//go:build unix

package authn

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

	"golang.org/x/oauth2"

	"gitea.alexandru.macocian.me/amacocian/sherlock/internal/keyring"
)

const (
	githubWebURL = "https://github.com"
	githubAPIURL = "https://api.github.com"
)

var (
	// ErrGitHubUnauthorized means the cached GitHub token is no longer
	// usable and the next request should run a fresh browser login.
	ErrGitHubUnauthorized = errors.New("authn: GitHub token is unauthorized")
	// ErrGitHubOrganization means the authenticated user does not meet
	// the configured organization-membership requirement.
	ErrGitHubOrganization = errors.New("authn: GitHub organization membership denied")
)

func githubEndpoint(issuer string) oauth2.Endpoint {
	base := strings.TrimRight(issuer, "/")
	if base == "" {
		base = githubWebURL
	}
	return oauth2.Endpoint{
		AuthURL:   base + "/login/oauth/authorize",
		TokenURL:  base + "/login/oauth/access_token",
		AuthStyle: oauth2.AuthStyleInParams,
	}
}

func githubAPIBase(issuer string) string {
	if issuer == "" || strings.TrimRight(issuer, "/") == githubWebURL {
		return githubAPIURL
	}
	return strings.TrimRight(issuer, "/")
}

func providerScopes(cfg Config) []string {
	if cfg.ProviderType != ProviderGitHub {
		return append([]string(nil), cfg.Scopes...)
	}
	var scopes []string
	add := func(scope string) {
		for _, existing := range scopes {
			if existing == scope {
				return
			}
		}
		scopes = append(scopes, scope)
	}
	for _, scope := range cfg.Scopes {
		switch scope {
		case "openid":
		case "profile":
			add("read:user")
		case "email":
			add("user:email")
		default:
			add(scope)
		}
	}
	add("read:user")
	add("user:email")
	if cfg.Organization != "" {
		add("read:org")
	}
	add("offline_access")
	return scopes
}

func exchangeAndIdentifyGitHub(ctx context.Context, cfg Config, oauthCfg *oauth2.Config, pkce PKCE, code string, client *http.Client) (Result, error) {
	tok, err := oauthCfg.Exchange(ctx, code,
		oauth2.SetAuthURLParam("code_verifier", pkce.Verifier),
	)
	if err != nil {
		return Result{}, fmt.Errorf("authn: GitHub token exchange: %w", err)
	}
	if tok.AccessToken == "" {
		return Result{}, errors.New("authn: GitHub token response missing access_token")
	}
	ts := keyring.TokenSet{
		AccessToken:  tok.AccessToken,
		RefreshToken: tok.RefreshToken,
		AccessExpAt:  tok.Expiry,
		RefreshExpAt: refreshExpiry(tok),
		ProviderType: ProviderGitHub,
		Organization: cfg.Organization,
		Issuer:       githubIssuer(cfg.Issuer),
		ClientID:     cfg.ClientID,
		ClientSecret: cfg.ClientSecret,
		Scopes:       githubGrantedScopes(tok, providerScopes(cfg)),
	}
	if tok.RefreshToken == "" {
		ts.RefreshExpAt = time.Time{}
	}
	enriched, err := validateGitHubSession(ctx, cfg, ts, client)
	if err != nil {
		return Result{}, err
	}
	return Result{Tokens: enriched}, nil
}

func validateAndPersistGitHub(ctx context.Context, store keyring.Store, service string, cfg Config, ts keyring.TokenSet, client *http.Client) (keyring.TokenSet, error) {
	enriched, err := validateGitHubSession(ctx, cfg, ts, client)
	if err != nil {
		return ts, err
	}
	if err := store.Set(service, enriched); err != nil {
		return ts, fmt.Errorf("authn: persist GitHub session validation: %w", err)
	}
	return enriched, nil
}

func validateGitHubSession(ctx context.Context, cfg Config, ts keyring.TokenSet, client *http.Client) (keyring.TokenSet, error) {
	if client == nil {
		client = &http.Client{Timeout: 30 * time.Second}
	}
	apiBase := githubAPIBase(cfg.Issuer)
	var user struct {
		ID    int64  `json:"id"`
		Login string `json:"login"`
		Name  string `json:"name"`
		Email string `json:"email"`
	}
	if err := githubGet(ctx, client, ts.AccessToken, apiBase+"/user", &user); err != nil {
		return ts, fmt.Errorf("authn: GitHub user lookup: %w", err)
	}
	if user.ID == 0 || user.Login == "" {
		return ts, errors.New("authn: GitHub user response is missing id or login")
	}
	if user.Email == "" {
		var emails []struct {
			Email    string `json:"email"`
			Primary  bool   `json:"primary"`
			Verified bool   `json:"verified"`
		}
		if err := githubGet(ctx, client, ts.AccessToken, apiBase+"/user/emails", &emails); err != nil {
			return ts, fmt.Errorf("authn: GitHub email lookup: %w", err)
		}
		for _, email := range emails {
			if email.Primary && email.Verified {
				user.Email = email.Email
				break
			}
		}
		if user.Email == "" {
			for _, email := range emails {
				if email.Verified {
					user.Email = email.Email
					break
				}
			}
		}
	}

	if cfg.Organization != "" {
		var membership struct {
			State string `json:"state"`
		}
		endpoint := apiBase + "/user/memberships/orgs/" + url.PathEscape(cfg.Organization)
		if err := githubGet(ctx, client, ts.AccessToken, endpoint, &membership); err != nil {
			var statusErr *githubHTTPError
			if errors.As(err, &statusErr) && statusErr.Code == http.StatusNotFound {
				return ts, fmt.Errorf("%w: organization %q membership was not found", ErrGitHubOrganization, cfg.Organization)
			}
			return ts, fmt.Errorf("authn: GitHub organization %q membership check failed: %w", cfg.Organization, err)
		}
		if membership.State != "active" {
			return ts, fmt.Errorf("%w: GitHub user %q is not an active member of organization %q", ErrGitHubOrganization, user.Login, cfg.Organization)
		}
	}

	ts.ProviderType = ProviderGitHub
	ts.Organization = cfg.Organization
	ts.Issuer = githubIssuer(cfg.Issuer)
	ts.Subject = strconv.FormatInt(user.ID, 10)
	ts.Name = user.Name
	if ts.Name == "" {
		ts.Name = user.Login
	}
	ts.Email = user.Email
	ts.ValidatedAt = time.Now()
	return ts, nil
}

func githubGet(ctx context.Context, client *http.Client, token, endpoint string, out any) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
	if err != nil {
		return err
	}
	req.Header.Set("Authorization", "Bearer "+token)
	req.Header.Set("Accept", "application/vnd.github+json")
	req.Header.Set("X-GitHub-Api-Version", "2026-03-10")
	resp, err := client.Do(req)
	if err != nil {
		return err
	}
	defer func() { _ = resp.Body.Close() }()
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		body, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
		if resp.StatusCode == http.StatusUnauthorized {
			return fmt.Errorf("%w: HTTP %d: %s", ErrGitHubUnauthorized, resp.StatusCode, strings.TrimSpace(string(body)))
		}
		return &githubHTTPError{Code: resp.StatusCode, Body: strings.TrimSpace(string(body))}
	}

	if err := json.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(out); err != nil {
		return fmt.Errorf("decode response: %w", err)
	}
	return nil
}

type githubHTTPError struct {
	Code int
	Body string
}

func (e *githubHTTPError) Error() string {
	return fmt.Sprintf("HTTP %d: %s", e.Code, e.Body)
}

func githubIssuer(issuer string) string {
	if issuer != "" {
		return strings.TrimRight(issuer, "/")
	}
	return githubWebURL
}

func githubGrantedScopes(tok *oauth2.Token, requested []string) []string {
	if scope, ok := tok.Extra("scope").(string); ok && scope != "" {
		return strings.FieldsFunc(scope, func(r rune) bool {
			return r == ' ' || r == ','
		})
	}
	return requested
}