Interview question for: http://www.heyswegner.com/2015/04/good-interview-questions.html

FlightAssignmentInterview.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;

public class Flight
{
    public string FeatureArea { get; private set; }
    public int TrafficPercent { get; private set; }

    public Flight(string featureArea, int trafficPercent)
    {
        FeatureArea = featureArea;
        TrafficPercent = trafficPercent;
    }
}

public class User
{
    public Guid Id { get; private set; }
    public User() { Id = Guid.NewGuid(); }
}

public class FlightAssignment
{
    private Dictionary<string, Flight[]> _flightAssignments = new Dictionary<string, Flight[]>();

    public void Add(Flight flight)
    {
        Flight[] trafficBuckets;
        if (!_flightAssignments.TryGetValue(flight.FeatureArea, out trafficBuckets))
        {
            trafficBuckets = new Flight[100];
            _flightAssignments.Add(flight.FeatureArea, trafficBuckets);
        }

        int trafficToAssign = flight.TrafficPercent;
        for (int i = 0; i < 100; i++)
        {
            if (trafficBuckets[i] == null)
            {
                trafficBuckets[i] = flight;
                trafficToAssign--;
                if (trafficToAssign == 0) break;
            }
        }
    }

    public void Remove(Flight flight)
    {
        var trafficBuckets = _flightAssignments[flight.FeatureArea];
        var buckets = trafficBuckets;
        for (int i = 0; i < 100; i++)
        {
            if (buckets[i] == flight)
                buckets[i] = null;
        }
    }

    public ISet<Flight> GetFlights(User user)
    {
        int trafficBucket = Math.Abs(user.Id.GetHashCode()) % 100;
        var assignments = new HashSet<Flight>();

        foreach (var trafficBuckets in _flightAssignments.Values)
        {
            var flight = trafficBuckets[trafficBucket];
            if (flight != null)
                assignments.Add(flight);
        }

        return assignments;
    }
}

[TestClass]
public class FlightAssignmentTests
{
    private FlightAssignment _assignmentService = new FlightAssignment();
    private User _user = new User();

    [TestMethod]
    public void BasicAssignment()
    {
        Assert.AreEqual(0, _assignmentService.GetFlights(_user).Count);

        var flight = new Flight("UI", trafficPercent: 100);
        _assignmentService.Add(flight);

        var assignments = _assignmentService.GetFlights(_user);
        Assert.AreEqual(1, assignments.Count);
        Assert.AreEqual(flight, assignments.Single());
    }

    [TestMethod]
    public void MultipleAssignment()
    {
        _assignmentService.Add(new Flight("backend", trafficPercent: 100));
        _assignmentService.Add(new Flight("frontend", trafficPercent: 100));

        var assignments = _assignmentService.GetFlights(_user);
        Assert.AreEqual(2, assignments.Count);
    }

    [TestMethod]
    public void MutualExclusiveFeatureFlights()
    {
        _assignmentService.Add(new Flight("UI", trafficPercent: 50));
        _assignmentService.Add(new Flight("UI", trafficPercent: 50));

        var assignments = _assignmentService.GetFlights(_user);
        Assert.AreEqual(1, assignments.Count);
    }
}