XSL iterate over attribute and ignore some elements - xslt-1.0

Question

I need to do rather simple thing of iterating over elements in my XML(more explanation under the XML)
<form>
<field index="1" name="field_X_1" group="firstGroup" type="String">
<value>Value of Field X 1 of first group</value>
</field>
<field index="1" name="field_Y_1" group="firstGroup" type="String">
<value>Value of Field Y 1 of first group</value>
</field>
<field index="2" name="field_X_2" group="firstGroup" type="String">
<value>Value of Field X 2 of first group</value>
</field>
<field index="2" name="field_Y_2" group="firstGroup" type="String">
<value>Value of Field Y 2 of first group</value>
</field>
<field index="1" name="field_A_1" group="secondGroup" type="String">
<value>Value of Field A 1 of second group</value>
</field>
<field index="1" name="field_B_1" group="secondGroup" type="String">
<value>Value of Field B 1 of second group</value>
</field>
<field index="2" name="field_A_2" group="secondGroup" type="String">
<value>Value of Field A 2 of second group</value>
</field>
<field index="2" name="field_B_2" group="secondGroup" type="String">
<value>Value of Field B 2 of second group</value>
</field>
</form>
Right now I am iterating over ALL fields of firstGroup and everytime I found that index has changed(by comparing index to index of sibbling) I add new line character).
Problem is I need only to iterate over index (but still keep distiction between firstGroup and secondGroup).
My output should look like
Value of Field X 1 of first group, Value of Field Y 1 of first group
Value of Field X 2 of first group, Value of Field Y 2 of first group
Value of Field A 1 of second group, Value of Field B 1 of second group
Value of Field A 2 of second group, Value of Field B 2 of second group
etc
My XSL now looks like
<xsl:for-each select='/form/field[#group="firstGroup"]'>
<xsl:sort select="#index" order="ascending"></xsl:sort>
<xsl:if test='preceding-sibling::*[#group="firstGroup"][1]/#index != #index and count(preceding-sibling::*) != 0 '>
<xsl:text>
</xsl:text>
</xsl:if>
<xsl:value-of select="//field[#name=concat('field_X_', #index,')]/value"/>
<xsl:value-of select="//field[#name=concat('field_Y_', #index,')]/value"/>
</xsl:for-each>
<!-- Differente iteration for second group-->
<xsl:text>
</xsl:text>
<xsl:for-each select='/form/field[#group="firstGroup"]'>
<xsl:sort select="#index" order="ascending"></xsl:sort>
<xsl:if test='preceding-sibling::*[#group="firstGroup"][1]/#index != #index and count(preceding-sibling::*) != 0 '>
<xsl:text>
</xsl:text>
</xsl:if>
<xsl:value-of select="//field[#name=concat('field_A_', #index,')]/value"/>
<xsl:value-of select="//field[#name=concat('field_B_', #index,')]/value"/>
</xsl:for-each>

Answer 1

<xsl:output method="text" />
<xsl:template match="form">
<!--* find all the groups *-->
<xsl:variable name="groups">
<xsl:call-template name="get-group-names">
<xsl:with-param name="nodes" select="field" />
</xsl:call-template>
</xsl:variable>
<xsl:call-template name="process-all-groups">
<xsl:with-param name="group-names" select="$groups" />
</xsl:call-template>
</xsl:template>
<xsl:template name="get-group-names">
<xsl:param name="nodes" />
<xsl:param name="result-so-far" />
<xsl:choose>
<xsl:when test="not($nodes)">
<xsl:value-of select="$result-so-far" />
</xsl:when>
<xsl:when test="contains(
concat(' ', $result-so-far),
concat(' ', $nodes[1]/#group, ' '))" >
<xsl:call-template name="get-group-names">
<xsl:with-param name="nodes" select="$nodes[position() > 1]" />
<xsl:with-param name="result-so-far" select="$result-so-far" />
</xsl:call-template>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="get-group-names">
<xsl:with-param name="nodes" select="$nodes[position() > 1]" />
<xsl:with-param name="result-so-far"
select="concat($result-so-far, $nodes[1]/#group, ' ')" />
</xsl:call-template>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<xsl:template name="process-all-groups">
<xsl:param name="group-names" />
<xsl:variable name="group" select="substring-before($group-names, ' ')"/>
<xsl:if test="not(string-length($group) = 0)">
<xsl:call-template name="index">
<xsl:with-param name="n" select="1" />
<xsl:with-param name="group" select="$group" />
</xsl:call-template>
<xsl:call-template name="process-all-groups">
<xsl:with-param name="group-names"
select="substring-after($group-names, ' ')" />
</xsl:call-template>
</xsl:if>
</xsl:template>
<xsl:template name="index">
<xsl:param name="n" select="1" />
<xsl:param name="group" />
<xsl:variable name="with-n"
select="/form/field[#group = $group][#index = $n]" />
<xsl:if test="$with-n">
<xsl:for-each select="$with-n">
<xsl:sort use="#index" order="ascending" />
<xsl:value-of select="value" />
<xsl:if test="not(position() = last())">
<xsl:text>,</xsl:text>
</xsl:if>
</xsl:for-each>
<xsl:text>
</xsl:text>
<xsl:call-template name="index">
<xsl:with-param name="n" select="$n + 1" />
<xsl:with-param name="group" select="$group" />
</xsl:call-template>
</xsl:if>
</xsl:template>
<xsl:template match="field"></xsl:template>