dedent.go

/*
The MIT License (MIT)

Copyright (c) 2018 Peter Lithammer

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

package dedent

import (
	"bytes"
	"regexp"
	"strings"
)

var (
	whitespaceOnly    = regexp.MustCompile("(?m)^[ \t]+$")
	leadingWhitespace = regexp.MustCompile("(?m)(^[ \t]*)(?:[^ \t\n])")
)

// Dedent removes any common leading whitespace from every line in text.
//
// This can be used to make multiline strings to line up with the left edge of
// the display, while still presenting them in the source code in indented
// form.
func Dedent(text string) string {
	if strings.HasPrefix(text, "\n") {
		text = strings.TrimPrefix(text, "\n")
	}

	var margin string

	text = whitespaceOnly.ReplaceAllString(text, "")
	indents := leadingWhitespace.FindAllStringSubmatch(text, -1)

	// Look for the longest leading string of spaces and tabs common to all
	// lines.
	for i, indent := range indents {
		if i == 0 {
			margin = indent[1]
		} else if strings.HasPrefix(indent[1], margin) {
			// Current line more deeply indented than previous winner:
			// no change (previous winner is still on top).
			continue
		} else if strings.HasPrefix(margin, indent[1]) {
			// Current line consistent with and no deeper than previous winner:
			// it's the new winner.
			margin = indent[1]
		} else {
			// Current line and previous winner have no common whitespace:
			// there is no margin.
			margin = ""
			break
		}
	}

	if margin != "" {
		text = regexp.MustCompile("(?m)^"+margin).ReplaceAllString(text, "")
	}
	return text
}

// Bytes is like Dedent but for []bytes.
func Bytes(text []byte) []byte {
	if bytes.HasPrefix(text, []byte("\n")) {
		text = bytes.TrimPrefix(text, []byte("\n"))
	}

	var margin []byte

	text = whitespaceOnly.ReplaceAll(text, []byte{})
	indents := leadingWhitespace.FindAllSubmatch(text, -1)

	// Look for the longest leading string of spaces and tabs common to all
	// lines.
	for i, indent := range indents {
		if i == 0 {
			margin = indent[1]
		} else if bytes.HasPrefix(indent[1], margin) {
			// Current line more deeply indented than previous winner:
			// no change (previous winner is still on top).
			continue
		} else if bytes.HasPrefix(margin, indent[1]) {
			// Current line consistent with and no deeper than previous winner:
			// it's the new winner.
			margin = indent[1]
		} else {
			// Current line and previous winner have no common whitespace:
			// there is no margin.
			margin = []byte{}
			break
		}
	}

	if len(margin) > 0 {
		text = regexp.MustCompile("(?m)^"+string(margin)).ReplaceAll(text, []byte{})
	}
	return text
}