Lesson 01 · Splitting text into lines

Splitting text into lines

A diff tool compares two documents line by line, so the very first thing we need is a way to turn a blob of text into a sequence of lines. Today you build that splitter - the input to everything that follows.

The goal

Split a text string into a sequence of lines, dropping the empty trailing element a final newline leaves behind.

Start here - the target

TO DO

Scenario: Text becomes a clean sequence of lines

Giventhe text "a\nb\nc\n"

WhenLines is called on it

Thenit returns the three lines ["a", "b", "c"] (the trailing newline does not add a fourth empty line)

AndLines("a\nb") also returns ["a", "b"], and Lines("") returns an empty sequence

Background

Every diff you have ever read is line-based: it talks about lines added, lines removed, lines kept. So the atom of our whole library is the line, and the first job is to chop a document into an ordered sequence of them. From here on, “a document” means a []string of lines, and every algorithm we build compares two such sequences.

The one subtlety is the trailing newline. A well-formed text file usually ends with \n, and naively splitting "a\nb\nc\n" on \n yields a phantom empty string at the end. We drop that trailing empty element so "a\nb\nc\n" is three lines, not four. A document that does not end in a newline (like "a\nb") keeps its last line as-is. We are throwing away the information about whether the file ended in a newline for now - a later lesson brings it back when it matters for output.

Make it work

func Lines(text string) []string {
  if text == "" {
    return []string{}
  }
  parts := strings.Split(text, "\n")
  // a trailing newline leaves a final "" element - drop it
  if len(parts) > 0 && parts[len(parts)-1] == "" {
    parts = parts[:len(parts)-1]
  }
  return parts
}

CheckpointDONE

You can turn any document into the sequence of lines the diff will compare. Commit and stop here.