/**
 * Upstream request builder — constructs the forwarded HTTP request.
 *
 * **`format` semantics**: This is the *upstream* format — the format used to
 * talk to the upstream LLM provider, not the client's inbound format. Both
 * ZCode coding-plan and start-plan routes post an Anthropic-format upstream
 * (since PR #34, 2026-08-29 — start-plan's old OpenAI gateway is retired);
 * OpenAI clients are translated before this builder is called.
 *
 * @see _reverse/NOTEPAD.md "How Credential is Used for LLM Calls"
 */
import type { Format } from "../translator/types.js";
import type { ProviderDef } from "../provider/types.js";
import type { Credential } from "../auth/types.js";
import type { ProxyIdentity } from "../config/types.js";
import { credentialString } from "../auth/types.js";
import { buildLlmIdentityHeaders } from "./identity.js";
import { buildZcodeTraceHeaders } from "./trace-headers.js";
import { sessionIdForHeader, shouldUseExactTraceHeaders } from "./session-context.js";

export interface UpstreamClientSession {
  source?: "none" | "explicit" | "lineage";
  action: "off" | "observe" | "enforce";
  sessionId?: string;
  upstreamSessionId?: string;
  /** Explicit x-zcode-session-type override (wins over sessionId-prefix derivation). */
  sessionType?: "main" | "subagent" | "other";
  requestId?: string;
  traceId?: string;
  queryId?: string;
}

export type UpstreamHeaderPair = [string, string];

const ANTHROPIC_VERSION = "2023-06-01";

/**
 * SDK UA suffix the real client appends to its User-Agent on LLM calls
 * (bundle `Cm` merges `ai-sdk/anthropic/${k0o}` into the UA; k0o = "3.0.81").
 * Only the LLM request path carries it — control-plane fetches (endpoint
 * routing, signing gate, claim/billing) keep the bare `ZCode/…` UA.
 */
const ANTHROPIC_SDK_UA = "ai-sdk/anthropic/3.0.81";

const STARTPLAN_ANTHROPIC_BASE = "https://zcode.z.ai/api/v1/zcode-plan";

/**
 * Build the upstream URL based on format + plan + provider.
 *
 * The `format` parameter is the *upstream* format — callers in handler.ts
 * pass the format the upstream will receive, which may differ from the
 * client's inbound format when the proxy is in compatibility mode.
 */
export function buildUpstreamURL(format: Format, provider: ProviderDef, plan: "coding-plan" | "start-plan" = "coding-plan"): string {
  if (plan === "start-plan") {
    // Anthropic-messages gateway used by the live ZCode desktop client for
    // start-plan (incl. claimed trial plans like the weekend package).
    // The legacy OpenAI route `${STARTPLAN_ANTHROPIC_BASE}/chat/completions`
    // returns "404 page not found" since the server-side rollout of this
    // endpoint (observed 2026-08-28).
    return `${STARTPLAN_ANTHROPIC_BASE}/anthropic/v1/messages`;
  }
  if (format === "anthropic") {
    return `${provider.anthropicBaseURL}/v1/messages`;
  }
  return `${provider.openaiBaseURL}/chat/completions`;
}

/**
 * Build auth + identity + trace headers for the upstream LLM request.
 *
 * The `format` parameter is the *upstream* format — selects auth scheme:
 * - Anthropic upstream, coding-plan → `x-api-key: {cred}` AND
 *   `Authorization: Bearer {cred}` (dual headers, bundle `ebo`) +
 *   `anthropic-version`
 * - Anthropic upstream, start-plan  → `Authorization: Bearer {jwt}` + `anthropic-version`
 * - OpenAI upstream (any plan)      → `Authorization: Bearer {cred|jwt}`
 *
 * Identity headers come from `buildLlmIdentityHeaders` (bundle 3.12.3 `g6n`
 * shape); the User-Agent carries the `ai-sdk/anthropic` SDK suffix (bundle
 * `Cm`/`k0o`).
 *
 * Trace/attribution headers mirror the bundle's `Bdt`
 * ("createModelRequestAttributionHeaders") when an explicit/enforced trace
 * context exists. Default enforce mode forwards the inferred conversation
 * session id; sessions without a resolvable id fall back to a synthesized
 * one (coding-plan must always carry x-session-id — signing requires it).
 */
export function buildAuthHeaders(
  format: Format,
  cred: Credential,
  identity: ProxyIdentity,
  plan: "coding-plan" | "start-plan" = "coding-plan",
  clientSession?: UpstreamClientSession,
): Record<string, string> {
  const credStr = plan === "start-plan" && cred.jwt ? cred.jwt : credentialString(cred);
  const base: Record<string, string> = {
    // LLM requests mirror the bundle's 3.12.3 source-headers builder (`g6n`,
    // no wrapper) — language/timezone always sent with "unknown" fallback,
    // X-ZCode-Agent inline 8th before the platform headers, no X-Device-Mid.
    // Context/control-plane fetches use buildIdentityHeaders (`TV`).
    ...buildLlmIdentityHeaders(identity),
    ...buildTraceHeaders(plan, clientSession),
  };
  // Bundle `Cm`/`k0o`: the anthropic SDK merges its identity into the UA —
  // real LLM requests arrive as `ZCode/{ver} ai-sdk/anthropic/3.0.81`.
  base["User-Agent"] = `${base["User-Agent"]} ${ANTHROPIC_SDK_UA}`;

  if (format === "anthropic") {
    if (plan === "start-plan" && cred.jwt) {
      base["authorization"] = `Bearer ${cred.jwt}`;
    } else {
      // Bundle `ebo` (coding-plan, anthropic): x-api-key from the SDK config
      // AND `Authorization: Bearer {credential}` from the custom-header merge
      // — both headers, same value.
      base["x-api-key"] = credStr;
      base["authorization"] = `Bearer ${credStr}`;
    }
    base["anthropic-version"] = ANTHROPIC_VERSION;
  } else {
    base["authorization"] = `Bearer ${credStr}`;
  }

  return base;
}

function buildTraceHeaders(plan: "coding-plan" | "start-plan", clientSession?: UpstreamClientSession): Record<string, string> {
  if (shouldUseExactTraceHeaders(plan, clientSession)) {
    // Degenerate bodies (lineage failed) and requestId-only requests resolve
    // to an exact context with NO session id — coding-plan would then ship
    // without x-session-id and signing would skip (client-signing requires
    // it), violating the "coding-plan 必签" invariant. Synthesize like the
    // fallback branch; start-plan is never signed and keeps the omission.
    const sessionId = sessionIdForHeader(clientSession)
      ?? (plan === "coding-plan" ? crypto.randomUUID() : undefined);
    return buildZcodeTraceHeaders({
      requestId: clientSession?.requestId,
      traceId: clientSession?.traceId,
      queryId: clientSession?.queryId,
      sessionId,
      ...(clientSession?.sessionType ? { sessionType: clientSession.sessionType } : {}),
    });
  }

  const headers: Record<string, string> = {
    "x-request-id": crypto.randomUUID(),
    // ZCode 3.9.1 attributes every model request; a forwarded conversation turn is the main-agent loop.
    "x-zcode-session-type": "main",
    "x-zcode-trace-id": crypto.randomUUID(),
  };
  if (plan !== "start-plan") {
    headers["x-query-id"] = crypto.randomUUID();
    headers["x-session-id"] = crypto.randomUUID();
  }
  return headers;
}

function collectPassthroughHeaders(req: Request): Record<string, string> {
  const result: Record<string, string> = {};
  for (const [key, value] of req.headers.entries()) {
    const lower = key.toLowerCase();
    // Explicit allowlist: client auth, routing auth, trace and V4 signing
    // headers are never forwarded. Their trusted values are rebuilt below.
    if (lower === "anthropic-beta") {
      result[lower] = value;
    }
  }
  return result;
}

export function buildUpstreamHeaderPairs(
  clientReq: Request,
  format: Format,
  cred: Credential,
  identity: ProxyIdentity,
  plan: "coding-plan" | "start-plan" = "coding-plan",
  extraHeaders?: Record<string, string>,
  clientSession?: UpstreamClientSession,
): UpstreamHeaderPair[] {
  // Forward the client's Accept-Encoding so the upstream compresses only when
  // the client can decode it. Hardcoding "gzip" broke clients (e.g. some Tauri
  // builds) that send `accept-encoding: identity` and cannot auto-decompress.
  const clientAcceptEncoding = clientReq.headers.get("accept-encoding") ?? "gzip";
  return [
    ["content-type", "application/json"],
    ["accept-encoding", clientAcceptEncoding],
    ...Object.entries(collectPassthroughHeaders(clientReq)),
    ...Object.entries(buildAuthHeaders(format, cred, identity, plan, clientSession)),
    ...Object.entries(extraHeaders ?? {}),
  ];
}

export function buildUpstreamRequest(
  clientReq: Request,
  format: Format,
  provider: ProviderDef,
  cred: Credential,
  body: string | undefined,
  identity: ProxyIdentity,
  plan: "coding-plan" | "start-plan" = "coding-plan",
  extraHeaders?: Record<string, string>,
  clientSession?: UpstreamClientSession,
): Request {
  const url = buildUpstreamURL(format, provider, plan);
  const headerPairs = buildUpstreamHeaderPairs(clientReq, format, cred, identity, plan, extraHeaders, clientSession);

  const init: RequestInit = {
    method: "POST",
    headers: Object.fromEntries(headerPairs),
  };

  if (body !== undefined) {
    init.body = body;
  }

  return new Request(url, init);
}