how to calculate needed result - xslt

Question

Please help with this xml:
<document>
<sheet id="0" name="Sheet1">
<line id="0">
<field id="0"><![CDATA[Calculate]]></field>
</line>
<line id="1">
<field id="0"><![CDATA[Quantity]]></field>
<field id="1"><![CDATA[Value]]></field>
</line>
<line id="2">
<field id="0"><![CDATA[3]]></field>
<field id="1"><![CDATA[2]]></field>
</line>
<line id="3">
<field id="0"><![CDATA[2]]></field>
<field id="1"><![CDATA[7]]></field>
</line>
<line id="4">
<field id="0"></field>
<field id="1"></field>
</line>
</sheet>
</document>
I need to get Sum of field[#id=1] before multiply this field with field[#id=0]. Line[#id=4] is empty so there should be condition to eliminate this line.
correct result should be:
<document>
<id>Calculate</id>
<line>
<sum>20</sum>
</line>
</document>

Answer 1

Here is an XSLT 2.0 solution that can be run with XSLT 2.0 processors like Saxon 9, XQSharp, AltovaXML:
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
version="2.0">
<xsl:output indent="yes"/>
<xsl:template match="document">
<xsl:copy>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template match="sheet">
<id><xsl:value-of select="line[#id = 0]/field"/></id>
<line>
<sum>
<xsl:value-of select="sum(line[field[#id = 1] castable as xs:double and field[#id = 0] castable as xs:double]/(field[#id = 1] * field[#id = 0]))"/>
</sum>
</line>
</xsl:template>
</xsl:stylesheet>
[edit] I am adding an XSLT 1.0 stylesheet with a named recursive template below:
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
<xsl:output indent="yes"/>
<xsl:template match="document">
<xsl:copy>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template name="sum">
<xsl:param name="lines"/>
<xsl:param name="total" select="0"/>
<xsl:choose>
<xsl:when test="not($lines)">
<xsl:value-of select="$total"/>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="sum">
<xsl:with-param name="lines" select="$lines[position() > 1]"/>
<xsl:with-param name="total" select="$total + $lines[1]/field[#id = 1] * $lines[1]/field[#id = 0]"/>
</xsl:call-template>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<xsl:template match="sheet">
<id><xsl:value-of select="line[#id = 0]/field"/></id>
<line>
<sum>
<xsl:call-template name="sum">
<xsl:with-param name="lines"
select="line[number(field[#id = 1]) = number(field[#id = 1])
and number(field[#id = 0]) = number(field[#id = 0])]"/>
</xsl:call-template>
</sum>
</line>
</xsl:template>
</xsl:stylesheet>