package extsvc

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math/rand/v2"
	"net/http"
	"time"
)

// ErrBreakerOpen is returned without touching the network when the circuit
// is open.
var ErrBreakerOpen = errors.New("circuit breaker open")

// ServiceError is a non-2xx response from the external service, carrying
// its §5 error envelope when present.
type ServiceError struct {
	Status  int
	Code    string
	Message string
}

func (e *ServiceError) Error() string {
	return fmt.Sprintf("service responded %d: %s %s", e.Status, e.Code, e.Message)
}

// Client is a bearer-token JSON client with retry (×2 on 5xx/network with
// jitter, §12) and a circuit breaker, shared by the two service clients.
type Client struct {
	Name    string
	BaseURL func() string // resolved per call (admin override support)
	Token   string
	HTTP    *http.Client
	Breaker *Breaker
}

func NewClient(name string, baseURL func() string, token string, timeout time.Duration) *Client {
	return &Client{
		Name:    name,
		BaseURL: baseURL,
		Token:   token,
		HTTP:    &http.Client{Timeout: timeout},
		Breaker: NewBreaker(),
	}
}

func (c *Client) BreakerState() string { return c.Breaker.State() }

// Healthy probes /healthz without breaker accounting (used by readyz and the
// admin panel).
func (c *Client) Healthy(ctx context.Context) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.BaseURL()+"/healthz", nil)
	if err != nil {
		return err
	}
	resp, err := c.HTTP.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("%s healthz: status %d", c.Name, resp.StatusCode)
	}
	return nil
}

// Call POSTs/GETs JSON through the breaker with retries. A nil out skips
// decoding.
func (c *Client) Call(ctx context.Context, method, path string, in, out any) error {
	if !c.Breaker.Allow() {
		return fmt.Errorf("%s: %w", c.Name, ErrBreakerOpen)
	}
	err := c.callOnce(ctx, method, path, in, out)
	c.Breaker.Record(err)
	return err
}

func (c *Client) callOnce(ctx context.Context, method, path string, in, out any) error {
	var body []byte
	if in != nil {
		var err error
		if body, err = json.Marshal(in); err != nil {
			return fmt.Errorf("encode request: %w", err)
		}
	}

	var lastErr error
	for attempt := 0; attempt < 3; attempt++ {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return ctx.Err()
			case <-time.After(time.Duration(attempt)*time.Second + time.Duration(rand.IntN(500))*time.Millisecond):
			}
		}
		req, err := http.NewRequestWithContext(ctx, method, c.BaseURL()+path, bytes.NewReader(body))
		if err != nil {
			return err
		}
		req.Header.Set("Content-Type", "application/json")
		req.Header.Set("Authorization", "Bearer "+c.Token)
		resp, err := c.HTTP.Do(req)
		if err != nil {
			lastErr = fmt.Errorf("%s %s: %w", c.Name, path, err)
			continue // network error → retry
		}
		data, err := io.ReadAll(io.LimitReader(resp.Body, 10<<20))
		resp.Body.Close()
		if err != nil {
			lastErr = fmt.Errorf("%s %s: read body: %w", c.Name, path, err)
			continue
		}
		if resp.StatusCode >= 500 {
			lastErr = &ServiceError{Status: resp.StatusCode, Message: string(truncate(data, 200))}
			continue // 5xx → retry
		}
		if resp.StatusCode >= 400 {
			se := &ServiceError{Status: resp.StatusCode}
			var env struct {
				Error struct {
					Code    string `json:"code"`
					Message string `json:"message"`
				} `json:"error"`
			}
			if json.Unmarshal(data, &env) == nil {
				se.Code, se.Message = env.Error.Code, env.Error.Message
			}
			return se // 4xx is not retryable
		}
		if out == nil {
			return nil
		}
		if err := json.Unmarshal(data, out); err != nil {
			return fmt.Errorf("%s %s: decode response: %w", c.Name, path, err)
		}
		return nil
	}
	return lastErr
}

func truncate(b []byte, n int) []byte {
	if len(b) <= n {
		return b
	}
	return b[:n]
}