// 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 (
	"bytes"
	"context"
	"encoding/json"
	"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

	// maxRetryAttempts is the number of times doRequest will attempt a request.
	// The retry backoff slice must have length maxRetryAttempts-1.
	maxRetryAttempts = 3
)

// 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)"
	}
	// 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
}

// 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

	// reviewPageSize overrides reviewsPerPage for testing. Zero means use default.
	reviewPageSize int
	// reviewMaxPages overrides maxReviewPages for testing. Zero means use default.
	reviewMaxPages int
}

// defaultCheckRedirect is the redirect policy used by NewClient and SetHTTPClient(nil).
// It rejects HTTPS→HTTP protocol downgrades (to prevent plaintext leakage) and strips
// the Authorization header on cross-host redirects to prevent credential leakage to
// third-party hosts (e.g. CDN redirects from GitHub).
func defaultCheckRedirect(req *http.Request, via []*http.Request) error {
	if len(via) >= 10 {
		return fmt.Errorf("stopped after 10 redirects")
	}
	// Guard: net/http guarantees len(via) >= 1 but this is undocumented;
	// defend against zero-length to avoid panic on index out of range.
	if len(via) == 0 {
		return nil
	}
	prev := via[len(via)-1]
	// Reject protocol downgrade: HTTPS→HTTP leaks request metadata over plaintext.
	if prev.URL.Scheme == "https" && req.URL.Scheme == "http" {
		return fmt.Errorf("refusing redirect from HTTPS to HTTP (%s → %s)", prev.URL.Host, req.URL.Host)
	}
	// Strip Authorization on cross-host redirect to avoid leaking credentials
	// to third-party hosts (GitHub legitimately redirects to CDN hosts).
	if req.URL.Host != prev.URL.Host {
		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.
// The slice must have exactly maxRetryAttempts-1 entries (one delay per retry gap).
// In production the default {1s, 2s} applies.
func (c *Client) SetRetryBackoff(d []time.Duration) error {
	if len(d) != maxRetryAttempts-1 {
		return fmt.Errorf("github: backoff length %d does not match maxRetryAttempts-1 (%d)", len(d), maxRetryAttempts-1)
	}
	c.retryBackoff = d
	return nil
}

// SetReviewPagination overrides the page size and max pages for ListReviews.
// Intended for testing only; must be called before any goroutines issue requests.
func (c *Client) SetReviewPagination(pageSize, maxPages int) {
	c.reviewPageSize = pageSize
	c.reviewMaxPages = maxPages
}

// requestOptions holds per-request configuration for doRequestCore.
type requestOptions struct {
	// bodyFn returns a fresh io.Reader for the request body on each attempt.
	// Must be non-nil for any request that carries a body (POST, PUT, PATCH,
	// or DELETE when a body is required by the API).
	// Returning a fresh reader on each call allows retries to re-send the body.
	bodyFn func() io.Reader

	// accept overrides the default Accept header. Empty means "application/vnd.github+json".
	accept string

	// extraHeaders are additional headers to set on each request attempt.
	extraHeaders map[string]string
}

// doRequestCore is the shared implementation for all HTTP requests with retry
// on 429 rate limit responses. It respects the Retry-After header when present
// (capped at maxRetryAfter). Transport errors are not retried.
func (c *Client) doRequestCore(ctx context.Context, method, reqURL string, opts requestOptions) ([]byte, error) {
	const maxRetryAfter = 120 * 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

	// backoff holds per-attempt delays: backoff[i] is the delay before attempt i+1.
	// Length must be maxRetryAttempts-1 (one entry per retry gap).
	// SetRetryBackoff validates at configuration time; the default is always valid.
	defaultBackoff := []time.Duration{1 * time.Second, 2 * time.Second}
	var backoff []time.Duration
	if c.retryBackoff != nil && len(c.retryBackoff) == maxRetryAttempts-1 {
		backoff = make([]time.Duration, len(c.retryBackoff))
		copy(backoff, c.retryBackoff)
	} else {
		backoff = make([]time.Duration, len(defaultBackoff))
		copy(backoff, defaultBackoff)
	}

	// 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 < 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() // no-op after fire; kept for symmetry with the ctx.Done case
				case <-ctx.Done():
					timer.Stop()
					return nil, ctx.Err()
				}
			}
		}

		var body io.Reader
		if opts.bodyFn != nil {
			body = opts.bodyFn()
		}
		req, err := http.NewRequestWithContext(ctx, method, reqURL, body)
		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 opts.accept != "" {
			req.Header.Set("Accept", opts.accept)
		} else {
			req.Header.Set("Accept", "application/vnd.github+json")
		}
		for k, v := range opts.extraHeaders {
			req.Header.Set(k, v)
		}

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

		// Capture response metadata before handleResponse takes body ownership.
		respStatus := resp.StatusCode
		retryAfterHeader := resp.Header.Get("Retry-After")

		respBody, done, handleErr := c.handleResponse(resp, maxResponseBytes, maxErrorBodyBytes)
		if done {
			return respBody, handleErr
		}
		lastErr = handleErr

		// Retry on 429 rate limit
		if respStatus == 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 := retryAfterHeader; 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
}

// 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) {
	return c.doRequestCore(ctx, method, reqURL, requestOptions{accept: accept})
}

// handleResponse reads and closes the response body, returning the result.
// It uses defer to ensure the body is always closed regardless of code path.
// Returns (body, done, err) where done=true means the caller should return immediately.
func (c *Client) handleResponse(resp *http.Response, maxRespBytes int, maxErrBytes int) ([]byte, bool, error) {
	defer resp.Body.Close()

	if resp.StatusCode >= 200 && resp.StatusCode < 300 {
		body, err := io.ReadAll(io.LimitReader(resp.Body, int64(maxRespBytes)+1))
		if err != nil {
			return nil, true, fmt.Errorf("read response body: %w", err)
		}
		if len(body) > maxRespBytes {
			return nil, true, fmt.Errorf("response body exceeded %d bytes", maxRespBytes)
		}
		return body, true, nil
	}

	errBody, readErr := io.ReadAll(io.LimitReader(resp.Body, int64(maxErrBytes)))
	if readErr != nil && len(errBody) == 0 {
		errBody = []byte(fmt.Sprintf("[error reading response body: %v]", readErr))
	}
	return nil, false, &APIError{StatusCode: resp.StatusCode, Body: string(errBody)}
}

// 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, "")
}

// doRequestWithBody is like doRequest but sends a request body.
// It accepts the raw body bytes and sets Content-Type to application/json.
// Retry semantics match doRequest (retries on 429 with Retry-After support).
func (c *Client) doRequestWithBody(ctx context.Context, method, reqURL string, reqBody []byte) ([]byte, error) {
	var opts requestOptions
	if reqBody != nil {
		opts.bodyFn = func() io.Reader { return bytes.NewReader(reqBody) }
		opts.extraHeaders = map[string]string{"Content-Type": "application/json"}
	}
	return c.doRequestCore(ctx, method, reqURL, opts)
}

// doJSONRequest performs an HTTP request with a JSON body and returns the response body.
// It delegates retry/backoff/429 handling to doRequestWithBody.
// This is a general-purpose helper used by any method that needs to send JSON payloads
// (e.g. PostReview, DismissReview).
func (c *Client) doJSONRequest(ctx context.Context, method, reqURL string, payload any) ([]byte, error) {
	jsonBody, err := json.Marshal(payload)
	if err != nil {
		return nil, fmt.Errorf("marshal request body: %w", err)
	}
	return c.doRequestWithBody(ctx, method, reqURL, jsonBody)
}