// Package github provides a client for the GitHub API.
// It supports pull request operations, file content retrieval, CI status checks,
// and directory listing for both github.com and GitHub Enterprise.
package github

import (
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"
)

const (
	defaultBaseURL = "https://api.github.com"
	userAgent      = "review-bot/1.0"

	// maxResponseBytes limits successful response body reads to 10 MiB.
	maxResponseBytes = 10 * 1024 * 1024
)

// APIError represents an HTTP error response from the GitHub API.
// It carries the status code so callers can distinguish between
// different failure modes (e.g. 404 vs 500).
//
// The Body field stores up to 4 KiB of the raw response for programmatic
// inspection. Error() truncates to 200 bytes for safe logging, but callers
// should avoid logging or propagating Body directly in production since it may
// contain sensitive details from the upstream server.
type APIError struct {
	StatusCode int
	Body       string
}

func (e *APIError) Error() string {
	body := e.Body
	if len(body) > 200 {
		body = body[:200] + "...(truncated)"
	}
	return fmt.Sprintf("HTTP %d: %s", e.StatusCode, body)
}

// IsNotFound reports whether an error is an API 404 response.
func IsNotFound(err error) bool {
	if apiErr, ok := asAPIError(err); ok {
		return apiErr.StatusCode == http.StatusNotFound
	}
	return false
}

// IsUnauthorized reports whether an error is an API 401 response.
func IsUnauthorized(err error) bool {
	if apiErr, ok := asAPIError(err); ok {
		return apiErr.StatusCode == http.StatusUnauthorized
	}
	return false
}

func asAPIError(err error) (*APIError, bool) {
	if err == nil {
		return nil, false
	}
	var target *APIError
	if errors.As(err, &target) {
		return target, true
	}
	return nil, false
}

// clientConfig holds optional configuration for NewClient.
type clientConfig struct {
	allowInsecureHTTP bool
}

// ClientOption configures optional behavior of NewClient.
type ClientOption func(*clientConfig)

// AllowInsecureHTTP permits the client to use HTTP (non-TLS) base URLs.
// This should only be used for trusted internal deployments or testing.
func AllowInsecureHTTP() ClientOption {
	return func(c *clientConfig) {
		c.allowInsecureHTTP = true
	}
}

// Client interacts with the GitHub API.
// A Client is safe for concurrent use by multiple goroutines.
// SetHTTPClient and SetRetryBackoff are intended for test setup only and must
// be called before any goroutines issue requests; they have no synchronization.
type Client struct {
	baseURL           string
	token             string
	allowInsecureHTTP bool
	httpClient        *http.Client

	// retryBackoff defines the delays between retry attempts for 429 responses.
	// retryBackoff[i] is the delay before attempt i+1 (after attempt i fails).
	// If nil, defaults to {1s, 2s}. Set to shorter durations in tests via SetRetryBackoff.
	retryBackoff []time.Duration
}

// defaultCheckRedirect is the redirect policy used by NewClient and SetHTTPClient(nil).
// It strips the Authorization header on cross-host redirects or protocol downgrades
// (HTTPS→HTTP) to prevent credential leakage, while still following the redirect.
func defaultCheckRedirect(req *http.Request, via []*http.Request) error {
	if len(via) >= 10 {
		return fmt.Errorf("stopped after 10 redirects")
	}
	// Strip Authorization on cross-host redirect or protocol downgrade (https→http).
	prev := via[len(via)-1]
	if req.URL.Host != prev.URL.Host || (prev.URL.Scheme == "https" && req.URL.Scheme == "http") {
		req.Header.Del("Authorization")
	}
	return nil
}

// NewClient creates a new GitHub API client.
// If baseURL is empty, it defaults to https://api.github.com.
// For GitHub Enterprise, pass the API base URL (e.g. https://github.concur.com/api/v3).
// The baseURL must use HTTPS; pass AllowInsecureHTTP() as an option to permit HTTP
// for trusted internal deployments (e.g. local testing).
func NewClient(token, baseURL string, opts ...ClientOption) *Client {
	if baseURL == "" {
		baseURL = defaultBaseURL
	}
	cfg := clientConfig{}
	for _, o := range opts {
		o(&cfg)
	}
	return &Client{
		baseURL:           strings.TrimRight(baseURL, "/"),
		allowInsecureHTTP: cfg.allowInsecureHTTP,
		token:             token,
		httpClient: &http.Client{
			Timeout:       30 * time.Second,
			CheckRedirect: defaultCheckRedirect,
		},
	}
}

// SetHTTPClient sets the underlying HTTP client used for requests.
// This is intended for test setup only to inject mock transports; it must be
// called before any goroutines issue requests.
//
// Passing nil restores the default client (30s timeout + auth-stripping
// CheckRedirect policy matching NewClient).
//
// Callers providing a non-nil client are responsible for configuring a safe
// CheckRedirect policy. Without one, the default net/http behavior will follow
// redirects and may forward the Authorization header to untrusted hosts.
func (c *Client) SetHTTPClient(hc *http.Client) {
	if hc == nil {
		hc = &http.Client{
			Timeout:       30 * time.Second,
			CheckRedirect: defaultCheckRedirect,
		}
	}
	c.httpClient = hc
}

// SetRetryBackoff configures the retry backoff durations for testing.
// It must be called before any goroutines issue requests.
// In production the default {1s, 2s} applies.
func (c *Client) SetRetryBackoff(d []time.Duration) {
	c.retryBackoff = d
}

// doRequest performs an HTTP request with retry on 429 rate limit responses.
// It respects the Retry-After header when present (capped at maxRetryAfter).
// Transport errors (network failures, context cancellation) are not retried.
func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept string) ([]byte, error) {
	const maxAttempts = 3
	const maxRetryAfter = 120 * time.Second

	var backoff []time.Duration
	if c.retryBackoff != nil {
		backoff = make([]time.Duration, len(c.retryBackoff))
		copy(backoff, c.retryBackoff)
	} else {
		backoff = []time.Duration{1 * time.Second, 2 * time.Second}
	}

	// maxErrorBodyBytes limits how much of an error response body is stored.
	// Kept small (4 KiB) to reduce the risk of sensitive data leakage if callers
	// log APIError.Body directly. Error() further truncates to 200 bytes.
	const maxErrorBodyBytes = 4 * 1024

	// Reject non-HTTPS URLs early since the URL is immutable across retries.
	if c.token != "" && !c.allowInsecureHTTP {
		parsed, err := url.Parse(reqURL)
		if err != nil {
			return nil, fmt.Errorf("parse request URL: %w", err)
		}
		if !strings.EqualFold(parsed.Scheme, "https") {
			return nil, fmt.Errorf("refusing to send credentials over non-HTTPS URL %q (use AllowInsecureHTTP option for trusted networks)", reqURL)
		}
	}

	var lastErr error
	for attempt := 0; attempt < maxAttempts; attempt++ {
		if attempt > 0 {
			var delay time.Duration
			if attempt-1 < len(backoff) {
				delay = backoff[attempt-1]
			}
			if delay > 0 {
				timer := time.NewTimer(delay)
				select {
				case <-timer.C:
					timer.Stop() // no-op after fire, releases runtime resources promptly
				case <-ctx.Done():
					timer.Stop()
					return nil, ctx.Err()
				}
			}
		}

		req, err := http.NewRequestWithContext(ctx, method, reqURL, nil)
		if err != nil {
			return nil, fmt.Errorf("create request: %w", err)
		}
		if c.token != "" {
			// Bearer is the OAuth2 standard and is accepted by GitHub for both
			// classic PATs and fine-grained tokens. The alternative "token" scheme
			// is GitHub-specific and offers no additional compatibility.
			req.Header.Set("Authorization", "Bearer "+c.token)
		}
		req.Header.Set("User-Agent", userAgent)
		if accept != "" {
			req.Header.Set("Accept", accept)
		} else {
			req.Header.Set("Accept", "application/vnd.github+json")
		}

		resp, err := c.httpClient.Do(req)
		if err != nil {
			return nil, fmt.Errorf("do request: %w", err)
		}

		if resp.StatusCode >= 200 && resp.StatusCode < 300 {
			body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBytes))
			resp.Body.Close()
			if err != nil {
				return nil, fmt.Errorf("read response body: %w", err)
			}
			if len(body) >= maxResponseBytes {
				return nil, fmt.Errorf("response body exceeded %d bytes (truncated)", maxResponseBytes)
			}
			return body, nil
		}

		errBody, _ := io.ReadAll(io.LimitReader(resp.Body, maxErrorBodyBytes))
		resp.Body.Close()

		lastErr = &APIError{StatusCode: resp.StatusCode, Body: string(errBody)}

		// Retry on 429 rate limit
		if resp.StatusCode == http.StatusTooManyRequests && attempt < maxAttempts-1 {
			// Check for Retry-After header and override backoff if present.
			// Supports both integer seconds (common) and HTTP-date format (RFC 7231).
			if ra := resp.Header.Get("Retry-After"); ra != "" {
				if seconds, err := strconv.Atoi(ra); err == nil && seconds > 0 {
					delay := time.Duration(seconds) * time.Second
					if delay > maxRetryAfter {
						delay = maxRetryAfter
					}
					if attempt < len(backoff) {
						backoff[attempt] = delay
					}
				} else if retryAt, err := http.ParseTime(ra); err == nil {
					delay := time.Until(retryAt)
					if delay < 0 {
						delay = 0
					}
					if delay > maxRetryAfter {
						delay = maxRetryAfter
					}
					if attempt < len(backoff) {
						backoff[attempt] = delay
					}
				}
			}
			continue
		}

		// Don't retry other errors
		return nil, lastErr
	}

	return nil, lastErr
}

// doGet is a convenience wrapper for GET requests with the default Accept header.
func (c *Client) doGet(ctx context.Context, reqURL string) ([]byte, error) {
	return c.doRequest(ctx, http.MethodGet, reqURL, "")
}