<?php

/**
 * Solr data helper
 *
 * @category Asm
 * @package Asm_Solr
 * @author Ingo Renner <ingo@infielddesign.com>
 */
class Asm_Solr_Helper_Data extends Mage_Core_Helper_Abstract
{

	public function getProductDocumentId($productId) {
		// TODO replace ### with site hash
		$documentId = '###/' . Mage_Catalog_Model_Product::ENTITY . '/' . $productId;

		return $documentId;
	}

	/**
	 * Gets the site hash for a domain
	 *
	 * @param string $domain Domain to calculate the site hash for.
	 * @return string site hash for $domain
	 */
	public function getSiteHashForDomain($domain) {
		$encryptionKey = Mage::getStoreConfig('global/crypt/key');

		$siteHash = sha1(
			$domain .
			$encryptionKey .
			'Asm_Solr'
		);

		return $siteHash;
	}

}

<?php

/**
 * Solr data helper
 *
 * @category Asm
 * @package Asm_Solr
 * @author Ingo Renner <ingo@infielddesign.com>
 */
class Asm_Solr_Helper_Data extends Mage_Core_Helper_Abstract
{

	public function getProductDocumentId($productId) {
		// TODO replace ### with site hash
		$documentId = '###/' . Mage_Catalog_Model_Product::ENTITY . '/' . $productId;

		return $documentId;
	}

}

--- 
+++ 
@@ -17,4 +17,22 @@
 		return $documentId;
 	}
 
+	/**
+	 * Gets the site hash for a domain
+	 *
+	 * @param string $domain Domain to calculate the site hash for.
+	 * @return string site hash for $domain
+	 */
+	public function getSiteHashForDomain($domain) {
+		$encryptionKey = Mage::getStoreConfig('global/crypt/key');
+
+		$siteHash = sha1(
+			$domain .
+			$encryptionKey .
+			'Asm_Solr'
+		);
+
+		return $siteHash;
+	}
+
 }
Add method to generate a site hash for a domain
