[router] add reasoning parser readme (#9837)

sgl-router/src/reasoning_parser/README.md

+# Reasoning Parser Architecture
+
+## 1. Executive Summary
+
+### High-Level Overview
+
+The reasoning parser layer provides a unified interface for detecting and extracting reasoning content from Large Language Model (LLM) outputs, particularly from models that support Chain-of-Thought (CoT) reasoning with explicit thinking blocks. The architecture follows a trait-based design pattern enabling pluggable parser implementations while maintaining consistent APIs across different model families that use various reasoning token formats.
+
+**Key Components:**
+- **Factory Pattern**: Registry-based creation and pooling of model-specific parsers
+- **Trait System**: `ReasoningParser` trait for implementation flexibility
+- **Parser Pooling**: Efficient reuse of parser instances across concurrent requests
+- **Streaming Support**: Incremental parsing with partial token buffering
+- **Model Detection**: Pattern-based matching for automatic parser selection
+- **State Management**: Stateful parsing for streaming scenarios with buffer management
+- **Thread Safety**: Arc<Mutex> based sharing for high-concurrency environments
+- **Extensibility**: Easy addition of new model-specific parsers
+
+**Data Flow:**
+1. Request → Factory (model detection) → Pooled Parser Retrieval
+2. One-Shot: Text → Parser → ParserResult (normal + reasoning text)
+3. Streaming: Chunks → Parser (stateful) → Incremental ParserResult
+4. Buffer Management: Partial Tokens → Buffer → Complete Token Detection
+5. Reset: Parser State → Clear Buffers → Ready for Reuse
+
+### Architecture Highlights
+
+- **Model-Specific Parsers**: DeepSeek-R1, Qwen3, Kimi, GLM45, Step3 variants
+- **Parser Pooling**: Singleton instances per model type for memory efficiency
+- **High Concurrency**: Mutex-protected parsers handle 1000+ req/sec
+- **Buffer Overflow Protection**: Configurable max buffer size (default 64KB)
+- **Partial Token Detection**: Intelligent buffering for incomplete delimiters
+- **Passthrough Mode**: Graceful fallback for unknown models
+- **Zero-Copy Where Possible**: Efficient string handling in hot paths
+
+## 2. Mermaid Diagrams
+
+### Component Flow Diagram
+
+```mermaid
+graph TB
+    subgraph Input
+        R[Request] --> MID[Model ID]
+    end
+
+    subgraph Factory Layer
+        MID --> PF[ParserFactory]
+        PF --> REG[ParserRegistry]
+        REG --> PM[Pattern Matching]
+        PM --> PP[Parser Pool]
+    end
+
+    subgraph Parser Pool
+        PP --> DS[DeepSeek-R1]
+        PP --> QW[Qwen3]
+        PP --> QWT[Qwen3-Thinking]
+        PP --> KM[Kimi]
+        PP --> GL[GLM45]
+        PP --> S3[Step3]
+        PP --> PT[Passthrough]
+    end
+
+    subgraph Parser Instance
+        DS --> BP[BaseReasoningParser]
+        QW --> BP
+        KM --> BP
+        GL --> BP
+        S3 --> BP
+    end
+
+    subgraph Processing
+        BP --> DAP[detect_and_parse]
+        BP --> PSI[parse_streaming]
+        BP --> RST[reset]
+    end
+
+    subgraph State Management
+        BP --> BUF[Buffer]
+        BP --> IR[in_reasoning flag]
+        BP --> STS[stripped_think_start]
+    end
+
+    subgraph Output
+        DAP --> PR[ParserResult]
+        PSI --> PR
+        PR --> NT[normal_text]
+        PR --> RT[reasoning_text]
+    end
+```
+
+### Sequence Flow Diagram
+
+```mermaid
+sequenceDiagram
+    participant C as Client
+    participant F as ParserFactory
+    participant R as Registry
+    participant P as Parser Pool
+    participant BP as BaseParser
+    participant PR as ParserResult
+
+    C->>F: get_pooled("deepseek-r1-model")
+    F->>R: find_pooled_parser_for_model()
+    R->>R: pattern_match("deepseek-r1")
+    R->>P: get_pooled_parser("deepseek_r1")
+
+    alt Parser exists in pool
+        P-->>F: Arc<Mutex<Parser>>
+    else Create new parser
+        P->>BP: new DeepSeekR1Parser()
+        P->>P: insert into pool
+        P-->>F: Arc<Mutex<Parser>>
+    end
+
+    F-->>C: PooledParser
+
+    C->>BP: lock().parse_reasoning_streaming_incremental()
+    loop streaming chunks
+        C->>BP: parse_reasoning_streaming_incremental(chunk)
+        BP->>BP: buffer.push_str(chunk)
+        BP->>BP: check partial tokens
+
+        alt Complete token found
+            BP->>PR: create result
+            BP->>BP: clear buffer
+            BP-->>C: ParserResult
+        else Partial token
+            BP->>BP: keep buffering
+            BP-->>C: ParserResult::default()
+        end
+    end
+
+    C->>BP: reset()
+    BP->>BP: clear buffers & flags
+    C->>BP: unlock()
+```
+
+### Class/Type Diagram
+
+```mermaid
+classDiagram
+    class ReasoningParser {
+        <<trait>>
+        +detect_and_parse_reasoning(&mut self, text: &str) Result~ParserResult~
+        +parse_reasoning_streaming_incremental(&mut self, text: &str) Result~ParserResult~
+        +reset(&mut self)
+        +model_type(&self) &str
+    }
+
+    class ParserResult {
+        +normal_text: String
+        +reasoning_text: String
+        +new(normal: String, reasoning: String) Self
+        +normal(text: String) Self
+        +reasoning(text: String) Self
+        +is_empty() bool
+    }
+
+    class ParserConfig {
+        +think_start_token: String
+        +think_end_token: String
+        +stream_reasoning: bool
+        +max_buffer_size: usize
+        +initial_in_reasoning: bool
+        +default() Self
+    }
+
+    class BaseReasoningParser {
+        -config: ParserConfig
+        -in_reasoning: bool
+        -buffer: String
+        -stripped_think_start: bool
+        -model_type: String
+        +new(config: ParserConfig) Self
+        +with_model_type(model: String) Self
+        -is_partial_token(&self, text: &str) bool
+    }
+
+    class DeepSeekR1Parser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class Qwen3Parser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class QwenThinkingParser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class KimiParser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class Glm45Parser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class Step3Parser {
+        -base: BaseReasoningParser
+        +new() Self
+    }
+
+    class ParserFactory {
+        -registry: ParserRegistry
+        +new() Self
+        +get_pooled(model_id: &str) PooledParser
+        +create(model_id: &str) Result~Box~dyn ReasoningParser~~
+        +clear_pool()
+    }
+
+    class ParserRegistry {
+        -creators: Arc~RwLock~HashMap~~
+        -pool: Arc~RwLock~HashMap~~
+        -patterns: Arc~RwLock~Vec~~
+        +register_parser(name: &str, creator: F)
+        +register_pattern(pattern: &str, parser_name: &str)
+        +get_pooled_parser(name: &str) Option~PooledParser~
+        +find_pooled_parser_for_model(model: &str) Option~PooledParser~
+    }
+
+    ReasoningParser <|.. BaseReasoningParser
+    ReasoningParser <|.. DeepSeekR1Parser
+    ReasoningParser <|.. Qwen3Parser
+    ReasoningParser <|.. QwenThinkingParser
+    ReasoningParser <|.. KimiParser
+    ReasoningParser <|.. Glm45Parser
+    ReasoningParser <|.. Step3Parser
+
+    DeepSeekR1Parser o-- BaseReasoningParser
+    Qwen3Parser o-- BaseReasoningParser
+    QwenThinkingParser o-- BaseReasoningParser
+    KimiParser o-- BaseReasoningParser
+    Glm45Parser o-- BaseReasoningParser
+    Step3Parser o-- BaseReasoningParser
+
+    BaseReasoningParser o-- ParserConfig
+    ParserFactory o-- ParserRegistry
+    ParserRegistry o-- ReasoningParser
+```
+
+## 3. Module-by-Module Deep Dive
+
+### 3.1 mod.rs (Main Module)
+
+**Key Responsibilities:**
+- Module organization and public API surface
+- Re-exports for convenient access to core types
+- Separation of concerns across submodules
+
+**Module Structure:**
+- `factory`: Parser creation and pooling logic
+- `parsers`: Concrete parser implementations
+- `traits`: Core trait definitions and types
+
+### 3.2 traits.rs (Trait Definitions)
+
+**ParserResult Methods**:
+- `new()`: Create with both normal and reasoning text
+- `normal()`: Create with only normal text (convenience)
+- `reasoning()`: Create with only reasoning text (convenience)
+- `is_empty()`: Check if result contains any text
+
+**ReasoningParser Trait**:
+- **`detect_and_parse_reasoning`**: One-shot parsing for complete text
+- **`parse_reasoning_streaming_incremental`**: Stateful streaming parser
+- **`reset`**: Clear state for parser reuse
+- **`model_type`**: Identify parser variant for debugging
+
+**ParserConfig Defaults**:
+- Default tokens: `<think>` and `</think>`
+- Stream reasoning: true (immediate output)
+- Max buffer: 65536 bytes (64KB)
+- Initial state: false (explicit reasoning blocks)
+
+### 3.3 factory.rs (Parser Creation & Pooling)
+
+**ParserRegistry Methods**:
+
+1. **`register_parser`**:
+   - Register creator function for parser type
+   - Lazy instantiation when requested
+   - Thread-safe registration
+
+2. **`register_pattern`**:
+   - Map model ID patterns to parser names
+   - First-match-wins ordering
+   - Case-insensitive matching
+
+3. **`get_pooled_parser`**:
+   - Check pool for existing instance
+   - Create and pool if not present
+   - Return Arc<Mutex> for sharing
+
+4. **`find_pooled_parser_for_model`**:
+   - Pattern match against model ID
+   - Delegate to get_pooled_parser
+   - Case-insensitive comparison
+
+**ParserFactory Methods**:
+
+1. **`new()`**:
+   - Register all built-in parsers
+   - Setup model pattern mappings
+   - Initialize empty pool
+
+2. **`get_pooled`**:
+   - Primary API for getting parsers
+   - Automatic passthrough fallback
+   - Guaranteed non-null return
+
+3. **`create`**:
+   - Create fresh parser instance
+   - No pooling (for testing/isolation)
+   - Returns Result for error handling
+
+**Registered Parsers**:
+- `base`: Generic configurable parser
+- `deepseek_r1`: DeepSeek-R1 (initial_in_reasoning=true)
+- `qwen3`: Qwen3 base model (initial_in_reasoning=false)
+- `qwen3_thinking`: Qwen3 thinking variant (initial_in_reasoning=true)
+- `kimi`: Kimi with Unicode tokens
+- `glm45`: GLM-4.5 parser
+- `step3`: Step3 parser
+- `passthrough`: No-op fallback parser
+
+**Model Pattern Mappings**:
+```
+"deepseek-r1" → "deepseek_r1"
+"qwen3-thinking" → "qwen3_thinking"
+"qwen-thinking" → "qwen3_thinking"
+"qwen3" → "qwen3"
+"qwen" → "qwen3"
+"glm45" → "glm45"
+"kimi" → "kimi"
+"step3" → "step3"
+```
+
+### 3.4 parsers/base.rs (Base Implementation)
+
+**Key Methods:**
+
+**`detect_and_parse_reasoning`**:
+```
+Algorithm:
+1. Check buffer overflow protection
+2. Detect reasoning presence (in_reasoning OR contains start_token)
+3. If no reasoning → return as normal text
+4. Remove start token and trim
+5. If no end token → assume truncated reasoning
+6. Split on end token
+7. Extract reasoning and normal portions
+```
+
+**`parse_reasoning_streaming_incremental`**:
+```
+Algorithm:
+1. Check buffer capacity
+2. Append text to buffer
+3. Check if buffer is partial token prefix
+4. If partial → buffer and return empty
+5. Strip start token if present
+6. Find end token position
+7. Handle based on state:
+   - In reasoning + end found → split and return both
+   - In reasoning + streaming → return accumulated reasoning
+   - Not in reasoning → return as normal text
+   - In reasoning + no end → continue buffering
+```
+
+**Critical Features:**
+
+1. **Partial Token Detection**:
+   - Prevents premature token matching
+   - Buffers incomplete delimiters
+   - Essential for streaming correctness
+
+2. **Buffer Management**:
+   - Overflow protection
+   - Accumulation for partial content
+   - Clear on complete token detection
+
+3. **State Tracking**:
+   - `in_reasoning`: Current parsing state
+   - `stripped_think_start`: Prevent double processing
+   - `buffer`: Accumulated partial content
+
+
+## 4. Extensibility Guide
+
+### Adding a New Parser
+
+**Step 1: Create Parser Implementation**
+
+```rust
+// src/reasoning_parser/parsers/mymodel.rs
+use crate::reasoning_parser::parsers::BaseReasoningParser;
+use crate::reasoning_parser::traits::{ParserConfig, ReasoningParser};
+
+pub struct MyModelParser {
+    base: BaseReasoningParser,
+}
+
+impl MyModelParser {
+    pub fn new() -> Self {
+        let config = ParserConfig {
+            think_start_token: "<reasoning>".to_string(),
+            think_end_token: "</reasoning>".to_string(),
+            stream_reasoning: true,
+            max_buffer_size: 65536,
+            initial_in_reasoning: false, // or true for implicit
+        };
+
+        Self {
+            base: BaseReasoningParser::new(config)
+                .with_model_type("mymodel".to_string()),
+        }
+    }
+}
+
+impl ReasoningParser for MyModelParser {
+    // Delegate to base or implement custom logic
+    fn detect_and_parse_reasoning(&mut self, text: &str)
+        -> Result<ParserResult, ParseError> {
+        self.base.detect_and_parse_reasoning(text)
+    }
+
+    // ... other trait methods
+}
+```
+
+**Step 2: Register in Factory**
+
+```rust
+// In factory.rs ParserFactory::new()
+registry.register_parser("mymodel", || {
+    Box::new(MyModelParser::new())
+});
+
+// Register patterns
+registry.register_pattern("my-model", "mymodel");
+registry.register_pattern("mymodel", "mymodel");
+```
+
+**Step 3: Export from Module**
+
+```rust
+// In parsers/mod.rs
+pub use self::mymodel::MyModelParser;
+
+// In reasoning_parser/mod.rs
+pub use parsers::MyModelParser;
+```
+
+### Custom Parsing Logic
+
+For parsers requiring custom logic beyond configuration:
+
+```rust
+impl ReasoningParser for CustomParser {
+    fn parse_reasoning_streaming_incremental(&mut self, text: &str)
+        -> Result<ParserResult, ParseError> {
+        // Custom state machine
+        // Custom token detection
+        // Custom buffering strategy
+        // Return appropriate ParserResult
+    }
+}
+```