Script to fix the root viewbox on simple svg diagrams

svg-fix-viewbox.rb

#!/usr/bin/env ruby

require 'xml'
require 'pp'
require 'bigdecimal'

unless ARGV.first
  puts "No file"
  exit 1
end

parser = XML::Parser.file(ARGV.first)
doc = parser.parse

# All bounding boxes are {:x1, :x2, :y1, :y2}.

# Merge two bounding boxes
def merge2(a, b)
  {x1: [a[:x1], b[:x1]].min,
   x2: [a[:x2], b[:x2]].max,
   y1: [a[:y1], b[:y1]].min,
   y2: [a[:y2], b[:y2]].max}
end

# Merge a bunch of boxes
def merge(boxes)
  boxes.reject(&:nil?).reduce { |a, b| merge2(a, b) }
end

# Make a decimal
def d(x)
  BigDecimal.new x
end

# Computes the bounding box of the given node.
def bbox(node, offset)
  x1, x2, y1, y2 = nil
  case node.name
    when "defs"
      return nil
    when "g"
      if t = node["transform"]
        # Parse a translation out of there, god this is such a hack
        if t =~ /translate\(([\d\.]+),([\d\.]+)\)/
          offset = {x: offset[:x] + d($1),
                    y: offset[:y] + d($2)}
          p "offset", offset
        end
      end
      return merge(node.children.map { |n| bbox(n, offset) })
    when 'line'
      x1 = d(node[:x1])
      x2 = d(node[:x2])
      y1 = d(node[:y1])
      y2 = d(node[:y2])
    when 'rect'
      x1 = d(node[:x])
      x2 = d(node[:x]) + d(node[:width])
      y1 = d(node[:y])
      y2 = d(node[:y]) + d(node[:height])
    when "script"
      return nil
    when "svg"
      return merge(node.children.map { |n| bbox(n, offset) })
    when 'text'
      # cool story: this is literally impossible
      x = d(node[:x])
      y = d(node[:y])

      # guess font height and width
      node[:style] =~ /font-size: ([\d\.]+)/
      height = d($1) # font sizes are an em: roughly descender-to-ascender
      width = node.content.length * 0.6 * height # greetings

      node[:style] =~ /text-anchor: (\w+)/
      case $1
      when 'middle'
        x1 = x - (width / 2)
        x2 = x + (width / 2) 
      when 'left', 'start', nil
        x1 = x
        x2 = x + width
      when 'right', 'end'
        x1 = x - width
        x2 = x
      else
        pp node
        raise RuntimeError, "what kind of anchor is #{$1}?"
      end

      node[:style] =~ /alignment-baseline: (\w+)/
      case $1
      when 'middle'
        y1 = y - (height / 2)
        y2 = y + (height / 2)
      when 'top'
        y1 = y
        y2 = y + height
      when 'bottom' # not part of spec, maybe I shouldn't emit this
        y1 = y - height
        y2 = y
      when nil
        # ehhhh maybe?
        y1 = y - (height * 0.7)
        y2 = y + (height * 0.3)
      else
        pp node
        raise RuntimeError, "what kind of baseline is #{$1}?"
      end

    else
      pp node
      raise RuntimeError, "what's a #{node.name}?"
  end

  {x1: x1 + offset[:x],
   x2: x2 + offset[:x],
   y1: y1 + offset[:y],
   y2: y2 + offset[:y]}
end

b = bbox(doc.root, {x: 0, y: 0})
doc.root['viewBox'] = [b[:x1].to_f,
                       b[:y1].to_f,
                       (b[:x2] - b[:x1]).to_f,
                       (b[:y2] - b[:y1]).to_f].join(' ')
doc.save(ARGV.first, indent: true)