textedit/lib.go

// Package textedit provides a non-regexp, streaming editor to apply basic text manipulation.
package textedit

import "io"

// Editor instances can be used to edit a text stream. It is expected from the implementations to be reusable
// with fresh state after the Flush was called on the enclosing writer.
type Editor interface {

	// Edit takes an input rune and the current state of the editor, and returns zero or more resulting runes
	// and the updated state.
	Edit(r rune, state any) ([]rune, any)

	// On flushing the editing writer, the writer will call ReleaseState on each provided editor in the
	// configured sequence with their latest associated state, and if there is any pending text to be written
	// out, the editors should return it at that point.
	ReleaseState(state any) []rune
}

type editorFunc[S any] struct {
	edit         func(rune, S) ([]rune, S)
	releaseState func(S) []rune
}

// Writer implements the io.Writer interface, passing the input to every editor in the configured sequence, and
// writing out the edited text to the underlying io.Writer.
type Writer struct {
	out    io.Writer
	editor Editor
	state  any
	err    error
}

func (e editorFunc[S]) Edit(r rune, state any) ([]rune, any) {
	s, _ := state.(S)
	return e.edit(r, s)
}

func (e editorFunc[S]) ReleaseState(state any) []rune {
	s, _ := state.(S)
	r := e.releaseState(s)
	return r
}

// Func can be used to define an Editor providing only the edit and releaseState functions.
func Func[S any](edit func(r rune, state S) ([]rune, S), releaseState func(S) []rune) Editor {
	if edit == nil {
		edit = func(r rune, s S) ([]rune, S) { return []rune{r}, s }
	}

	if releaseState == nil {
		releaseState = func(S) []rune { return nil }
	}

	return editorFunc[S]{edit: edit, releaseState: releaseState}
}

// Replace is a built-in editor for simple replace operations. The arguments are mapped such that the ones at
// odd positions will be the sequences to match in the input, and the immediately following ones will be the
// sequences replacing the matched sequences. Empty replacement sequences can be used to deleted the matched
// ones. In case of odd number of arguments, an empty arg is automatically appended.
func Replace(a ...string) Editor {
	return replace(a...)
}

// Escape can be used for basic escaping, where the found characters from the chars argument are replaced with
// the same character prefixed by the escape char in the esc argument. The escape char will be escaped
// automatically.
func Escape(esc rune, chars ...rune) Editor {
	return escape(esc, chars...)
}

// Indent applies indentation to multi-line text. Duplicate, leading, trailing and non-space whitespaces, other
// than the newline character, are collapsed into a single space.
func Indent(first, rest string) Editor {
	return wrapIndent([]rune(first), []rune(rest), 0, 0)
}

// Wrap wrap wraps multiline text to define maximual text width. Duplicate, leading, trailing and non-space
// whitespaces are collapsed into a single space.
func Wrap(firstWidth, restWidth int) Editor {
	return wrapIndent(nil, nil, firstWidth, restWidth)
}

// WrapIndent is like Wrap, but applies indentation.
func WrapIndent(firstIndent, restIndent string, firstWidth, restWidth int) Editor {
	return wrapIndent([]rune(firstIndent), []rune(restIndent), firstWidth, restWidth)
}

// SingleLine is like Wrap, but with infinite text width.
func SingleLine() Editor {
	return sequence(
		replace("\n", " "),
		wrapIndent(nil, nil, 0, 0),
	)
}

// New initializes an editing writer. The editor instances will be called in the order they are passed in to
// New.
func New(out io.Writer, e ...Editor) *Writer {
	return &Writer{
		out:    out,
		editor: sequence(e...),
	}
}

func (w *Writer) write(r []rune) error {
	if w.err != nil {
		return w.err
	}

	for _, ri := range r {
		rr, s := w.editor.Edit(ri, w.state)
		if _, err := w.out.Write([]byte(string(rr))); err != nil {
			w.err = err
			return w.err
		}

		w.state = s
	}

	return nil
}

func (w *Writer) flush() error {
	if w.err != nil {
		return w.err
	}

	r := w.editor.ReleaseState(w.state)
	w.state = nil
	if _, err := w.out.Write([]byte(string(r))); err != nil {
		w.err = err
		return w.err
	}

	return nil
}

// Write calls the configured editors with the input and forwards their output to the underlying io.Writer. It
// always returns len(p) as the number bytes written. It only returns an error when the underlying writer
// returns and error.
func (w *Writer) Write(p []byte) (int, error) {
	return len(p), w.write([]rune(string(p)))
}

// WriteRune calls the configured editors with the input and forwards their output to the underlying io.Writer.
// It always returns byte length of the input rune as the number bytes written. It only returns an error when
// the underlying writer returns and error.
func (w *Writer) WriteRune(r rune) (int, error) {
	return len([]byte(string(r))), w.write([]rune{r})
}

// Flush makes the underlying editors to release their associated state, and writes out the resulting text to
// the underlying io.Writer, but first passes it to the subsequent editors for editing. When the used editor
// instances comply with the expectations of the Editor interface, the writer will have a fresh state and can be
// reused for further editing.
func (w *Writer) Flush() error {
	return w.flush()
}