Print_server/Inspectron.Epson.Tests/CompositeDiscoveryServiceTests.cs

using Inspectron.Epson;

namespace Inspectron.Epson.Tests;

public class CompositeDiscoveryServiceTests
{
    private sealed class StubDiscovery : IDiscoveryService
    {
        private readonly List<DiscoveredPrinter> _printers;
        private readonly Exception? _throws;

        public StubDiscovery(params string[] ips)
        {
            _printers = ips.Select(ip => new DiscoveredPrinter { IPAddress = ip }).ToList();
        }

        public StubDiscovery(Exception throws)
        {
            _printers = new List<DiscoveredPrinter>();
            _throws = throws;
        }

        public StubDiscovery(List<DiscoveredPrinter> printers)
        {
            _printers = printers;
        }

        public Task<List<DiscoveredPrinter>> DiscoverPrintersAsync(
            TimeSpan? timeout = null,
            Action<DiscoveredPrinter>? onPrinterDiscovered = null)
        {
            if (_throws is not null)
            {
                throw _throws;
            }

            foreach (var p in _printers)
            {
                onPrinterDiscovered?.Invoke(p);
            }

            return Task.FromResult(_printers);
        }
    }

    private sealed class SlowStubDiscovery : IDiscoveryService
    {
        private readonly TimeSpan _delay;
        private readonly string _ip;

        public SlowStubDiscovery(TimeSpan delay, string ip)
        {
            _delay = delay;
            _ip = ip;
        }

        public async Task<List<DiscoveredPrinter>> DiscoverPrintersAsync(
            TimeSpan? timeout = null,
            Action<DiscoveredPrinter>? onPrinterDiscovered = null)
        {
            await Task.Delay(_delay);
            return new List<DiscoveredPrinter> { new DiscoveredPrinter { IPAddress = _ip } };
        }
    }

    [Fact]
    public async Task Merges_results_from_every_child()
    {
        var composite = new CompositeDiscoveryService(new IDiscoveryService[]
        {
            new StubDiscovery("10.0.0.1"),
            new StubDiscovery("10.0.0.2", "10.0.0.3")
        });

        var result = await composite.DiscoverPrintersAsync(TimeSpan.FromMilliseconds(50));

        Assert.Equal(3, result.Count);
        Assert.Contains(result, p => p.IPAddress == "10.0.0.1");
        Assert.Contains(result, p => p.IPAddress == "10.0.0.2");
        Assert.Contains(result, p => p.IPAddress == "10.0.0.3");
    }

    [Fact]
    public async Task Deduplicates_by_ip_keeping_the_first_child_that_reported_it()
    {
        // The two stubs report the same IP but are distinguishable, so this
        // proves first-wins rather than merely proving that one of them survived.
        var first = new StubDiscovery(new List<DiscoveredPrinter>
        {
            new DiscoveredPrinter { IPAddress = "10.0.0.1", ModelName = "TM-T30III", ServiceType = "ENPC" }
        });
        var second = new StubDiscovery(new List<DiscoveredPrinter>
        {
            new DiscoveredPrinter { IPAddress = "10.0.0.1", ModelName = "POS895", ServiceType = "SPRT" }
        });

        var composite = new CompositeDiscoveryService(new IDiscoveryService[] { first, second });

        var result = await composite.DiscoverPrintersAsync(TimeSpan.FromMilliseconds(50));

        var only = Assert.Single(result);
        Assert.Equal("TM-T30III", only.ModelName);
        Assert.Equal("ENPC", only.ServiceType);
    }

    [Fact]
    public async Task Runs_children_concurrently_not_sequentially()
    {
        // Three children that each take ~300ms. Sequential execution needs >=900ms;
        // concurrent should land near 300ms. The 700ms bound leaves generous headroom
        // for a loaded machine while still failing a sequential regression widely.
        var children = new IDiscoveryService[]
        {
            new SlowStubDiscovery(TimeSpan.FromMilliseconds(300), "10.0.0.1"),
            new SlowStubDiscovery(TimeSpan.FromMilliseconds(300), "10.0.0.2"),
            new SlowStubDiscovery(TimeSpan.FromMilliseconds(300), "10.0.0.3")
        };

        var composite = new CompositeDiscoveryService(children);

        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
        var result = await composite.DiscoverPrintersAsync(TimeSpan.FromSeconds(5));
        stopwatch.Stop();

        Assert.Equal(3, result.Count);
        Assert.True(stopwatch.ElapsedMilliseconds < 700,
            $"expected concurrent execution near 300ms but took {stopwatch.ElapsedMilliseconds}ms, " +
            "which indicates the children ran sequentially");
    }

    [Fact]
    public async Task One_child_throwing_does_not_suppress_the_others()
    {
        var composite = new CompositeDiscoveryService(new IDiscoveryService[]
        {
            new StubDiscovery(new InvalidOperationException("boom")),
            new StubDiscovery("10.0.0.7")
        });

        var result = await composite.DiscoverPrintersAsync(TimeSpan.FromMilliseconds(50));

        Assert.Single(result);
        Assert.Equal("10.0.0.7", result[0].IPAddress);
    }

    [Fact]
    public async Task Forwards_the_discovered_callback()
    {
        var seen = new List<string>();
        var composite = new CompositeDiscoveryService(new IDiscoveryService[]
        {
            new StubDiscovery("10.0.0.1", "10.0.0.2")
        });

        await composite.DiscoverPrintersAsync(TimeSpan.FromMilliseconds(50),
            p => seen.Add(p.IPAddress));

        Assert.Equal(2, seen.Count);
    }

    [Fact]
    public async Task Callback_fires_once_per_ip_even_when_two_children_report_it()
    {
        var seen = new List<string>();
        var composite = new CompositeDiscoveryService(new IDiscoveryService[]
        {
            new StubDiscovery("10.0.0.1"),
            new StubDiscovery("10.0.0.1")
        });

        await composite.DiscoverPrintersAsync(TimeSpan.FromMilliseconds(50),
            p => seen.Add(p.IPAddress));

        Assert.Single(seen);
    }
}