23. Splicing the Text of a Document in Place

• After change 20, most of an edit's cost was the text: Edit copied the code points into a new slice, converted them back to a string, and rebuilt the code-point-to-byte offset table by decoding the whole string. It now replaces the edited range in the code points (or bytes) in place, builds the new string by concatenating substrings of the old one, and splices the offset table, adding the byte difference to the entries after the edit (input.replace). Nodes keep referring to the old string, which does not change. TestDocumentEditText checks after random edits with multi-byte characters and invalid UTF-8, in both units, that the text and offsets equal those of the edited text read from scratch.
• Effect (the program in the streaming guide, 100,000 lines, Apple M3 Max): Edit 12.3 → 2.7 ms (one rule per line), 16.5 → 3.4 ms (Operators Rule lines), 9.2 → 0.3 ms (no memo entries); JSON document of 283 KB 6.6 → 2.7 ms per edit.