Preserve redo log for mutable objects

Replay it during time travel debugging

src/runtime/object.js

@@ -0,0 +1,68 @@
+import {Multiversion, rollback_if_needed, wrap_methods, mutate} from './multiversion.js'
+
+export function create_object(initial, cxt, index, literals) {
+  const multiversion = new Multiversion(cxt)
+
+  let latest = {...initial}
+  const redo_log = []
+
+  function rollback_if_needed() {
+    if(multiversion.needs_rollback()) {
+      latest = {...initial}
+      for(let i = 0; i < redo_log.length; i++) {
+        const log_item = redo_log[i]
+        if(log_item.version_number > multiversion.cxt.version_counter) {
+          break
+        }
+        if(log_item.type == 'set') {
+          latest[log_item.prop] = log_item.value
+        } else if(log_item.type == 'delete') {
+          delete latest[log_item.prop]
+        } else {
+          throw new Error('illegal type')
+        }
+      }
+    }
+  }
+
+  const handler = {
+    get(target, prop, receiver) {
+      rollback_if_needed()
+      return latest[prop]
+    },
+
+    has(target, prop) {
+      rollback_if_needed()
+      return prop in latest
+    },
+
+    set(obj, prop, value) {
+      rollback_if_needed()
+      const version_number = ++multiversion.cxt.version_counter
+      if(multiversion.is_created_during_current_expansion()) {
+        redo_log.push({ type: 'set', prop, value, version_number })
+      }
+      latest[prop] = value
+      return true
+    },
+
+    ownKeys(target) {
+      rollback_if_needed()
+      return Object.keys(latest)
+    },
+
+    getOwnPropertyDescriptor(target, prop) {
+      rollback_if_needed()
+      return { 
+        configurable: true, 
+        enumerable: true, 
+        value: latest[prop],
+      };
+    },
+
+    // TODO delete property handler
+  }
+  const result = new Proxy(initial, handler)
+  literals.set(index, result)
+  return result
+}