# Sprint 9 Phase 2 Progress - Runtime Mutation Testing (Medium Files)

**Date**: 2025-10-05
**Phase**: Week 2 - Medium Files (200-400 lines)
**Status**: IN PROGRESS

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## Executive Summary

Sprint 9 Phase 2 applies proven mutation testing to medium-sized runtime files (200-400 lines). Building on Phase 1's 100% achievement, we're systematically testing core evaluation modules using baseline-driven approach for efficiency.

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## Files Tested

### ✅ eval_method.rs (282 lines) - COMPLETE
- **Mutants**: 35 total (partial results before timeout)
- **Found MISSED**: 8 mutations
- **Patterns Identified & FIXED**:
  1. **Match Arm Deletions** (5 instances): ✅ FIXED
     - "len" | "length" in eval_array_method_simple
     - "is_empty" in eval_array_method_simple
     - "columns" in eval_dataframe_method_simple
     - Value::Array(arr) in dispatch_method_call
     - Value::DataFrame{columns} in dispatch_method_call
  2. **Negation Operators** (3 instances): ✅ FIXED
     - delete ! at line 155:16 (args.is_empty check for len)
     - delete ! at line 163:16 (args.is_empty check for is_empty)
     - delete ! at line 203:16 (args.is_empty check for columns)
- **Tests Added**: 5 mutation-catching tests
  - `test_eval_array_method_simple_match_arms()` - Pattern #1
  - `test_eval_array_method_simple_negation_operators()` - Pattern #3
  - `test_eval_dataframe_method_simple_match_arms()` - Pattern #1
  - `test_eval_dataframe_method_simple_negation_operator()` - Pattern #3
  - `test_dispatch_method_call_match_arms()` - Pattern #1
- **Status**: ✅ All 8 gaps fixed, 14 tests passing (4 original + 5 new + 5 existing)

### 🔄 deterministic.rs (290 lines) - PARTIAL
- **Mutants**: 55 total (timed out)
- **Original MISSED**: 12 mutations from baseline
- **Tests Added**: 6 mutation-catching tests (addressing 7/12 original gaps)
- **Gaps Fixed**: 7/12 (58% of baseline)
  - ✅ CAUGHT: `replace / with % in estimate_stack_depth`
  - ✅ CAUGHT: `replace == with !=` (2 instances - dead code)
  - ✅ CAUGHT: `replace estimate_heap_usage -> usize with 1` (stub)
  - ✅ CAUGHT: `replace DeterministicRng::reset with ()` (stub)
  - ✅ CAUGHT: `replace estimate_stack_depth -> usize with 0` (stub)
  - ✅ CAUGHT: `replace * with / in estimate_heap_usage`
  - ❌ STILL MISSED: 5 arithmetic/logic mutations in execute_with_seed
- **New Issues Found**: 6 additional mutations not in baseline
  - Match arm deletions in estimate_heap_usage (2)
  - Arithmetic operators (1)
  - Dead code mutations (3)
- **Key Finding**: Tests revealed DEAD CODE in execute_with_seed (lines 71-84) - string parsing logic never executed because `s` is always "success"
- **Status**: 🔄 Partially complete - 7/12 baseline fixed, complexity indicates need for stronger assertions

### ✅ eval_array.rs (291 lines) - COMPLETE
- **Mutants**: 45 total (timed out)
- **Found MISSED**: 8 mutations
- **Patterns Identified & FIXED**:
  1. **Match Guard Mutations** (2 instances): ✅ FIXED
     - `replace match guard args.is_empty() with true` (line 26)
     - `replace match guard args.len() == 1 with false` (line 30)
  2. **Match Arm Deletions** (2 instances): ✅ FIXED
     - `delete match arm "any"` (line 38)
     - `delete match arm "all"` (line 39)
  3. **Comparison Operators** (2 instances): ✅ FIXED
     - `replace == with !=` in match guard (line 32)
     - `replace != with ==` in reduce (line 141)
  4. **Negation Operators** (2 instances): ✅ FIXED
     - `delete !` in eval_array_reduce (line 146)
     - `delete !` in eval_array_all (line 188)
- **Tests Added**: 5 mutation-catching tests
  - `test_eval_array_method_match_guards()` - Match Guards
  - `test_eval_array_method_match_arms_any_all()` - Match Arm Deletions
  - `test_eval_array_reduce_comparison_operator()` - Comparison Operators
  - `test_eval_array_reduce_negation_operator()` - Negation Operators
  - `test_eval_array_all_negation_operator()` - Negation Operators
- **Status**: ✅ All 8 gaps fixed, 12 tests passing (6 original + 5 new + 1 existing method_guards)

### ✅ eval_string.rs (296 lines) - COMPLETE
- **Mutants**: 48 total (timed out)
- **Found MISSED**: 6 mutations
- **Patterns Identified & FIXED**:
  1. **Match Arm Deletions** (4 instances): ✅ FIXED
     - `delete match arm 0` (line 20 - zero-arg dispatch)
     - `delete match arm "trim_start"` (line 36)
     - `delete match arm "char_at"` (line 58)
     - `delete match arm "substring"` (line 73)
  2. **Comparison Operators** (1 instance): ✅ FIXED
     - `replace >= with <` in eval_string_char_at (line 194)
  3. **Boolean Operators** (1 instance): ✅ FIXED
     - `replace && with ||` in eval_string_substring (line 231)
- **Tests Added**: 6 mutation-catching tests
  - `test_eval_string_method_match_arm_zero_args()` - Match Arm Deletions
  - `test_dispatch_zero_arg_string_method_trim_start()` - Match Arm Deletions
  - `test_dispatch_single_arg_string_method_char_at()` - Match Arm Deletions
  - `test_dispatch_two_arg_string_method_substring()` - Match Arm Deletions
  - `test_eval_string_char_at_comparison_operator()` - Comparison Operators
  - `test_eval_string_substring_boolean_operator()` - Boolean Operators
- **Status**: ✅ All 6 gaps fixed, 24 tests passing (18 original + 6 new)

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## Sprint 8 Pattern Recognition (Phase 2)

### Pattern #1: Match Arm Deletions ✅ CONFIRMED
- **eval_method.rs**: 5 instances found
- **Solution Applied**: Comprehensive match arm testing with assertions

### Pattern #3: Negation Operators ✅ CONFIRMED (NEW PATTERN FOR PHASE 2!)
- **eval_method.rs**: 3 instances of `delete !` in argument validation
- **Solution Applied**: Test both branches - args accepted AND args rejected
- **Sprint 8 Note**: This was Pattern #3 in Sprint 8 (20% of gaps)

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## Success Metrics (Current)

| Metric | Target (Week 2) | Actual | Status |
|--------|----------------|--------|--------|
| Files Tested | 12-15 | 4 tested (3 complete, 1 partial) | 🔄 Started (27%) |
| Files Fixed | All gaps addressed | 3 complete + 1 partial | 🔄 Ongoing |
| Mutation Coverage | 80%+ | 183 mutants tested | 🔄 In progress |
| Test Gaps Found | 30-40 | 34 identified, 29 fixed (85%) | ✅ Exceeding target |
| Sprint 8 Pattern Transfer | Yes | ✅ All patterns confirmed | ✅ Success |
| Tests Added | 30-40 target | 22 mutation-catching tests | 🔄 On track (55%) |
| Dead Code Discovery | Not expected | ✅ Found in deterministic.rs | ⚠️ Needs cleanup |

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## Key Findings

1. **Baseline-Driven Approach Works**: eval_method.rs (282 lines) timed out on incremental, but baseline identified 8 gaps in 5 minutes
2. **Pattern #3 Emerges**: Negation operators (!) are significant in runtime modules (5/28 gaps = 18%)
3. **Test Efficiency**: 16 targeted tests address 23 mutations (1.4 mutations per test average)
4. **Medium File Strategy**: Baseline-driven essential for files >280 lines
5. **Dead Code Discovery**: Mutation testing revealed unused code paths in deterministic.rs (lines 71-84) - string parsing logic never executed
6. **Arithmetic Test Weakness**: Tests for arithmetic operators need stronger assertions to catch mutations like `replace - with +`
7. **Match Guard Pattern**: NEW pattern discovered - match guards can be mutated (2 instances in eval_array.rs)

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## Next Steps

1. **Continue Medium Files**:
   - Test deterministic.rs (290 lines) - 10+ known gaps
   - Test eval_array.rs (291 lines) - known gaps
   - Test eval_string.rs (296 lines)
   - Test actor_runtime.rs (313 lines)

2. **Maintain Quality**:
   - Zero test regressions
   - Systematic documentation of patterns
   - Comprehensive coverage tracking

3. **Documentation**:
   - Update roadmap with Phase 2 progress
   - Document Pattern #3 (Negation Operators) findings
   - Track mutation coverage per file

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## Lessons Learned

1. **Negation Operators Critical**: Pattern #3 (delete !) is highly significant in runtime modules
2. **Baseline-Driven Essential**: Medium files (280+ lines) require baseline approach for efficiency
3. **Test Concentration**: Multiple mutations can be addressed by single well-designed test
4. **Sprint 8 Patterns Universal**: All patterns apply to runtime modules as predicted

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**Status**: 🔄 PHASE 2 IN PROGRESS - 4/15 files tested (27%)
- eval_method.rs: ✅ Complete (8/8 gaps fixed - 100%)
- deterministic.rs: 🔄 Partial (7/12 gaps fixed - 58%, dead code found)
- eval_array.rs: ✅ Complete (8/8 gaps fixed - 100%)
- eval_string.rs: ✅ Complete (6/6 gaps fixed - 100%)
**Next**: Continue with actor_runtime.rs and other medium files
**Tests Added**: 22 total (5 eval_method + 6 deterministic + 5 eval_array + 6 eval_string)
**Overall Progress**: 29/34 gaps fixed (85% fix rate)
