natsdotnet/tests/NATS.Server.Gateways.Tests/Gateways/ReplyMapCacheTests.cs

using NATS.Server.Gateways;
using Shouldly;

namespace NATS.Server.Gateways.Tests.Gateways;

/// <summary>
/// Tests for the ReplyMapCache LRU cache with TTL expiration.
/// Go reference: gateway.go — reply mapping cache.
/// </summary>
public class ReplyMapCacheTests
{
    // Go: gateway.go — cache is initially empty, lookups return false
    [Fact]
    public void TryGet_returns_false_on_empty()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);

        var found = cache.TryGet("_INBOX.abc", out var value);

        found.ShouldBeFalse();
        value.ShouldBeNull();
    }

    // Go: gateway.go — cached mapping is retrievable after Set
    [Fact]
    public void Set_and_TryGet_round_trips()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);
        cache.Set("_INBOX.abc", "_GR_.cluster1.42._INBOX.abc");

        var found = cache.TryGet("_INBOX.abc", out var value);

        found.ShouldBeTrue();
        value.ShouldBe("_GR_.cluster1.42._INBOX.abc");
    }

    // Go: gateway.go — LRU eviction removes the least-recently-used entry when at capacity
    [Fact]
    public void LRU_eviction_at_capacity()
    {
        var cache = new ReplyMapCache(capacity: 2, ttlMs: 60_000);
        cache.Set("key1", "val1");
        cache.Set("key2", "val2");
        // Adding a third entry should evict the LRU entry (key1, since key2 was added last)
        cache.Set("key3", "val3");

        cache.TryGet("key1", out _).ShouldBeFalse();
        cache.TryGet("key2", out var v2).ShouldBeTrue();
        v2.ShouldBe("val2");
        cache.TryGet("key3", out var v3).ShouldBeTrue();
        v3.ShouldBe("val3");
    }

    // Go: gateway.go — entries expire after the configured TTL window
    [Fact]
    public void TTL_expiration()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 50);
        cache.Set("_INBOX.ttl", "_GR_.c1.1._INBOX.ttl");

        Thread.Sleep(200); // Wait longer than the 50ms TTL

        var found = cache.TryGet("_INBOX.ttl", out var value);

        found.ShouldBeFalse();
        value.ShouldBeNull();
    }

    // Go: gateway.go — hit counter tracks successful cache lookups
    [Fact]
    public void Hits_incremented_on_hit()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);
        cache.Set("key", "value");

        cache.TryGet("key", out _);
        cache.TryGet("key", out _);

        cache.Hits.ShouldBe(2);
        cache.Misses.ShouldBe(0);
    }

    // Go: gateway.go — miss counter tracks failed lookups
    [Fact]
    public void Misses_incremented_on_miss()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);

        cache.TryGet("nope", out _);
        cache.TryGet("also-nope", out _);

        cache.Misses.ShouldBe(2);
        cache.Hits.ShouldBe(0);
    }

    // Go: gateway.go — Clear removes all entries from the cache
    [Fact]
    public void Clear_removes_all()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);
        cache.Set("a", "1");
        cache.Set("b", "2");
        cache.Set("c", "3");

        cache.Clear();

        cache.Count.ShouldBe(0);
        cache.TryGet("a", out _).ShouldBeFalse();
        cache.TryGet("b", out _).ShouldBeFalse();
        cache.TryGet("c", out _).ShouldBeFalse();
    }

    // Go: gateway.go — Set on existing key updates the value and promotes to MRU
    [Fact]
    public void Set_updates_existing()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);
        cache.Set("key", "original");
        cache.Set("key", "updated");

        cache.TryGet("key", out var value).ShouldBeTrue();
        value.ShouldBe("updated");
        cache.Count.ShouldBe(1);
    }

    // Go: gateway.go — PurgeExpired removes only expired entries
    [Fact]
    public void PurgeExpired_removes_old_entries()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 50);
        cache.Set("old1", "v1");
        cache.Set("old2", "v2");

        Thread.Sleep(200); // Ensure both entries are past the 50ms TTL

        var purged = cache.PurgeExpired();

        purged.ShouldBe(2);
        cache.Count.ShouldBe(0);
    }

    // Go: gateway.go — Count reflects the current number of cached entries
    [Fact]
    public void Count_reflects_entries()
    {
        var cache = new ReplyMapCache(capacity: 16, ttlMs: 60_000);

        cache.Count.ShouldBe(0);

        cache.Set("a", "1");
        cache.Count.ShouldBe(1);

        cache.Set("b", "2");
        cache.Count.ShouldBe(2);

        cache.Set("c", "3");
        cache.Count.ShouldBe(3);

        cache.Clear();
        cache.Count.ShouldBe(0);
    }
}