docs/code-style.md

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# Paimon C++ Code Style Guide

This document defines the coding conventions for the paimon-cpp project. All pull requests are expected to follow these rules. Automated tooling (clang-format, clang-tidy, cpplint, pre-commit) enforces many of them.

---

## Language Standard

- **C++17** is the target standard.
- Do **not** use C++20 or later features.
- Only the **x86_64** architecture is currently supported.

---

## Formatting

Formatting is based on **Google C++ Style** with the following overrides (defined in `.clang-format`):

| Setting | Value |
|---------|-------|
| `ColumnLimit` | 100 |
| `IndentWidth` | 4 |
| `AccessModifierOffset` | -3 |
| `AllowShortFunctionsOnASingleLine` | Empty |

---

## Naming Conventions

| Category | Style | Example |
|----------|-------|---------|
| Class / Struct | PascalCase | `TableScan`, `FileUtils` |
| Method | PascalCase | `CreatePlan()`, `ReadFile()` |
| Member variable | snake_case with trailing `_` | `schema_id_`, `commit_user_` |
| Local variable / parameter | snake_case | `table_path`, `json_str` |
| Constant (new code) | `k` prefix + PascalCase | `kFieldVersion`, `kMaxRetryCount` |
| Constant (existing code) | Match surrounding style | `FIELD_VERSION` if that is the local convention |
| Namespace | lowercase | `paimon`, `paimon::test` |
| File name | snake_case | `table_scan.cpp`, `file_utils.h` |
| Test file | `*_test.cpp` next to source | `table_scan_test.cpp` |

---

## File Layout

### License Header

Every file **must** start with the Apache 2.0 license header.

```cpp
/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
```

### Header Guard

Use `#pragma once`. Do **not** use `#ifndef` / `#define` guards.

### Include Order

Group includes in the following order, separated by blank lines:

1. **Corresponding header** — e.g. `snapshot.cpp` includes `paimon/core/snapshot.h` first.
2. **C standard headers** — `<cstdint>`, `<cstring>`, etc.
3. **C++ standard headers** — `<string>`, `<vector>`, `<memory>`, etc.
4. **Third-party headers** — `fmt/format.h`, `rapidjson/document.h`, etc.
5. **Project headers** — `paimon/status.h`, `paimon/result.h`, etc.

### Namespace

All code lives inside `namespace paimon { }`. Close with a comment:

```cpp
namespace paimon {
// ...
}  // namespace paimon
```

Test code uses `namespace paimon::test`.

---

## Error Handling

### `Status` / `Result<T>` — No Exceptions

All fallible functions return `Status` (no value) or `Result<T>` (with value). **Exceptions are not used** for error propagation in production code.

### Error Propagation Macros

| Macro | Purpose |
|-------|---------|
| `PAIMON_RETURN_NOT_OK(status)` | Propagate a `Status` error |
| `PAIMON_ASSIGN_OR_RAISE(lhs, rexpr)` | Extract value from `Result<T>`, return on error |
| `PAIMON_RETURN_NOT_OK_FROM_ARROW(arrow_status)` | Bridge Apache Arrow `Status` |
| `PAIMON_ASSIGN_OR_RAISE_FROM_ARROW(lhs, rexpr)` | Bridge Apache Arrow `Result` |

### `PAIMON_ASSIGN_OR_RAISE` and `PAIMON_ASSIGN_OR_RAISE_FROM_ARROW` — Always Use Explicit Types

```cpp
// ✅ Good — explicit type
PAIMON_ASSIGN_OR_RAISE(std::unique_ptr<TableScan> scan,
                       TableScan::Create(std::move(ctx)));

// ❌ Bad — auto
PAIMON_ASSIGN_OR_RAISE(auto scan, TableScan::Create(std::move(ctx)));
```

If you need a `shared_ptr` from a function returning `unique_ptr`, write the target type directly — the macro already moves:

```cpp
PAIMON_ASSIGN_OR_RAISE(std::shared_ptr<T> ret, FuncReturningUniquePtr());
```

---

## Class Design

### Factory Pattern — `static Create()` + Private Constructor

When construction can fail, use a factory method:

```cpp
class TableScan {
 public:
    static Result<std::unique_ptr<TableScan>> Create(std::unique_ptr<ScanContext> context);
    ~TableScan() = default;

 private:
    explicit TableScan(std::unique_ptr<ScanContext> context);
};
```

- `Create()` returns `Result<std::unique_ptr<T>>`.
- The private constructor does only trivial member assignment.
- All fallible initialization goes in `Create()`, **after** construction.
- If construction cannot fail (POD, data-only classes), a public constructor is fine.

### Utility Classes

Static-only utility classes delete their constructors and deconstructors:

```cpp
class StringUtils {
 public:
    StringUtils() = delete;
    ~StringUtils() = delete;

    static std::vector<std::string> Split(const std::string& s, char delim);
    // ...
};
```

### Interface Classes

Pure-virtual base classes use a virtual default destructor:

```cpp
virtual ~ClassName() = default;
```

---

## Smart Pointers & Memory

- Prefer `std::unique_ptr` for sole ownership.
- Use `std::shared_ptr` only when shared ownership is genuinely needed.
- **No raw `new` / `delete`** except inside factory `Create()` methods that call a private constructor.
- Use `std::optional<T>` for optional values.
- Use `ScopeGuard` (in `src/paimon/common/utils/`) for RAII cleanup.

---

## Comments & Documentation

- Use `///` (Doxygen) for classes and public methods.
- Use `@param` and `@return` for parameters and return values.
- Do **not** comment obvious code.
- Inline comments go **above** the line, not at the end.

```cpp
/// Create a scan plan from the current snapshot.
///
/// @param snapshot_id The snapshot to scan.
/// @return A Plan on success, or an error Status.
Result<std::shared_ptr<Plan>> CreatePlan(int64_t snapshot_id);
```

### TODO Comments

TODOs are allowed but **must** include an author tag:

```cpp
// TODO(alice): Support changelog manifest list size validation
```

---

## String Formatting

Use `fmt::format()` for string formatting. Do **not** concatenate with `std::to_string()` + `+`.

```cpp
// ✅ Good
auto msg = fmt::format("Snapshot {} not found in {}", snapshot_id, path);

// ❌ Bad
auto msg = "Snapshot " + std::to_string(snapshot_id) + " not found in " + path;
```

---

## Public API Visibility

Mark all public API symbols with `PAIMON_EXPORT`:

```cpp
class PAIMON_EXPORT Timestamp { ... };
```

---

## Reuse Existing Utilities

Before writing new helper code, check whether the project already provides what you need. The tables below list the most commonly used utilities.

### General Utilities (`src/paimon/common/utils/`)

| Class | Purpose |
|-------|---------|
| `StringUtils` | Split, Replace, StartsWith, EndsWith, Trim, etc. |
| `ObjectUtils` | Collection helpers (ContainsAll, etc.) |
| `DateTimeUtils` | Date / time conversions |
| `RapidJsonUtil` | JSON serialization / deserialization |
| `StreamUtils` | Stream read / write helpers |
| `OptionsUtils` | Configuration parsing |
| `PathUtil` | Path joining and manipulation |
| `Preconditions` | Precondition checks |
| `ScopeGuard` | RAII cleanup guard |
| `BloomFilter` / `BloomFilter64` | Bloom filters |
| `MurmurHashUtils` | Hashing |
| `CRC32C` | Checksums |
| `BitSet` | Bit set |
| `RoaringBitmap32` / `RoaringBitmap64` | Roaring bitmaps |
| `SerializationUtils` | Serialization helpers |
| `DataConverterUtils` | Data conversion |
| `DecimalUtils` | Decimal arithmetic |
| `FieldTypeUtils` | Field type helpers |
| `BinPacking` | Bin-packing algorithm |
| `ConcurrentHashMap` | Thread-safe hash map |
| `ThreadsafeQueue` | Thread-safe queue |
| `LinkedHashMap` | Insertion-ordered hash map |
| `LongCounter` | Atomic counter |
| `UUID` | UUID generation |

### Arrow Utilities (`src/paimon/common/utils/arrow/`)

| File | Purpose |
|------|---------|
| `ArrowUtils` | Arrow format helpers |
| `MemUtils` | Arrow memory helpers |
| `status_utils.h` | Arrow ↔ Paimon status bridge macros |

### Core Utilities (`src/paimon/core/utils/`)

| Class | Purpose |
|-------|---------|
| `FileUtils` | File listing, version file operations |
| `FileStorePathFactory` | Path factory for data / manifest files |
| `SnapshotManager` | Snapshot management |
| `TagManager` | Tag management |
| `BranchManager` | Branch management |
| `PartitionPathUtils` | Partition path helpers |
| `FieldMapping` | Field mapping |
| `FieldsComparator` | Field comparison |
| `PrimaryKeyTableUtils` | Primary-key table helpers |

### Concurrency (`src/paimon/common/executor/`)

| Function | Purpose |
|----------|---------|
| `CollectAll()` | Collect results from multiple futures |
| `Wait()` | Wait for multiple void futures |

### Testing Utilities (`src/paimon/testing/utils/`)

| Class | Purpose |
|-------|---------|
| `TestHelper` | End-to-end helper: create tables, write & commit data, scan & read results |
| `DataGenerator` | Generate `RecordBatch` data, split by partition/bucket, extract partial rows |
| `BinaryRowGenerator` | Generate `BinaryRow`, `InternalRow`, `SimpleStats`, and `BinaryArray` for tests |
| `KeyValueChecker` | Assert expected vs. actual `KeyValue` results in primary-key table tests |
| `ReadResultCollector` | Collect `KeyValue` or Arrow `ChunkedArray` results from a `BatchReader` |
| `DictArrayConverter` | Convert Arrow dictionary-encoded arrays to plain arrays |
| `UniqueTestDirectory` | RAII helper that creates a unique temp directory and cleans it up on destruction |
| `TimezoneGuard` | RAII guard that sets `TZ` environment variable and restores it on destruction |
| `IOExceptionHelper` | Inject I/O errors via `IOHook` for fault-injection testing |
| `testharness.h` | Test macros: `ASSERT_OK`, `ASSERT_NOK`, `ASSERT_NOK_WITH_MSG`, `ASSERT_RAISES` |

### Mock Objects (`src/paimon/testing/mock/`)

| Class | Purpose |
|-------|---------|
| `MockFileSystem` / `MockFileSystemFactory` | Mock file system and its factory for unit tests |
| `MockFileFormat` / `MockFileFormatFactory` | Mock file format and its factory |
| `MockFormatReaderBuilder` / `MockFormatWriterBuilder` | Mock reader/writer builders |
| `MockFormatWriter` / `MockFileBatchReader` | Mock format writer and batch reader |
| `MockIndexPathFactory` | Mock index path factory |
| `MockKeyValueDataFileRecordReader` | Mock key-value data file reader holding in-memory data |
| `MockStatsExtractor` | Mock statistics extractor |

---

## Testing

- Test files live **next to** the source file they test, named `*_test.cpp`.
- Use **Google Test** (`gtest`).
- Test classes go in `namespace paimon::test`.
- Use project test macros: `ASSERT_OK`, `ASSERT_NOK`, `ASSERT_NOK_WITH_MSG`.
- **Prefer `ASSERT_*` over `EXPECT_*`** — `ASSERT_*` stops the test immediately on failure, preventing cascading errors.
  - **Exception**: In non-void helper functions, use `EXPECT_*` because `ASSERT_*` expands to `return;` which is incompatible with non-void return types.

---

## Tooling

### Pre-commit Hooks (Recommended)

The easiest way to stay compliant is to install [pre-commit](https://pre-commit.com/):

```bash
pip install pre-commit
pre-commit install
```

This installs Git hooks that automatically run on every commit:

| Hook | What it checks |
|------|----------------|
| `clang-format` | C++ formatting (`.clang-format`) |
| `cpplint` | Google C++ lint rules |
| `cmake-format` | CMake file formatting (`.cmake-format.py`) |
| `codespell` | Spelling errors |
| `trailing-whitespace` | Trailing whitespace |
| `end-of-file-fixer` | Missing newline at end of file |
| `check-added-large-files` | Files > 5 MB |

To run all hooks on all files manually:

```bash
pre-commit run --all-files
```

### clang-tidy

Static analysis is configured in `.clang-tidy`. Key enabled check groups:

- `bugprone-*` — Common bug patterns
- `google-*` — Google style checks
- `modernize-*` — C++ modernization suggestions
- `clang-analyzer-*` — Clang static analyzer

### Building & Testing

```bash
# Configure (from project root)
mkdir -p build && cd build
cmake ../ \
    -DCMAKE_BUILD_TYPE=debug \
    -DPAIMON_BUILD_TESTS=ON

# Build
make -j$(nproc)

# Run a specific test suite
./debug/paimon-core-test --gtest_filter="CoreOptionsTest*"
```

---

## Pull Request Checklist

Before submitting a PR, please verify:

- [ ] Code compiles without warnings under `-Wall` (enabled by default in the build system).
- [ ] `pre-commit run --all-files` passes (or individual tools: clang-format, cpplint, codespell).
- [ ] New / modified public APIs are marked with `PAIMON_EXPORT`.
- [ ] Every new file has the Apache 2.0 license header.
- [ ] Error handling uses `Status` / `Result<T>` — no exceptions.
- [ ] New utility code checks for existing helpers before reinventing.
- [ ] Tests are added or updated for the changed functionality.
- [ ] `ASSERT_*` is preferred over `EXPECT_*` in tests.
- [ ] No raw `new` / `delete` outside factory methods.
- [ ] PR description follows the [template](.github/PULL_REQUEST_TEMPLATE.md).