Kata 04: The Zero-Allocation JSON Parser

Target Idioms: Performance Optimization, json.RawMessage, Streaming Parsers, Buffer Reuse Difficulty: 🟡 Intermediate

🧠 The "Why**

Developers from dynamic languages often parse JSON by unmarshaling entire documents into map[string]interface{} or generic structs. In high-throughput Go services, this creates:

Massive memory churn (GC pressure)
Unnecessary allocations for unused fields
Lost type safety

The Go way: Parse only what you need, reuse everything. This kata teaches you to treat JSON as a stream, not a document.

🎯 The Scenario

You're processing 10MB/s of IoT sensor data with JSON like:

{"sensor_id": "temp-1", "timestamp": 1234567890, "readings": [22.1, 22.3, 22.0], "metadata": {...}}

You only need sensor_id and the first reading value. Traditional unmarshal would allocate for all fields and the entire readings array.

🛠 The Challenge

Implement SensorParser that extracts specific fields without full unmarshaling.

1. Functional Requirements

Parse sensor_id (string) and first readings value (float64) from JSON stream
Process io.Reader input (could be HTTP body, file, or network stream)
Handle malformed JSON gracefully (skip bad records, continue parsing)
Benchmark under 100ns per object and 0 allocations per parse

2. The "Idiomatic" Constraints (Pass/Fail Criteria)

NO encoding/json.Unmarshal: Use json.Decoder with Token() streaming
Reuse Buffers: Use sync.Pool for bytes.Buffer or json.Decoder
Early Exit: Stop parsing once required fields are found
Type Safety: Return concrete struct SensorData{sensorID string, value float64}, not interface{}
Memory Limit: Process arbitrarily large streams in constant memory (<1MB heap)

🧪 Self-Correction (Test Yourself)

The Allocation Test:
```
go test -bench=. -benchmem -count=5
```
Pass: allocs/op = 0 for parsing loop Fail: Any allocations in hot path
The Stream Test:
- Pipe 1GB of JSON through your parser (mock with repeating data)
- Pass: Memory usage flatlines after warm-up
- Fail: Memory grows linearly with input size
The Corruption Test:
- Input: {"sensor_id": "a"} {"bad json here (malformed second object)
- Pass: Returns first object, logs/skips second, doesn't panic
- Fail: Parser crashes or stops processing entirely

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