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Postbox/ItemCacheMetaTable.swift

@@ -0,0 +1,80 @@
+import Foundation
+
+public typealias ItemCacheCollectionId = Int8
+
+public struct ItemCacheCollectionSpec {
+    public let lowWaterItemCount: Int32
+    public let highWaterItemCount: Int32
+    
+    public init(lowWaterItemCount: Int32, highWaterItemCount: Int32) {
+        self.lowWaterItemCount = lowWaterItemCount
+        self.highWaterItemCount = highWaterItemCount
+    }
+}
+
+struct ItemCacheCollectionState: Coding {
+    let nextAccessIndex: Int32
+    
+    init(decoder: Decoder) {
+        self.nextAccessIndex = decoder.decodeInt32ForKey("i")
+    }
+    
+    func encode(_ encoder: Encoder) {
+        encoder.encodeInt32(self.nextAccessIndex, forKey: "i")
+    }
+}
+
+final class ItemCacheMetaTable: Table {
+    static func tableSpec(_ id: Int32) -> ValueBoxTable {
+        return ValueBoxTable(id: id, keyType: .int64)
+    }
+    
+    private let sharedKey = ValueBoxKey(length: 8)
+    
+    private var cachedCollectionStates: [ItemCacheCollectionId: ItemCacheCollectionState] = [:]
+    private var updatedCollectionStateIds = Set<ItemCacheCollectionId>()
+    
+    private func key(_ id: ItemCacheCollectionId) -> ValueBoxKey {
+        self.sharedKey.setInt64(0, value: Int64(id))
+        return self.sharedKey
+    }
+    
+    private func get(_ id: ItemCacheCollectionId) -> ItemCacheCollectionState? {
+        if let cached = self.cachedCollectionStates[id] {
+            return cached
+        } else {
+            if let value = self.valueBox.get(self.table, key: self.key(id)), let state = Decoder(buffer: value).decodeRootObject() as? ItemCacheCollectionState {
+                self.cachedCollectionStates[id] = state
+                return state
+            } else {
+                return nil
+            }
+        }
+    }
+    
+    private func set(_ id: ItemCacheCollectionId, state: ItemCacheCollectionState) {
+        self.cachedCollectionStates[id] = state
+        self.updatedCollectionStateIds.insert(id)
+    }
+    
+    override func clearMemoryCache() {
+        self.cachedCollectionStates.removeAll()
+        self.updatedCollectionStateIds.removeAll()
+    }
+    
+    override func beforeCommit() {
+        if !self.updatedCollectionStateIds.isEmpty {
+            let sharedEncoder = Encoder()
+            for id in self.updatedCollectionStateIds {
+                if let state = self.cachedCollectionStates[id] {
+                    sharedEncoder.reset()
+                    sharedEncoder.encodeRootObject(state)
+                    self.valueBox.set(self.table, key: self.key(id), value: sharedEncoder.readBufferNoCopy())
+                }
+            }
+        }
+        
+        self.updatedCollectionStateIds.removeAll()
+    }
+}
+