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37 lines
1.6 KiB
Markdown
37 lines
1.6 KiB
Markdown
# Kata 11: The NDJSON Reader That Survives Long Lines
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**Target Idioms:** Streaming I/O (`io.Reader`), `bufio.Reader` vs `Scanner`, Handling `ErrBufferFull`, Low Allocation
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**Difficulty:** 🟡 Intermediate
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## 🧠 The "Why"
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Seasoned devs reach for `bufio.Scanner` and it “works”… until production sends a line > 64K and you get:
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`bufio.Scanner: token too long`.
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This kata forces you to implement a streaming reader that can handle **arbitrarily large lines** without falling over.
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## 🎯 The Scenario
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You ingest NDJSON logs from stdin or a file. Lines can be huge (hundreds of KB). You must process line-by-line.
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## 🛠 The Challenge
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Implement:
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- `func ReadNDJSON(ctx context.Context, r io.Reader, handle func([]byte) error) error`
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### 1. Functional Requirements
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- [ ] Call `handle(line)` for each line (without the trailing newline).
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- [ ] Stop immediately on `handle` error.
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- [ ] Stop immediately on `ctx.Done()`.
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### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
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- [ ] **Must NOT** rely on default `bufio.Scanner` behavior.
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- [ ] **Must** use `bufio.Reader` and correctly handle `ReadSlice('\n')` returning `ErrBufferFull`.
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- [ ] **Must** avoid per-line allocations where possible (reuse buffers).
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- [ ] Wrap errors with line number context using `%w`.
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## 🧪 Self-Correction (Test Yourself)
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- **If a 200KB line crashes with “token too long”:** you failed.
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- **If cancellation doesn’t stop promptly:** you failed.
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- **If you allocate a new buffer each line:** you failed the low-allocation goal.
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## 📚 Resources
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- https://pkg.go.dev/bufio
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- https://pkg.go.dev/io
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