mcpctl/src/mcplocal/src/llm/pagination.ts

import { randomUUID } from 'node:crypto';
import type { ProviderRegistry } from '../providers/registry.js';
import { estimateTokens } from './token-counter.js';

// --- Configuration ---

export interface PaginationConfig {
  /** Character threshold above which responses get paginated (default 80_000) */
  sizeThreshold: number;
  /** Characters per page (default 40_000) */
  pageSize: number;
  /** Max cached results (LRU eviction) (default 64) */
  maxCachedResults: number;
  /** TTL for cached results in ms (default 300_000 = 5 min) */
  ttlMs: number;
  /** Max tokens for the LLM index generation call (default 2048) */
  indexMaxTokens: number;
}

export const DEFAULT_PAGINATION_CONFIG: PaginationConfig = {
  sizeThreshold: 80_000,
  pageSize: 40_000,
  maxCachedResults: 64,
  ttlMs: 300_000,
  indexMaxTokens: 2048,
};

// --- Cache Entry ---

interface PageInfo {
  /** 0-based page index */
  index: number;
  /** Start character offset in the raw string */
  startChar: number;
  /** End character offset (exclusive) */
  endChar: number;
  /** Approximate token count */
  estimatedTokens: number;
}

interface CachedResult {
  resultId: string;
  toolName: string;
  raw: string;
  pages: PageInfo[];
  index: PaginationIndex;
  createdAt: number;
}

// --- Index Types ---

export interface PageSummary {
  page: number;
  startChar: number;
  endChar: number;
  estimatedTokens: number;
  summary: string;
}

export interface PaginationIndex {
  resultId: string;
  toolName: string;
  totalSize: number;
  totalTokens: number;
  totalPages: number;
  pageSummaries: PageSummary[];
  indexType: 'smart' | 'simple';
}

// --- The MCP response format ---

export interface PaginatedToolResponse {
  content: Array<{
    type: 'text';
    text: string;
  }>;
}

// --- LLM Prompt ---

export const PAGINATION_INDEX_SYSTEM_PROMPT = `You are a document indexing assistant. Given a large tool response split into pages, generate a concise summary for each page describing what data it contains.

Rules:
- For each page, write 1-2 sentences describing the key content
- Be specific: mention entity names, IDs, counts, or key fields visible on that page
- If it's JSON, describe the structure and notable entries
- If it's text, describe the topics covered
- Output valid JSON only: an array of objects with "page" (1-based number) and "summary" (string)
- Example output: [{"page": 1, "summary": "Configuration nodes and global settings (inject, debug, function nodes 1-15)"}, {"page": 2, "summary": "HTTP request nodes and API integrations (nodes 16-40)"}]`;

/**
 * Handles transparent pagination of large MCP tool responses.
 *
 * When a tool response exceeds the size threshold, it is cached and an
 * index is returned instead. The LLM can then request specific pages
 * via _page/_resultId parameters on subsequent tool calls.
 *
 * If an LLM provider is available, the index includes AI-generated
 * per-page summaries. Otherwise, simple byte-range descriptions are used.
 */
export class ResponsePaginator {
  private cache = new Map<string, CachedResult>();
  private readonly config: PaginationConfig;

  constructor(
    private providers: ProviderRegistry | null,
    config: Partial<PaginationConfig> = {},
    private modelOverride?: string,
  ) {
    this.config = { ...DEFAULT_PAGINATION_CONFIG, ...config };
  }

  /**
   * Check if a raw response string should be paginated.
   */
  shouldPaginate(raw: string): boolean {
    return raw.length >= this.config.sizeThreshold;
  }

  /**
   * Paginate a large response: cache it and return the index.
   * Returns null if the response is below threshold.
   */
  async paginate(toolName: string, raw: string): Promise<PaginatedToolResponse | null> {
    if (!this.shouldPaginate(raw)) return null;

    const resultId = randomUUID();
    const pages = this.splitPages(raw);
    let index: PaginationIndex;

    try {
      index = await this.generateSmartIndex(resultId, toolName, raw, pages);
    } catch (err) {
      console.error(`[pagination] Smart index failed for ${toolName}, falling back to simple:`, err instanceof Error ? err.message : String(err));
      index = this.generateSimpleIndex(resultId, toolName, raw, pages);
    }

    // Store in cache
    this.evictExpired();
    this.evictLRU();
    this.cache.set(resultId, {
      resultId,
      toolName,
      raw,
      pages,
      index,
      createdAt: Date.now(),
    });

    return this.formatIndexResponse(index);
  }

  /**
   * Serve a specific page from cache.
   * Returns null if the resultId is not found (cache miss / expired).
   */
  getPage(resultId: string, page: number | 'all'): PaginatedToolResponse | null {
    this.evictExpired();
    const entry = this.cache.get(resultId);
    if (!entry) return null;

    if (page === 'all') {
      return {
        content: [{ type: 'text', text: entry.raw }],
      };
    }

    // Pages are 1-based in the API
    const pageInfo = entry.pages[page - 1];
    if (!pageInfo) {
      return {
        content: [{
          type: 'text',
          text: `Error: page ${String(page)} is out of range. This result has ${String(entry.pages.length)} pages (1-${String(entry.pages.length)}).`,
        }],
      };
    }

    const pageContent = entry.raw.slice(pageInfo.startChar, pageInfo.endChar);
    return {
      content: [{
        type: 'text',
        text: `[Page ${String(page)}/${String(entry.pages.length)} of result ${resultId}]\n\n${pageContent}`,
      }],
    };
  }

  /**
   * Check if a tool call has pagination parameters (_page / _resultId).
   * Returns the parsed pagination request, or null if not a pagination request.
   */
  static extractPaginationParams(
    args: Record<string, unknown>,
  ): { resultId: string; page: number | 'all' } | null {
    const resultId = args['_resultId'];
    const pageParam = args['_page'];
    if (typeof resultId !== 'string' || pageParam === undefined) return null;

    if (pageParam === 'all') return { resultId, page: 'all' };

    const page = Number(pageParam);
    if (!Number.isInteger(page) || page < 1) return null;

    return { resultId, page };
  }

  // --- Private methods ---

  private splitPages(raw: string): PageInfo[] {
    const pages: PageInfo[] = [];
    let offset = 0;
    let pageIndex = 0;

    while (offset < raw.length) {
      const end = Math.min(offset + this.config.pageSize, raw.length);
      // Try to break at a newline boundary if we're not at the end
      let breakAt = end;
      if (end < raw.length) {
        const lastNewline = raw.lastIndexOf('\n', end);
        if (lastNewline > offset) {
          breakAt = lastNewline + 1;
        }
      }

      pages.push({
        index: pageIndex,
        startChar: offset,
        endChar: breakAt,
        estimatedTokens: estimateTokens(raw.slice(offset, breakAt)),
      });

      offset = breakAt;
      pageIndex++;
    }

    return pages;
  }

  private async generateSmartIndex(
    resultId: string,
    toolName: string,
    raw: string,
    pages: PageInfo[],
  ): Promise<PaginationIndex> {
    const provider = this.providers?.getProvider('fast');
    if (!provider) {
      return this.generateSimpleIndex(resultId, toolName, raw, pages);
    }

    // Build a prompt with page previews (first ~500 chars of each page)
    const previews = pages.map((p, i) => {
      const preview = raw.slice(p.startChar, Math.min(p.startChar + 500, p.endChar));
      const truncated = p.endChar - p.startChar > 500 ? '\n[...]' : '';
      return `--- Page ${String(i + 1)} (chars ${String(p.startChar)}-${String(p.endChar)}, ~${String(p.estimatedTokens)} tokens) ---\n${preview}${truncated}`;
    }).join('\n\n');

    const result = await provider.complete({
      messages: [
        { role: 'system', content: PAGINATION_INDEX_SYSTEM_PROMPT },
        { role: 'user', content: `Tool: ${toolName}\nTotal size: ${String(raw.length)} chars, ${String(pages.length)} pages\n\n${previews}` },
      ],
      maxTokens: this.config.indexMaxTokens,
      temperature: 0,
      ...(this.modelOverride ? { model: this.modelOverride } : {}),
    });

    // LLMs often wrap JSON in ```json ... ``` fences — strip them
    const cleaned = result.content.replace(/^```(?:json)?\s*\n?/i, '').replace(/\n?```\s*$/i, '').trim();
    const summaries = JSON.parse(cleaned) as Array<{ page: number; summary: string }>;

    return {
      resultId,
      toolName,
      totalSize: raw.length,
      totalTokens: estimateTokens(raw),
      totalPages: pages.length,
      indexType: 'smart',
      pageSummaries: pages.map((p, i) => ({
        page: i + 1,
        startChar: p.startChar,
        endChar: p.endChar,
        estimatedTokens: p.estimatedTokens,
        summary: summaries.find((s) => s.page === i + 1)?.summary ?? `Page ${String(i + 1)}`,
      })),
    };
  }

  private generateSimpleIndex(
    resultId: string,
    toolName: string,
    raw: string,
    pages: PageInfo[],
  ): PaginationIndex {
    return {
      resultId,
      toolName,
      totalSize: raw.length,
      totalTokens: estimateTokens(raw),
      totalPages: pages.length,
      indexType: 'simple',
      pageSummaries: pages.map((p, i) => ({
        page: i + 1,
        startChar: p.startChar,
        endChar: p.endChar,
        estimatedTokens: p.estimatedTokens,
        summary: `Page ${String(i + 1)}: characters ${String(p.startChar)}-${String(p.endChar)} (~${String(p.estimatedTokens)} tokens)`,
      })),
    };
  }

  private formatIndexResponse(index: PaginationIndex): PaginatedToolResponse {
    const lines = [
      `This response is too large to return directly (${String(index.totalSize)} chars, ~${String(index.totalTokens)} tokens).`,
      `It has been split into ${String(index.totalPages)} pages.`,
      '',
      'To retrieve a specific page, call this same tool again with additional arguments:',
      `  "_resultId": "${index.resultId}"`,
      `  "_page": <page_number>       (1-${String(index.totalPages)})`,
      '  "_page": "all"               (returns the full response)',
      '',
      `--- Page Index${index.indexType === 'smart' ? ' (AI-generated summaries)' : ''} ---`,
    ];

    for (const page of index.pageSummaries) {
      lines.push(`  Page ${String(page.page)}: ${page.summary}`);
    }

    return {
      content: [{ type: 'text', text: lines.join('\n') }],
    };
  }

  private evictExpired(): void {
    const now = Date.now();
    for (const [id, entry] of this.cache) {
      if (now - entry.createdAt > this.config.ttlMs) {
        this.cache.delete(id);
      }
    }
  }

  private evictLRU(): void {
    while (this.cache.size >= this.config.maxCachedResults) {
      const oldest = this.cache.keys().next();
      if (oldest.done) break;
      this.cache.delete(oldest.value);
    }
  }

  /** Exposed for testing. */
  get cacheSize(): number {
    return this.cache.size;
  }

  /** Clear all cached results. */
  clearCache(): void {
    this.cache.clear();
  }
}