import { spawn } from "node:child_process";
import { z } from "zod";
import type { ResolvedMetadataGenerationModel } from "./metadata-generation-models.js";

const MAX_HELPER_OUTPUT_BYTES = 64 * 1024;
const helperOutputSchema = z.discriminatedUnion("ok", [
  z.object({ ok: z.literal(true), content: z.string(), usage: z.object({ input: z.number().int().nonnegative(), cacheRead: z.number().int().nonnegative(), output: z.number().int().nonnegative() }).strict().optional() }).strict(),
  z.object({ ok: z.literal(false), error: z.literal("provider_failure") }).strict(),
]);

export interface ModelCompletionMessage {
  role: "system" | "user";
  content: string;
}

export interface ModelCompletionRequest {
  model: ResolvedMetadataGenerationModel;
  messages: readonly ModelCompletionMessage[];
  signal: AbortSignal;
}
export interface ModelCompletionUsage { input: number; cacheRead: number; output: number; }
export interface ModelCompletionResult { content: string; usage: ModelCompletionUsage; }

/** The provider boundary used by Description Generation. */
export interface ModelCompleter {
  complete(request: ModelCompletionRequest): Promise<string | ModelCompletionResult>;
}

export class ModelCompletionProviderError extends Error {
  constructor() {
    super("model completion failed");
    this.name = "ModelCompletionProviderError";
  }
}

export class ModelCompletionCancelledError extends Error {
  constructor() {
    super("model completion cancelled");
    this.name = "ModelCompletionCancelledError";
  }
}

export class PythonModelCompleter implements ModelCompleter {
  constructor(private readonly options: {
    pythonExecutable: string;
    cwd: string;
    helperModule?: string;
    timeoutMs?: number;
    terminationGraceMs?: number;
  }) {}

  async complete(request: ModelCompletionRequest): Promise<ModelCompletionResult> {
    if (request.signal.aborted) throw new ModelCompletionCancelledError();
    const payload = {
      model: `${request.model.provider}/${request.model.model}`,
      ...(request.model.apiKey === undefined ? {} : { api_key: request.model.apiKey }),
      messages: request.messages.map((message) => ({ ...message })),
      ...(request.model.endpoint?.baseUrl === undefined
        ? {}
        : { api_base: request.model.endpoint.baseUrl }),
      ...(request.model.endpoint?.apiVersion === undefined
        ? {}
        : { api_version: request.model.endpoint.apiVersion }),
      ...(request.model.disableThinking === true ? { disable_thinking: true } : {}),
    };

    return await new Promise<ModelCompletionResult>((resolve, reject) => {
      const child = spawn(
        this.options.pythonExecutable,
        ["-m", this.options.helperModule ?? "tht.internal.litellm_completion"],
        {
          cwd: this.options.cwd,
          stdio: ["pipe", "pipe", "pipe"],
        },
      );
      let stdout = "";
      let settled = false;
      let timeout: ReturnType<typeof setTimeout> | undefined;
      let killFallback: ReturnType<typeof setTimeout> | undefined;
      let terminatingWith: Error | undefined;
      const cleanup = () => {
        if (timeout) clearTimeout(timeout);
        if (killFallback) clearTimeout(killFallback);
        request.signal.removeEventListener("abort", cancel);
      };
      const fail = (error: Error = new ModelCompletionProviderError()) => {
        if (settled) return;
        settled = true;
        cleanup();
        reject(error);
      };
      const terminate = (error: Error) => {
        if (settled || terminatingWith) return;
        terminatingWith = error;
        if (timeout) clearTimeout(timeout);
        try {
          child.kill("SIGTERM");
        } catch {
          fail(error);
          return;
        }
        killFallback = setTimeout(() => {
          if (settled || child.exitCode !== null || child.signalCode !== null) return;
          try {
            child.kill("SIGKILL");
          } catch {
            fail(error);
          }
        }, this.options.terminationGraceMs ?? 250);
      };
      const cancel = () => {
        terminate(new ModelCompletionCancelledError());
      };
      timeout = setTimeout(() => {
        terminate(new ModelCompletionProviderError());
      }, this.options.timeoutMs ?? 120_000);
      request.signal.addEventListener("abort", cancel, { once: true });
      if (request.signal.aborted) cancel();

      child.stdout.setEncoding("utf8");
      child.stdout.on("data", (chunk: string) => {
        if (terminatingWith) return;
        stdout += chunk;
        if (Buffer.byteLength(stdout, "utf8") > MAX_HELPER_OUTPUT_BYTES) {
          terminate(new ModelCompletionProviderError());
        }
      });
      // Helper and provider diagnostics are deliberately not copied into application logs.
      child.stderr.resume();
      child.once("error", () => fail(terminatingWith ?? new ModelCompletionProviderError()));
      child.once("close", (code) => {
        if (settled) return;
        if (terminatingWith) return fail(terminatingWith);
        try {
          if (code !== 0) return fail();
          const output = helperOutputSchema.parse(JSON.parse(stdout));
          if (!output.ok) return fail();
          settled = true;
          cleanup();
          resolve({ content: output.content, usage: output.usage ?? { input: 0, cacheRead: 0, output: 0 } });
        } catch {
          fail();
        }
      });
      child.stdin.once("error", () => {
        if (terminatingWith) return;
        terminate(new ModelCompletionProviderError());
      });
      child.stdin.end(JSON.stringify(payload));
    });
  }
}