Property-Based Test Generator

Design and generate property-based tests for changed files, with self-scoring to ensure quality (24/30+ on the evaluation rubric).

Constraints

• Black-box only — never depend on internal implementation details.
• No trivial properties (type-check-only, no-exception-only are forbidden).
• Minimize assume/filter — express constraints in generators instead.
• Always ensure seed + minimal counterexample reproducibility.

Workflow

1. Detect changed files — identify PBT candidates from git diff
2. Extract specifications — read each file, document inputs/outputs/constraints
3. Design properties — minimum 5 per function, following the property type hierarchy
4. Build generators — express input domains with edge cases and good shrinking
5. Implement tests — write test files in the project's framework
6. Self-score — evaluate against rubric, improve if below 24/30
7. Report — present results to user

Step 1 — Detect Changed Files

Identify PBT candidates from the current branch diff:

git diff --name-only --diff-filter=ACMR $(git merge-base main HEAD) HEAD

Filter to source files (.ts, .tsx, .js, .jsx, .py, .rs), excluding test files, config, styles, and assets.

Good PBT candidates (prioritize these):

• Pure functions (no side effects, deterministic)
• Validators / type guards (is*, validate*)
• Parsers / serializers (encode/decode, parse/stringify)
• Formatters (data → string transformations)
• Reducers / state transitions
• Sorting / filtering / transformation utilities

Poor PBT candidates (skip these):

• React components (use integration tests instead)
• Side-effectful functions (API calls, file I/O)
• Simple getters/setters with no logic

Step 2 — Extract Specifications

For each candidate function, document:

1. Inputs — types, ranges, constraints
2. Outputs — types, expected relationships to inputs
3. State — mutable state involved, if any
4. Requirements — business rules as bullet points
5. Preconditions — what must be true about inputs

Step 3 — Design Properties (min 5)

Design properties in this priority order:

Type	Description	When to use
Invariant	Output always satisfies a condition	Length preservation, range bounds, type guarantees
Round-trip	decode(encode(x)) === x	Parsers, serializers, codecs
Idempotence	f(f(x)) === f(x)	Normalizers, formatters, canonicalizers
Metamorphic	Relationship between f(x) and f(transform(x))	Sort, filter, math operations
Monotonicity	x ≤ y → f(x) ≤ f(y)	Scoring, ranking, pricing
Reference model	optimized(x) === naive(x)	Optimized reimplementations

Each property MUST include:

• Natural-language description
• Corresponding requirement from Step 2
• One buggy implementation example that this property would catch

Step 4 — Build Generators

Determine the project language and select the library:

• TypeScript/JavaScript → fast-check — see references/fast-check.md
• Python → hypothesis — see references/hypothesis.md
• Rust → proptest — see references/proptest.md

Generator design rules:

• Express constraints via generator composition, not filter/assume.
• Target filter rejection rate < 10%.
• Explicitly include edge cases: empty, zero, boundary, max-size, duplicates, skewed distributions.
• Prefer base Arbitrary/Strategy combinations for natural shrinking.
• Set explicit size limits to control generation cost.

Step 5 — Implement Tests

Write test files following project conventions:

• Read existing *.property.test.* files for style reference.
• Read test config and setup files.
• File naming: *.property.test.ts (TS/JS), test_*_property.py (Python), or #[cfg(test)] mod tests / tests/ (Rust). Follow project convention.

Each test must include:

1. Descriptive property name
2. Generator definition
3. Test body (arrange/act/assert)
4. Seed output on failure
5. Reproduction instructions (as comment)

Step 6 — Self-Score

After implementation, evaluate against the rubric in references/evaluation.md.

Score 15 criteria (A1-A5, B1-B6, C1-C4) at 0-2 points each.

• 24+ points: proceed to report.
• < 24 points: identify weak criteria, improve properties/generators/diagnostics, re-score. Repeat up to 2 times.

Step 7 — Report

Output in this order:

1. Requirements summary — extracted specifications
2. Property list — natural language + requirement mapping + buggy impl example
3. Generator strategies — with edge case rationale
4. Test implementation — actual test code (not pseudocode)
5. Reproduction instructions — how to re-run with seed
6. Score table + improvement log — final self-assessment