tdd

name	tdd
description	Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development.

Test-Driven Development

Project context

This codebase has two test surfaces:

• C# (RacePredictor.Core.Tests, RaceDataDownloader.Tests) — xUnit + FluentAssertions + NSubstitute + Verify (snapshot tests) + RichardSzalay.MockHttp. Tests are named {Class}Should.{Behavior}. The MockFileSystemBuilder and MockRacingDataDownloader helpers exist to wire up the standard system-boundary mocks. Verified snapshots live alongside tests as .verified.txt files.
• Python (Data/) — feature-engineering and prediction notebooks. There is no formal test runner today; the pragmatic equivalent is running run.ps1 end-to-end and inspecting the generated Horse_Stats.csv, Jockey_Stats.csv, Race_Features.csv, Predictions.json. When adding non-trivial Python logic, pull it into a pure function that takes a DataFrame in and returns a DataFrame out, so it can be exercised with pytest against a small fixture CSV.

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. The command-handler tests in this project are the model — they call RunAsync and assert on the CSV content captured by the mock filesystem. They describe what the system does, not how.

Bad tests are coupled to implementation. They mock internal collaborators (the parsers, the node finder), assert on Received(n) call counts for things that aren't true boundaries, or independently invoke a parser to verify a handler.

See tests.md for good vs bad examples and mocking.md for where the system boundaries actually are (IHtmlLoader, IFileSystem, IClock, IRacingDataDownloader).

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" — treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

• Tests written in bulk test imagined behavior, not actual behavior
• You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
• Tests become insensitive to real changes — they pass when behavior breaks, fail when behavior is fine
• You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

1. Planning

Before writing any code:

• Confirm with user what interface changes are needed (e.g. a new command handler under RaceDataDownloader/Commands/, or a new parser in RacePredictor.Core.RacingPost/)
• Confirm with user which behaviors to test (prioritize)
• Identify opportunities for deep modules (small interface, deep implementation — the existing parsers are good examples)
• Design interfaces for testability
• List the behaviors to test (not implementation steps)
• Get user approval on the plan

Ask: "What should the public interface look like? Which behaviors are most important to test?"

You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.

2. Tracer Bullet

Write ONE test that confirms ONE thing about the system:

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

For a new command handler, the tracer bullet is usually the happy-path "downloads and writes the expected CSV" test, verified via await Verify(_mockFileSystemBuilder.GetContent(...)).

3. Incremental Loop

For each remaining behavior:

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

• One test at a time
• Only enough code to pass current test
• Don't anticipate future tests
• Keep tests focused on observable behavior

4. Refactor

After all tests pass, look for refactor candidates:

• Extract duplication (often into FileSystemExtensions, StringExtensions, or a new helper)
• Deepen modules (move complexity behind simple interfaces)
• Apply SOLID principles where natural
• Trim comments — keep only non-obvious why; delete any that restate the code (AGENTS.md "Comments")
• Consider what new code reveals about existing code
• Run dotnet test (or .\run.ps1 for the full pipeline) after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only (e.g. RunAsync, Parse, build_*)
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added