Simon Willison’s Weblog

Zed Decoded: Rope & SumTree (via) Text editors like Zed need in-memory data structures that are optimized for handling large strings where text can be inserted or deleted at any point without needing to copy the whole string.

Ropes are a classic, widely used data structure for this.

Zed have their own implementation of ropes in Rust, but it's backed by something even more interesting: a SumTree, described here as a thread-safe, snapshot-friendly, copy-on-write B+ tree where each leaf node contains multiple items and a Summary for each Item, and internal tree nodes contain a Summary of the items in its subtree.

These summaries allow for some very fast traversal tree operations, such as turning an offset in the file into a line and row coordinate and vice-versa. The summary itself can be anything, so each application of SumTree in Zed collects different summary information.

Uses in Zed include tracking highlight regions, code folding state, git blame information, project file trees and more - over 20 different classes and counting.

Zed co-founder Nathan Sobo calls SumTree "the soul of Zed".

Also notable: this detailed article is accompanied by an hour long video with a four-way conversation between Zed maintainers providing a tour of these data structures in the Zed codebase.