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06-testing-quality/15-testing-parallel-fuzz-harness/README.md

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+# Kata 15: The Go Test Harness (Subtests, Parallel, Fuzz)
+**Target Idioms:** Table-Driven Tests, `t.Run`, `t.Parallel`, Fuzzing (`go test -fuzz`)  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+Developers often write:
+- one-off tests with repetition,
+- unsafe parallel subtests (loop variable capture),
+- no fuzz testing for parsers/sanitizers.
+
+Idiomatic Go testing is:
+- table-driven,
+- readable failures,
+- parallel where safe,
+- fuzzed for edge cases.
+
+## 🎯 The Scenario
+You’re implementing a sanitizer:
+- `func NormalizeHeaderKey(s string) (string, error)`
+  Rules:
+- only ASCII letters/digits/hyphen allowed
+- normalize to canonical header form (e.g., `content-type` -> `Content-Type`)
+- reject invalid input
+
+## 🛠 The Challenge
+Write:
+1) The implementation, and
+2) A test suite that proves it’s solid.
+
+### 1. Functional Requirements
+- [ ] Canonicalize valid inputs.
+- [ ] Reject invalid characters.
+- [ ] Stable behavior (same input => same output).
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] Tests must be **table-driven** with `t.Run`.
+- [ ] Use **parallel subtests** correctly (no loop var capture bugs).
+- [ ] Include a **fuzz test** that:
+    - never panics,
+    - never returns a string containing invalid characters,
+    - roundtrips canonical form (calling Normalize twice is idempotent).
+
+## 🧪 Self-Correction (Test Yourself)
+- **If parallel subtests flake:** you likely captured the loop variable.
+- **If fuzzing finds panics:** you missed an edge case.
+
+## 📚 Resources
+- https://go.dev/blog/subtests
+- https://go.dev/wiki/TableDrivenTests
+- https://go.dev/doc/security/fuzz/
+- https://go.dev/doc/tutorial/fuzz