LibraryApp/LibraryApp.Domain/Services/LendingService.cs

using LibraryApp.Domain.Entities;
using LibraryApp.Domain.Repositories;

namespace LibraryApp.Domain.Services;

public class LendingService
{
    private readonly ILendRecordRepository _lendRecords;

    public LendingService(ILendRecordRepository lendRecords)
    {
        _lendRecords = lendRecords;
    }

    public async Task<Result<LendRecord>> LendBookAsync(BookInstance bookInstance, Member member)
    {
        if (bookInstance.Status != BookInstanceStatus.OnShelf)
            return Result<LendRecord>.Failure($"Book is not available for lending. Current status: {bookInstance.Status}.");

        var code = await GenerateUniqueLendCodeAsync();
        bookInstance.MarkAsLent();
        var record = new LendRecord(bookInstance.Id, member.Id, code);
        return Result<LendRecord>.Success(record);
    }

    public Task<Result> ReturnBookAsync(BookInstance bookInstance, LendRecord lendRecord)
    {
        if (bookInstance.Status != BookInstanceStatus.Lent)
            return Task.FromResult(Result.Failure($"Book is not currently lent out. Current status: {bookInstance.Status}."));

        bookInstance.ReturnToShelf();
        lendRecord.MarkAsReturned();
        return Task.FromResult(Result.Success());
    }

    private async Task<LendCode> GenerateUniqueLendCodeAsync()
    {
        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        var random = Random.Shared;

        while (true)
        {
            var code = new string(Enumerable.Range(0, 6).Select(_ => chars[random.Next(chars.Length)]).ToArray());
            var lendCode = new LendCode(code);
            if (!await _lendRecords.ExistsWithCodeAsync(lendCode))
                return lendCode;
        }
    }
}