using System.Text.Json; public class User { public string name { get; } public IDictionary owes { get; set; } public IDictionary owed_by { get; set; } public double balance => owed_by.Sum(x => x.Value) - owes.Sum(x => x.Value); public User(string name) { this.name = name; owes = new Dictionary(); owed_by = new Dictionary(); } public void Lend(User borrower, double amount) { var remaining = amount; if (owes.ContainsKey(borrower.name)) { remaining = owes[borrower.name] - amount; if (remaining > 0) { owes[borrower.name] = remaining; return; } owes.Remove(borrower.name); remaining *= -1; } if (remaining > 0) { if (owed_by.ContainsKey(borrower.name)) { owed_by[borrower.name] += remaining; } else { owed_by.Add(borrower.name, remaining); owed_by = owed_by.OrderBy(x => x.Key).ToDictionary(x => x.Key, x => x.Value); } } } public void Borrow(User lender, double amount) { var remaining = amount; if (owed_by.ContainsKey(lender.name)) { remaining = owed_by[lender.name] - amount; if (remaining > 0) { owed_by[lender.name] = remaining; return; } owed_by.Remove(lender.name); remaining *= -1; } if (remaining > 0) { if (owes.ContainsKey(lender.name)) { owes[lender.name] += remaining; } else { owes.Add(lender.name, remaining); owes = owes.OrderBy(x => x.Key).ToDictionary(x => x.Key, x => x.Value); } } } } public class RestApi { private IList users; public RestApi(string database) { users = JsonSerializer.Deserialize>(database)!; } public string Get(string url, string? payload = null) { if (payload != null) { var values = JsonSerializer.Deserialize>>(payload)!; var requestedUsers = values["users"]; return JsonSerializer.Serialize(users.Where(x => requestedUsers.Contains(x.name))); } return JsonSerializer.Serialize(users); } public string Post(string url, string payload) { if (url == "/add") { var values = JsonSerializer.Deserialize>(payload); var newUser = new User(values!["user"]); users.Add(newUser); return JsonSerializer.Serialize(newUser); } else if (url == "/iou") { var values = JsonSerializer.Deserialize>(payload)!; var lender = users.First(x => x.name.Equals(values["lender"].ToString())); var borrower = users.First(x => x.name.Equals(values["borrower"].ToString())); var amount = double.Parse(values["amount"].ToString()!); lender.Lend(borrower, amount); borrower.Borrow(lender, amount); return JsonSerializer.Serialize(new[] { lender, borrower }.OrderBy(x => x.name)); } return string.Empty; } }