This is a Vim plugin that provides keybindings (gS/gJ) to split single-line code constructs into multiple lines and join them back, covering 30+ languages. The architecture uses a callback-based system where per-filetype scripts register split/join callbacks in buffer-local variables, and the main engine (autoload/sj.vim) iterates through them. The codebase is modular and well-structured, with reusable utility functions for text replacement, cursor management, and parsing (e.g., sj#argparser). Each language has dedicated autoload files that implement specific transformations (e.g., hashes, blocks, if-clauses). The plugin supports customization via many settings and alignment through external plugins. It is thoroughly tested with RSpec/Vimrunner and is actively maintained.