2 files changed, 53 insertions, 0 deletions
diff --git a/src/data/composite/wiki-data/index.js b/src/data/composite/wiki-data/index.js
index 3ccfa75d..b4cf6d13 100644
--- a/src/data/composite/wiki-data/index.js
+++ b/src/data/composite/wiki-data/index.js
@@ -13,3 +13,4 @@ export {default as withResolvedReferenceList} from './withResolvedReferenceList.
 export {default as withReverseContributionList} from './withReverseContributionList.js';
 export {default as withReverseReferenceList} from './withReverseReferenceList.js';
 export {default as withThingsSortedAlphabetically} from './withThingsSortedAlphabetically.js';
+export {default as withUniqueReferencingThing} from './withUniqueReferencingThing.js';
diff --git a/src/data/composite/wiki-data/withUniqueReferencingThing.js b/src/data/composite/wiki-data/withUniqueReferencingThing.js
new file mode 100644
index 00000000..ce04f838
--- /dev/null
+++ b/src/data/composite/wiki-data/withUniqueReferencingThing.js
@@ -0,0 +1,52 @@
+// Like withReverseReferenceList, but this is specifically for special "unique"
+// references, meaning this thing is referenced by exactly one or zero things
+// in the data list.
+
+import {input, templateCompositeFrom} from '#composite';
+
+import {exitWithoutDependency, raiseOutputWithoutDependency}
+  from '#composite/control-flow';
+
+import inputWikiData from './inputWikiData.js';
+import withReverseReferenceList from './withReverseReferenceList.js';
+
+export default templateCompositeFrom({
+  annotation: `withUniqueReferencingThing`,
+
+  inputs: {
+    data: inputWikiData({allowMixedTypes: false}),
+    list: input({type: 'string'}),
+  },
+
+  outputs: ['#uniqueReferencingThing'],
+
+  steps: () => [
+    // withReverseRefernceList does this check too, but it early exits with
+    // an empty array. That's no good here!
+    exitWithoutDependency({
+      dependency: input('data'),
+      mode: input.value('empty'),
+    }),
+
+    withReverseReferenceList({
+      data: input('data'),
+      list: input('list'),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: '#reverseReferenceList',
+      mode: input.value('empty'),
+      output: input.value({'#uniqueReferencingThing': null}),
+    }),
+
+    {
+      dependencies: ['#reverseReferenceList'],
+      compute: (continuation, {
+        ['#reverseReferenceList']: reverseReferenceList,
+      }) => continuation({
+        ['#uniqueReferencingThing']:
+          reverseReferenceList[0],
+      }),
+    },
+  ],
+});