Test Driven Development

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

Requirements:

• One behavior
• Clear name
• Real code (no fakes unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

zig build test

Confirm:

• Test fails (not compile errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

fn retryOperation(config: RetryConfig) ![]const u8 { // YAGNI - over-engineered }

Over-engineered
</Bad>

Don't add features, refactor other code, or "improve" beyond the test.

### Verify GREEN - Watch It Pass

**MANDATORY.**

```bash
zig build test

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Common Rationalizations

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: Empty mermaid code block causes crash

RED

test "empty mermaid code block returns null diagram" {
    const allocator = std.testing.allocator;
    const result = try detector.detect(allocator, "");
    try std.testing.expect(result == null);
}

Verify RED

$ zig build test
1/1 test "empty mermaid code block returns null diagram"...FAIL
Expected null, found .flowchart

GREEN

pub fn detect(allocator: Allocator, content: []const u8) !?DiagramType {
    if (content.len == 0) return null;
    // ... existing detection logic
}

Verify GREEN

$ zig build test
All 42 tests passed.

REFACTOR Extract validation for multiple input checks if needed.

Zig Testing Patterns

Basic test structure

const std = @import("std");
const testing = std.testing;

test "descriptive name of behavior" {
    // Arrange
    const allocator = testing.allocator; // Detects leaks!

    // Act
    const result = try myFunction(allocator, input);
    defer allocator.free(result);

    // Assert
    try testing.expectEqual(expected, result);
    try testing.expectEqualStrings("expected", actual);
    try testing.expect(condition);
}

Common assertions

try testing.expectEqual(expected, actual);       // Value equality
try testing.expectEqualStrings("abc", slice);    // String comparison
try testing.expect(boolean_condition);            // Boolean check
try testing.expectError(error.OutOfMemory, expr); // Error expected
try testing.expectFmt("formatted", "{}", .{val}); // Format check

Testing with allocators

Zig's testing.allocator automatically detects memory leaks — use it for all tests that allocate.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass (zig build test)
• Output pristine (no errors, warnings)
• Tests use real code (fakes only if unavoidable)
• Edge cases and errors covered
• No memory leaks (testing.allocator catches these)

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When writing tests, read @testing-anti-patterns.md to avoid common pitfalls:

• Testing setup behavior instead of real behavior
• Adding test-only methods to production structs
• Over-complex test fixtures

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.