NVM Advanced Storage Objects

These are example implementations of the PackedList and DynamicList structures in Python.

• PackedList
• DynamicList

Background

NVM prevents arrays from holding more than 1024 items.

The PackedList is used to store more than 1024 items in a structure in NVM smart contracts.

The PackedList is limited that it can only hold 7 layers. That caps its storage at:

(1024 * 7 - 6)

or

7,162

total items.

A further implementation is the DynamicList storage object, that dynamically creates new PackedList's as needed. It can store 1024 PackedList objects for a total storage of:

1024 * (1024 * 7 - 6)

or

7,333,888

total items.

PackedList Implementation

The PackedList has the follow structure:

packed = {
  "array": [],
  "items": 0
}

Each time an item is added or removed from the array, the items count is changed. When the array becomes full, it is wrapped in a new array.

For example, if the maximum array length was 2:

[]            // Starting
[1]           // Add 1
[1, 2]        // Add 2
[[1, 2], 3]   // Add 3

To remove an item, just swap it with the last item and remove the last item

[1, 3]        // Remove 2

DynamicList Implementation

The DynamicList has the follow structure:

dynamic = {
  "packed": [],
  "items": 0
}

Each time an item is added or removed from the array, the items count is changed. When all of the PackedList objects in the packed array become full, a new PackedList is allocated and added.