Implements ChangePack, CheckPoint

yorkie/src/main/kotlin/dev/yorkie/document/change/ChangePack.kt

@@ -0,0 +1,34 @@
+package dev.yorkie.document.change
+
+import com.google.protobuf.ByteString
+import dev.yorkie.document.time.TimeTicket
+
+/**
+ * [ChangePack] is a unit for delivering changes in a document to the remote.
+ *
+ * @property documentKey the key of the document.
+ * @property checkPoint It is used to determine the client received changes.
+ * @property snapshot a byte array that encodes the document.
+ * @property minSyncedTicket the minimum logical time taken by clients who attach to the document.
+ * It is used to collect garbage on the replica on the client.
+ */
+internal class ChangePack(
+    val documentKey: String,
+    val checkPoint: CheckPoint,
+    val changes: List<Change>,
+    val snapshot: ByteString?,
+    val minSyncedTicket: TimeTicket?,
+) {
+
+    /**
+     * Returns the whether this [ChangePack] has changes or not.
+     */
+    val hasChanges
+        get() = changes.isNotEmpty()
+
+    /**
+     * Returns the whether this [ChangePack] has a snapshot or not.
+     */
+    val hasSnapshot
+        get() = snapshot?.isEmpty == false
+}

yorkie/src/main/kotlin/dev/yorkie/document/change/CheckPoint.kt

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+package dev.yorkie.document.change
+
+import kotlin.math.max
+
+/**
+ * [CheckPoint] is used to determine the changes sent and received by the client.
+ * This is immutable.
+ */
+internal data class CheckPoint(
+    val serverSeq: Long,
+    val clientSeq: Int,
+) {
+
+    /**
+     * Creates a new instance with increased client sequence.
+     */
+    fun increaseClientSeq(increase: Int): CheckPoint {
+        return if (increase == 0) {
+            this
+        } else {
+            CheckPoint(serverSeq, clientSeq + increase)
+        }
+    }
+
+    /**
+     * Creates a new instance with the given [CheckPoint] if it is greater
+     * than the values of internal properties.
+     */
+    fun forward(checkPoint: CheckPoint): CheckPoint {
+        return if (this == checkPoint) {
+            this
+        } else {
+            val serverSeq = max(serverSeq, checkPoint.serverSeq)
+            val clientSeq = max(clientSeq, checkPoint.clientSeq)
+            CheckPoint(serverSeq, clientSeq)
+        }
+    }
+
+    companion object {
+        val InitialCheckPoint = CheckPoint(0, 0)
+    }
+}