MeterVault/src/Infrastructure/Normalization/AnalysisDataWriter.cs

using MeterVault.Core.Analysis;
using MeterVault.Core.Analysis.Rollups;
using MeterVault.Infrastructure.Persistence;
using MeterVault.Infrastructure.Persistence.Analysis;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.ChangeTracking;

namespace MeterVault.Infrastructure.Normalization;

/// <summary>
/// Writes one meter's analysis tables (D-12) — day and month rollups, coverage runs and rollup state — by diff
/// against what is stored: a row that is still right is not touched, a changed one is updated in place, a new
/// one added and a vanished one removed. Stages the changes; the caller's <c>SaveChanges</c> writes them in its
/// transaction, together with the meter's consumption.
/// </summary>
/// <remarks>
/// <para>
/// A recompute runs on every live reading, and almost all of a meter's history comes out of it unchanged. A
/// diff keeps that to the rows the new reading actually moved — usually the current day and month and the
/// last coverage run — instead of rewriting years of rows each time.
/// </para>
/// <para>
/// The change tracker, not only the database, is the truth here: one context may recompute a meter several
/// times before it saves (an import touching a meter twice, a worker ingesting two readings in one scope).
/// Rows staged by an earlier pass are still tracked — added, modified or deleted — and the diff builds on
/// them rather than tripping over their keys.
/// </para>
/// </remarks>
internal sealed class AnalysisDataWriter(MeterVaultDbContext db)
{
    /// <summary>Every analysis table is keyed by meter first, under this property.</summary>
    private const string MeterIdProperty = nameof(MeterRollupState.MeterId);

    private readonly MeterVaultDbContext _db = db;

    /// <summary>
    /// Stages the meter's rollups and coverage, and its state row. <see cref="MeterRollupState.BuiltAt"/> moves to
    /// <paramref name="now"/> only when something about the meter's analysis data changed.
    /// </summary>
    /// <returns>True when any row was added, changed or removed.</returns>
    public async Task<bool> WriteAsync(
        int meterId,
        MeterRollups rollups,
        IReadOnlyList<CoverageRun> coverage,
        MeterRollupState state,
        DateTimeOffset now,
        CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(rollups);
        ArgumentNullException.ThrowIfNull(coverage);
        ArgumentNullException.ThrowIfNull(state);

        // Listing tracked entries detects changes across the whole context by default — every consumption row
        // the recompute just staged, several times over. Nothing here depends on it: the diff compares values
        // itself, and SaveChanges detects the edits it makes.
        var autoDetect = _db.ChangeTracker.AutoDetectChangesEnabled;
        _db.ChangeTracker.AutoDetectChangesEnabled = false;
        try
        {
            return await WriteCoreAsync(meterId, rollups, coverage, state, now, cancellationToken).ConfigureAwait(false);
        }
        finally
        {
            _db.ChangeTracker.AutoDetectChangesEnabled = autoDetect;
        }
    }

    private async Task<bool> WriteCoreAsync(
        int meterId,
        MeterRollups rollups,
        IReadOnlyList<CoverageRun> coverage,
        MeterRollupState state,
        DateTimeOffset now,
        CancellationToken cancellationToken)
    {
        var changed = false;

        var days = await CurrentAsync(_db.ConsumptionRollups, meterId, cancellationToken).ConfigureAwait(false);
        changed |= Sync(
            days, rollups.Days, d => (d.Day, d.Kind), b => (b.Start, b.Kind),
            (row, bucket) => row.Assign(bucket), bucket => ConsumptionRollup.From(meterId, bucket));

        var months = await CurrentAsync(_db.ConsumptionRollupMonths, meterId, cancellationToken).ConfigureAwait(false);
        changed |= Sync(
            months, rollups.Months, m => (m.Month, m.Kind), b => (b.Start, b.Kind),
            (row, bucket) => row.Assign(bucket), bucket => ConsumptionRollupMonth.From(meterId, bucket));

        var runs = await CurrentAsync(_db.MeterCoverage, meterId, cancellationToken).ConfigureAwait(false);
        changed |= Sync(
            runs, coverage, r => r.SpanFrom, r => r.From.ToUniversalTime(),
            (row, run) => row.Assign(run), run => MeterCoverageRun.From(meterId, run));

        var states = await CurrentAsync(_db.MeterRollupStates, meterId, cancellationToken).ConfigureAwait(false);
        var entry = states.SingleOrDefault();
        if (entry is null)
        {
            _db.MeterRollupStates.Add(new MeterRollupState
            {
                MeterId = meterId,
                Revision = state.Revision,
                Zone = state.Zone,
                NormalizedUnit = state.NormalizedUnit,
                Kind = state.Kind,
                LastReadingAt = state.LastReadingAt,
                BuiltAt = now.ToUniversalTime(),
            });
            return true;
        }

        var stored = entry.Entity;
        var resurrected = entry.State == EntityState.Deleted;
        if (resurrected)
        {
            entry.State = EntityState.Modified;
        }

        var stateChanged = resurrected
            || stored.Revision != state.Revision
            || !string.Equals(stored.Zone, state.Zone, StringComparison.Ordinal)
            || !string.Equals(stored.NormalizedUnit, state.NormalizedUnit, StringComparison.Ordinal)
            || stored.Kind != state.Kind
            || !Nullable.Equals(stored.LastReadingAt, state.LastReadingAt);
        if (changed || stateChanged)
        {
            stored.Revision = state.Revision;
            stored.Zone = state.Zone;
            stored.NormalizedUnit = state.NormalizedUnit;
            stored.Kind = state.Kind;
            stored.LastReadingAt = state.LastReadingAt;
            stored.BuiltAt = now.ToUniversalTime();
        }

        return changed || stateChanged;
    }

    /// <summary>
    /// The meter's rows as they stand for this context: the stored rows (tracked, so identity resolution hands
    /// back anything an earlier pass already holds) plus rows an earlier pass added and has not saved, minus the
    /// ones it removed, which keep their entry (state Deleted) so they can be brought back.
    /// </summary>
    /// <remarks>
    /// A tracked row the database no longer has — removed behind the tracker's back, as <c>ExecuteDelete</c> or a
    /// cascade does — is detached, as the consumption rows are: updating or deleting it would fail the save.
    /// </remarks>
    private async Task<List<EntityEntry<T>>> CurrentAsync<T>(DbSet<T> set, int meterId, CancellationToken cancellationToken)
        where T : class
    {
        var stored = await set.Where(e => EF.Property<int>(e, MeterIdProperty) == meterId)
            .ToListAsync(cancellationToken).ConfigureAwait(false);
        var inDatabase = new HashSet<T>(stored, ReferenceEqualityComparer.Instance);

        var entries = _db.ChangeTracker.Entries<T>()
            .Where(e => e.State != EntityState.Detached && (int)e.Property(MeterIdProperty).CurrentValue! == meterId)
            .ToList();
        foreach (var stale in entries.Where(e => e.State != EntityState.Added && !inDatabase.Contains(e.Entity)))
        {
            stale.State = EntityState.Detached;
        }

        return [.. entries.Where(e => e.State != EntityState.Detached)];
    }

    /// <summary>Reconciles the tracked rows of one table with the desired values; true when anything changed.</summary>
    private bool Sync<TEntity, TDesired, TKey>(
        List<EntityEntry<TEntity>> current,
        IEnumerable<TDesired> desired,
        Func<TEntity, TKey> entityKey,
        Func<TDesired, TKey> desiredKey,
        Func<TEntity, TDesired, bool> assign,
        Func<TDesired, TEntity> create)
        where TEntity : class
        where TKey : notnull
    {
        var changed = false;
        var byKey = current.ToDictionary(e => entityKey(e.Entity));
        var wanted = new HashSet<TKey>();

        foreach (var item in desired)
        {
            var key = desiredKey(item);
            wanted.Add(key);
            if (!byKey.TryGetValue(key, out var entry))
            {
                _db.Add(create(item));
                changed = true;
                continue;
            }

            if (entry.State == EntityState.Deleted)
            {
                // Removed by an earlier pass that has not been saved: the row is wanted again, so it stays
                // and is updated with whatever it holds now.
                assign(entry.Entity, item);
                entry.State = EntityState.Modified;
                changed = true;
                continue;
            }

            changed |= assign(entry.Entity, item);
        }

        foreach (var (key, entry) in byKey)
        {
            if (wanted.Contains(key) || entry.State == EntityState.Deleted)
            {
                continue;
            }

            // An added row is simply dropped from tracking; a stored one is deleted.
            _db.Remove(entry.Entity);
            changed = true;
        }

        return changed;
    }
}