package httpkit

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

const (
	defaultTimeout = 30 * time.Second
	maxBackoff     = 30 * time.Second
)

// Config configures an httpkit Client.
type Config struct {
	// Timeout for each HTTP request attempt. Default: 30s.
	// Ignored if HTTPClient is provided.
	Timeout time.Duration

	// MaxRetries is the number of retry attempts after the initial request.
	// 0 means no retries (single attempt). Default: 0.
	MaxRetries int

	// Headers are applied to every request. Per-request headers in Request
	// override these on key collision.
	Headers map[string]string

	// Logger for retry and error events. Default: slog.Default().
	Logger *slog.Logger

	// HTTPClient allows injecting a pre-configured http.Client.
	// When set, Timeout is ignored. Use this to share connection pools
	// across multiple httpkit Clients.
	HTTPClient *http.Client
}

// Client is a configured HTTP client with retry and error classification.
type Client struct {
	http       *http.Client
	maxRetries int
	headers    map[string]string
	logger     *slog.Logger
}

// NewClient creates a new httpkit Client.
func NewClient(cfg Config) *Client {
	httpClient := cfg.HTTPClient
	if httpClient == nil {
		timeout := cfg.Timeout
		if timeout == 0 {
			timeout = defaultTimeout
		}
		httpClient = &http.Client{Timeout: timeout}
	}

	logger := cfg.Logger
	if logger == nil {
		logger = slog.Default()
	}

	return &Client{
		http:       httpClient,
		maxRetries: cfg.MaxRetries,
		headers:    cfg.Headers,
		logger:     logger,
	}
}

// Request describes an HTTP request to execute.
type Request struct {
	Method  string
	URL     string
	Body    any               // JSON-marshaled if non-nil
	Headers map[string]string // Per-request headers (override defaults)
}

// Do executes an HTTP request with automatic retry on retryable errors.
//
// On 2xx: returns (responseBody, nil).
// On non-2xx after retries: returns (nil, *APIError).
// On transport error after retries: returns (nil, error).
//
// The retry loop uses exponential backoff with full jitter and respects
// context cancellation between attempts.
func (c *Client) Do(ctx context.Context, req Request) ([]byte, error) {
	var lastErr error

	for attempt := 0; attempt <= c.maxRetries; attempt++ {
		if attempt > 0 {
			if !IsRetryable(lastErr) {
				return nil, lastErr
			}

			backoff := c.backoff(attempt)
			c.logger.Info("retrying request",
				"attempt", attempt,
				"max_retries", c.maxRetries,
				"url", req.URL,
				"backoff_ms", backoff.Milliseconds(),
			)

			select {
			case <-time.After(backoff):
			case <-ctx.Done():
				return nil, ctx.Err()
			}
		}

		body, err := c.doOnce(ctx, req)
		if err == nil {
			return body, nil
		}
		lastErr = err

		// Don't retry if parent context is done.
		if ctx.Err() != nil {
			return nil, lastErr
		}
	}

	if c.maxRetries > 0 {
		return nil, fmt.Errorf("failed after %d attempts: %w", c.maxRetries+1, lastErr)
	}
	return nil, lastErr
}

// doOnce executes a single HTTP request attempt.
func (c *Client) doOnce(ctx context.Context, req Request) ([]byte, error) {
	var bodyReader io.Reader
	if req.Body != nil {
		data, err := json.Marshal(req.Body)
		if err != nil {
			return nil, fmt.Errorf("marshal request body: %w", err)
		}
		bodyReader = bytes.NewReader(data)
	}

	httpReq, err := http.NewRequestWithContext(ctx, req.Method, req.URL, bodyReader)
	if err != nil {
		return nil, fmt.Errorf("create request: %w", err)
	}

	// Apply default headers first.
	for k, v := range c.headers {
		httpReq.Header.Set(k, v)
	}
	// Apply per-request headers (override defaults).
	for k, v := range req.Headers {
		httpReq.Header.Set(k, v)
	}
	// Auto-set Content-Type for JSON bodies.
	if req.Body != nil && httpReq.Header.Get("Content-Type") == "" {
		httpReq.Header.Set("Content-Type", "application/json")
	}

	resp, err := c.http.Do(httpReq)
	if err != nil {
		if errors.Is(err, context.DeadlineExceeded) || isTimeoutError(err) {
			return nil, fmt.Errorf("%w: %v", ErrTimeout, err)
		}
		return nil, err
	}

	body, err := io.ReadAll(resp.Body)
	resp.Body.Close()
	if err != nil {
		return nil, fmt.Errorf("read response body: %w", err)
	}

	if resp.StatusCode >= 200 && resp.StatusCode < 300 {
		return body, nil
	}

	return nil, &APIError{
		StatusCode: resp.StatusCode,
		Body:       string(body),
		sentinel:   ClassifyStatus(resp.StatusCode),
	}
}

// backoff returns the sleep duration for a retry attempt using
// exponential backoff with full jitter.
func (c *Client) backoff(attempt int) time.Duration {
	base := time.Second
	ceiling := min(base*time.Duration(1<<uint(attempt)), maxBackoff) // 2s, 4s, 8s, ...
	// Full jitter: uniform random in [0, ceiling).
	return time.Duration(rand.Int63n(int64(ceiling)))
}

// isTimeoutError checks for net.Error timeout, which http.Client uses
// for its Timeout field.
func isTimeoutError(err error) bool {
	type timeouter interface {
		Timeout() bool
	}
	var te timeouter
	if errors.As(err, &te) {
		return te.Timeout()
	}
	return false
}