Add facet count endpoint

src/plone/restapi/services/configure.zcml

@@ -19,6 +19,7 @@
   <include package=".copymove" />
   <include package=".database" />
   <include package=".discussion" />
+  <include package=".facet_count" />
   <include package=".groups" />
   <include package=".navigation" />
   <include package=".contextnavigation" />

src/plone/restapi/services/facet_count/configure.zcml

@@ -0,0 +1,16 @@
+<configure
+    xmlns="http://namespaces.zope.org/zope"
+    xmlns:cache="http://namespaces.zope.org/cache"
+    xmlns:plone="http://namespaces.plone.org/plone"
+    xmlns:zcml="http://namespaces.zope.org/zcml"
+    >
+
+  <plone:service
+      method="POST"
+      factory=".get.FacetCountGet"
+      for="Products.CMFPlone.interfaces.IPloneSiteRoot"
+      permission="zope2.View"
+      name="@facet-count"
+      />
+
+</configure>

src/plone/restapi/services/facet_count/get.py

@@ -0,0 +1,54 @@
+from plone.restapi.deserializer import json_body
+from plone.restapi.services import Service
+from plone.restapi.services.querystringsearch.get import QuerystringSearch
+from Products.CMFCore.utils import getToolByName
+import json
+
+
+class FacetCountGet(Service):
+    """Returns facet count."""
+
+    def reply(self):
+        facet_count = {}
+        ctool = getToolByName(self.context, "portal_catalog")
+        body = json_body(self.request)
+
+        facet = body.get("facet", None)
+        query = body.get("query", None)
+
+        try:
+            index = ctool._catalog.getIndex(facet)
+        except:
+            index = None
+
+        if query:
+            body["query"] = [qs for qs in query if qs["i"] != facet]
+            self.request.set("BODY", json.dumps(body))
+
+        brains = QuerystringSearch(self.context, self.request).getResults()
+
+        brains_rids = set(brain.getRID() for brain in brains)
+        index_rids = [rid for rid in index.documentToKeyMap()] if index else []
+
+        rids = brains_rids.intersection(index_rids)
+
+        for rid in rids:
+            for key in index.keyForDocument(rid):
+                if key not in facet_count:
+                    facet_count[key] = 0
+                facet_count[key] += 1
+
+        return {
+            "@id": "%s/@facet-count" % self.context.absolute_url(),
+            "facets": {
+                facet: {
+                    "count": len(rids),
+                    "data": [
+                        {"value": key, "count": value}
+                        for key, value in facet_count.items()
+                    ],
+                }
+            }
+            if facet
+            else {},
+        }