package main

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// Client is an HTTP client for the PokeAPI with disk caching and concurrency limiting.
type Client struct {
	baseURL    string
	httpClient *http.Client
	cacheDir   string
	sem        chan struct{} // concurrency limiter
}

// NewClient creates a new PokeAPI client.
func NewClient(baseURL, cacheDir string, concurrency int) *Client {
	return &Client{
		baseURL: strings.TrimRight(baseURL, "/"),
		httpClient: &http.Client{
			Timeout: 2 * time.Minute,
		},
		cacheDir: cacheDir,
		sem:      make(chan struct{}, concurrency),
	}
}

// Get fetches the given endpoint, using disk cache when available.
func (c *Client) Get(ctx context.Context, endpoint string) ([]byte, error) {
	// Check cache first (no semaphore needed for disk reads)
	safeName := strings.NewReplacer("/", "_", "?", "_").Replace(endpoint) + ".json"
	cachePath := filepath.Join(c.cacheDir, safeName)

	if data, err := os.ReadFile(cachePath); err == nil {
		return data, nil
	}

	// Acquire semaphore for HTTP request
	select {
	case c.sem <- struct{}{}:
		defer func() { <-c.sem }()
	case <-ctx.Done():
		return nil, ctx.Err()
	}

	url := c.baseURL + "/" + endpoint

	var data []byte
	maxRetries := 3
	for attempt := range maxRetries {
		req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
		if err != nil {
			return nil, fmt.Errorf("creating request for %s: %w", endpoint, err)
		}

		resp, err := c.httpClient.Do(req)
		if err != nil {
			if attempt < maxRetries-1 {
				backoff := time.Duration(1<<uint(attempt)) * time.Second
				time.Sleep(backoff)
				continue
			}
			return nil, fmt.Errorf("fetching %s: %w", endpoint, err)
		}

		body, readErr := io.ReadAll(resp.Body)
		resp.Body.Close()

		if resp.StatusCode != http.StatusOK {
			if attempt < maxRetries-1 && resp.StatusCode >= 500 {
				backoff := time.Duration(1<<uint(attempt)) * time.Second
				time.Sleep(backoff)
				continue
			}
			return nil, fmt.Errorf("fetching %s: status %d", endpoint, resp.StatusCode)
		}

		if readErr != nil {
			return nil, fmt.Errorf("reading response for %s: %w", endpoint, readErr)
		}

		data = body
		break
	}

	// Write to cache
	if err := os.MkdirAll(c.cacheDir, 0o755); err != nil {
		return nil, fmt.Errorf("creating cache dir: %w", err)
	}
	if err := os.WriteFile(cachePath, data, 0o644); err != nil {
		return nil, fmt.Errorf("writing cache for %s: %w", endpoint, err)
	}

	return data, nil
}

// ClearCache removes the cache directory.
func (c *Client) ClearCache() error {
	return os.RemoveAll(c.cacheDir)
}