claude-hooks/lib/context-monitor.js

/**
 * Context Monitor - Estimates context usage and triggers backups
 */

const fs = require('fs-extra');
const path = require('path');

class ContextMonitor {
  constructor() {
    this.estimatedTokens = 0;
    this.promptCount = 0;
    this.toolExecutions = 0;
    this.sessionStartTime = Date.now();
    
    // Configuration
    this.maxContextTokens = 200000; // Conservative estimate
    this.backupThreshold = 0.85; // 85% of max context
    this.timeThresholdMinutes = 30; // 30 minutes
    this.toolThreshold = 25; // 25 tool executions
    
    this.lastBackupTime = Date.now();
    this.lastBackupToolCount = 0;
  }
  
  /**
   * Update context estimates from user prompt
   */
  updateFromPrompt(promptData) {
    this.promptCount++;
    
    // Estimate tokens from prompt
    const prompt = promptData.prompt || '';
    const estimatedPromptTokens = this._estimateTokens(prompt);
    
    this.estimatedTokens += estimatedPromptTokens;
    
    // Add context size if provided
    if (promptData.context_size) {
      this.estimatedTokens += promptData.context_size;
    }
  }
  
  /**
   * Update context estimates from tool usage
   */
  updateFromToolUse(toolData) {
    this.toolExecutions++;
    
    // Estimate tokens from tool parameters and output
    const parameters = JSON.stringify(toolData.parameters || {});
    const output = toolData.output || '';
    const error = toolData.error || '';
    
    const toolTokens = this._estimateTokens(parameters + output + error);
    this.estimatedTokens += toolTokens;
    
    // File operations add more context
    if (toolData.tool === 'Read' || toolData.tool === 'Edit') {
      this.estimatedTokens += 2000; // Typical file size estimate
    } else if (toolData.tool === 'Bash') {
      this.estimatedTokens += 500; // Command output estimate
    }
  }
  
  /**
   * Check if backup should be triggered
   */
  checkBackupTriggers(hookType, data) {
    const decisions = [];
    
    // Context threshold trigger
    const contextRatio = this.getContextUsageRatio();
    if (contextRatio > this.backupThreshold) {
      decisions.push({
        shouldBackup: true,
        reason: `Context usage ${(contextRatio * 100).toFixed(1)}%`,
        urgency: 'high'
      });
    }
    
    // Time-based trigger
    const sessionMinutes = (Date.now() - this.sessionStartTime) / (1000 * 60);
    const timeSinceBackup = (Date.now() - this.lastBackupTime) / (1000 * 60);
    
    if (timeSinceBackup > this.timeThresholdMinutes) {
      decisions.push({
        shouldBackup: true,
        reason: `${this.timeThresholdMinutes} minutes since last backup`,
        urgency: 'medium'
      });
    }
    
    // Tool-based trigger
    const toolsSinceBackup = this.toolExecutions - this.lastBackupToolCount;
    if (toolsSinceBackup >= this.toolThreshold) {
      decisions.push({
        shouldBackup: true,
        reason: `${this.toolThreshold} tools since last backup`,
        urgency: 'medium'
      });
    }
    
    // Return highest priority decision
    if (decisions.length > 0) {
      const urgencyOrder = { high: 3, medium: 2, low: 1 };
      decisions.sort((a, b) => urgencyOrder[b.urgency] - urgencyOrder[a.urgency]);
      return decisions[0];
    }
    
    return { shouldBackup: false };
  }
  
  /**
   * Get current context usage ratio (0.0 to 1.0)
   */
  getContextUsageRatio() {
    return Math.min(1.0, this.estimatedTokens / this.maxContextTokens);
  }
  
  /**
   * Mark that a backup was performed
   */
  markBackupPerformed() {
    this.lastBackupTime = Date.now();
    this.lastBackupToolCount = this.toolExecutions;
  }
  
  /**
   * Estimate tokens from text (rough approximation)
   */
  _estimateTokens(text) {
    if (!text) return 0;
    
    // Rough estimate: ~4 characters per token for English text
    // Add some buffer for formatting and special tokens
    return Math.ceil(text.length / 3.5);
  }
  
  /**
   * Get context usage statistics
   */
  getStats() {
    const sessionMinutes = (Date.now() - this.sessionStartTime) / (1000 * 60);
    
    return {
      estimatedTokens: this.estimatedTokens,
      contextUsageRatio: this.getContextUsageRatio(),
      promptCount: this.promptCount,
      toolExecutions: this.toolExecutions,
      sessionMinutes: Math.round(sessionMinutes),
      lastBackupMinutesAgo: Math.round((Date.now() - this.lastBackupTime) / (1000 * 60))
    };
  }
}

module.exports = { ContextMonitor };