Change one variables values in large XML file to being incremental beginning at 1

So I have a giant XML file, in the following format:

<?xml version="1.0" encoding="UTF-8"?>
<furnidata>
  <roomitemtypes>
    <furnitype id="13" classname="shelves_norja">
      <revision>49500</revision>
      <defaultdir>0</defaultdir>
      <xdim>1</xdim>
      <ydim>1</ydim>
      <partcolors>
        <color>#ffffff</color>
        <color>#F7EBBC</color>
      </partcolors>
      <name>Beige Bookcase</name>
      <description>For nic naks and books.</description>
      <adurl/>
      <offerid>1871</offerid>
      <buyout>1</buyout>
      <rentofferid>-1</rentofferid>
      <rentbuyout>0</rentbuyout>
      <bc>1</bc>
      <excludeddynamic>0</excludeddynamic>
      <customparams/>
      <specialtype>1</specialtype>
      <canstandon>0</canstandon>
      <cansiton>0</cansiton>
      <canlayon>0</canlayon>
    </furnitype>
    <furnitype id="14" classname="shelves_polyfon">
      <revision>48082</revision>
      <defaultdir>0</defaultdir>
      <xdim>2</xdim>
      <ydim>1</ydim>
      <partcolors>
        <color>0</color>
        <color>0</color>
        <color>0</color>
      </partcolors>
      <name>Bookcase</name>
      <description>Make your books look sleek.</description>
      <adurl/>
      <offerid>2123</offerid>
      <buyout>1</buyout>
      <rentofferid>-1</rentofferid>
      <rentbuyout>0</rentbuyout>
      <bc>1</bc>
      <excludeddynamic>0</excludeddynamic>
      <customparams/>
      <specialtype>1</specialtype>
      <canstandon>0</canstandon>
      <cansiton>0</cansiton>
      <canlayon>0</canlayon>
    </furnitype>
  </roomitemtypes>
</furnidata>

So theres about 1000 different sets of 'furnitypes' and to keep it simple, I want (from top to bottom), the 'offerid' of each furnitype to start at 1 for the first one at the top, and increase by 1 for every furnitype.

So the first offerid for the first furnitype would be 1, 2 for the second, 3 for the third, and so forth. Right now the offerid's for each furnitype are random numbers.

Theres way too many for me to manually edit them 1 by 1.