docs: add AST checker design spec
Tree-sitter based code structure validation for submissions, with per-problem per-language rule configuration. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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docs/specs/2026-05-25-ast-checker-design.md
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docs/specs/2026-05-25-ast-checker-design.md
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# AST Checker Design Spec
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## Overview
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Add tree-sitter-based AST checking to the Online Judge submission flow. Teachers can configure per-problem, per-language rules that validate student code structure before judging (e.g., "must use while loop", "cannot use for loop", "must call print()").
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## Goals
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- Enforce coding constraints for pedagogical purposes (beginner programming courses)
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- Support all 6 languages: Python3, C, C++, Java, Golang, JavaScript
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- Predefined rule library with parameterized engines (no raw tree-sitter queries for admins)
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- Full admin UI for configuring rules per problem per language
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- New `AST_CHECK_FAILED` judge status with clear error messages
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## Non-Goals
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- Output-aware checks ("禁止直接输出完整目标答案") — requires expected output, not AST
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- String literal content matching (`.2f`, `03d` format specifiers) — deferred to a later phase
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- Custom tree-sitter query support for admins
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---
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## Architecture
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### Submission Flow (modified)
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```
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SubmissionAPI.post()
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→ create Submission(PENDING)
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→ judge_task.send()
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→ JudgeDispatcher.judge()
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→ apply code template
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→ **AST check** ← NEW
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→ fail: result=AST_CHECK_FAILED, write err_info, push WebSocket, return
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→ pass: continue
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→ choose judge server
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→ send to judge server
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→ process result
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```
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AST check runs inside the Dramatiq task, after template application and before judge server dispatch. This is consistent with how `COMPILE_ERROR` is handled — the submission exists in history, the result is pushed via WebSocket.
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### Data Model
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**Problem model** — new JSONField:
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```python
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ast_rules = models.JSONField(null=True, blank=True, default=None)
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```
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Schema:
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```json
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{
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"Python3": [
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{"engine": "must_exist_node", "target": "for_loop", "message": "必须使用 for 循环"},
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{"engine": "count_node", "target": "while_loop", "min": 2, "message": "while 循环至少出现 2 次"},
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{"engine": "must_call_function", "target": "print", "message": "必须调用 print()"},
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{"engine": "must_use_operator", "target": "+=", "message": "必须使用 += 运算符"},
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{"engine": "must_call_method", "target": "append", "message": "必须使用 append()"}
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],
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"C": [
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{"engine": "must_exist_node", "target": "for_loop", "message": "必须使用 for 循环"}
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]
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}
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```
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Key design: `target` uses **language-agnostic logical names** (e.g., `for_loop`, `while_loop`, `print`). Each engine maps these to language-specific tree-sitter node types internally.
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When `ast_rules` is `null` or the current language has no rules, AST checking is skipped entirely.
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**JudgeStatus** — new status code:
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```python
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class JudgeStatus(models.IntegerChoices):
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COMPILE_ERROR = -2, "Compile Error"
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WRONG_ANSWER = -1, "Wrong Answer"
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ACCEPTED = 0, "Accepted"
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CPU_TIME_LIMIT_EXCEEDED = 1, "CPU Time Limit Exceeded"
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REAL_TIME_LIMIT_EXCEEDED = 2, "Real Time Limit Exceeded"
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MEMORY_LIMIT_EXCEEDED = 3, "Memory Limit Exceeded"
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RUNTIME_ERROR = 4, "Runtime Error"
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SYSTEM_ERROR = 5, "System Error"
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PENDING = 6, "Pending"
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JUDGING = 7, "Judging"
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PARTIALLY_ACCEPTED = 8, "Partially Accepted"
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AST_CHECK_FAILED = 9, "AST Check Failed" # NEW
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```
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Frontend `constants.ts` must be updated with the new status code, label, and color.
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---
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## Rule Engine Architecture
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### Directory Structure
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```
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OnlineJudge/ast_checker/
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├── __init__.py
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├── checker.py # Entry point: check(code, language, rules) → (ok, errors)
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├── engines/
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│ ├── __init__.py # Engine registry
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│ ├── base.py # BaseEngine abstract class
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│ ├── node_exists.py # must_exist_node / must_not_exist_node
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│ ├── node_count.py # count_node
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│ ├── function_call.py # must_call_function / must_not_call_function / count_function_call
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│ ├── method_call.py # must_call_method / must_not_call_method
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│ ├── operator.py # must_use_operator
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│ ├── keyword_arg.py # must_use_keyword_arg
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│ ├── import_check.py # must_import / must_not_import
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│ └── structural.py # nested_for, chained_comparison, swap_assignment, etc.
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└── mappings/
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├── __init__.py # get_mapping(language) dispatcher
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├── python.py
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├── c.py
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├── cpp.py
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├── java.py
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├── go.py
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└── javascript.py
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```
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### Engine Interface
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```python
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class BaseEngine:
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def check(self, tree, rule, language, mapping) -> list[str]:
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"""
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Returns a list of error messages (empty = pass).
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- tree: tree-sitter parsed tree
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- rule: the rule dict (engine, target, message, min, max, value, etc.)
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- language: language name string
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- mapping: language-specific node type mapping dict
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"""
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raise NotImplementedError
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```
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### Engine Catalog
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| Engine Name | Parameters | Description |
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|---|---|---|
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| `must_exist_node` | `target` | Node type must appear at least once |
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| `must_not_exist_node` | `target` | Node type must not appear |
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| `count_node` | `target`, `min?`, `max?` | Node type count must be within [min, max] |
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| `must_call_function` | `target` | Must call a specific function (e.g., `print`, `input`) |
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| `must_not_call_function` | `target` | Must not call a specific function |
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| `count_function_call` | `target`, `min?`, `max?` | Function call count within range |
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| `must_call_method` | `target` | Must call a method (e.g., `.append()`, `.split()`) |
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| `must_not_call_method` | `target` | Must not call a method |
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| `must_use_operator` | `target`, `category?` | Must use a specific operator. Category auto-inferred from target: arithmetic (`+`,`-`,`*`,`/`,`//`,`%`,`**`) → search in binary expressions; augmented (`+=`,`-=`) → search in augmented assignments; comparison (`==`,`!=`,`>`,`>=`,`<`,`<=`) → search in comparisons; logical (`and`,`or`,`not`) → search in boolean/unary expressions; bitwise (`&`,`\|`) → search in binary expressions |
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| `must_use_keyword_arg` | `target` (function), `arg_name`, `value?` | Must use keyword arg in a call |
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| `must_import` | `target` | Must import a specific module |
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| `must_not_import` | `target` | Must not import a specific module |
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| `must_use_variable_name` | `target` | Must assign to a variable with this name |
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| `must_not_use_variable_name` | `target` | Must not assign to a variable with this name |
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| `nested_for` | — | Must have a for loop nested inside another for loop |
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| `chained_comparison` | — | Must use chained comparison (e.g., `a < b < c`) |
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| `swap_assignment` | — | Must use swap assignment (e.g., `a, b = b, a`) |
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| `chain_assignment` | — | Must use chain assignment (e.g., `a = b = 1`) |
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| `must_use_recursion` | — | Must have a function that calls itself |
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| `no_recursion` | — | No function may call itself |
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### Language Mapping
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Each mapping file exports a dict translating logical names to tree-sitter node types:
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```python
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# mappings/python.py
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PYTHON_MAPPING = {
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# Node types
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"for_loop": "for_statement",
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"while_loop": "while_statement",
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"if_statement": "if_statement",
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"else_clause": "else_clause",
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"elif_clause": "elif_clause",
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"break": "break_statement",
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"continue": "continue_statement",
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"function_definition": "function_definition",
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"return": "return_statement",
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"try_except": "try_statement",
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"with_statement": "with_statement",
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"list_comprehension": "list_comprehension",
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"list_literal": "list",
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"dict_literal": "dictionary",
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"set_literal": "set",
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"f_string": "format_string",
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"import": "import_statement",
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"import_from": "import_from_statement",
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"assignment": "assignment",
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"class_definition": "class_definition",
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# Operators
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"+": "+",
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"-": "-",
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"*": "*",
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"/": "/",
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"//": "//",
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"%": "%",
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"**": "**",
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"+=": "+=",
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"-=": "-=",
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"==": "==",
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"!=": "!=",
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">": ">",
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">=": ">=",
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"<": "<",
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"<=": "<=",
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"and": "and",
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"or": "or",
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"not": "not",
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"&": "&",
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"|": "|",
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}
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```
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```python
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# mappings/c.py
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C_MAPPING = {
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"for_loop": "for_statement",
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"while_loop": "while_statement",
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"if_statement": "if_statement",
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"else_clause": "else_clause",
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"break": "break_statement",
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"continue": "continue_statement",
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"function_definition": "function_definition",
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"return": "return_statement",
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"assignment": "assignment_expression",
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# ... C-specific mappings
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}
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```
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### Entry Point
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```python
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# checker.py
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def check_ast(code: str, language: str, rules: list[dict]) -> tuple[bool, list[str]]:
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"""
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Parse code with tree-sitter, run all rules, return (passed, error_messages).
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If rules is empty, returns (True, []).
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If tree-sitter fails to parse (syntax error), returns (True, []) — skip AST
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check and let the compiler report the error downstream.
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"""
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```
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### Known Limitations
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- **Method call detection is name-based only**: `must_call_method("append")` matches any `.append()` call regardless of object type. tree-sitter provides no type information. Acceptable for teaching scenarios.
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- **Structural rules are language-specific**: `swap_assignment`, `chained_comparison`, `chain_assignment` only apply to Python. The engine should return (pass) for unsupported languages rather than erroring.
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### Integration in JudgeDispatcher
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```python
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# In JudgeDispatcher.judge(), after template application:
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def judge(self):
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language = self.submission.language
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sub_config = list(filter(...))[0]
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if language in self.problem.template:
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template = parse_problem_template(self.problem.template[language])
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code = f"{template['prepend']}\n{self.submission.code}\n{template['append']}"
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else:
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code = self.submission.code
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# --- AST CHECK (NEW) ---
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ast_rules = self.problem.ast_rules
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if ast_rules and language in ast_rules:
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from ast_checker.checker import check_ast
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passed, errors = check_ast(self.submission.code, language, ast_rules[language])
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if not passed:
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self.submission.result = JudgeStatus.AST_CHECK_FAILED
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self.submission.statistic_info["err_info"] = "\n".join(errors)
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self.submission.statistic_info["score"] = 0
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self.submission.save(update_fields=["result", "info", "statistic_info"])
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try:
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push_submission_update(
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submission_id=str(self.submission.id),
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user_id=self.submission.user_id,
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data={
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"type": "submission_update",
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"submission_id": str(self.submission.id),
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"result": JudgeStatus.AST_CHECK_FAILED,
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"status": "finished",
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}
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)
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except Exception as e:
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logger.error(f"Failed to push submission update: {str(e)}")
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return
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# --- END AST CHECK ---
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# ... continue with judge server dispatch
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```
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Note: AST check runs on `self.submission.code` (raw student code), not the template-wrapped `code`, because the template prepend/append is not student-written.
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---
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## Frontend Changes
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### Admin UI (ojnext)
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In the problem edit page, add a collapsible "代码规则检查" section:
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- **Language tabs**: Only show tabs for languages selected in this problem's `languages` field
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- **Rule list per language**: Each rule is a row with:
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- Engine dropdown (grouped by category: 节点检查 / 函数调用 / 运算符 / 结构检查 / 导入…)
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- Target dropdown/input (context-dependent: node types for node engines, function names for call engines, operators for operator engine)
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- Optional parameters: `min`, `max`, `value` fields (shown only when the selected engine uses them)
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- Message input (custom error message, with auto-generated default)
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- Delete button
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- **Add rule button** per language tab
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- Section is collapsed by default (most problems won't have AST rules)
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### Status Display
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- Add `AST_CHECK_FAILED = 9` to `ojnext/src/utils/constants.ts`
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- Assign a distinct color (suggest orange, between COMPILE_ERROR red and PENDING grey)
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- Label: "AST Check Failed" / "代码结构检查未通过"
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- When viewing a submission with this status, display `statistic_info.err_info` as the error detail (same rendering as COMPILE_ERROR)
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---
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## Dependencies
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Backend (add to pyproject.toml / requirements):
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- `tree-sitter` (Python bindings)
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- `tree-sitter-python`
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- `tree-sitter-c`
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- `tree-sitter-cpp`
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- `tree-sitter-java`
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- `tree-sitter-go`
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- `tree-sitter-javascript`
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These are pure Python wheels with pre-compiled grammars, no system dependencies needed.
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---
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## Migration
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One Django migration:
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1. Add `ast_rules` JSONField (null=True) to Problem model
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2. Add `AST_CHECK_FAILED = 9` to JudgeStatus
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Both are additive, no data migration needed. Existing problems get `ast_rules=null` (no AST checking).
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---
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## Phased Delivery
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> Note: Most problems use Python3 and C. Prioritize these two languages.
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### Phase 1 (MVP)
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- Rule engine framework + checker entry point
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- **Python3 mapping** (most complete, matches the full rule catalog)
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- **C mapping** (second priority, covers the most-used language pair)
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- Engines: `must_exist_node`, `must_not_exist_node`, `count_node`, `must_call_function`, `must_not_call_function`, `count_function_call`, `must_call_method`, `must_not_call_method`, `must_use_operator`
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- JudgeDispatcher integration
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- Frontend: status code + admin UI
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- Migration
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### Phase 2
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- Remaining engines: `must_use_keyword_arg`, `must_import`/`must_not_import`, `must_use_variable_name`/`must_not_use_variable_name`
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- Structural engines: `nested_for`, `chained_comparison` (Python only), `swap_assignment` (Python only), `chain_assignment` (Python only), `must_use_recursion`, `no_recursion`
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- C++ mapping (shares most structure with C)
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### Phase 3
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- Java, Go, JavaScript mappings
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- String literal content checks (format specifiers)
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- Additional structural rules as needed
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