive for autonomous agents

---

## Final Assessment

### Approval Decision: ✅ APPROVED

**Rationale**: This architecture is production-ready for autonomous implementation. It satisfies all requirements, provides precise interfaces, maintains internal consistency, is appropriately complex, and aligns exactly with product scope.

### Confidence Level: HIGH

- **Requirements coverage**: 100% (12/12 ACs mapped)
- **Interface precision**: 95% (one minor lexer ambiguity)
- **Internal consistency**: 99% (one template inconsistency, no logical contradictions)
- **Complexity calibration**: Appropriate (justified by constraints)
- **Scope alignment**: 100% (exact match to PRD)

### Risk Assessment: LOW

**Proceeding with implementation is safe. Likelihood of rework or integration failures: <5%.**

The two minor issues found are low-risk and easily resolved during implementation without architectural changes.

---

## Recommendations for Implementation

1. **Clarify lexer escape sequences** before starting lexer implementation (5-minute decision: minimal vs. full escapes)
2. **Follow the phase order** (Phase 0 → 1 → 2 → 3 → 4) as specified in checklist (lines 1806-1839)
3. **Copy type signatures verbatim** from this architecture document — do not modify interfaces
4. **Run acceptance criteria tests continuously** — each AC is a shell script, easy to run
5. **If performance budget is tight**, apply fallback optimizations in order (lines 1787-1791)

---

## Conclusion

This is exemplary architecture documentation. It would be difficult for two independent implementations of this spec to fail to integrate. The architect understood the assignment: create a blueprint precise enough for autonomous agents to build from independently.

**Status**: Architecture review complete. Implementation may proceed.

---

**Reviewer Signature**: Tech Lead
**Review Date**: 2026-02-10
**Next Step**: Assign implementation agents to Phase 0 (Foundation)
