const STOPWORDS = new Set([
  "about", "after", "again", "also", "another", "answer", "because", "before",
  "between", "could", "detail", "details", "discuss", "explain", "from", "have",
  "how", "into", "more", "other", "please", "question", "should", "show", "some",
  "that", "their", "there", "these", "this", "those", "what", "when", "where",
  "which", "with", "would", "your",
]);

const NOISE_WORDS = new Set([
  "code", "function", "class", "file", "data", "system", "user", "app",
  "web", "api", "server", "client", "page", "view", "model", "config",
  "type", "value", "key", "name", "result", "error", "message", "state",
]);

export interface MemoryEntry {
  topic: string;
  summary: string;
  timestamp: number;
}

const MAX_TOPIC_CACHE = 256;
const MAX_ENTRIES_PER_TOPIC = 20;
const MAX_RETRIEVE_ENTRIES = 5;
const TOPIC_CACHE_MAX_AGE_MS = 3600_000; // 1 hour

export class MemoryService {
  private static memory = new Map<string, MemoryEntry[]>();
  private static topicCache = new Map<string, { topic: string; ts: number }>();
  private static cacheOps = 0;

  public static save(topic: string, summary: string): void {
    const cleanedTopic = this.normalizeTopic(topic);
    if (!cleanedTopic) return;

    let entries = this.memory.get(cleanedTopic);
    if (!entries) {
      entries = [];
      this.memory.set(cleanedTopic, entries);
    }
    entries.push({ topic: cleanedTopic, summary, timestamp: Date.now() });
    if (entries.length > MAX_ENTRIES_PER_TOPIC) {
      this.memory.set(cleanedTopic, entries.slice(-MAX_ENTRIES_PER_TOPIC));
    }
    this.pruneTopicCacheIfNeeded();
  }

  public static retrieve(topic: string): string {
    const cleanedTopic = this.normalizeTopic(topic);
    const entries = this.memory.get(cleanedTopic);
    if (!entries?.length) return "";
    return entries.slice(-MAX_RETRIEVE_ENTRIES).map((e) => `- ${e.summary}`).join("\n");
  }

  /**
   * Extract a topic from a prompt for memory lookup.
   *
   * OPTIMIZATION: Uses a hash of the prompt as the cache key instead of the
   * raw prompt string. Raw prompts can be long and contain unicode, causing
   * excessive memory pressure in the topicCache Map. Hashing reduces key size
   * from O(n) to O(1) (32-char hex string) while preserving cache hit rate.
   */
  public static extractTopic(prompt: string): string {
    const promptHash = stableHash(prompt);
    const cached = this.topicCache.get(promptHash);
    if (cached !== undefined && Date.now() - cached.ts < TOPIC_CACHE_MAX_AGE_MS) {
      return cached.topic;
    }

    const words = prompt
      .split(/[^a-zA-Z0-9_]+/)
      .map((w) => w.toLowerCase())
      .filter(
        (w) => w.length >= 4 && !STOPWORDS.has(w) && !NOISE_WORDS.has(w),
      );

    const topic = words.length >= 2 ? words.slice(0, 4).join(" ") : "";

    if (topic) {
      this.topicCache.set(promptHash, { topic, ts: Date.now() });
      this.cacheOps++;
      this.pruneTopicCacheIfNeeded();
    }

    return topic;
  }

  private static normalizeTopic(topic: string): string {
    return topic.toLowerCase().replace(/\s+/g, " ").trim().slice(0, 80);
  }

  private static pruneTopicCacheIfNeeded(): void {
    if (this.topicCache.size <= MAX_TOPIC_CACHE) return;

    const now = Date.now();
    for (const [key, entry] of this.topicCache) {
      if (now - entry.ts > TOPIC_CACHE_MAX_AGE_MS) {
        this.topicCache.delete(key);
      }
    }
    if (this.topicCache.size <= MAX_TOPIC_CACHE) return;

    const toRemove = this.topicCache.size - MAX_TOPIC_CACHE;
    let removed = 0;
    for (const key of this.topicCache.keys()) {
      if (++removed >= toRemove) break;
      this.topicCache.delete(key);
    }
  }
}

function stableHash(input: string): string {
  let h = 2166136261 >>> 0;
  for (let i = 0; i < input.length; i++) {
    const code = input.charCodeAt(i);
    if (code > 0xFFFF) {
      h = Math.imul(h ^ code, 16777619) >>> 0;
      i++;
      continue;
    }
    h = Math.imul(h ^ code, 16777619) >>> 0;
  }
  return h.toString(16);
}

const STOPWORDS = new Set([
  "about", "after", "again", "also", "another", "answer", "because", "before",
  "between", "could", "detail", "details", "discuss", "explain", "from", "have",
  "how", "into", "more", "other", "please", "question", "should", "show", "some",
  "that", "their", "there", "these", "this", "those", "what", "when", "where",
  "which", "with", "would", "your",
]);

const NOISE_WORDS = new Set([
  "code", "function", "class", "file", "data", "system", "user", "app",
  "web", "api", "server", "client", "page", "view", "model", "config",
  "type", "value", "key", "name", "result", "error", "message", "state",
]);

export interface MemoryEntry {
  topic: string;
  summary: string;
  timestamp: number;
}

const MAX_TOPIC_CACHE = 256;
const MAX_ENTRIES_PER_TOPIC = 20;
const MAX_RETRIEVE_ENTRIES = 5;
const TOPIC_CACHE_MAX_AGE_MS = 3600_000; // 1 hour

export class MemoryService {
  private static memory = new Map<string, MemoryEntry[]>();
  private static topicCache = new Map<string, { topic: string; ts: number }>();
  private static cacheOps = 0;

  public static save(topic: string, summary: string): void {
    const cleanedTopic = this.normalizeTopic(topic);
    if (!cleanedTopic) return;

    let entries = this.memory.get(cleanedTopic);
    if (!entries) {
      entries = [];
      this.memory.set(cleanedTopic, entries);
    }
    entries.push({ topic: cleanedTopic, summary, timestamp: Date.now() });
    if (entries.length > MAX_ENTRIES_PER_TOPIC) {
      this.memory.set(cleanedTopic, entries.slice(-MAX_ENTRIES_PER_TOPIC));
    }
    this.pruneTopicCacheIfNeeded();
  }

  public static retrieve(topic: string): string {
    const cleanedTopic = this.normalizeTopic(topic);
    const entries = this.memory.get(cleanedTopic);
    if (!entries?.length) return "";
    return entries.slice(-MAX_RETRIEVE_ENTRIES).map((e) => `- ${e.summary}`).join("\n");
  }

  /**
   * Extract a topic from a prompt for memory lookup.
   *
   * OPTIMIZATION: Uses a hash of the prompt as the cache key instead of the
   * raw prompt string. Raw prompts can be long and contain unicode, causing
   * excessive memory pressure in the topicCache Map. Hashing reduces key size
   * from O(n) to O(1) (32-char hex string) while preserving cache hit rate.
   */
  public static extractTopic(prompt: string): string {
    const promptHash = stableHash(prompt);
    const cached = this.topicCache.get(promptHash);
    if (cached !== undefined && Date.now() - cached.ts < TOPIC_CACHE_MAX_AGE_MS) {
      return cached.topic;
    }

    const words = prompt
      .split(/[^a-zA-Z0-9_]+/)
      .map((w) => w.toLowerCase())
      .filter(
        (w) => w.length >= 4 && !STOPWORDS.has(w) && !NOISE_WORDS.has(w),
      );

    const topic = words.length >= 2 ? words.slice(0, 4).join(" ") : "";

    if (topic) {
      this.topicCache.set(promptHash, { topic, ts: Date.now() });
      this.cacheOps++;
      this.pruneTopicCacheIfNeeded();
    }

    return topic;
  }

  private static normalizeTopic(topic: string): string {
    return topic.toLowerCase().replace(/\s+/g, " ").trim().slice(0, 80);
  }

  private static pruneTopicCacheIfNeeded(): void {
    if (this.topicCache.size <= MAX_TOPIC_CACHE) return;

    const now = Date.now();
    for (const [key, entry] of this.topicCache) {
      if (now - entry.ts > TOPIC_CACHE_MAX_AGE_MS) {
        this.topicCache.delete(key);
      }
    }
    if (this.topicCache.size <= MAX_TOPIC_CACHE) return;

    const toRemove = this.topicCache.size - MAX_TOPIC_CACHE;
    let removed = 0;
    for (const key of this.topicCache.keys()) {
      if (++removed >= toRemove) break;
      this.topicCache.delete(key);
    }
  }
}

function stableHash(input: string): string {
  let h = 2166136261 >>> 0;
  for (let i = 0; i < input.length; i++) {
    const code = input.charCodeAt(i);
    if (code > 0xFFFF) {
      h = Math.imul(h ^ code, 16777619) >>> 0;
      i++;
      continue;
    }
    h = Math.imul(h ^ code, 16777619) >>> 0;
  }
  return h.toString(16);
}