hamt

Overview ¶

Package hamt implements a Hash Array Mapped Trie over ipfs merkledag nodes. It is implemented mostly as described in the wikipedia article on HAMTs, however the table size is variable (usually 256 in our usages) as opposed to 32 as suggested in the article. The hash function used is currently Murmur3, but this value is configurable (the datastructure reports which hash function its using).

The one algorithmic change we implement that is not mentioned in the wikipedia article is the collapsing of empty shards. Given the following tree: ( '[' = shards, '{' = values ) [ 'A' ] -> [ 'B' ] -> { "ABC" }

|       L-> { "ABD" }
L-> { "ASDF" }

If we simply removed "ABC", we would end up with a tree where shard 'B' only has a single child. This causes two issues, the first, is that now we have an extra lookup required to get to "ABD". The second issue is that now we have a tree that contains only "ABD", but is not the same tree that we would get by simply inserting "ABD" into a new tree. To address this, we always check for empty shard nodes upon deletion and prune them to maintain a consistent tree, independent of insertion order.