Lesson 25 · Loading a file - Build a Text Editor

Loading a file

An editor needs something to edit, so it has to read text in from the outside. Today you load a stream of bytes into the buffer, giving the piece table its immutable original.

The goal

Load text from a reader into the editor, replacing the buffer and resetting the cursor to the start.

Start here - the target

TO DO

Scenario: Loading text from a reader

Givenan editor and a reader yielding "line1\nline2"

Whenthe editor loads from the reader

Thenthe buffer Text() is "line1\nline2", LineCount() is 2, and the cursor is at Row 0, Col 0

Andloading from an empty reader gives Text() "" and LineCount() 1

Background

So far the buffer’s text came from a string literal in a test; a real editor reads it from a file - or any byte stream, which is why loading takes a generic reader rather than a filename. The bytes it reads become the piece table’s immutable original, exactly the role that buffer was built for: it is the never-changing baseline, and every edit you make afterward lives in the add buffer as pieces layered on top.

Loading replaces the buffer wholesale and resets the cursor to the origin, because you are now editing a different document. Reading through an interface rather than opening a path keeps this testable with an in-memory reader and keeps the file system - the genuinely OS-specific part - out of the core, to be wired up in the finalize pass. An empty stream still yields a one-line buffer, honoring the “always at least one line” rule from chapter one.

Make it work

func (e *Editor) Load(r io.Reader) error {
  data, err := io.ReadAll(r)
  if err != nil { return err }
  e.Buf = New(string(data)) // the loaded bytes become the original
  e.Row, e.Col = 0, 0
  return nil
}

CheckpointDONE

The editor can load text from any reader. Commit and stop here.