using System;
using System.Collections.Generic;
using System.Linq;
using LaDOSE.Business.Interface;
using LaDOSE.Entity;
using LaDOSE.Entity.Context;
using Microsoft.EntityFrameworkCore;

namespace LaDOSE.Business.Service
{
    public class UserService : IUserService
    {
        private LaDOSEDbContext _context;

        public UserService(LaDOSEDbContext context)
        {
            _context = context;
        }

        public ApplicationUser Authenticate(string username, string password)
        {
            if (string.IsNullOrEmpty(username) || string.IsNullOrEmpty(password))
                return null;
            var user = _context.ApplicationUser
                .Include(x => x.UserRoles).ThenInclude(ur => ur.Role)
                .SingleOrDefault(x => x.Username == username);

            // check if username exists
            if (user == null)
                return null;

            // check if password is correct
            if (!VerifyPasswordHash(password, user.PasswordHash, user.PasswordSalt))
                return null;
    
            // authentication successful
            return user;
        }

        public IEnumerable<ApplicationUser> GetAll()
        {
            return _context.ApplicationUser
                .Include(x => x.UserRoles).ThenInclude(ur => ur.Role)
                .ToList();
        }

        /// <summary>
        /// Roles are included because authorization reads them on every authenticated
        /// request (see the OnTokenValidated handler in Startup).
        /// </summary>
        public ApplicationUser GetById(int id)
        {
            return _context.ApplicationUser
                .Include(x => x.UserRoles).ThenInclude(ur => ur.Role)
                .SingleOrDefault(x => x.Id == id);
        }

        public IEnumerable<ApplicationRole> GetAllRoles()
        {
            return _context.ApplicationRole.OrderBy(x => x.Name).ToList();
        }

        public ApplicationUser Create(ApplicationUser user, string password, IEnumerable<string> roleNames = null)
        {
            // validation
            if (string.IsNullOrWhiteSpace(user?.Username))
                throw new Exception("Username is required");

            if (string.IsNullOrWhiteSpace(password))
                throw new Exception("Password is required");

            if (_context.ApplicationUser.Any(x => x.Username == user.Username))
                throw new Exception("Username \"" + user.Username + "\" is already taken");

            // EF fills userid/roleid on the join rows from these navigations when it saves.
            user.UserRoles = ResolveRoles(roleNames)
                .Select(role => new ApplicationUserRole { Role = role })
                .ToList();

            byte[] passwordHash, passwordSalt;
            CreatePasswordHash(password, out passwordHash, out passwordSalt);

            user.PasswordHash = passwordHash;
            user.PasswordSalt = passwordSalt;

            _context.ApplicationUser.Add(user);
            _context.SaveChanges();

            return user;
        }

        /// <summary>
        /// Turns role names into the existing rows of <c>applicationrole</c>. An unknown
        /// name is an error rather than a new role, so a typo cannot quietly produce an
        /// account with no privileges — or, worse, a second spelling of "Admin".
        /// </summary>
        private List<ApplicationRole> ResolveRoles(IEnumerable<string> roleNames)
        {
            var wanted = (roleNames ?? Enumerable.Empty<string>())
                .Where(name => !string.IsNullOrWhiteSpace(name))
                .Select(name => name.Trim())
                .Distinct(StringComparer.OrdinalIgnoreCase)
                .ToList();

            if (wanted.Count == 0)
                return new List<ApplicationRole>();

            var known = _context.ApplicationRole.ToList();
            var resolved = new List<ApplicationRole>();
            foreach (var name in wanted)
            {
                var role = known.FirstOrDefault(r => string.Equals(r.Name, name, StringComparison.OrdinalIgnoreCase));
                if (role == null)
                    throw new Exception($"Unknown role \"{name}\"");
                resolved.Add(role);
            }

            return resolved;
        }

        public void Update(ApplicationUser userParam, string password = null)
        {
            var user = _context.ApplicationUser.Find(userParam.Id);

            if (user == null)
                throw new Exception("User not found");

            if (userParam.Username != user.Username)
            {
                // username has changed so check if the new username is already taken
                if (_context.ApplicationUser.Any(x => x.Username == userParam.Username))
                    throw new Exception("Username " + userParam.Username + " is already taken");
            }

            // update user properties
            user.FirstName = userParam.FirstName;
            user.LastName = userParam.LastName;
            user.Username = userParam.Username;

            // update password if it was entered
            if (!string.IsNullOrWhiteSpace(password))
            {
                byte[] passwordHash, passwordSalt;
                CreatePasswordHash(password, out passwordHash, out passwordSalt);

                user.PasswordHash = passwordHash;
                user.PasswordSalt = passwordSalt;
            }

            _context.ApplicationUser.Update(user);
            _context.SaveChanges();
        }

        public void Delete(int id)
        {
            // applicationuserrole's foreign keys are ON DELETE RESTRICT, so the join rows
            // have to go first — clearing the collection makes EF delete them.
            var user = _context.ApplicationUser
                .Include(x => x.UserRoles).ThenInclude(ur => ur.Role)
                .SingleOrDefault(x => x.Id == id);

            if (user == null)
                return;

            user.UserRoles?.Clear();
            _context.ApplicationUser.Remove(user);
            _context.SaveChanges();
        }

        // private helper methods

        private static void CreatePasswordHash(string password, out byte[] passwordHash, out byte[] passwordSalt)
        {
            if (password == null) throw new ArgumentNullException("password");
            if (string.IsNullOrWhiteSpace(password)) throw new ArgumentException("Value cannot be empty or whitespace only string.", "password");

            using (var hmac = new System.Security.Cryptography.HMACSHA512())
            {
                passwordSalt = hmac.Key;
                passwordHash = hmac.ComputeHash(System.Text.Encoding.UTF8.GetBytes(password));
            }
        }

        private static bool VerifyPasswordHash(string password, byte[] storedHash, byte[] storedSalt)
        {
            if (password == null) throw new ArgumentNullException("password");
            if (string.IsNullOrWhiteSpace(password)) throw new ArgumentException("Value cannot be empty or whitespace only string.", "password");
            if (storedHash.Length != 64) throw new ArgumentException("Invalid length of password hash (64 bytes expected).", "passwordHash");
            if (storedSalt.Length != 128) throw new ArgumentException("Invalid length of password salt (128 bytes expected).", "passwordHash");

            using (var hmac = new System.Security.Cryptography.HMACSHA512(storedSalt))
            {
                var computedHash = hmac.ComputeHash(System.Text.Encoding.UTF8.GetBytes(password));
                for (int i = 0; i < computedHash.Length; i++)
                {
                    if (computedHash[i] != storedHash[i]) return false;
                }
            }

            return true;
        }
    }
}