add fp3 tests

csharp-fp3.Tests/WithdrawMoneyTests.cs

@@ -0,0 +1,110 @@
+using CsharpFp3.Application;
+using CsharpFp3.Domain;
+using CsharpFp3.Infrastructure;
+using CsharpFp3.Library;
+
+namespace CsharpFp3.Tests;
+
+/// Tests covering the application-layer use case surface:
+/// what the WithdrawMoney feature does from the caller's perspective.
+public class WithdrawMoneyTests
+{
+    private static (
+        Func<Guid, decimal, Result<Account, AccountError>> withdraw,
+        Repository repo
+    ) BuildHandler()
+    {
+        var repo = InMemoryAccountRepository.Create();
+        var withdraw = AccountApplication.CreateWithdrawMoney(repo).Value;
+        return (withdraw, repo);
+    }
+
+    [Fact]
+    public void Withdrawing_from_an_account_reduces_its_balance_by_the_withdrawn_amount()
+    {
+        var (withdraw, repo) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repo.SaveAccount(AccountDomain.Open(accountId, new Money(200m)).Value);
+
+        withdraw(accountId, 75m);
+
+        var account = repo.LoadAccount(accountId).Value;
+        Assert.Equal(125m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_the_entire_balance_leaves_the_account_at_zero()
+    {
+        var (withdraw, repo) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repo.SaveAccount(AccountDomain.Open(accountId, new Money(100m)).Value);
+
+        withdraw(accountId, 100m);
+
+        var account = repo.LoadAccount(accountId).Value;
+        Assert.Equal(0m, account.Balance.Amount);
+    }
+
+    [Fact]
+    public void Withdrawing_from_a_non_existent_account_returns_AccountNotFound()
+    {
+        var (withdraw, _) = BuildHandler();
+
+        var result = withdraw(Guid.NewGuid(), 50m);
+
+        Assert.IsType<AccountError.AccountNotFound>(result.Error);
+    }
+
+    [Fact]
+    public void Withdrawing_more_than_the_available_balance_returns_InsufficientBalance()
+    {
+        var (withdraw, repo) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+
+        var result = withdraw(accountId, 100m);
+
+        Assert.IsType<AccountError.InsufficientBalance>(result.Error);
+    }
+
+    [Fact]
+    public void AccountNotFound_error_reports_the_requested_account_id()
+    {
+        var (withdraw, _) = BuildHandler();
+        var accountId = Guid.NewGuid();
+
+        var error = Assert.IsType<AccountError.AccountNotFound>(
+            withdraw(accountId, 50m).Error
+        );
+
+        Assert.Equal(accountId, error.AccountId);
+    }
+
+    [Fact]
+    public void InsufficientBalance_error_reports_the_current_balance_and_attempted_amount()
+    {
+        var (withdraw, repo) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+
+        var error = Assert.IsType<AccountError.InsufficientBalance>(
+            withdraw(accountId, 120m).Error
+        );
+
+        Assert.Equal(50m, error.Balance.Amount);
+        Assert.Equal(120m, error.Amount.Amount);
+    }
+
+    [Fact]
+    public void After_a_failed_withdrawal_the_balance_is_unchanged()
+    {
+        var (withdraw, repo) = BuildHandler();
+        var accountId = Guid.NewGuid();
+        repo.SaveAccount(AccountDomain.Open(accountId, new Money(50m)).Value);
+
+        withdraw(accountId, 999m);
+
+        var account = repo.LoadAccount(accountId).Value;
+        Assert.Equal(50m, account.Balance.Amount);
+    }
+}