gfx/code_editor.h

Why a plugin has one

A plugin that runs a user's script -- a Lua patch, a formula for a waveshaper, a live-coded sequencer -- or that lets somebody edit the XML of a preset or a mapping by hand needs a place to type that is not a text field. Code is long, indented, line-based and read by its colours; a TextArea that wraps it, eats its tabs and redraws all of it per keystroke fails at every one of those.

Lines, not a buffer

TextArea keeps one vector of characters and wraps it again on change, which is right for a paragraph. A source file is a hundred thousand lines: inserting a character in one flat buffer moves megabytes, and every question a caret asks becomes a search for line starts. CodeDocument keeps a string per line, so an edit touches its own line and a line number is an index. Absolute offsets -- what find and a listener speak -- come from line starts computed lazily and invalidated only from the edited line down: an edit near the end of a long file does not recount its beginning.

Line endings

Each line remembers how it ended -- LF, CR LF or a lone CR -- so a file with mixed endings saves byte for byte as it loaded, and a diff of an edit shows the line edited rather than every line. A line break is ONE character in every offset whatever bytes it is saved as, so a caret never stands between a CR and its LF. New breaks -- typed, pasted, inserted -- take the document's newline, which loading sets to the file's commonest ending; normaliseLineEndings() brings the rest into line, undoably.

Undo is commands here, not snapshots

undo.h keeps whole states, and says why that is right for a plugin's state. For a document of megabytes a snapshot per keystroke is the wrong trade, so every edit is recorded as the text it inserted or removed, grouped into transactions (newTransaction), adjacent typing merged into one record, and the history bounded by a budget of characters.

Colouring a line at a time

A tokeniser reads ONE line, given the state the line before it ended in -- inside a block comment, a raw string, a string continued with a backslash -- and returns the state this line ends in. The editor keeps those states, one per line, valid down to the first line edited since: painting the bottom of a file scans the lines above it once, without building tokens, and an edit re-scans from its own line. Tokens are only built for the lines on screen.

Monospaced, on a grid

Every character stands in a column -- a tab reaches the next stop, an East Asian wide character takes two, a combining mark none -- and is drawn at its column's x. A caret on a grid is arithmetic, and a font that is not monospaced still lines up, drawn a character at a time. The price is that no bidirectional reordering or Arabic joining happens: a grid of cells cannot show those, and a caret moving over the cells could not follow them.