XSLT : Cummulative Sum (Conditional) - xslt

Question

I need to do a conditional sum using XSLT. The sum of 'Oty' for each 'SKU' should be calculated only for providers listed within the 'Provider' node. In the provided example, the Qty for providerCode 4 should be skipped as its not in the 'Providers' list. I'm restricted to using XSLT 1.0.
I would appreciate any help. Thanks!
Here is the sample XML.
<?xml version="1.0" encoding="UTF-8"?>
<Root>
<Providers>
<ProviderCode>1</ProviderCode>
<ProviderCode>2</ProviderCode>
<ProviderCode>3</ProviderCode>
</Providers>
<SKU>
<SKU>XYZ</SKU>
<Description>XYZ Description</Description>
<Provider>
<ProviderCode>1</ProviderCode>
<Qty>100</Qty>
</Provider>
<Provider>
<ProviderCode>2</ProviderCode>
<Qty>67</Qty>
</Provider>
<Provider>
<ProviderCode>3</ProviderCode>
<Qty>74</Qty>
</Provider>
<Provider>
<ProviderCode>4</ProviderCode>
<Qty>62</Qty>
</Provider>
</SKU>
<SKU>
<SKU>ABC</SKU>
<Description>ABC Description</Description>
<Provider>
<ProviderCode>1</ProviderCode>
<Qty>20</Qty>
</Provider>
<Provider>
<ProviderCode>2</ProviderCode>
<Qty>77</Qty>
</Provider>
<Provider>
<ProviderCode>3</ProviderCode>
<Qty>42</Qty>
</Provider>
<Provider>
<ProviderCode>4</ProviderCode>
<Qty>631</Qty>
</Provider>
</SKU>
</Root>
Here is the required output.
<?xml version="1.0" encoding="UTF-8"?>
<Root>
<SKU>
<SKU>XYZ</SKU>
<Qty>241</Qty>
</SKU>
<SKU>
<SKU>ABC</SKU>
<Qty>139</Qty>
</SKU>
</Root>

Answer 1

You can simply use sum on the nodes you want, either by comparing sum(Provider[ProviderCode = //Providers/ProviderCode]/Qty) or by using a key:
<xsl:stylesheet
version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output indent="yes"/>
<xsl:key name="prov" match="Providers/ProviderCode" use="."/>
<xsl:template match="Root">
<xsl:copy>
<xsl:apply-templates select="SKU"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Root/SKU">
<xsl:copy>
<xsl:copy-of select="SKU"/>
<Qty><xsl:value-of select="sum(Provider[key('prov', ProviderCode)]/Qty)"/></Qty>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>