import type { LanguageModelV3Middleware, LanguageModelV3Prompt } from '@ai-sdk/provider';

/**
 * Creates middleware that adds Anthropic prompt caching directives.
 * Marks the last system message and last user message with ephemeral cache control,
 * reducing input token cost and latency on repeated calls.
 */
export function createAnthropicCachingMiddleware(): LanguageModelV3Middleware {
  return {
    specificationVersion: 'v3',
    transformParams: async ({ params }) => {
      const messages = [...params.prompt] as Array<Record<string, unknown>>;

      // Find the last system message and last user message
      let lastSystemIdx = -1;
      let lastUserIdx = -1;
      for (let i = 0; i < messages.length; i++) {
        if (messages[i].role === 'system') lastSystemIdx = i;
        if (messages[i].role === 'user') lastUserIdx = i;
      }

      const targets = [lastSystemIdx, lastUserIdx].filter(i => i >= 0);
      for (const idx of targets) {
        const msg = { ...messages[idx] };
        msg.providerOptions = {
          ...(msg.providerOptions as Record<string, unknown> || {}),
          anthropic: {
            ...((msg.providerOptions as Record<string, unknown>)?.anthropic as Record<string, unknown> || {}),
            cacheControl: { type: 'ephemeral' },
          },
        };
        messages[idx] = msg;
      }

      return { ...params, prompt: messages as unknown as LanguageModelV3Prompt };
    },
  };
}