# SHDL Comprehensive Test Report

**Date:** 2025-01-XX  
**Test File:** `tests/test_shdl_comprehensive.py`  
**Tests Passed:** 62/62 (100%)  
**Test Duration:** ~16.5 seconds

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## Executive Summary

This test suite was created based on the **documented intended behavior** of SHDL (Simple Hardware Description Language), not by reverse-engineering the implementation. All 62 tests pass successfully, verifying that the SHDL compiler and simulator behave according to specification.

---

## Test Categories

### 1. Primitive Gates (6 tests)
Tests for the 4 primitive logic gates and 2 constant sources defined in the Base SHDL specification:

| Gate | Test | Status |
|------|------|--------|
| AND | Full truth table (00→0, 01→0, 10→0, 11→1) | ✅ PASS |
| OR | Full truth table (00→0, 01→1, 10→1, 11→1) | ✅ PASS |
| NOT | Full truth table (0→1, 1→0) | ✅ PASS |
| XOR | Full truth table (00→0, 01→1, 10→1, 11→0) | ✅ PASS |
| __VCC__ | Always outputs 1 | ✅ PASS |
| __GND__ | Always outputs 0 | ✅ PASS |

### 2. Derived Gates (2 tests)
Tests for gates built from primitives:

| Gate | Definition | Status |
|------|------------|--------|
| NAND | NOT(AND(A,B)) | ✅ PASS |
| NOR | NOT(OR(A,B)) | ✅ PASS |

### 3. Propagation Tests (3 tests)
Tests for signal propagation through gates:

- **Single gate propagation:** Signals propagate through a single gate with one `step()` call ✅
- **Chain of 3 NOTs:** Signals propagate through 3 gates in 3 `step()` calls ✅
- **Deep circuit (5 NOTs):** Requires 5+ `step()` calls for full propagation ✅

**Key Finding:** The simulator uses a per-gate propagation model. Each `step()` advances signals by one gate level.

### 4. Half Adder (1 test)
Exhaustive test of half adder (4 input combinations):

| A | B | Sum | Carry |
|---|---|-----|-------|
| 0 | 0 | 0 | 0 | ✅ |
| 0 | 1 | 1 | 0 | ✅ |
| 1 | 0 | 1 | 0 | ✅ |
| 1 | 1 | 0 | 1 | ✅ |

### 5. Full Adder (1 test)
Exhaustive test of full adder (8 input combinations):

All 8 combinations of (A, B, Cin) tested and verified ✅

### 6. Multi-bit Adder (4 tests)
4-bit ripple carry adder tests:

| Test Case | Result |
|-----------|--------|
| 0 + 0 = 0 | ✅ PASS |
| 1 + 1 = 2 | ✅ PASS |
| 15 + 1 = 0 (overflow) | ✅ PASS |
| Various sample cases | ✅ PASS |

### 7. Multi-bit Signals (3 tests)
8-bit signal operations:

| Operation | Status |
|-----------|--------|
| 8-bit pass-through | ✅ PASS |
| 8-bit bitwise NOT | ✅ PASS |
| 8-bit bitwise AND | ✅ PASS |

### 8. Generators (3 tests)
Generator expression tests (`>i[N]{ ... }`):

| Test | Status |
|------|--------|
| Generator creates 8 instances | ✅ PASS |
| Generator with range [2:5] | ✅ PASS |
| Arithmetic substitution in generators | ✅ PASS |

### 9. Constants (4 tests)
Constant definition tests:

| Format | Example | Status |
|--------|---------|--------|
| Implicit width | `N = 7` | ✅ PASS |
| Explicit width | `N[4] = 7` | ✅ PASS |
| Binary | `N = 0b1010` | ✅ PASS |
| Hexadecimal | `N = 0xFF` | ✅ PASS |

### 10. Hierarchy (2 tests)
Component nesting tests:

| Test | Status |
|------|--------|
| Simple nested components | ✅ PASS |
| Hierarchical behavior verification | ✅ PASS |

### 11. Multiplexer (2 tests)
2-to-1 multiplexer tests:

| Sel | Output | Status |
|-----|--------|--------|
| 0 | Input A | ✅ PASS |
| 1 | Input B | ✅ PASS |

### 12. Parsing (5 tests)
Parser validation tests:

| Feature | Status |
|---------|--------|
| Simple component parsing | ✅ PASS |
| Multi-bit port parsing | ✅ PASS |
| Generator parsing | ✅ PASS |
| Import statement parsing | ✅ PASS |
| Constant definition parsing | ✅ PASS |

### 13. Error Handling (3 tests)
Error detection tests:

| Error Type | Status |
|------------|--------|
| Lexer error (invalid token) | ✅ PASS |
| Parse error (missing arrow) | ✅ PASS |
| Parse error (unclosed brace) | ✅ PASS |

### 14. Decoder (1 test)
2-to-4 decoder test with all 4 input combinations ✅

### 15. Propagation Analysis (3 tests)
Detailed propagation depth analysis:

| Test | Status |
|------|--------|
| Single gate depth | ✅ PASS |
| Two-gate depth | ✅ PASS |
| Propagation consistency | ✅ PASS |

### 16. Edge Cases (3 tests)
Boundary condition tests:

| Test | Status |
|------|--------|
| 16-bit max values | ✅ PASS |
| Alternating patterns (0xAAAA, 0x5555) | ✅ PASS |
| Single bit isolation | ✅ PASS |

### 17. Base SHDL Formatting (2 tests)
Flattener output tests:

| Test | Status |
|------|--------|
| Simple component formatting | ✅ PASS |
| Generator expansion | ✅ PASS |

### 18. 8-Bit Adder (6 tests)
Full 8-bit adder with carry chain tests:

| Test | Status |
|------|--------|
| 0 + 0 = 0 | ✅ PASS |
| 255 + 0 = 255 | ✅ PASS |
| 255 + 1 = overflow | ✅ PASS |
| 0xFF + 0xFF carry propagation | ✅ PASS |
| Carry input functionality | ✅ PASS |
| Various sample additions | ✅ PASS |

### 19. Bit Range Subscripts (2 tests)
Bit extraction tests:

| Test | Status |
|------|--------|
| High nibble extraction | ✅ PASS |
| Low nibble extraction | ✅ PASS |

### 20. Component Imports (1 test)
File import functionality test ✅

### 21. Reset Behavior (2 tests)
Circuit reset tests:

| Test | Status |
|------|--------|
| Reset clears state | ✅ PASS |
| Multiple reset cycles | ✅ PASS |

### 22. Signal Width Validation (2 tests)
Boundary value tests:

| Test | Status |
|------|--------|
| 8-bit boundaries | ✅ PASS |
| 16-bit boundaries | ✅ PASS |

### 23. Deep Hierarchy (1 test)
4-level component nesting test ✅

---

## Key Findings

### 1. Propagation Model
The simulator uses a **step-based propagation model** where:
- Each `step(1)` call advances signals through exactly one gate level
- Deep circuits require multiple steps: `step(N)` where N ≥ circuit depth
- For reliable results, over-stepping is safe (extra steps don't change stable output)

**Recommendation:** Use `step(10)` for simple circuits, `step(20-30)` for 8-bit adders.

### 2. Bit Indexing
SHDL uses **1-based indexing** where:
- LSB is bit 1 (not bit 0)
- For an 8-bit signal `A[8]`: bits are numbered A[1] through A[8]
- A[1] = LSB, A[8] = MSB

### 3. API Usage
The `Circuit` class requires careful handling:
- When passing source code, use file paths (not inline strings)
- The `poke(signal, value)` method sets inputs
- The `peek(signal)` method reads outputs
- The `step(n)` method advances simulation by n cycles
- The `reset()` method clears internal state

### 4. Comment Syntax
**IMPORTANT:** SHDL does **not** support `//` or `/* */` comments in the source code. Comments cause parse errors.

### 5. Flattener API
The `Flattener` class:
- `load_file(path)` returns a `Module` object
- `load_source(source)` returns a `Module` object  
- `flatten(component_name)` returns flattened `Component`
- `flatten_to_base_shdl(component_name)` returns Base SHDL string

---

## Discrepancies Between Documentation and Implementation

### No Discrepancies Found
All tested behaviors match the documented specification:
- Primitive gates work as documented
- Bit indexing is 1-based as specified
- Generators expand correctly
- Hierarchy flattening works properly
- Error handling produces appropriate exceptions

---

## Test Coverage Areas

### Well Covered ✅
- All primitive gates
- Basic arithmetic circuits (half adder, full adder, n-bit adders)
- Multi-bit signals
- Generator expressions
- Constants (all formats)
- Component hierarchy
- Parsing and error handling
- Signal propagation

### Not Covered ❌
- Clock/register behavior (sequential circuits)
- Debugger breakpoints
- Waveform generation
- Advanced ALU operations (from ALU/ directory)
- Memory circuits (from memory/ directory)
- CPU components (from CPU/ directory)

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## Files Created

### Test Circuits (`tests/circuits/`)
- `test_gates.shdl` - Primitive gate wrappers
- `test_half_full_adder.shdl` - HalfAdder and FullAdder
- `test_adder4.shdl` - 4-bit ripple carry adder
- `test_adder8.shdl` - 8-bit ripple carry adder
- `test_mux.shdl` - Multiplexer circuits
- `test_decoder.shdl` - Decoder circuits
- `test_generators.shdl` - Generator pattern tests
- `test_bitwise.shdl` - Bitwise operations

### Test Suite
- `test_shdl_comprehensive.py` - ~1900 lines, 62 tests, 23 test classes

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## Recommendations

1. **Add Comment Support:** Consider adding `//` line comments to the lexer for better code documentation.

2. **Document step() Requirements:** Add guidance on how many `step()` calls are needed for different circuit depths.

3. **Sequential Circuit Tests:** Extend tests to cover clock-based circuits when debugging those features.

4. **Property-Based Testing:** Consider adding hypothesis-based tests for exhaustive arithmetic verification.

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## Conclusion

The SHDL compiler and simulator implementation correctly matches the documented specification. All 62 tests pass, verifying:
- Correct primitive gate behavior
- Proper multi-bit signal handling
- Generator expansion
- Component hierarchy flattening
- Arithmetic circuit correctness
- Error handling

The test suite provides comprehensive coverage of the documented features and can serve as a regression test baseline.
