using Microsoft.CodeAnalysis.CSharp.Scripting;
using Microsoft.CodeAnalysis.Scripting;
using ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;

namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.Scripts;

/// <summary>
/// Tests for <see cref="SiteScriptCompileCache"/>.
/// </summary>
/// <remarks>
/// <para>
/// <b>Deferred-work register row 100 — the cache is no longer static.</b> Every test here builds
/// its OWN <see cref="SiteScriptCompileCache"/>, so <see cref="SiteScriptCompileCache.Hits"/> and
/// <see cref="SiteScriptCompileCache.Count"/> count this test's compiles and nothing else. That
/// retires three workarounds this file used to carry:
/// </para>
/// <list type="bullet">
///   <item>the <c>[Collection("SiteScriptCompileCache")]</c> serialisation with
///   <c>ScriptCompilationServiceTests</c> and <c>DeploymentWarmThenGateTests</c> — those three
///   classes shared a mutable global and had to be kept off each other; they no longer touch any
///   shared state and run fully in parallel again;</item>
///   <item>the per-test GUID key token (register row 91) — no other class can reach this cache at
///   all, so keys need not be globally unique;</item>
///   <item>the one-sided counter bounds and the <c>ForeignEntrySlack = 512</c> allowance on
///   <c>Count</c> — foreign inserts are impossible now, so every assertion below is exact.</item>
/// </list>
/// <para>
/// The old escape hatch, <c>Clear()</c>, is gone from the type: it had no production caller and
/// existed only to reset the global between tests. Constructing a cache is the reset.
/// </para>
/// </remarks>
public class SiteScriptCompileCacheTests
{
    /// <summary>The cache under test. Private to this test instance — xunit builds one per test.</summary>
    private readonly SiteScriptCompileCache _cache = new();

    private static ScriptCompilationResult Ok() =>
        ScriptCompilationResult.Succeeded(CSharpScript.Create<object?>("1", ScriptOptions.Default, typeof(ScriptGlobals)));

    [Fact]
    public void GetOrAdd_SameCodeAndGlobals_ComputesOnce()
    {
        var factoryCalls = 0;
        ScriptCompilationResult Factory() { factoryCalls++; return Ok(); }

        var r1 = _cache.GetOrAdd("return 1;", typeof(ScriptGlobals), Factory);
        var r2 = _cache.GetOrAdd("return 1;", typeof(ScriptGlobals), Factory);

        Assert.Equal(1, factoryCalls);
        Assert.Same(r1, r2);
        Assert.Equal(1, _cache.Hits);
        Assert.Equal(1, _cache.Count);
    }

    [Fact]
    public void GetOrAdd_SameCode_DifferentGlobals_AreSeparateEntries()
    {
        var scriptCompiles = 0;
        var triggerCompiles = 0;

        var script = _cache.GetOrAdd("1 > 0", typeof(ScriptGlobals),
            () => { scriptCompiles++; return Ok(); });
        var trigger = _cache.GetOrAdd("1 > 0", typeof(TriggerExpressionGlobals),
            () => { triggerCompiles++; return Ok(); });

        Assert.Equal(1, scriptCompiles);
        Assert.Equal(1, triggerCompiles);
        Assert.NotSame(script, trigger);

        // Two distinct entries and no cross-globals hit — exact, because this cache is ours.
        Assert.Equal(2, _cache.Count);
        Assert.Equal(0, _cache.Hits);

        // Each entry stays independently addressable afterwards.
        Assert.Same(script, _cache.GetOrAdd("1 > 0", typeof(ScriptGlobals),
            () => throw new InvalidOperationException("script entry was not cached")));
        Assert.Same(trigger, _cache.GetOrAdd("1 > 0", typeof(TriggerExpressionGlobals),
            () => throw new InvalidOperationException("trigger entry was not cached")));
    }

    /// <summary>
    /// Register row 100: two caches are genuinely independent stores. This is the property the
    /// whole change exists for — one test class's cache activity can no longer evict, serve, or
    /// be counted by another's. It is the direct replacement for the old
    /// <c>Clear()</c>-under-a-serialized-collection arrangement.
    /// </summary>
    [Fact]
    public void SeparateCaches_DoNotShareEntriesOrCounters()
    {
        var other = new SiteScriptCompileCache();

        var mine = _cache.GetOrAdd("return 1;", typeof(ScriptGlobals), Ok);
        _cache.GetOrAdd("return 1;", typeof(ScriptGlobals), Ok);   // a hit on OUR cache

        // The other cache has neither the entry nor the hit.
        Assert.Equal(0, other.Count);
        Assert.Equal(0, other.Hits);

        // Same key, other store: it must compute its own value, and get a distinct instance.
        var theirCompiles = 0;
        var theirs = other.GetOrAdd("return 1;", typeof(ScriptGlobals),
            () => { theirCompiles++; return Ok(); });
        Assert.Equal(1, theirCompiles);
        Assert.NotSame(mine, theirs);

        // …and filling the other cache past its bound — the closest thing left to the old
        // wholesale Clear() — leaves ours completely untouched.
        for (var i = 0; i <= SiteScriptCompileCache.MaxEntries; i++)
            other.GetOrAdd($"return {i};", typeof(ScriptGlobals), Ok);

        Assert.Equal(1, _cache.Count);
        Assert.Equal(1, _cache.Hits);
        Assert.Same(mine, _cache.GetOrAdd("return 1;", typeof(ScriptGlobals),
            () => throw new InvalidOperationException("our entry was evicted by another cache")));
    }

    [Fact]
    public void Overflow_StaysWithinTheBound()
    {
        for (var i = 0; i <= SiteScriptCompileCache.MaxEntries; i++)
            _cache.GetOrAdd($"return {i};", typeof(ScriptGlobals), Ok);

        // MaxEntries + 1 distinct inserts, single-threaded: the sweep ran and the ceiling holds
        // exactly (no foreign inserts can land here any more — register row 100).
        Assert.True(_cache.Count <= SiteScriptCompileCache.MaxEntries,
            $"cache grew past its bound: {_cache.Count} vs max {SiteScriptCompileCache.MaxEntries}");

        var recomputed = 0;
        for (var i = 0; i < 16; i++)
            _cache.GetOrAdd($"return {i};", typeof(ScriptGlobals), () => { recomputed++; return Ok(); });

        Assert.True(recomputed > 0, "no oldest-batch entry was evicted — the overflow sweep did not run");
    }

    // ── WP3.1: approximate LRU replaced the wholesale-Clear overflow cliff ──────────

    /// <summary>
    /// WP3.1 (test group 8): overflow must evict only the OLDEST batch, keeping hot entries.
    /// The old behaviour cleared all 1024 entries, so the very next deploy or Instance-Actor
    /// start paid a full recompile storm on actor threads.
    /// </summary>
    [Fact]
    public void Overflow_EvictsOldestBatch_AndKeepsRecentlyTouchedEntries()
    {
        // Fill to the bound. Entries 1..N are inserted oldest-first.
        for (var i = 0; i < SiteScriptCompileCache.MaxEntries; i++)
            _cache.GetOrAdd($"return {i};", typeof(ScriptGlobals), Ok);

        // Touch entry 0 so it carries the NEWEST access stamp despite being inserted first.
        var hot = _cache.GetOrAdd("return 0;", typeof(ScriptGlobals), Ok);
        var hitsAfterTouch = _cache.Hits;
        Assert.Equal(1, hitsAfterTouch);   // exactly one hit so far: the touch

        // Push well past the bound so at least one eviction sweep definitely runs.
        for (var i = 0; i < 200; i++)
            _cache.GetOrAdd($"return overflow{i};", typeof(ScriptGlobals), Ok);

        // Nothing was cleared wholesale: the cache is still most of the way full and the hit
        // counter survived (a wholesale clear used to reset it to 0).
        Assert.True(_cache.Count > SiteScriptCompileCache.MaxEntries / 2,
            $"cache collapsed to {_cache.Count} entries — this looks like a wholesale clear");
        Assert.Equal(hitsAfterTouch, _cache.Hits);   // 200 distinct inserts, none of them hits

        // The touched entry survived the sweep — a hit, not a recompile. This is the LRU
        // property: recency, not insertion order, decides what stays.
        var again = _cache.GetOrAdd("return 0;", typeof(ScriptGlobals),
            () => throw new InvalidOperationException("hot entry was evicted — LRU is not keeping recently-used entries"));
        Assert.Same(hot, again);
        Assert.Equal(hitsAfterTouch + 1, _cache.Hits);

        // …and untouched old entries genuinely were evicted, so the sweep really ran.
        var recomputed = 0;
        for (var i = 1; i <= 16; i++)
            _cache.GetOrAdd($"return {i};", typeof(ScriptGlobals), () => { recomputed++; return Ok(); });

        Assert.True(recomputed > 0, "no old entry was evicted — the overflow sweep did not run");
    }

    /// <summary>
    /// WP3.1: concurrent inserts crossing the bound together must not blow past it — the
    /// eviction sweep is double-checked under its own lock precisely so a stampede performs
    /// one sweep rather than one per racing thread.
    /// </summary>
    [Fact]
    public async Task ConcurrentGetOrAddStorm_KeepsTheBound()
    {
        const int perTask = 400;
        const int tasks = 8;

        await Task.WhenAll(Enumerable.Range(0, tasks).Select(t => Task.Run(() =>
        {
            for (var i = 0; i < perTask; i++)
                _cache.GetOrAdd($"return {t}_{i};", typeof(ScriptGlobals), Ok);
        })));

        // A small transient overshoot is by design: threads that lose the double-checked
        // eviction race still insert their own entry afterwards, so the ceiling is "bounded",
        // not "never exceeded by one". The slack is bounded by the number of racing threads
        // (register row 100 — it used to be 512, to absorb inserts from other test classes
        // sharing the process-global cache; nothing foreign can reach this instance).
        // The property under test is that 3200 distinct inserts do not accumulate — before
        // WP3.1 this was a wholesale clear, and a per-entry eviction bug would show up here as
        // unbounded growth, not as a handful of extra rows.
        Assert.True(_cache.Count <= SiteScriptCompileCache.MaxEntries + tasks,
            $"cache grew past its bound: {_cache.Count} vs max {SiteScriptCompileCache.MaxEntries} " +
            $"(+{tasks} racing-insert slack) after {tasks * perTask} distinct inserts");
    }
}