using ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites;
using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
using ZB.MOM.WW.ScadaBridge.Commons.Types.Flattening;

namespace ZB.MOM.WW.ScadaBridge.DeploymentManager;

/// <summary>
/// Memoises the repository reads that <see cref="FlatteningPipeline"/> performs,
/// for the lifetime of ONE flatten/validate session.
///
/// <para>
/// Without a session every instance flatten re-walks its whole template
/// inheritance chain (one query per link), re-loads the compositions of every
/// template in that chain and of every composed chain it reaches, and re-issues
/// the three session-global queries — shared scripts, the shared-schema library,
/// and the target site's data connections. For a bulk deploy of N instances off
/// the same template that is N× the same work; even a single instance re-loads a
/// composed chain once per composing template.
/// </para>
///
/// <para>
/// <b>Cache key.</b> Templates are keyed on <c>(id, ITemplateGraphWatermark
/// version)</c> and chain MEMBERSHIP additionally on the watermark's
/// <see cref="ITemplateGraphWatermark.StructureVersion"/>, so a template edited
/// mid-session is re-read rather than served from the memo. The session is also
/// short-lived by construction — it is created per deploy/validate operation and
/// discarded with it — so a cached graph can never outlive the operation that
/// captured it. That is the design's "template state is captured at the time of
/// flatten" guarantee, unchanged: the session narrows the capture window, it does
/// not widen it.
/// </para>
///
/// <para>
/// <b>Threading.</b> A session is NOT thread-safe and must not be shared across
/// threads. It caches entities materialised by one scoped <c>DbContext</c>, which
/// is itself single-threaded, and it hands out tracked entities that the
/// in-flight bundle importer relies on observing. <c>DeploySiteAsync</c> honours
/// this by running the whole prepare (flatten) phase serially on one scope and
/// parallelising only the site round-trips, which touch no repository at all.
/// </para>
/// </summary>
public sealed class FlattenSession
{
    private readonly ITemplateGraphWatermark _watermark;

    private readonly Dictionary<int, (long Version, Template Template)> _templates = new();
    private readonly Dictionary<int, CachedChain> _chains = new();
    private readonly Dictionary<int, (long Version, IReadOnlyList<TemplateComposition> Compositions)> _compositions = new();
    private readonly Dictionary<int, IReadOnlyDictionary<int, DataConnection>> _dataConnections = new();

    private IReadOnlyList<ResolvedScript>? _sharedScripts;
    private IReadOnlyDictionary<string, string>? _schemaLibrary;

    /// <summary>Initializes a session bound to the process-wide graph watermark.</summary>
    /// <param name="watermark">Watermark supplying the per-template versions used as cache keys.</param>
    public FlattenSession(ITemplateGraphWatermark watermark)
    {
        _watermark = watermark ?? throw new ArgumentNullException(nameof(watermark));
    }

    /// <summary>
    /// Number of template-chain walks that actually hit the repository. Reset
    /// never; used by tests to assert that N instances sharing a template load
    /// that chain exactly once.
    /// </summary>
    public int ChainLoads { get; private set; }

    /// <summary>Number of single-template repository reads that missed the memo.</summary>
    public int TemplateLoads { get; private set; }

    /// <summary>Number of per-template composition queries that missed the memo.</summary>
    public int CompositionLoads { get; private set; }

    /// <summary>Number of times the session-global queries (shared scripts, schema library) ran.</summary>
    public int GlobalLoads { get; private set; }

    /// <summary>Number of per-site data-connection queries that missed the memo.</summary>
    public int DataConnectionLoads { get; private set; }

    /// <summary>
    /// Every template id this session resolved — the union of all inheritance and
    /// composed chains it walked. The staleness fast path records the watermark
    /// version of exactly this set, so a change to ANY template that fed a
    /// flatten invalidates the memoised revision hash.
    /// </summary>
    public IReadOnlyCollection<int> VisitedTemplateIds => _templates.Keys;

    /// <summary>
    /// Every visited template id paired with the watermark version this session
    /// READ it at — the pre-flatten watermark the staleness fast path must memoise
    /// against.
    ///
    /// <para>
    /// Re-reading the versions after the flatten instead is a correctness bug, not
    /// an optimisation detail: an edit that lands mid-flatten pairs the PRE-edit
    /// content (and therefore the pre-edit hash) with the POST-edit version, and
    /// the memo then answers "not stale" until something else moves the watermark.
    /// Recording the version each template was read at can only ever cause an extra
    /// flatten, never a stale answer.
    /// </para>
    /// </summary>
    public IReadOnlyDictionary<int, long> VisitedTemplateVersions =>
        _templates.ToDictionary(entry => entry.Key, entry => entry.Value.Version);

    /// <summary>
    /// Returns the inheritance chain for <paramref name="templateId"/> (the
    /// template itself, then each ancestor), loading it through
    /// <paramref name="loadTemplate"/> on a miss.
    /// </summary>
    /// <param name="templateId">Root of the chain to build.</param>
    /// <param name="loadTemplate">Repository read for one template with its children.</param>
    /// <returns>The chain, ordered leaf-first.</returns>
    public async Task<IReadOnlyList<Template>> GetChainAsync(
        int templateId,
        Func<int, Task<Template?>> loadTemplate)
    {
        var structure = _watermark.StructureVersion;
        if (_chains.TryGetValue(templateId, out var cached) && IsChainCurrent(cached, structure))
            return cached.Chain;

        var chain = new List<Template>();
        var memberVersions = new Dictionary<int, long>();
        var currentId = (int?)templateId;

        // Guard against a cyclic parent edge. Acyclicity is enforced on save, but
        // this walk must not hang if a bad row ever reaches the database.
        var seen = new HashSet<int>();
        while (currentId.HasValue && seen.Add(currentId.Value))
        {
            memberVersions[currentId.Value] = _watermark.GetTemplateVersion(currentId.Value);
            var template = await GetTemplateAsync(currentId.Value, loadTemplate);
            if (template == null) break;
            chain.Add(template);
            currentId = template.ParentTemplateId;
        }

        ChainLoads++;
        _chains[templateId] = new CachedChain(structure, memberVersions, chain);
        return chain;
    }

    /// <summary>
    /// A memoised chain plus the exact watermark readings it was built under.
    /// </summary>
    /// <param name="Structure">Structure version, covering chain MEMBERSHIP.</param>
    /// <param name="MemberVersions">Per-template version of every link, covering chain CONTENTS.</param>
    /// <param name="Chain">The chain itself, ordered leaf-first.</param>
    private sealed record CachedChain(
        long Structure,
        IReadOnlyDictionary<int, long> MemberVersions,
        IReadOnlyList<Template> Chain);

    /// <summary>
    /// A cached chain is current only when BOTH the graph shape and every member's
    /// own version are unchanged.
    ///
    /// <para>
    /// Both halves are load-bearing. Structure alone would miss an ordinary member
    /// edit (an attribute added to a template already in the chain), which changes
    /// what the flattener produces without changing which templates it walks.
    /// Member versions alone would miss a re-parent, which changes membership
    /// without touching any surviving member. Checking only one would let a session
    /// serve a chain that no longer reflects the database.
    /// </para>
    /// </summary>
    private bool IsChainCurrent(CachedChain cached, long structure)
    {
        if (cached.Structure != structure)
            return false;

        foreach (var (templateId, version) in cached.MemberVersions)
        {
            if (_watermark.GetTemplateVersion(templateId) != version)
                return false;
        }

        return true;
    }

    /// <summary>
    /// Returns one template with its children, loading it through
    /// <paramref name="loadTemplate"/> on a miss or when its watermark version moved.
    /// </summary>
    /// <param name="templateId">Template to read.</param>
    /// <param name="loadTemplate">Repository read for one template with its children.</param>
    /// <returns>The template, or <see langword="null"/> when it does not exist.</returns>
    public async Task<Template?> GetTemplateAsync(int templateId, Func<int, Task<Template?>> loadTemplate)
    {
        var version = _watermark.GetTemplateVersion(templateId);
        if (_templates.TryGetValue(templateId, out var cached) && cached.Version == version)
            return cached.Template;

        var template = await loadTemplate(templateId);
        TemplateLoads++;
        if (template != null)
            _templates[templateId] = (version, template);
        return template;
    }

    /// <summary>
    /// Returns the composition rows declared directly on <paramref name="templateId"/>.
    /// </summary>
    /// <param name="templateId">Template whose compositions are read.</param>
    /// <param name="loadCompositions">Repository read for one template's compositions.</param>
    /// <returns>The composition rows.</returns>
    public async Task<IReadOnlyList<TemplateComposition>> GetCompositionsAsync(
        int templateId,
        Func<int, Task<IReadOnlyList<TemplateComposition>>> loadCompositions)
    {
        var version = _watermark.GetTemplateVersion(templateId);
        if (_compositions.TryGetValue(templateId, out var cached) && cached.Version == version)
            return cached.Compositions;

        var compositions = await loadCompositions(templateId);
        CompositionLoads++;
        _compositions[templateId] = (version, compositions);
        return compositions;
    }

    /// <summary>
    /// Returns the site's data connections keyed by id. Cached per site for the
    /// session — a bulk site deploy resolves them once for the whole batch.
    /// </summary>
    /// <param name="siteId">Site whose connections are read.</param>
    /// <param name="loadConnections">Repository read for a site's data connections.</param>
    /// <returns>The site's connections keyed by database id.</returns>
    public async Task<IReadOnlyDictionary<int, DataConnection>> GetDataConnectionsAsync(
        int siteId,
        Func<int, Task<IReadOnlyDictionary<int, DataConnection>>> loadConnections)
    {
        if (_dataConnections.TryGetValue(siteId, out var cached))
            return cached;

        var connections = await loadConnections(siteId);
        DataConnectionLoads++;
        _dataConnections[siteId] = connections;
        return connections;
    }

    /// <summary>
    /// Returns the shared-script set used for semantic validation. Hoisted to the
    /// session so a bulk deploy issues this query once instead of once per instance.
    /// </summary>
    /// <param name="loadSharedScripts">Repository read for all shared scripts.</param>
    /// <returns>The resolved shared scripts.</returns>
    public async Task<IReadOnlyList<ResolvedScript>> GetSharedScriptsAsync(
        Func<Task<IReadOnlyList<ResolvedScript>>> loadSharedScripts)
    {
        if (_sharedScripts != null)
            return _sharedScripts;

        _sharedScripts = await loadSharedScripts();
        GlobalLoads++;
        return _sharedScripts;
    }

    /// <summary>
    /// Returns the shared JSON-Schema library as a name → schema-JSON map, hoisted
    /// to the session for the same reason as the shared scripts.
    /// </summary>
    /// <param name="loadSchemaLibrary">Repository read building the schema library map.</param>
    /// <returns>The schema library.</returns>
    public async Task<IReadOnlyDictionary<string, string>> GetSchemaLibraryAsync(
        Func<Task<IReadOnlyDictionary<string, string>>> loadSchemaLibrary)
    {
        if (_schemaLibrary != null)
            return _schemaLibrary;

        _schemaLibrary = await loadSchemaLibrary();
        GlobalLoads++;
        return _schemaLibrary;
    }
}