# ReefVM Architectural Improvement Ideas

This document contains architectural ideas for improving ReefVM. These focus on enhancing the VM's capabilities through structural improvements rather than just adding new opcodes.

## 1. Scope Resolution Optimization

**Current Issue**: Variable lookups are O(n) through the scope chain on every `LOAD`. This becomes expensive in deeply nested closures.

**Architectural Solution**: Implement **static scope analysis** with **lexical addressing**:

```typescript
// Instead of: LOAD x  (runtime scope chain walk)
// Compile to: LOAD_FAST 2 1  (scope depth 2, slot 1 - O(1) lookup)

class Scope {
  locals: Map<string, Value>
  parent?: Scope

  // NEW: Add indexed slots for fast access
  slots: Value[]  // Direct array access
  nameToSlot: Map<string, number>  // Compile-time mapping
}
```

**Benefits**:
- O(1) variable access instead of O(n)
- Critical for hot loops and deeply nested functions
- Compiler can still fall back to named lookup for dynamic cases

---

## 2. Module System Architecture

**Current Gap**: No way to organize code across multiple files or create reusable libraries.

**Architectural Solution**: Add first-class module support:

```typescript
// New opcodes: IMPORT, EXPORT, MAKE_MODULE
// New bytecode structure:
type Bytecode = {
  instructions: Instruction[]
  constants: Constant[]
  exports?: Map<string, number>  // Exported symbols
  imports?: Import[]               // Import declarations
}

type Import = {
  modulePath: string
  symbols: string[]  // [] means import all
  alias?: string
}
```

**Pattern**:
```
MAKE_MODULE .module_body
EXPORT add
EXPORT subtract
HALT

.module_body:
  MAKE_FUNCTION (x y) .add_impl
  RETURN
```

**Benefits**:
- Code organization and reusability
- Circular dependency detection at load time
- Natural namespace isolation
- Enables standard library architecture

---

## 3. Source Map Integration

**Current Issue**: Runtime errors show bytecode addresses, not source locations.

**Architectural Solution**: Add source mapping layer:

```typescript
type Bytecode = {
  instructions: Instruction[]
  constants: Constant[]
  sourceMap?: SourceMap  // NEW
}

type SourceMap = {
  file?: string
  mappings: SourceMapping[]  // Instruction index → source location
}

type SourceMapping = {
  instruction: number
  line: number
  column: number
  source?: string  // Original source text
}
```

**Benefits**:
- Meaningful error messages with line/column
- Debugger can show original source
- Stack traces map to source code
- Critical for production debugging

---

## 4. Debugger Hook Architecture

**Current Gap**: No way to pause execution, inspect state, or step through code.

**Architectural Solution**: Add debug event system:

```typescript
class VM {
  debugger?: Debugger

  async execute(instruction: Instruction) {
    // Before execution
    await this.debugger?.onInstruction(this.pc, instruction, this)

    // Execute
    switch (instruction.op) { ... }

    // After execution
    await this.debugger?.afterInstruction(this.pc, this)
  }
}

interface Debugger {
  breakpoints: Set<number>
  onInstruction(pc: number, instruction: Instruction, vm: VM): Promise<void>
  afterInstruction(pc: number, vm: VM): Promise<void>
  onCall(fn: Value, args: Value[]): Promise<void>
  onReturn(value: Value): Promise<void>
  onException(error: Value): Promise<void>
}
```

**Benefits**:
- Step-through debugging
- Breakpoints at any instruction
- State inspection at any point
- Non-invasive (no bytecode modification)
- Can build IDE integrations

---

## 5. Bytecode Optimization Pass Framework

**Current Gap**: Bytecode is emitted directly, no optimization.

**Architectural Solution**: Add optimization pipeline:

```typescript
type Optimizer = (bytecode: Bytecode) => Bytecode

// Framework for composable optimization passes
class BytecodeOptimizer {
  passes: Optimizer[] = []

  add(pass: Optimizer): this {
    this.passes.push(pass)
    return this
  }

  optimize(bytecode: Bytecode): Bytecode {
    return this.passes.reduce((bc, pass) => pass(bc), bytecode)
  }
}

// Example passes:
const optimizer = new BytecodeOptimizer()
  .add(constantFolding)      // PUSH 2; PUSH 3; ADD → PUSH 5
  .add(deadCodeElimination)  // Remove unreachable code after HALT/RETURN
  .add(jumpChaining)         // JUMP .a → .a: JUMP .b → JUMP .b directly
  .add(peepholeOptimization) // DUP; POP → (nothing)
```

**Benefits**:
- Faster execution without changing compiler
- Can add passes without modifying VM
- Composable and testable
- Enables aggressive optimizations (inlining, constant folding, etc.)

---

## 6. Value Memory Management Architecture

**Current Issue**: No tracking of memory usage, no GC hooks, unbounded growth.

**Architectural Solution**: Add memory management layer:

```typescript
class MemoryManager {
  allocatedBytes: number = 0
  maxBytes?: number

  allocateValue(value: Value): Value {
    const size = this.sizeOf(value)
    if (this.maxBytes && this.allocatedBytes + size > this.maxBytes) {
      throw new Error('Out of memory')
    }
    this.allocatedBytes += size
    return value
  }

  sizeOf(value: Value): number {
    // Estimate memory footprint
  }

  // Hook for custom GC
  gc?: () => void
}

class VM {
  memory: MemoryManager

  // All value-creating operations check memory
  push(value: Value) {
    this.memory.allocateValue(value)
    this.stack.push(value)
  }
}
```

**Benefits**:
- Memory limits for sandboxing
- Memory profiling
- Custom GC strategies
- Prevents runaway memory usage

---

## 7. Instruction Profiler Architecture

**Current Gap**: No way to identify performance bottlenecks in bytecode.

**Architectural Solution**: Add instrumentation layer:

```typescript
class Profiler {
  instructionCounts: Map<number, number> = new Map()
  instructionTime: Map<number, number> = new Map()
  hotFunctions: Map<number, FunctionProfile> = new Map()

  recordInstruction(pc: number, duration: number) {
    this.instructionCounts.set(pc, (this.instructionCounts.get(pc) || 0) + 1)
    this.instructionTime.set(pc, (this.instructionTime.get(pc) || 0) + duration)
  }

  getHotSpots(): HotSpot[] {
    // Identify most-executed instructions
  }

  generateReport(): ProfileReport {
    // Human-readable performance report
  }
}

class VM {
  profiler?: Profiler

  async execute(instruction: Instruction) {
    const start = performance.now()
    // ... execute ...
    const duration = performance.now() - start
    this.profiler?.recordInstruction(this.pc, duration)
  }
}
```

**Benefits**:
- Identify hot loops and functions
- Guide optimization efforts
- Measure impact of changes
- Can feed into JIT compiler (future)

---

## 8. Standard Library Plugin Architecture

**Current Issue**: Native functions registered manually, no standard library structure.

**Architectural Solution**: Module-based native libraries:

```typescript
interface NativeModule {
  name: string
  exports: Record<string, any>
  init?(vm: VM): void
}

class VM {
  modules: Map<string, NativeModule> = new Map()

  registerModule(module: NativeModule) {
    this.modules.set(module.name, module)
    module.init?.(this)

    // Auto-register exports to global scope
    for (const [name, value] of Object.entries(module.exports)) {
      this.set(name, value)
    }
  }

  loadModule(name: string): NativeModule {
    return this.modules.get(name) || throw new Error(`Module ${name} not found`)
  }
}

// Example usage:
const mathModule: NativeModule = {
  name: 'math',
  exports: {
    sin: Math.sin,
    cos: Math.cos,
    sqrt: Math.sqrt,
    PI: Math.PI
  }
}

vm.registerModule(mathModule)
```

**Benefits**:
- Organized standard library
- Lazy loading of modules
- Third-party plugin system
- Clear namespace boundaries

---

## 9. Streaming Bytecode Execution

**Current Limitation**: Must load entire bytecode before execution.

**Architectural Solution**: Incremental bytecode loading:

```typescript
class StreamingBytecode {
  chunks: BytecodeChunk[] = []

  append(chunk: BytecodeChunk) {
    // Remap addresses, merge constants
    this.chunks.push(chunk)
  }

  getInstruction(pc: number): Instruction | undefined {
    // Resolve across chunks
  }
}

class VM {
  async runStreaming(stream: ReadableStream<BytecodeChunk>) {
    for await (const chunk of stream) {
      this.bytecode.append(chunk)
      await this.continue()  // Execute new chunk
    }
  }
}
```

**Benefits**:
- Execute before full load (faster startup)
- Network streaming of bytecode
- Incremental compilation
- Better REPL experience

---

## 10. Type Annotation System (Optional Runtime Types)

**Current Gap**: All values dynamically typed, no way to enforce types.

**Architectural Solution**: Optional type metadata:

```typescript
type TypedValue = Value & {
  typeAnnotation?: TypeAnnotation
}

type TypeAnnotation =
  | { kind: 'number' }
  | { kind: 'string' }
  | { kind: 'array', elementType?: TypeAnnotation }
  | { kind: 'dict', valueType?: TypeAnnotation }
  | { kind: 'function', params: TypeAnnotation[], return: TypeAnnotation }

// New opcodes: TYPE_CHECK, TYPE_ASSERT
// Functions can declare parameter types:
MAKE_FUNCTION (x:number y:string) .body
```

**Benefits**:
- Catch type errors earlier
- Self-documenting code
- Enables static analysis tools
- Optional (doesn't break existing code)
- Can enable optimizations (known number type → skip toNumber())

---

## 11. VM State Serialization

**Current Gap**: Can't save/restore VM execution state.

**Architectural Solution**: Serializable VM state:

```typescript
class VM {
  serialize(): SerializedState {
    return {
      instructions: this.instructions,
      constants: this.constants,
      pc: this.pc,
      stack: this.stack.map(serializeValue),
      callStack: this.callStack.map(serializeFrame),
      scope: serializeScope(this.scope),
      handlers: this.handlers
    }
  }

  static deserialize(state: SerializedState): VM {
    const vm = new VM(/* ... */)
    vm.restore(state)
    return vm
  }
}
```

**Benefits**:
- Save/restore execution state
- Distributed computing (send state to workers)
- Crash recovery
- Time-travel debugging
- Checkpoint/restart

---

## 12. Async Iterator Support

**Current Gap**: Iterators work via break, but no async iteration.

**Architectural Solution**: First-class async iteration:

```typescript
// New value type:
type Value = ... | { type: 'async_iterator', value: AsyncIterableIterator<Value> }

// New opcodes: MAKE_ASYNC_ITERATOR, AWAIT_NEXT, YIELD_ASYNC

// Pattern:
for_await (item in asyncIterable) {
  // Compiles to AWAIT_NEXT loop
}
```

**Benefits**:
- Stream processing
- Async I/O without blocking
- Natural async patterns
- Matches JavaScript async iterators

---

## Priority Recommendations

### Tier 1 (Highest Impact):
1. **Source Map Integration** - Critical for usability
2. **Module System** - Essential for scaling beyond toy programs
3. **Scope Resolution Optimization** - Performance multiplier

### Tier 2 (High Value):
4. **Debugger Hook Architecture** - Developer experience game-changer
5. **Standard Library Plugin Architecture** - Enables ecosystem
6. **Bytecode Optimization Framework** - Performance without complexity

### Tier 3 (Nice to Have):
7. **Instruction Profiler** - Guides future optimization
8. **Memory Management** - Important for production use
9. **VM State Serialization** - Enables advanced use cases

### Tier 4 (Future/Experimental):
10. **Type Annotations** - Optional, doesn't break existing code
11. **Streaming Bytecode** - Mostly useful for large programs
12. **Async Iterators** - Specialized use case

---

## Design Principles

These improvements focus on:
- **Performance** (scope optimization, bytecode optimization)
- **Developer Experience** (source maps, debugger, profiler)
- **Scalability** (modules, standard library architecture)
- **Production Readiness** (memory management, serialization)

All ideas maintain ReefVM's core design philosophy of simplicity, orthogonality, and explicit behavior.