add Skip struct

src/structs/Skip.js

@@ -0,0 +1,58 @@
+
+import {
+  AbstractStruct,
+  AbstractUpdateEncoder, StructStore, Transaction, ID // eslint-disable-line
+} from '../internals.js'
+import * as error from 'lib0/error.js'
+
+export const structSkipRefNumber = 10
+
+/**
+ * @private
+ */
+export class Skip extends AbstractStruct {
+  get deleted () {
+    return true
+  }
+
+  delete () {}
+
+  /**
+   * @param {Skip} right
+   * @return {boolean}
+   */
+  mergeWith (right) {
+    if (this.constructor !== right.constructor) {
+      return false
+    }
+    this.length += right.length
+    return true
+  }
+
+  /**
+   * @param {Transaction} transaction
+   * @param {number} offset
+   */
+  integrate (transaction, offset) {
+    // skip structs cannot be integrated
+    error.unexpectedCase()
+  }
+
+  /**
+   * @param {AbstractUpdateEncoder} encoder
+   * @param {number} offset
+   */
+  write (encoder, offset) {
+    encoder.writeInfo(structSkipRefNumber)
+    encoder.writeLen(this.length - offset)
+  }
+
+  /**
+   * @param {Transaction} transaction
+   * @param {StructStore} store
+   * @return {null | number}
+   */
+  getMissing (transaction, store) {
+    return null
+  }
+}