less_conversion.rb

require_relative 'char_string_scanner'
require 'bootstrap-sass/version'

# This is the script used to automatically convert all of twbs/bootstrap LESS to Sass.
#
# Most differences are fixed by regexps and other forms of string substitution.
# There are Bootstrap-specific workarounds for the lack of parent selectors, recursion, mixin namespaces, extend within @media, etc in Sass 3.2.
class Converter
  module LessConversion
    # Some regexps for matching bits of SCSS:
    SELECTOR_CHAR               = '\[\]$\w\-{}#,.:&>@'
    # 1 selector (the part before the {)
    SELECTOR_RE                 = /[#{SELECTOR_CHAR}]+[#{SELECTOR_CHAR}\s]*/
    # 1 // comment
    COMMENT_RE                  = %r((?:^[ \t]*//[^\n]*\n))
    # 1 {, except when part of @{ and #{
    RULE_OPEN_BRACE_RE          = /(?<![@#\$])\{/
    # same as the one above, but in reverse (on a reversed string)
    RULE_OPEN_BRACE_RE_REVERSE  = /\{(?![@#\$])/
    # match closed brace, except when \w precedes }, or when }[.'"]. a heurestic to exclude } that are not selector body close }
    RULE_CLOSE_BRACE_RE         = /(?<!\w)\}(?![.'"])/
    RULE_CLOSE_BRACE_RE_REVERSE = /(?<![.'"])\}(?!\w)/
    # match any brace that opens or closes a properties body
    BRACE_RE                    = /#{RULE_OPEN_BRACE_RE}|#{RULE_CLOSE_BRACE_RE}/m
    BRACE_RE_REVERSE            = /#{RULE_OPEN_BRACE_RE_REVERSE}|#{RULE_CLOSE_BRACE_RE_REVERSE}/m
    # valid characters in mixin definitions
    SCSS_MIXIN_DEF_ARGS_RE      = /[\w\-,\s$:#%()]*/
    LESS_MIXIN_DEF_ARGS_RE      = /[\w\-,;.\s@:#%()]*/

    # These mixins will get vararg definitions in SCSS (not supported by LESS):
    NESTED_MIXINS               = {'#gradient' => 'gradient'}

    # These mixins will get vararg definitions in SCSS (not supported by LESS):
    VARARG_MIXINS               = %w(
      scale transition transition-duration transition-property transition-transform box-shadow
    )

    # A list of classes that will be extracted into mixins
    # Only the top-level selectors of form .CLASS { ... } are extracted. CLASS must not be used in any other rule definition.
    # This is a work-around for libsass @extend issues
    CLASSES_TO_MIXINS = %w(
      list-unstyled form-inline
    )

    # Convert a snippet of bootstrap LESS to Scss
    def convert_less(less)
      less = convert_to_scss(less)
      less = yield(less) if block_given?
      less
    end

    def shared_mixins
      @shared_mixins ||= begin
        log_status '  Reading shared mixins from mixins.less'
        CLASSES_TO_MIXINS + read_mixins(read_files('less', bootstrap_less_files.grep(/mixins\//)).values.join("\n"),
                                        nested: NESTED_MIXINS)
      end
    end

    def process_stylesheet_assets
      log_status 'Processing stylesheets...'
      files   = read_files('less', bootstrap_less_files)
      save_to = @save_to[:scss]

      log_status '  Converting LESS files to Scss:'
      files.each do |name, file|
        log_processing name
        # apply common conversions
        file = convert_less(file)
        file = replace_all file, %r{// stylelint-disable.*?\n+}, '', optional: true
        if name.start_with?('mixins/')
          file = varargify_mixin_definitions(file, *VARARG_MIXINS)
          %w(responsive-(in)?visibility input-size text-emphasis-variant bg-variant).each do |mixin|
            file = parameterize_mixin_parent_selector file, mixin if file =~ /#{mixin}/
          end
          NESTED_MIXINS.each do |sel, name|
            file = flatten_mixins(file, sel, name) if /#{Regexp.escape(sel)}/ =~ file
          end
          file = replace_all file, /(?<=[.-])\$state/, '#{$state}' if file =~ /[.-]\$state/
        end
        case name
          when 'mixins/buttons.less'
            file = replace_all file, /(\.dropdown-toggle)&/, '&\1'
          when 'mixins/list-group.less'
            file = replace_rules(file, '  .list-group-item-') { |rule| extract_nested_rule rule, 'a&' }
          when 'mixins/gradients.less'
            file = replace_ms_filters(file)
            file = deinterpolate_vararg_mixins(file)
          when 'mixins/vendor-prefixes.less'
            # remove second scale mixins as this is handled via vararg in the first one
            file = replace_rules(file, Regexp.escape('@mixin scale($ratioX, $ratioY...)')) { '' }
          when 'mixins/grid-framework.less'
            file = convert_grid_mixins file
          when 'component-animations.less'
            file = extract_nested_rule file, "#{SELECTOR_RE}&\\.in"
          when 'responsive-utilities.less'
            file = apply_mixin_parent_selector file, '\.(?:visible|hidden)'
          when 'variables.less'
            file = insert_default_vars(file)
            file = ['$bootstrap-sass-asset-helper: false !default;', file].join("\n")
            file = replace_all file, %r{(\$icon-font-path): \s*"(.*)" (!default);}, "\n" + unindent(<<-SCSS, 14)
              // [converter] If $bootstrap-sass-asset-helper if used, provide path relative to the assets load path.
              // [converter] This is because some asset helpers, such as Sprockets, do not work with file-relative paths.
              \\1: if($bootstrap-sass-asset-helper, "bootstrap/", "\\2bootstrap/") \\3;
            SCSS
          when 'breadcrumbs.less'
            file = replace_all file, /(.*)(\\00a0)/, unindent(<<-SCSS, 8) + "\\1\#{$nbsp}"
              // [converter] Workaround for https://github.com/sass/libsass/issues/1115
              $nbsp: "\\2";
            SCSS
          when 'close.less'
            # extract .close { button& {...} } rule
            file = extract_nested_rule file, 'button&'
          when 'dropdowns.less'
            file = replace_all file, /@extend \.dropdown-menu-right;/, 'right: 0; left: auto;'
            file = replace_all file, /@extend \.dropdown-menu-left;/, 'left: 0; right: auto;'
          when 'forms.less'
            file = extract_nested_rule file, 'textarea&'
            file = apply_mixin_parent_selector(file, '\.input-(?:sm|lg)')
            file = replace_rules file, /\.form-group-(?:sm|lg)/ do |rule|
              apply_mixin_parent_selector rule, '.form-control'
            end
          when 'navbar.less'
            file = replace_all file, /(\s*)\.navbar-(right|left)\s*\{\s*@extend\s*\.pull-(right|left);\s*/, "\\1.navbar-\\2 {\\1  float: \\2 !important;\\1"
          when 'tables.less'
            file = replace_all file, /(@include\s*table-row-variant\()(\w+)/, "\\1'\\2'"
          when 'thumbnails.less', 'labels.less', 'badges.less', 'buttons.less'
            file = extract_nested_rule file, 'a&'
          when 'glyphicons.less'
            file = replace_rules(file, /\s*@font-face/) { |rule| replace_asset_url rule, :font }
          when 'type.less'
            file = apply_mixin_parent_selector(file, '\.(text|bg)-(success|primary|info|warning|danger)')
            # .bg-primary will not get patched automatically as it includes an additional rule. fudge for now
            file = replace_all(file, "  @include bg-variant($brand-primary);\n}", "}\n@include bg-variant('.bg-primary', $brand-primary);")
        end

        path = File.join save_to, name.sub(/\.less$/, '.scss')
        path = File.join File.dirname(path), '_' + File.basename(path)
        save_file(path, file)
        log_processed File.basename(path)
      end

      # move bootstrap/_bootstrap.scss to _bootstrap.scss adjusting import paths
      main_from = "#{save_to}/_bootstrap.scss"
      main_to   = File.expand_path("#{save_to}/../_bootstrap.scss")
      save_file main_to, File.read(main_from).gsub(/ "/, ' "bootstrap/')
      File.delete(main_from)

      # generate variables template
      save_file 'templates/project/_bootstrap-variables.sass',
                "// Override Bootstrap variables here (defaults from bootstrap-sass v#{Bootstrap::VERSION}):\n\n" +
                    File.read("#{save_to}/_variables.scss").lines[1..-1].join.gsub(/^(?=\$)/, '// ').gsub(/ !default;/, '')
    end

    def bootstrap_less_files
      @bootstrap_less_files ||= get_paths_by_type('less', /\.less$/)
    end

    # apply general less to scss conversion
    def convert_to_scss(file)
      # get local mixin names before converting the definitions
      mixins = shared_mixins + read_mixins(file)
      file   = replace_vars(file)
      file   = replace_mixin_definitions(file)
      file   = replace_mixins(file, mixins)
      file   = extract_mixins_from_selectors(file, CLASSES_TO_MIXINS.inject({}) { |h, cl| h.update(".#{cl}" => cl) })
      file   = replace_spin(file)
      file   = replace_fadein(file)
      file   = replace_image_urls(file)
      file   = replace_escaping(file)
      file   = convert_less_ampersand(file)
      file   = deinterpolate_vararg_mixins(file)
      file   = replace_calculation_semantics(file)
      file   = replace_file_imports(file)
      file   = wrap_at_groups_with_at_root(file)
      file   = replace_division(file)
      file
    end

    def wrap_at_groups_with_at_root(file)
      replace_rules(file, /@(?:font-face|-ms-viewport)/) { |rule, _pos|
        %Q(@at-root {\n#{indent rule, 2}\n})
      }
    end

    def replace_division(less)
      re = /(?<!\w)\(\s*([^(]+?)\s+\/\s+([^)]+?)\s*\)/
      return less if less !~ re
      "@use \"sass:math\";\n" + less.gsub(re, 'math.div(\1, \2)')
    end

    def sass_fn_exists(fn)
      %Q{(#{fn}("") != unquote('#{fn}("")'))}
    end

    def replace_asset_url(rule, type)
      replace_all rule, /url\((.*?)\)/, "url(if($bootstrap-sass-asset-helper, twbs-#{type}-path(\\1), \\1))"
    end

    # convert recursively evaluated selector $list to @for loop
    def mixin_all_grid_columns(css, selector: raise('pass class'), from: 1, to: raise('pass to'))
      mxn_def = css.each_line.first.strip
      # inject local variables as default arguments
      # this is to avoid overwriting outer variables with the same name with Sass <= 3.3
      # see also: https://github.com/twbs/bootstrap-sass/issues/636
      locals = <<-SASS.strip
        $i: #{from}, $list: "#{selector}"
      SASS
      mxn_def.sub!(/(\(?)(\)\s*\{)/) {  "#{$1}#{', ' if $1.empty?}#{locals}#{$2}" }
      step_body = (css =~ /\$list \{\n(.*?)\n[ ]*\}/m) && $1
<<-SASS
// [converter] This is defined recursively in LESS, but Sass supports real loops
#{mxn_def}
  @for $i from (#{from} + 1) through #{to} {
    $list: "\#{$list}, #{selector}";
  }
  \#{$list} {
#{unindent step_body, 2}
  }
}
SASS
    end

    # convert grid mixins LESS when => Sass @if
    def convert_grid_mixins(file)
      file = replace_rules file, /@mixin make-grid-columns/, comments: false do |css, pos|
        mixin_all_grid_columns css, selector: '.col-xs-#{$i}, .col-sm-#{$i}, .col-md-#{$i}, .col-lg-#{$i}', to: '$grid-columns'
      end
      file = replace_rules file, /@mixin float-grid-columns/, comments: false do |css, pos|
        mixin_all_grid_columns css, selector: '.col-#{$class}-#{$i}', to: '$grid-columns'
      end
      file = replace_rules file, /@mixin calc-grid-column/ do |css|
        css = indent css.gsub(/.*when (.*?) {/, '@if \1 {').gsub(/(\$[\w-]+)\s+=\s+(\w+)/, '\1 == \2').gsub(/(?<=-)(\$[a-z]+)/, '#{\1}')
        if css =~ /== width/
          css = "@mixin calc-grid-column($index, $class, $type) {\n#{css}"
        elsif css =~ /== offset/
          css += "\n}"
        end
        css
      end
      file = replace_rules file, /@mixin loop-grid-columns/ do |css|
        unindent <<-SASS, 8
        // [converter] This is defined recursively in LESS, but Sass supports real loops
        @mixin loop-grid-columns($columns, $class, $type) {
          @for $i from 0 through $columns {
            @include calc-grid-column($i, $class, $type);
          }
        }
        SASS
      end
      file
    end


    # We need to keep a list of shared mixin names in order to convert the includes correctly
    # Before doing any processing we read shared mixins from a file
    # If a mixin is nested, it gets prefixed in the list (e.g. #gradient > .horizontal to 'gradient-horizontal')
    def read_mixins(mixins_file, nested: {})
      mixins = get_mixin_names(mixins_file, silent: true)
      nested.each do |selector, prefix|
        # we use replace_rules without replacing anything just to use the parsing algorithm
        replace_rules(mixins_file, selector) { |rule|
          mixins += get_mixin_names(unindent(unwrap_rule_block(rule)), silent: true).map { |name| "#{prefix}-#{name}" }
          rule
        }
      end
      mixins.uniq!
      mixins.sort!
      log_file_info "mixins: #{mixins * ', '}" unless mixins.empty?
      mixins
    end

    def get_mixin_names(file, opts = {})
      names = get_css_selectors(file).join("\n" * 2).scan(/^\.([\w-]+)\(#{LESS_MIXIN_DEF_ARGS_RE}\)(?: when.*?)?[ ]*\{/).map(&:first).uniq.sort
      log_file_info "mixin defs: #{names * ', '}" unless opts[:silent] || names.empty?
      names
    end

    # margin: a -b
    # LESS: sets 2 values
    # Sass: sets 1 value (a-b)
    # This wraps a and -b so they evaluates to 2 values in Sass
    def replace_calculation_semantics(file)
      # split_prop_val.call('(@navbar-padding-vertical / 2) -@navbar-padding-horizontal')
      # #=> ["(navbar-padding-vertical / 2)", "-navbar-padding-horizontal"]
      split_prop_val = proc { |val|
        s         = CharStringScanner.new(val)
        r         = []
        buff      = ''
        d         = 0
        prop_char = %r([\$\w\-/\*\+%!])
        while (token = s.scan_next(/([\)\(]|\s+|#{prop_char}+)/))
          buff << token
          case token
            when '('
              d += 1
            when ')'
              d -= 1
              if d == 0
                r << buff
                buff = ''
              end
            when /\s/
              if d == 0 && !buff.strip.empty?
                r << buff
                buff = ''
              end
          end
        end
        r << buff unless buff.empty?
        r.map(&:strip)
      }

      replace_rules file do |rule|
        replace_properties rule do |props|
          props.gsub /(?<!\w)([\w-]+):(.*?);/ do |m|
            prop, vals = $1, split_prop_val.call($2)
            next m unless vals.length >= 2 && vals.any? { |v| v =~ /^[\+\-]\$/ }
            transformed = vals.map { |v| v.strip =~ %r(^\(.*\)$) ? v : "(#{v})" }
            log_transform "property #{prop}: #{transformed * ' '}", from: 'wrap_calculation'
            "#{prop}: #{transformed * ' '};"
          end
        end
      end
    end

    # @import "file.less" to "#{target_path}file;"
    def replace_file_imports(less, target_path = '')
      less.gsub %r([@\$]import ["|']([\w\-/]+).less["|'];),
                %Q(@import "#{target_path}\\1";)
    end

    def replace_all(file, regex, replacement = nil, optional: false, &block)
      log_transform regex, replacement
      new_file = file.gsub(regex, replacement, &block)
      raise "replace_all #{regex}, #{replacement} NO MATCH" if !optional && file == new_file
      new_file
    end

    # @mixin a() { tr& { color:white } }
    # to:
    # @mixin a($parent) { tr#{$parent} { color: white } }
    def parameterize_mixin_parent_selector(file, rule_sel)
      log_transform rule_sel
      param = '$parent'
      replace_rules(file, '^\s*@mixin\s*' + rule_sel) do |mxn_css|
        mxn_css.sub! /(?=@mixin)/, "// [converter] $parent hack\n"
        # insert param into mixin def
        mxn_css.sub!(/(@mixin [\w-]+)\(([\$\w\-,\s]*)\)/) { "#{$1}(#{param}#{', ' if $2 && !$2.empty?}#{$2})" }
        # wrap properties in #{$parent} { ... }
        replace_properties(mxn_css) { |props|
          next props if props.strip.empty?
          spacer = ' ' * indent_width(props)
          "#{spacer}\#{#{param}} {\n#{indent(props.sub(/\s+\z/, ''), 2)}\n#{spacer}}"
        }
        # change nested& rules to nested#{$parent}
        replace_rules(mxn_css, /.*&[ ,:]/) { |rule| replace_in_selector rule, /&/, "\#{#{param}}" }
      end
    end

    # extracts rule immediately after it's parent, and adjust the selector
    # .x { textarea& { ... }}
    # to:
    # .x { ... }
    # textarea.x { ... }
    def extract_nested_rule(file, selector, new_selector = nil)
      matches = []
      # first find the rules, and remove them
      file    = replace_rules(file, "\s*#{selector}", comments: true) { |rule, pos, css|
        new_sel = new_selector || "#{get_selector(rule).gsub(/&/, selector_for_pos(css, pos.begin))}"
        matches << [rule, pos, new_sel]
        indent "// [converter] extracted #{get_selector(rule)} to #{new_sel}".tr("\n", ' ').squeeze(' '), indent_width(rule)
      }
      raise "extract_nested_rule: no such selector: #{selector}" if matches.empty?
      # replace rule selector with new_selector
      matches.each do |m|
        m[0].sub! /(#{COMMENT_RE}*)^(\s*).*?(\s*){/m, "\\1\\2#{m[2]}\\3{"
        log_transform selector, m[2]
      end
      replace_substrings_at file,
                            matches.map { |_, pos| close_brace_pos(file, pos.begin, 1) + 1 },
                            matches.map { |rule, _| "\n\n" + unindent(rule) }
    end

    # .visible-sm { @include responsive-visibility() }
    # to:
    # @include responsive-visibility('.visible-sm')
    def apply_mixin_parent_selector(file, rule_sel)
      log_transform rule_sel
      replace_rules file, '\s*' + rule_sel, comments: false do |rule, rule_pos, css|
        body = unwrap_rule_block(rule.dup).strip
        next rule unless body =~ /^@include \w+/m || body =~ /^@media/ && body =~ /\{\s*@include/
        rule =~ /(#{COMMENT_RE}*)([#{SELECTOR_CHAR}\s*]+?)#{RULE_OPEN_BRACE_RE}/
        cmt, sel = $1, $2.strip
        # take one up selector chain if this is an &. selector
        if sel.start_with?('&')
          parent_sel = selector_for_pos(css, rule_pos.begin)
          sel        = parent_sel + sel[1..-1]
        end
        # unwrap, and replace @include
        unindent unwrap_rule_block(rule).gsub(/(@include [\w-]+)\(?([\$\w\-,\s]*)\)?/) {
          name, args = $1, $2
          sel.gsub(/\s+/, ' ').split(/,\s*/ ).map { |sel_part|
            "#{cmt}#{name}('#{sel_part}'#{', ' if args && !args.empty?}#{args})"
          }.join(";\n")
        }
      end
    end

    # #gradient > { @mixin horizontal ... }
    # to:
    # @mixin gradient-horizontal
    def flatten_mixins(file, container, prefix)
      log_transform container, prefix
      replace_rules file, Regexp.escape(container) do |mixins_css|
        unindent unwrap_rule_block(mixins_css).gsub(/@mixin\s*([\w-]+)/, "@mixin #{prefix}-\\1")
      end
    end

    # .btn { ... } -> @mixin btn { ... }; .btn { @include btn }
    def extract_mixins_from_selectors(file, selectors_to_mixins)
      selectors_to_mixins.each do |selector, mixin|
        file = replace_rules file, Regexp.escape(selector), prefix: false do |selector_css|
          log_transform "#{selector} { ... } -> @mixin #{mixin} { ... }; #{selector} { @include #{mixin} } ", from: 'extract_mixins_from_selectors'
          <<-SCSS
// [converter] extracted from `#{selector}` for libsass compatibility
@mixin #{mixin} {#{unwrap_rule_block(selector_css)}
}
// [converter] extracted as `@mixin #{mixin}` for libsass compatibility
#{selector} {
  @include #{mixin};
}
          SCSS
        end
      end
      file
    end

    # @include and @extend from LESS:
    #  .mixin()             -> @include mixin()
    #  #scope > .mixin()    -> @include scope-mixin()
    #  &:extend(.mixin all) -> @include mixin()
    def replace_mixins(less, mixin_names)
      mixin_pattern = /(?<=^|\s)((?:[#|\.][\w-]+\s*>\s*)*)\.([\w-]+)\((.*)\)(?!\s\{)/

      less = less.gsub(mixin_pattern) do |_|
        scope, name, args = $1, $2, $3
        scope = scope.scan(/[\w-]+/).join('-') + '-' unless scope.empty?
        args = "(#{args.tr(';', ',')})" unless args.empty?
        if name && mixin_names.include?("#{scope}#{name}")
          "@include #{scope}#{name}#{args}"
        else
          "@extend .#{scope}#{name}"
        end
      end

      less.gsub /&:extend\((#{SELECTOR_RE})(?: all)?\)/ do
        selector = $1
        selector =~ /\.([\w-]+)/
        mixin    = $1
        if mixin && mixin_names.include?(mixin)
          "@include #{mixin}"
        else
          "@extend #{selector}"
        end
      end
    end

    # change Microsoft filters to Sass calling convention
    def replace_ms_filters(file)
      log_transform
      file.gsub(
          /filter: e\(%\("progid:DXImageTransform.Microsoft.gradient\(startColorstr='%d', endColorstr='%d', GradientType=(\d)\)", ?argb\(([\-$\w]+)\), ?argb\(([\-$\w]+)\)\)\);/,
          %Q(filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='\#{ie-hex-str(\\2)}', endColorstr='\#{ie-hex-str(\\3)}', GradientType=\\1);)
      )
    end

    # unwraps topmost rule block
    # #sel { a: b; }
    # to:
    # a: b;
    def unwrap_rule_block(css)
      css[(css =~ RULE_OPEN_BRACE_RE) + 1..-1].sub(/\n?}\s*\z/m, '')
    end

    def replace_mixin_definitions(less)
      less.gsub(/^(\s*)\.([\w-]+\(.*\))(\s*\{)/) { |match|
        "#{$1}@mixin #{$2.tr(';', ',')}#{$3}".sub(/,\)/, ')')
      }
    end

    def replace_vars(less)
      less = less.dup
      # skip header comment
      less =~ %r(\A/\*(.*?)\*/)m
      from           = $~ ? $~.to_s.length : 0
      less[from..-1] = less[from..-1].
          gsub(/(?!@mixin|@media|@page|@keyframes|@font-face|@-\w)@/, '$').
          # variables that would be ignored by gsub above: e.g. @page-header-border-color
          gsub(/@(page[\w-]+)/, '$\1')
      less
    end

    def replace_spin(less)
      less.gsub(/(?![\-$@.])spin(?!-)/, 'adjust-hue')
    end

    def replace_fadein(less)
      less.gsub(/(?![\-$@.])fadein\((.*?),\s*(.*?)%\)/) { "fade_in(#{$1}, #{$2.to_i / 100.0})" }
    end

    def replace_image_urls(less)
      less.gsub(/background-image: url\("?(.*?)"?\);/) { |s| replace_asset_url s, :image }
    end

    def replace_escaping(less)
      less = less.gsub(/~"([^"]+)"/, '\1').gsub(/~'([^']+)'/, '\1') # Get rid of ~"" escape
      less.gsub!(/\$\{([^}]+)\}/, '$\1') # Get rid of @{} escape
      # interpolate variables in strings, e.g. url("$file-1x") => url("#{$file-1x}")
      less.gsub!(/"[^"\n]*"/) { |str| str.gsub(/\$[^"\n$.\\]+/, '#{\0}') }
      less.gsub(/(\W)e\(%\("?([^"]*)"?\)\)/, '\1\2') # Get rid of e(%("")) escape
    end

    def insert_default_vars(scss)
      log_transform
      scss.gsub(/^(\$.+);/, '\1 !default;')
    end

    # Converts &-
    def convert_less_ampersand(less)
      regx = /^\.badge\s*\{[\s\/\w\(\)]+(&{1}-{1})\w.*?^}$/m

      tmp = ''
      less.scan(/^(\s*&)(-[\w\[\]]+\s*\{.+})$/) do |ampersand, css|
        tmp << ".badge#{css}\n"
      end

      less.gsub(regx, tmp)
    end

    # unindent by n spaces
    def unindent(txt, n = 2)
      txt.gsub /^[ ]{#{n}}/, ''
    end

    # indent by n spaces
    def indent(txt, n = 2)
      spaces = ' ' * n
      txt.gsub /^/, spaces
    end

    # get indent length from the first line of txt
    def indent_width(txt)
      txt.match(/\A\s*/).to_s.length
    end

    # @mixin transition($transition) {
    # to:
    # @mixin transition($transition...) {
    def varargify_mixin_definitions(scss, *mixins)
      scss = scss.dup
      replaced = []
      mixins.each do |mixin|
        if scss.gsub! /(@mixin\s*#{Regexp.quote(mixin)})\((#{SCSS_MIXIN_DEF_ARGS_RE})\)/, '\1(\2...)'
          replaced << mixin
        end
      end
      log_transform *replaced unless replaced.empty?
      scss
    end

    # @include transition(#{border-color ease-in-out .15s, box-shadow ease-in-out .15s})
    # to
    # @include transition(border-color ease-in-out .15s, box-shadow ease-in-out .15s)
    def deinterpolate_vararg_mixins(scss)
      scss = scss.dup
      VARARG_MIXINS.each do |mixin|
        if scss.gsub! /(@include\s*#{Regexp.quote(mixin)})\(\s*\#\{([^}]+)\}\s*\)/, '\1(\2)'
          log_transform mixin
        end
      end
      scss
    end

    # get full selector for rule_block
    def get_selector(rule_block)
      sel = /^\s*(#{SELECTOR_RE}?)\s*\{/.match(rule_block) && $1 && $1.strip
      sel.sub /\s*\{\n\s.*/m, ''
    end

    # replace CSS rule blocks matching rule_prefix with yield(rule_block, rule_pos)
    # will also include immediately preceding comments in rule_block
    #
    # option :comments -- include immediately preceding comments in rule_block
    #
    # replace_rules(".a{ \n .b{} }", '.b') { |rule, pos| ">#{rule}<"  } #=> ".a{ \n >.b{}< }"
    def replace_rules(less, selector = SELECTOR_RE, options = {}, &block)
      options       = {prefix: true, comments: true}.merge(options || {})
      less          = less.dup
      s             = CharStringScanner.new(less)
      rule_re       = if options[:prefix]
                        /(?:#{selector}[#{SELECTOR_CHAR})=(\s]*?#{RULE_OPEN_BRACE_RE})/
                      else
                        /#{selector}[\s]*#{RULE_OPEN_BRACE_RE}/
                      end
      rule_start_re = if options[:comments]
                        /(?:#{COMMENT_RE}*)^#{rule_re}/
                      else
                        /^#{rule_re}/
                      end

      positions = []
      while (rule_start = s.scan_next(rule_start_re))
        pos = s.pos
        positions << (pos - rule_start.length..close_brace_pos(less, pos - 1))
      end
      replace_substrings_at(less, positions, &block)
      less
    end

    # Get a all top-level selectors (with {)
    def get_css_selectors(css, opts = {})
      s         = CharStringScanner.new(css)
      selectors = []
      while s.scan_next(RULE_OPEN_BRACE_RE)
        brace_pos = s.pos
        def_pos   = css_def_pos(css, brace_pos+1, -1)
        sel       = css[def_pos.begin..brace_pos - 1].dup
        sel.strip! if opts[:strip]
        selectors << sel
        sel.dup.strip
        s.pos = close_brace_pos(css, brace_pos, 1) + 1
      end
      selectors
    end

    # replace in the top-level selector
    # replace_in_selector('a {a: {a: a} } a {}', /a/, 'b') => 'b {a: {a: a} } b {}'
    def replace_in_selector(css, pattern, sub)
      # scan for selector positions in css
      s        = CharStringScanner.new(css)
      prev_pos = 0
      sel_pos  = []
      while (brace = s.scan_next(RULE_OPEN_BRACE_RE))
        pos = s.pos
        sel_pos << (prev_pos .. pos - 1)
        s.pos    = close_brace_pos(css, s.pos - 1) + 1
        prev_pos = pos
      end
      replace_substrings_at(css, sel_pos) { |s| s.gsub(pattern, sub) }
    end


    # replace first level properties in the css with yields
    # replace_properties("a { color: white }") { |props| props.gsub 'white', 'red' }
    def replace_properties(css, &block)
      s = CharStringScanner.new(css)
      s.skip_until /#{RULE_OPEN_BRACE_RE}\n?/
      from = s.pos
      m = s.scan_next(/\s*#{SELECTOR_RE}#{RULE_OPEN_BRACE_RE}/) || s.scan_next(/\s*#{RULE_CLOSE_BRACE_RE}/)
      to = s.pos - m.length - 1
      replace_substrings_at css, [(from .. to)], &block
    end


    # immediate selector of css at pos
    def selector_for_pos(css, pos, depth = -1)
      css[css_def_pos(css, pos, depth)].dup.strip
    end

    # get the pos of css def at pos (search backwards)
    def css_def_pos(css, pos, depth = -1)
      to       = open_brace_pos(css, pos, depth)
      prev_def = to - (css[0..to].reverse.index(RULE_CLOSE_BRACE_RE_REVERSE) || to) + 1
      from     = prev_def + 1 + (css[prev_def + 1..-1] =~ %r(^\s*[^\s/]))
      (from..to - 1)
    end

    # next matching brace for brace at from
    def close_brace_pos(css, from, depth = 0)
      s = CharStringScanner.new(css[from..-1])
      while (b = s.scan_next(BRACE_RE))
        depth += (b == '}' ? -1 : +1)
        break if depth.zero?
      end
      raise "match not found for {" unless depth.zero?
      from + s.pos - 1
    end

    # opening brace position from +from+ (search backwards)
    def open_brace_pos(css, from, depth = 0)
      s = CharStringScanner.new(css[0..from].reverse)
      while (b = s.scan_next(BRACE_RE_REVERSE))
        depth += (b == '{' ? +1 : -1)
        break if depth.zero?
      end
      raise "matching { brace not found" unless depth.zero?
      from - s.pos + 1
    end

    # insert substitutions into text at positions (Range or Fixnum)
    # substitutions can be passed as array or as yields from the &block called with |substring, position, text|
    # position is a range (begin..end)
    def replace_substrings_at(text, positions, replacements = nil, &block)
      offset = 0
      positions.each_with_index do |p, i|
        p       = (p...p) if p.is_a?(Fixnum)
        from    = p.begin + offset
        to      = p.end + offset
        p       = p.exclude_end? ? (from...to) : (from..to)
        # block returns the substitution, e.g.: { |text, pos| text[pos].upcase }
        r       = replacements ? replacements[i] : block.call(text[p], p, text)
        text[p] = r
        # add the change in length to offset
        offset  += r.size - (p.end - p.begin + (p.exclude_end? ? 0 : 1))
      end
      text
    end
  end
end