import type {
  AgentBrowserCdpEvent,
  AgentBrowserCdpEventPage,
} from '../../shared/agent-browser';

type GapReason = NonNullable<AgentBrowserCdpEventPage['gap']>['reason'];

interface BufferedEvent {
  event: AgentBrowserCdpEvent;
  byteLength: number;
}

interface GapMarker {
  sequence: number;
  reason: GapReason;
}

export interface AgentBrowserEventBufferOptions {
  maxEvents?: number;
  maxBytes?: number;
}

const DEFAULT_MAX_EVENTS = 10_000;
const DEFAULT_MAX_BYTES = 16 * 1024 * 1024;

export class AgentBrowserEventBuffer {
  private readonly events: BufferedEvent[] = [];
  private readonly gaps: GapMarker[] = [];
  private readonly maxEvents: number;
  private readonly maxBytes: number;
  private bytes = 0;
  private sequence = 0;
  private evictedThrough = 0;

  constructor(options: AgentBrowserEventBufferOptions = {}) {
    this.maxEvents = options.maxEvents ?? DEFAULT_MAX_EVENTS;
    this.maxBytes = options.maxBytes ?? DEFAULT_MAX_BYTES;
  }

  get cursor(): number {
    return this.sequence;
  }

  push(event: Omit<AgentBrowserCdpEvent, 'sequence'>): AgentBrowserCdpEvent {
    const complete = { ...event, sequence: ++this.sequence };
    const byteLength = Buffer.byteLength(JSON.stringify(complete));
    this.events.push({ event: complete, byteLength });
    this.bytes += byteLength;
    this.evict();
    return complete;
  }

  markGap(reason: GapReason): number {
    const sequence = ++this.sequence;
    this.gaps.push({ sequence, reason });
    this.pruneGaps();
    return sequence;
  }

  read(after = 0, methods?: string[], limit = 100): AgentBrowserCdpEventPage {
    const safeAfter = Math.max(0, Math.trunc(after));
    const safeLimit = Math.min(Math.max(Math.trunc(limit), 1), 200);
    const methodSet = methods?.length ? new Set(methods) : null;
    const start = Math.max(safeAfter, this.evictedThrough);
    const selected: AgentBrowserCdpEvent[] = [];
    let scannedThrough = start;

    for (const buffered of this.events) {
      if (buffered.event.sequence <= start) continue;
      scannedThrough = buffered.event.sequence;
      if (!methodSet || methodSet.has(buffered.event.method)) {
        selected.push(buffered.event);
        if (selected.length >= safeLimit) break;
      }
    }

    if (selected.length < safeLimit) {
      scannedThrough = Math.max(scannedThrough, this.sequence);
    }

    const nextCursor = Math.max(safeAfter, scannedThrough);
    const hasMore = this.events.some(({ event }) =>
      event.sequence > nextCursor && (!methodSet || methodSet.has(event.method)));
    const oldestAvailable = this.events[0]?.event.sequence ?? this.sequence + 1;
    const gap = safeAfter < this.evictedThrough
      ? { reason: 'evicted' as const, oldestAvailable }
      : this.gaps.find((marker) =>
          marker.sequence > safeAfter && marker.sequence <= nextCursor);

    return {
      events: selected,
      nextCursor,
      hasMore,
      ...(gap
        ? {
            gap: {
              reason: gap.reason,
              oldestAvailable,
            },
          }
        : {}),
    };
  }

  clear(): void {
    this.events.length = 0;
    this.gaps.length = 0;
    this.bytes = 0;
    this.sequence = 0;
    this.evictedThrough = 0;
  }

  private evict(): void {
    while (this.events.length > this.maxEvents || this.bytes > this.maxBytes) {
      const removed = this.events.shift();
      if (!removed) break;
      this.bytes -= removed.byteLength;
      this.evictedThrough = Math.max(this.evictedThrough, removed.event.sequence);
    }
    this.pruneGaps();
  }

  private pruneGaps(): void {
    while (this.gaps[0] && this.gaps[0].sequence <= this.evictedThrough) {
      this.gaps.shift();
    }
  }
}