feat(batch13): port filestore skip-first-block query helpers

dotnet/src/ZB.MOM.NatsNet.Server/JetStream/FileStore.cs

@@ -2032,6 +2032,106 @@ public sealed class JetStreamFileStore : IStreamStore, IDisposable
         _lmb = _blks.Count > 0 ? _blks[^1] : null;
     }
 
+    // -----------------------------------------------------------------------
+    // Read/query helper methods (Batch 13)
+    // -----------------------------------------------------------------------
+
+    // Lock should be held by caller.
+    private (int Next, Exception? Error) CheckSkipFirstBlock(string filter, bool wc, int bi)
+    {
+        if (string.IsNullOrEmpty(filter) || filter == ">")
+            return (bi + 1, null);
+
+        var start = uint.MaxValue;
+        uint stop = 0;
+
+        if (_psim is { } psim)
+        {
+            if (wc)
+            {
+                psim.Match(Encoding.UTF8.GetBytes(filter), (_, psi) =>
+                {
+                    if (psi.Fblk < start)
+                        start = psi.Fblk;
+                    if (psi.Lblk > stop)
+                        stop = psi.Lblk;
+                    return true;
+                });
+            }
+            else
+            {
+                var (psi, ok) = psim.Find(Encoding.UTF8.GetBytes(filter));
+                if (ok && psi != null)
+                {
+                    start = psi.Fblk;
+                    stop = psi.Lblk;
+                }
+            }
+        }
+
+        if (start == uint.MaxValue)
+            return (-1, StoreErrors.ErrStoreEOF);
+
+        return SelectSkipFirstBlock(bi, start, stop);
+    }
+
+    // Lock should be held by caller.
+    private (int Next, Exception? Error) CheckSkipFirstBlockMulti(SimpleSublist? sl, int bi)
+    {
+        if (_psim == null || sl == null)
+            return (-1, StoreErrors.ErrStoreEOF);
+
+        var start = uint.MaxValue;
+        uint stop = 0;
+
+        _psim.IterFast((subj, psi) =>
+        {
+            var matched = false;
+            sl.Match(Encoding.UTF8.GetString(subj), _ => matched = true);
+            if (matched)
+            {
+                if (psi.Fblk < start)
+                    start = psi.Fblk;
+                if (psi.Lblk > stop)
+                    stop = psi.Lblk;
+            }
+            return true;
+        });
+
+        if (start == uint.MaxValue)
+            return (-1, StoreErrors.ErrStoreEOF);
+
+        return SelectSkipFirstBlock(bi, start, stop);
+    }
+
+    // Lock should be held by caller.
+    private (int Next, Exception? Error) SelectSkipFirstBlock(int bi, uint start, uint stop)
+    {
+        if (bi < 0 || bi >= _blks.Count)
+            return (-1, StoreErrors.ErrStoreEOF);
+
+        var mbi = _blks[bi].Index;
+        if (stop <= mbi)
+            return (-1, StoreErrors.ErrStoreEOF);
+
+        if (start > mbi && _bim.TryGetValue(start, out var mb) && mb != null)
+        {
+            var ni = -1;
+            for (var i = 0; i < _blks.Count; i++)
+            {
+                if (mb.Last.Seq <= _blks[i].Last.Seq)
+                {
+                    ni = i;
+                    break;
+                }
+            }
+
+            return (ni, null);
+        }
+
+        return (bi + 1, null);
+    }
+
     // -----------------------------------------------------------------------
     // IStreamStore — type / state
     // -----------------------------------------------------------------------