package service

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"time"

	"git.umbrella.haus/ae/notatest/internal/data"
	"github.com/go-chi/chi/v5"
	"github.com/golang-jwt/jwt/v5"
	"github.com/google/uuid"
	"github.com/gorilla/csrf"
	"github.com/jackc/pgx/v5/pgconn"
	"github.com/rs/zerolog/log"
	"golang.org/x/crypto/bcrypt"
)

const (
	accessTokenDuration  = 15 * time.Minute
	refreshTokenDuration = 7 * 24 * time.Hour
)

var (
	ErrInvalidToken      = errors.New("invalid token")
	ErrAuthHeaderInvalid = errors.New("token couldn't be parsed from Authorization header")
)

// User object context key for incoming requests (handled by middlewares). Only `*userClaims` type
// objects should be stored behind this key for consistency.
type userCtxKey struct{}

// DTO without sensitive data fields such as user's password hash.
type userResponse struct {
	ID        uuid.UUID  `json:"id"`
	Username  string     `json:"username"`
	IsAdmin   bool       `json:"is_admin"`
	CreatedAt *time.Time `json:"created_at"`
	UpdatedAt *time.Time `json:"updated_at"`
}

// Custom JWT claims (should always be handled in the middleware layer).
type userClaims struct {
	Admin                bool   `json:"admin"`
	TokenType            string `json:"type"` // "access" or "refresh"
	jwt.RegisteredClaims        // User's UUID should be stored in the subject claim
}

type tokenPair struct {
	AccessToken  string
	RefreshToken string
}

// Mockable token related database operations interface.
type TokenStore interface {
	CreateRefreshToken(ctx context.Context, arg data.CreateRefreshTokenParams) (data.RefreshToken, error)
	GetRefreshTokenByHash(ctx context.Context, tokenHash string) (data.RefreshToken, error)
	RevokeRefreshToken(ctx context.Context, tokenHash string) error
	RevokeAllUserRefreshTokens(ctx context.Context, id uuid.UUID) error
}

// Mockable user related database operations interface.
type UserStore interface {
	CreateUser(ctx context.Context, arg data.CreateUserParams) (data.User, error)
	ListUsers(ctx context.Context, arg data.ListUsersParams) ([]data.User, error)
	GetUserByID(ctx context.Context, id uuid.UUID) (data.User, error)
	GetUserByUsername(ctx context.Context, username string) (data.User, error)
	UpdatePassword(ctx context.Context, arg data.UpdatePasswordParams) error
	DeleteUser(ctx context.Context, id uuid.UUID) error
	RevokeAllUserRefreshTokens(ctx context.Context, id uuid.UUID) error
}

// Chi HTTP router for authentication/authorization related actions (users/tokens). In theory
// (especially in production) the `UserStore` and `TokenStore` will point to the same database
// handler, but for code readability they should be kept in separate structs.
type authResource struct {
	Config SvcConfig
	Users  UserStore
	Tokens TokenStore
}

func (rs authResource) Routes() chi.Router {
	r := chi.NewRouter()

	// Public routes
	r.Post("/signup", rs.Create) // POST /auth/signup - registration
	r.Post("/login", rs.Login)   // POST /auth/login - login

	// Protected routes (access token required)
	r.Group(func(r chi.Router) {
		r.Use(requireAccessToken(rs.Config.JWTSecret)) // JWT claims -> ctx.
		r.Get("/me", rs.Get)                           // GET /auth/me - current user data
		r.Post("/logout", rs.Logout)                   // POST /auth/logout - revoke all refresh cookies

		// Owner routes
		r.Route("/owner", func(r chi.Router) {
			r.Put("/", rs.UpdatePassword) // PUT /auth/owner - update user password
			r.Delete("/", rs.OwnerDelete) // DELETE /auth/owner - delete user (owner)
		})

		// Administration routes (admin claim required)
		r.Route("/admin", func(r chi.Router) {
			r.Use(adminOnlyMiddleware)
			r.Get("/all", rs.List) // GET /auth/admin/all - list all users
			r.Route(fmt.Sprintf("/{%s}", targetUserUUIDCtxParameter), func(r chi.Router) {
				r.Use(uuidCtx(targetUserUUIDCtxParameter))
				r.Delete("/", rs.AdminDelete) // DELETE /auth/admin/{id} - delete user (admin)
			})
		})
	})

	// Protected routes (refresh token required)
	r.Route("/cookie", func(r chi.Router) {
		// The refresh token httpOnly cookie is restricted to `/api/auth/cookie`, which is why this
		// is the only endpoint where CSRF must be taken into account (HTTPS requirement disabled
		// for local development)
		if rs.Config.IsProd {
			r.Use(csrf.Protect([]byte(rs.Config.CSRFSecret)))
		} else {
			r.Use(csrf.Protect([]byte(rs.Config.CSRFSecret), csrf.Secure(false)))
		}
		r.Use(requireRefreshToken(rs.Config.JWTSecret))
		r.Get("/csrf", rs.GetCSRFToken)           // GET /auth/cookie/csrf - get a new CSRF token in response headers
		r.Post("/refresh", rs.RefreshAccessToken) // POST /auth/cookie/refresh - convert refresh token to new token pair
	})

	return r
}

// Handler for new user creation. Will check the incoming JSON object's integrity, validate/normalize
// the username, and validate the password (check whether it's compromised via the HIBP API and
// calculate its entropy).
func (rs authResource) Create(w http.ResponseWriter, r *http.Request) {
	type request struct {
		Username *string `json:"username"`
		Password *string `json:"password"`
	}

	var req request
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil || req.Username == nil || req.Password == nil {
		respondError(w, http.StatusBadRequest, "Invalid request body")
		return
	}

	// Username normalization (to lowercase)
	normalizedUsername := normalizeUsername(*req.Username)
	if err := validateUsername(normalizedUsername); err != nil {
		respondError(w, http.StatusBadRequest, err.Error())
		return
	}

	// Password validation (length, HIBP API, and entropy)
	if err := validatePassword(*req.Password); err != nil {
		respondError(w, http.StatusBadRequest, err.Error())
		return
	}

	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(*req.Password), bcrypt.DefaultCost)
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to create user")
		return
	}

	user, err := rs.Users.CreateUser(r.Context(), data.CreateUserParams{
		Username:     normalizedUsername,
		PasswordHash: string(hashedPassword),
	})
	if err != nil {
		if isDuplicateEntry(err) {
			respondError(w, http.StatusConflict, "Username is already in use")
		} else {
			respondError(w, http.StatusInternalServerError, "Failed to create user")
		}
		return
	}

	respondJSON(w, http.StatusCreated, map[string]string{
		"id":       user.ID.String(),
		"username": user.Username,
	})
}

// Handler for logging in to an existing user account using a username-password credentials pair.
// Will check the incoming JSON object's integrity, use a normalized version of the username for a
// database lookup, and compare the given password's hash against the one stored in the database.
// By default only returns a fresh access tokens (and a refresh token as a httpOnly cookie), but
// if the `includeUser` parameter is set to `true` the user DTO will also be included.
func (rs authResource) Login(w http.ResponseWriter, r *http.Request) {
	type request struct {
		Username *string `json:"username"`
		Password *string `json:"password"`
	}

	var req request
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil || req.Username == nil || req.Password == nil {
		respondError(w, http.StatusBadRequest, "Invalid request body")
		return
	}

	user, err := rs.Users.GetUserByUsername(r.Context(), normalizeUsername(*req.Username))
	if err != nil {
		respondError(w, http.StatusUnauthorized, "Invalid credentials")
		return
	}

	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(*req.Password)); err != nil {
		respondError(w, http.StatusUnauthorized, "Invalid credentials")
		return
	}

	// Generate a new access/refresh token pair and store a SHA256 hash of the refresh token into
	// the database for further token rotations.
	tokenPair, err := rs.GenerateTokenPair(r.Context(), user.ID, user.IsAdmin)
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to generate tokens")
		return
	}

	// Set refresh token into a httpOnly cookie
	http.SetCookie(w, &http.Cookie{
		Name:     "notatest.refresh_token",
		Value:    tokenPair.RefreshToken,
		Path:     "/api/auth/cookie",
		MaxAge:   int(refreshTokenDuration.Seconds()),
		HttpOnly: true,
		Secure:   rs.Config.IsProd,
		SameSite: http.SameSiteStrictMode,
	})

	// Build response
	response := map[string]any{
		"access_token": tokenPair.AccessToken,
	}

	// Include user data DTO into the response if the `includeUser` parameter was set to `true`
	if includeUser, _ := strconv.ParseBool(r.URL.Query().Get("includeUser")); includeUser {
		response["user"] = userResponse{
			ID:        user.ID,
			Username:  user.Username,
			IsAdmin:   user.IsAdmin,
			CreatedAt: user.CreatedAt,
			UpdatedAt: user.UpdatedAt,
		}
	}

	respondJSON(w, http.StatusOK, response)
}

// Handler for getting full data of the current user as a DTO (database lookup) based on the JWT
// claims set into the request's context by a middleware.
func (rs authResource) Get(w http.ResponseWriter, r *http.Request) {
	user := rs.userFromCtxClaims(w, r)
	if user == nil {
		return
	}

	respondJSON(w, http.StatusOK, userResponse{
		ID:        user.ID,
		Username:  user.Username,
		IsAdmin:   user.IsAdmin,
		CreatedAt: user.CreatedAt,
		UpdatedAt: user.UpdatedAt,
	})
}

// Handler for updating the current user's password. Performs the same password strength checks as
// the registration handler (`rs.Create`) and revokes any existing refresh tokens the user has
// stored in the database.
func (rs authResource) UpdatePassword(w http.ResponseWriter, r *http.Request) {
	type request struct {
		OldPassword string `json:"old_password"`
		NewPassword string `json:"new_password"`
	}

	var req request
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		respondError(w, http.StatusBadRequest, "Invalid request body")
		return
	}

	user := rs.userFromCtxClaims(w, r)
	if user == nil {
		return
	}

	// Verify the old password before proceeding with the update
	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(req.OldPassword)); err != nil {
		respondError(w, http.StatusUnauthorized, "Invalid credentials")
		return
	}

	if err := validatePassword(req.NewPassword); err != nil {
		respondError(w, http.StatusBadRequest, err.Error())
		return
	}

	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(req.NewPassword), bcrypt.DefaultCost)
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to update password")
		return
	}

	if err := rs.Users.UpdatePassword(r.Context(), data.UpdatePasswordParams{
		ID:           user.ID,
		PasswordHash: string(hashedPassword),
	}); err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to update password")
		return
	}

	if err := rs.Users.RevokeAllUserRefreshTokens(r.Context(), user.ID); err != nil {
		log.Error().Msgf("Failed to revoke refresh tokens: %s", err)
	}

	w.WriteHeader(http.StatusNoContent)
}

// Handler for hard deleting the current user. Requires the user's password as JSON input as a precaution.
func (rs authResource) OwnerDelete(w http.ResponseWriter, r *http.Request) {
	type request struct {
		Password string `json:"password"`
	}

	var req request
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		respondError(w, http.StatusBadRequest, "Invalid request body")
		return
	}

	user := rs.userFromCtxClaims(w, r)
	if user == nil {
		return
	}

	// Verify the old password before allowing the deletion
	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(req.Password)); err != nil {
		respondError(w, http.StatusUnauthorized, "Invalid credentials")
		return
	}

	err := rs.Users.DeleteUser(r.Context(), user.ID)
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to delete user")
		return
	}

	// Clear the refresh token cookie
	http.SetCookie(w, &http.Cookie{
		Name:     "notatest.refresh_token",
		Value:    "",
		Path:     "/api/auth/cookie",
		MaxAge:   0, // Expires immediately
		HttpOnly: true,
		Secure:   rs.Config.IsProd,
		SameSite: http.SameSiteStrictMode,
	})

	if err := rs.Users.RevokeAllUserRefreshTokens(r.Context(), user.ID); err != nil {
		log.Error().Msgf("Failed to revoke refresh tokens: %s", err)
	}

	w.WriteHeader(http.StatusNoContent)
}

// Handler for listing all users stored in the database. Should only be allowed to be called by
// administrator level users.
func (rs authResource) List(w http.ResponseWriter, r *http.Request) {
	limit, offset := getPaginationParams(r)
	users, err := rs.Users.ListUsers(r.Context(), data.ListUsersParams{
		Limit:  limit,
		Offset: offset,
	})
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to retrieve users")
		return
	}

	// Output sanitization to DTO
	var output []userResponse
	for _, user := range users {
		output = append(output, userResponse{
			ID:        user.ID,
			Username:  user.Username,
			IsAdmin:   user.IsAdmin,
			CreatedAt: user.CreatedAt,
			UpdatedAt: user.UpdatedAt,
		})
	}

	respondJSON(w, http.StatusOK, output)
}

// Handler for deleting another user account based on their ID. Will check the existence of the
// user based on the given ID and additionally revoke all the stored refresh tokens on successful
// deletion. Should only be allowed to be called by administrator level users.
func (rs authResource) AdminDelete(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	targetID, ok := ctx.Value(uuidCtxKey{Name: targetUserUUIDCtxParameter}).(uuid.UUID)
	if !ok {
		respondError(w, http.StatusBadRequest, "Resource ID missing")
		return
	}

	if err := rs.Users.DeleteUser(r.Context(), targetID); err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to delete user")
		return
	}

	if err := rs.Users.RevokeAllUserRefreshTokens(r.Context(), targetID); err != nil {
		log.Error().Msgf("Failed to revoke refresh tokens: %s", err)
	}

	w.WriteHeader(http.StatusNoContent)
}

// Generate a new pair of access and refresh tokens (JWTs) with the user's UUID as the identifying
// `Subject` claim and custom claims for the user's administrator status (boolean) and token type
// ("refresh"/"access"). Stores a SHA256 hash of the refresh token into the database for further
// token rotations.
func (rs authResource) GenerateTokenPair(ctx context.Context, userID uuid.UUID, isAdmin bool) (*tokenPair, error) {
	tokenPair, err := generateTokenPair(userID.String(), isAdmin, rs.Config.JWTSecret)
	if err != nil {
		return nil, err
	}

	hash := sha256.Sum256([]byte(tokenPair.RefreshToken))
	tokenHash := hex.EncodeToString(hash[:])

	// Store the SHA256 hash of the refresh token with (almost) identical expiration timestamp
	expiresAt := time.Now().Add(refreshTokenDuration)
	_, err = rs.Tokens.CreateRefreshToken(ctx, data.CreateRefreshTokenParams{
		UserID:    userID,
		TokenHash: tokenHash,
		ExpiresAt: expiresAt,
	})
	if err != nil {
		return nil, err
	}

	return tokenPair, nil
}

// Revoke the given token from the database by calculating its SHA256 hash.
func (rs authResource) RevokeRefreshToken(ctx context.Context, token string) error {
	hash := sha256.Sum256([]byte(token))
	tokenHash := hex.EncodeToString(hash[:])
	return rs.Tokens.RevokeRefreshToken(ctx, tokenHash)
}

// Validate the given refresh token by performing a database lookup with its SHA256 hash. Returns
// the refresh token database object on successful lookup. Fails if the token has been revoked
// (soft database operation) or expired (i.e. the corresponding user has to log in again).
func (rs authResource) ValidateRefreshToken(ctx context.Context, token string) (*data.RefreshToken, error) {
	hash := sha256.Sum256([]byte(token))
	tokenHash := hex.EncodeToString(hash[:])

	dbToken, err := rs.Tokens.GetRefreshTokenByHash(ctx, tokenHash)
	if err != nil {
		return nil, err
	}

	// Check for soft revocation and/or expiration
	if dbToken.Revoked || time.Now().After(dbToken.ExpiresAt) {
		return nil, ErrInvalidToken
	}

	return &dbToken, nil
}

// Handler for returning the CSRF token in the `X-CSRF-Token` header. Notably this request doesn't
// need to contain a valid CSRF token as it uses "safe" (non-mutating) method GET, which means CSRF
// won't be enforced.
func (rs authResource) GetCSRFToken(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("X-CSRF-Token", csrf.Token(r))
	w.WriteHeader(http.StatusNoContent)
}

// Handler for performing a token rotation, i.e. invalidating the given refresh token (each refresh
// token is a single use utility) and exchanging it for a new pair of refresh and access tokens.
func (rs authResource) RefreshAccessToken(w http.ResponseWriter, r *http.Request) {
	// Get claims from context
	claims, ok := r.Context().Value(userCtxKey{}).(*userClaims)
	if !ok || claims.TokenType != "refresh" {
		respondError(w, http.StatusUnauthorized, "Invalid token")
		return
	}

	// Attempt to get the token from httpOnly cookie
	refreshToken, err := getTokenFromCookie(r)
	if err != nil {
		respondError(w, http.StatusUnauthorized, "Unauthorized")
		return
	}

	// Validate the refresh token in the database
	if _, err := rs.ValidateRefreshToken(r.Context(), refreshToken); err != nil {
		respondError(w, http.StatusUnauthorized, "Invalid refresh token")
		return
	}

	// Revoke the given (single use) refresh token
	if err := rs.RevokeRefreshToken(r.Context(), refreshToken); err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to revoke token")
		return
	}

	userID, err := uuid.Parse(claims.Subject)
	if err != nil {
		respondError(w, http.StatusBadRequest, "Invalid user ID")
		return
	}

	// Generate a new pair (access & refresh tokens)
	tokenPair, err := rs.GenerateTokenPair(r.Context(), userID, claims.Admin)
	if err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to generate tokens")
		return
	}

	// Set refresh token into a httpOnly cookie
	http.SetCookie(w, &http.Cookie{
		Name:     "notatest.refresh_token",
		Value:    tokenPair.RefreshToken,
		Path:     "/api/auth/cookie",
		MaxAge:   int(refreshTokenDuration.Seconds()),
		HttpOnly: true,
		Secure:   rs.Config.IsProd,
		SameSite: http.SameSiteStrictMode,
	})

	// Return the access token in the response body (it should be stored in browser's memory client-side)
	respondJSON(w, http.StatusOK, map[string]string{
		"access_token": tokenPair.AccessToken,
	})
}

// Handler for performing a logout process for the current user, i.e. replacing the current
// httpOnly `refresh_token` cookie with one that expires immediately. Theoretically the user
// will still be able to authenticate until the access token (stored client-side) expires,
// but that's up to the client to handle.
func (rs authResource) Logout(w http.ResponseWriter, r *http.Request) {
	claims, ok := r.Context().Value(userCtxKey{}).(*userClaims)
	if !ok {
		respondError(w, http.StatusUnauthorized, "Not authenticated")
		return
	}

	userID, err := uuid.Parse(claims.Subject)
	if err != nil {
		respondError(w, http.StatusBadRequest, "Invalid user ID")
		return
	}

	// Clear the refresh token cookie
	http.SetCookie(w, &http.Cookie{
		Name:     "notatest.refresh_token",
		Value:    "",
		Path:     "/api/auth/cookie",
		MaxAge:   0, // Expires immediately
		HttpOnly: true,
		Secure:   rs.Config.IsProd,
		SameSite: http.SameSiteStrictMode,
	})

	if err := rs.Tokens.RevokeAllUserRefreshTokens(r.Context(), userID); err != nil {
		respondError(w, http.StatusInternalServerError, "Failed to logout")
		return
	}

	w.WriteHeader(http.StatusNoContent)
}

// Parse JWT claims (`userClaims`) from the request's context, and perform a database lookup based
// on `Subject` (after parsing it to `uuid.UUID`) to fetch the corresponding user's data.
func (rs authResource) userFromCtxClaims(w http.ResponseWriter, r *http.Request) *data.User {
	claims, ok := r.Context().Value(userCtxKey{}).(*userClaims)
	if !ok {
		respondError(w, http.StatusUnauthorized, "Unauthorized")
		return nil
	}

	userID, err := uuid.Parse(claims.Subject)
	if err != nil {
		respondError(w, http.StatusBadRequest, "Invalid user ID")
		return nil
	}

	user, err := rs.Users.GetUserByID(r.Context(), userID)
	if err != nil {
		respondError(w, http.StatusNotFound, "User not found")
		return nil
	}

	return &user
}

// Helper function for generating the initial administrator level account if one doesn't already
// exists in the database.
func CreateAdminIfNotExists(ctx context.Context, q *data.Queries, username, password string) error {
	admins, err := q.ListAdmins(ctx)
	if err != nil {
		return err
	}
	if len(admins) > 0 {
		log.Debug().Msg("Admin accounts already exist, skipping creation")
		return nil
	}

	// Username normalization (to lowercase)
	normalizedUsername := normalizeUsername(username)
	if err := validateUsername(normalizedUsername); err != nil {
		return err
	}

	// Password validation (length, HIBP API, and entropy)
	if err := validatePassword(password); err != nil {
		return err
	}

	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
	if err != nil {
		return err
	}

	_, err = q.CreateAdmin(ctx, data.CreateAdminParams{
		Username:     normalizedUsername,
		PasswordHash: string(hashedPassword),
	})
	if err != nil {
		return err
	}

	log.Info().Msgf("Initial admin user '%s' created successfully", username)

	return nil
}

// Parse the JWT token from the request's Authorization header.
func getTokenFromHeader(r *http.Request) (string, error) {
	bearerToken := r.Header.Get("Authorization")
	bearerFields := strings.Fields(bearerToken)
	if len(bearerFields) == 2 && strings.ToLower(bearerFields[0]) == "bearer" {
		return bearerFields[1], nil
	}
	return "", ErrAuthHeaderInvalid
}

// Parse the JWT token from the request's cookies (httpOnly cookie).
func getTokenFromCookie(r *http.Request) (string, error) {
	cookie, err := r.Cookie("notatest.refresh_token")
	if err != nil {
		return "", err
	}
	return cookie.Value, nil
}

// Helper function for generating a new JWT token pair with the given specifications.
func generateTokenPair(userID string, isAdmin bool, jwtSecret string) (*tokenPair, error) {
	atClaims := userClaims{
		Admin:     isAdmin,
		TokenType: "access",
		RegisteredClaims: jwt.RegisteredClaims{
			Subject:   userID,
			ExpiresAt: jwt.NewNumericDate(time.Now().Add(accessTokenDuration)),
			IssuedAt:  jwt.NewNumericDate(time.Now()),
			NotBefore: jwt.NewNumericDate(time.Now()),
		},
	}
	accessToken := jwt.NewWithClaims(jwt.SigningMethodHS256, atClaims)

	t, err := accessToken.SignedString([]byte(jwtSecret))
	if err != nil {
		return nil, err
	}

	rtClaims := userClaims{
		Admin:     isAdmin,
		TokenType: "refresh",
		RegisteredClaims: jwt.RegisteredClaims{
			Subject:   userID,
			ExpiresAt: jwt.NewNumericDate(time.Now().Add(refreshTokenDuration)),
			IssuedAt:  jwt.NewNumericDate(time.Now()),
			NotBefore: jwt.NewNumericDate(time.Now()),
		},
	}
	refreshToken := jwt.NewWithClaims(jwt.SigningMethodHS256, rtClaims)

	rt, err := refreshToken.SignedString([]byte(jwtSecret))
	if err != nil {
		return nil, err
	}

	return &tokenPair{AccessToken: t, RefreshToken: rt}, nil
}

// Check if the given error is a PostgreSQL error for `unique_violation` (error code 23505), i.e.
// whether an entry with the given details already exists in the database table.
func isDuplicateEntry(err error) bool {
	var pgErr *pgconn.PgError
	if errors.As(err, &pgErr) {
		return pgErr.Code == "23505"
	}
	return false
}