1 files changed, 112 insertions, 0 deletions
diff --git a/src/Godosa.Core/Runs/Scripts/CheckpointCache.cs b/src/Godosa.Core/Runs/Scripts/CheckpointCache.cs
new file mode 100644
index 0000000..1e1dd3a
--- /dev/null
+++ b/src/Godosa.Core/Runs/Scripts/CheckpointCache.cs
@@ -0,0 +1,112 @@
+// Provenance: own design — shared run scripts (spec §5.4)
+using System.Buffers.Binary;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Godosa.Core.Runs.Scripts;
+
+public sealed record LabelState(string Name, long Start, string File, int Line);
+
+public sealed record RunnerState(long Frame, ulong Digest, List<ScopeState> Scopes, List<LabelState> Labels);
+
+public sealed record CachedCheckpoint(RunnerState State, byte[] Game);
+
+public static class CheckpointKeys
+{
+    public const string RunnerVersion = "run-scripts 1";
+
+    public static IReadOnlyDictionary<int, string> Compute(ScriptProgram p, string contentVersion, ulong seed)
+    {
+        var d = new StateDigest();
+        d.Add(RunnerVersion);
+        d.Add(contentVersion);
+        d.Add(seed);
+        d.Add((int)p.Mode);
+        foreach (var (k, v) in p.Vars.OrderBy(kv => kv.Key, StringComparer.Ordinal))
+            d.Add($"{k}={v}");
+        var keys = new Dictionary<int, string>();
+        for (var i = 0; i < p.Code.Count; i++)
+        {
+            var instr = p.Code[i];
+            d.Add((int)instr.Op);
+            d.Add(instr.Format);
+            d.Add(instr.At.File);
+            d.Add(instr.Text);
+            if (instr.Op == Op.Checkpoint)
+                keys[i] = d.Hex;
+        }
+        return keys;
+    }
+}
+
+public sealed class CheckpointCache(string dir, long capBytes)
+{
+    private static readonly byte[] Magic = "GRS1"u8.ToArray();
+    private static readonly JsonSerializerOptions Json = new() { Converters = { new JsonStringEnumConverter() } };
+
+    public CachedCheckpoint? Load(string key)
+    {
+        var path = Path.Combine(dir, key + ".snap");
+        if (!File.Exists(path))
+            return null;
+        byte[] bytes;
+        try
+        {
+            bytes = File.ReadAllBytes(path);
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException)
+        {
+            return null;
+        }
+        if (bytes.Length < 8 || !bytes.AsSpan(0, 4).SequenceEqual(Magic))
+            return null;
+        var length = BinaryPrimitives.ReadInt32LittleEndian(bytes.AsSpan(4));
+        if (length < 0 || 8 + length > bytes.Length)
+            return null;
+        RunnerState? state;
+        try
+        {
+            state = JsonSerializer.Deserialize<RunnerState>(Encoding.UTF8.GetString(bytes, 8, length), Json);
+        }
+        catch (JsonException)
+        {
+            return null; // a corrupt entry is a miss
+        }
+        return state == null ? null : new CachedCheckpoint(state, bytes[(8 + length)..]);
+    }
+
+    public void Save(string key, CachedCheckpoint cp)
+    {
+        Directory.CreateDirectory(dir);
+        var json = Encoding.UTF8.GetBytes(JsonSerializer.Serialize(cp.State, Json));
+        var path = Path.Combine(dir, key + ".snap");
+        var tmp = path + ".tmp";
+        using (var f = File.Create(tmp))
+        {
+            f.Write(Magic);
+            Span<byte> len = stackalloc byte[4];
+            BinaryPrimitives.WriteInt32LittleEndian(len, json.Length);
+            f.Write(len);
+            f.Write(json);
+            f.Write(cp.Game);
+        }
+        File.Move(tmp, path, overwrite: true);
+        Evict(path);
+    }
+
+    private void Evict(string keep)
+    {
+        var files = new DirectoryInfo(dir).GetFiles("*.snap").OrderBy(f => f.LastWriteTimeUtc).ToList();
+        var total = files.Sum(f => f.Length);
+        foreach (var f in files)
+        {
+            if (total <= capBytes)
+                break;
+            if (f.FullName == Path.GetFullPath(keep))
+                continue;
+            total -= f.Length;
+            f.Delete();
+        }
+    }
+}