// Package github provides a client for the GitHub API.
// It supports pull request operations, file content retrieval,
// and review submission for both github.com and GitHub Enterprise.
package github

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

const (
	defaultBaseURL = "https://api.github.com"

	// maxRetryAttempts is the number of times doRequest will attempt a request.
	maxRetryAttempts = 3

	// maxRetryAfter caps the maximum delay from a Retry-After header to prevent
	// a server from stalling the client indefinitely.
	maxRetryAfter = 60 * time.Second

	// maxErrorBodyBytes limits how much of an error response body we read
	// to protect against malicious servers sending unbounded data.
	maxErrorBodyBytes = 64 * 1024 // 64 KB

	// maxResponseBodyBytes limits how much of a successful response body we read
	// for defense-in-depth against servers returning excessively large payloads.
	maxResponseBodyBytes = 10 * 1024 * 1024 // 10 MB
)

// 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 64 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)"
	}
	// Sanitize newlines to prevent log injection from upstream response bodies.
	body = strings.ReplaceAll(body, "\n", " ")
	body = strings.ReplaceAll(body, "\r", " ")
	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
}

// 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 {
	// TODO: baseURL is populated by NewClient but not yet consumed by doRequest/doGet.
	// Higher-level exported methods (GetPullRequest, etc.) will use it to
	// construct request URLs; remove this field if those methods end up
	// accepting full URLs instead.
	baseURL string
	token   string
	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}.
	retryBackoff []time.Duration

	// now returns the current time. Defaults to time.Now.
	// Override in tests to control HTTP-date Retry-After calculations.
	now func() time.Time
}

// 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).
func NewClient(token, baseURL string) *Client {
	if baseURL == "" {
		baseURL = defaultBaseURL
	}
	return &Client{
		baseURL: strings.TrimRight(baseURL, "/"),
		token:   token,
		httpClient: &http.Client{Timeout: 30 * time.Second},
		now:     time.Now,
	}
}

// SetHTTPClient sets the underlying HTTP client used for requests.
// This is intended for testing to inject mock transports.
func (c *Client) SetHTTPClient(hc *http.Client) {
	c.httpClient = hc
}

// SetRetryBackoff sets the delays between retry attempts.
// This is intended for testing to speed up retry tests.
//
// Note: if an empty non-nil slice is provided, Retry-After delays parsed from
// server responses will be computed and capped but not applied (because
// attempt < len(backoff) is always false). This is acceptable for the
// test-only use case but callers should be aware of this edge case.
func (c *Client) SetRetryBackoff(backoff []time.Duration) {
	c.retryBackoff = backoff
}

// parseRetryAfter parses a Retry-After header value, supporting both integer
// seconds (e.g. "120") and HTTP-date format (e.g. "Thu, 01 Dec 2025 16:00:00 GMT")
// as specified in RFC 7231 §7.1.3.
//
// For integer values, it returns the duration directly.
// For HTTP-date values, it computes the delay as the difference between the
// parsed time and now. If the date is in the past, it returns 0.
//
// Returns (0, false) if the value cannot be parsed as either format.
func (c *Client) parseRetryAfter(value string) (time.Duration, bool) {
	value = strings.TrimSpace(value)

	// Try integer seconds first (most common from GitHub).
	// RFC 7231 allows delta-seconds of 0 to indicate immediate retry.
	if seconds, err := strconv.Atoi(value); err == nil && seconds >= 0 {
		return time.Duration(seconds) * time.Second, true
	}

	// Try HTTP-date format (RFC 7231 §7.1.3).
	// http.ParseTime handles RFC 1123, RFC 850, and ASCTIME formats.
	if retryAt, err := http.ParseTime(value); err == nil {
		delay := retryAt.Sub(c.now())
		if delay < 0 {
			delay = 0
		}
		return delay, true
	}

	return 0, false
}

// doRequest performs an HTTP request with retry on 429 rate limit responses.
// It respects the Retry-After header when present, supporting both integer
// seconds and HTTP-date formats (capped at maxRetryAfter).
func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept string) ([]byte, error) {
	var backoff []time.Duration
	if c.retryBackoff != nil {
		backoff = append([]time.Duration(nil), c.retryBackoff...)
	} else {
		backoff = []time.Duration{1 * time.Second, 2 * time.Second}
	}

	var lastErr error
	for attempt := 0; attempt < maxRetryAttempts; 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()
				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)
		}
		req.Header.Set("Authorization", "Bearer "+c.token)
		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, maxResponseBodyBytes))
			resp.Body.Close()
			if err != nil {
				return nil, fmt.Errorf("read response body: %w", err)
			}
			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 < maxRetryAttempts-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 delay, ok := c.parseRetryAfter(ra); ok {
					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, url string) ([]byte, error) {
	return c.doRequest(ctx, http.MethodGet, url, "")
}