// schedulerclient.go：gateway → scheduler 内部 HTTP 契约客户端。
//
// 内部契约（W2/W3 对齐面，详见 impl-receipt-w2 §4）：
//
//	POST {base}/enqueue  body=contract.JobEnvelopeExt  → 200 {ok,job_id,request_id,queued_position}
//	  失败：503 {error:{code:"unavailable"}} / 429 {error:{code:"rate_limited"}}
//	GET  {base}/result/{request_id} → 200 {request_id,status,position?,envelope?}
//
// 失败语义：连接失败/5xx 一律返回 *APIError（gateway 转 503+Retry-After，零落盘）。
package gateway

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
	"time"

	"onesvm.com/onesvm/browser-server/internal/contract"
)

// EnqueueTimeout enqueue 单次请求超时（brief：3s）。
const EnqueueTimeout = 3 * time.Second

// APIError scheduler 侧错误（HTTP 状态 + 错误码）。
type APIError struct {
	HTTPStatus int
	Code       string
	Message    string
}

// Error 实现 error。
func (e *APIError) Error() string {
	return fmt.Sprintf("scheduler: %d %s: %s", e.HTTPStatus, e.Code, e.Message)
}

// SchedulerClient scheduler HTTP 客户端（enqueue 3s 超时 + 1 次重试；result 轮询）。
type SchedulerClient struct {
	base string
	hc   *http.Client
}

// NewSchedulerClient 构造。base 形如 http://scheduler:8641。
func NewSchedulerClient(base string) *SchedulerClient {
	base = strings.TrimRight(base, "/")
	return &SchedulerClient{
		base: base,
		hc: &http.Client{
			Timeout: EnqueueTimeout,
			Transport: &http.Transport{
				MaxIdleConns:          8,
				IdleConnTimeout:       30 * time.Second,
				ResponseHeaderTimeout: EnqueueTimeout,
			},
		},
	}
}

// Enqueue 入队（3s 超时 + 1 次重试）。重试仅针对网络错误/503（幂等：request_id 唯一约束兜底）。
func (c *SchedulerClient) Enqueue(ctx context.Context, job *contract.JobEnvelopeExt) (*enqueueResult, error) {
	body, err := json.Marshal(job)
	if err != nil {
		return nil, fmt.Errorf("gateway: job 序列化: %w", err)
	}
	var lastErr error
	for attempt := 0; attempt < 2; attempt++ {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return nil, ctx.Err()
			case <-time.After(200 * time.Millisecond):
			}
		}
		res, err := c.postEnqueue(ctx, body)
		if err == nil {
			return res, nil
		}
		lastErr = err
		// 4xx（除 503/429 限流类）不重试：请求本身有问题
		if ae, ok := err.(*APIError); ok && ae.HTTPStatus != http.StatusServiceUnavailable &&
			ae.HTTPStatus != http.StatusTooManyRequests {
			return nil, err
		}
	}
	return nil, lastErr
}

// postEnqueue 单次 enqueue 调用。
func (c *SchedulerClient) postEnqueue(ctx context.Context, body []byte) (*enqueueResult, error) {
	cctx, cancel := context.WithTimeout(ctx, EnqueueTimeout)
	defer cancel()
	req, err := http.NewRequestWithContext(cctx, http.MethodPost, c.base+"/enqueue", bytes.NewReader(body))
	if err != nil {
		return nil, fmt.Errorf("gateway: enqueue 请求构造: %w", err)
	}
	req.Header.Set("Content-Type", "application/json")
	resp, err := c.hc.Do(req)
	if err != nil {
		return nil, &APIError{HTTPStatus: http.StatusServiceUnavailable,
			Code: contract.CodeUnavailable, Message: "scheduler 不可达"}
	}
	defer resp.Body.Close()
	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 64*1024))
	switch {
	case resp.StatusCode == http.StatusOK:
		var res enqueueResult
		if err := json.Unmarshal(raw, &res); err != nil {
			return nil, fmt.Errorf("gateway: enqueue 响应解析: %w", err)
		}
		return &res, nil
	case resp.StatusCode == http.StatusServiceUnavailable || resp.StatusCode == http.StatusTooManyRequests:
		ae := &APIError{HTTPStatus: resp.StatusCode, Code: contract.CodeUnavailable,
			Message: "scheduler 队列满"}
		var eb struct {
			Error *contract.ErrBody `json:"error"`
		}
		if json.Unmarshal(raw, &eb) == nil && eb.Error != nil {
			ae.Code = eb.Error.Code
			ae.Message = eb.Error.Message
		}
		return nil, ae
	default:
		return nil, &APIError{HTTPStatus: resp.StatusCode, Code: contract.CodeUpstream,
			Message: fmt.Sprintf("enqueue 异常状态 %d", resp.StatusCode)}
	}
}

// Result 查询任务状态（轮询路径；短超时防轮询拖垮）。
func (c *SchedulerClient) Result(ctx context.Context, requestID string) (*resultState, error) {
	cctx, cancel := context.WithTimeout(ctx, 3*time.Second)
	defer cancel()
	req, err := http.NewRequestWithContext(cctx, http.MethodGet,
		c.base+"/result/"+requestID, nil)
	if err != nil {
		return nil, fmt.Errorf("gateway: result 请求构造: %w", err)
	}
	resp, err := c.hc.Do(req)
	if err != nil {
		// 轮询途中的瞬时不可达：让上层重试下一跳，不立即判死
		return nil, &APIError{HTTPStatus: http.StatusServiceUnavailable,
			Code: contract.CodeUnavailable, Message: "scheduler 不可达"}
	}
	defer resp.Body.Close()
	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 16*1024*1024))
	// 202 = scheduler 侧 queued/running 非终态（ITER-1 F1 统一形状，golden：
	// contract.ResultShapeAccepted）：解析 status 返回，由 waitResult 继续轮询。
	if resp.StatusCode == http.StatusAccepted {
		var st resultState
		if err := json.Unmarshal(raw, &st); err != nil {
			return nil, fmt.Errorf("gateway: result 202 响应解析: %w", err)
		}
		return &st, nil
	}
	if resp.StatusCode != http.StatusOK {
		return nil, &APIError{HTTPStatus: resp.StatusCode, Code: contract.CodeUpstream,
			Message: fmt.Sprintf("result 异常状态 %d", resp.StatusCode)}
	}
	var st resultState
	if err := json.Unmarshal(raw, &st); err != nil {
		return nil, fmt.Errorf("gateway: result 响应解析: %w", err)
	}
	return &st, nil
}

// Pressure 查询背压（/readyz 用）。
func (c *SchedulerClient) Pressure(ctx context.Context) error {
	cctx, cancel := context.WithTimeout(ctx, 2*time.Second)
	defer cancel()
	req, err := http.NewRequestWithContext(cctx, http.MethodGet, c.base+"/pressure", nil)
	if err != nil {
		return err
	}
	resp, err := c.hc.Do(req)
	if err != nil {
		return fmt.Errorf("scheduler /pressure 不可达: %w", err)
	}
	defer resp.Body.Close()
	_, _ = io.Copy(io.Discard, io.LimitReader(resp.Body, 4*1024))
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("scheduler /pressure 状态 %d", resp.StatusCode)
	}
	return nil
}