3 files changed, 113 insertions, 0 deletions
diff --git a/src/data/composite/data/index.js b/src/data/composite/data/index.js
index 0fc84061..c80bb350 100644
--- a/src/data/composite/data/index.js
+++ b/src/data/composite/data/index.js
@@ -25,6 +25,8 @@ export {default as withPropertiesFromList} from './withPropertiesFromList.js';
 export {default as withFlattenedList} from './withFlattenedList.js';
 export {default as withUnflattenedList} from './withUnflattenedList.js';
 
+export {default as withIndexInList} from './withIndexInList.js';
+export {default as withNearbyItemFromList} from './withNearbyItemFromList.js';
 
 // Utilities which act on slightly more particular data forms
 // (probably, containers of particular kinds of values)
diff --git a/src/data/composite/data/withIndexInList.js b/src/data/composite/data/withIndexInList.js
new file mode 100644
index 00000000..b1af2033
--- /dev/null
+++ b/src/data/composite/data/withIndexInList.js
@@ -0,0 +1,38 @@
+// Gets the index of the provided item in the provided list. Note that this
+// will output -1 if the item is not found, and this may be detected using
+// any availability check with type: 'index'. If the list includes the item
+// twice, the output index will be of the first match.
+//
+// Both the list and item must be provided.
+//
+// See also:
+//  - withNearbyItemFromList
+//  - exitWithoutDependency
+//  - raiseOutputWithoutDependency
+//
+
+import {input, templateCompositeFrom} from '#composite';
+
+export default templateCompositeFrom({
+  annotation: `withIndexInList`,
+
+  inputs: {
+    list: input({acceptsNull: false, type: 'array'}),
+    item: input({acceptsNull: false}),
+  },
+
+  outputs: ['#index'],
+
+  steps: () => [
+    {
+      dependencies: [input('list'), input('item')],
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('item')]: item,
+      }) => continuation({
+        ['#index']:
+          list.indexOf(item),
+      }),
+    },
+  ],
+});
diff --git a/src/data/composite/data/withNearbyItemFromList.js b/src/data/composite/data/withNearbyItemFromList.js
new file mode 100644
index 00000000..83a8cc21
--- /dev/null
+++ b/src/data/composite/data/withNearbyItemFromList.js
@@ -0,0 +1,73 @@
+// Gets a nearby (typically adjacent) item in a list, meaning the item which is
+// placed at a particular offset compared to the provided item. This is null if
+// the provided list doesn't include the provided item at all, and also if the
+// offset would read past either end of the list - except if configured:
+//
+//  - If the 'wrap' input is provided (as true), the offset will loop around
+//    and continue from the opposing end.
+//
+//  - If the 'valuePastEdge' input is provided, that value will be output
+//    instead of null.
+//
+// Both the list and item must be provided.
+//
+// See also:
+//  - withIndexInList
+//
+
+import {input, templateCompositeFrom} from '#composite';
+import {atOffset} from '#sugar';
+
+import {raiseOutputWithoutDependency} from '#composite/control-flow';
+
+import withIndexInList from './withIndexInList.js';
+
+export default templateCompositeFrom({
+  annotation: `withNearbyItemFromList`,
+
+  inputs: {
+    list: input({acceptsNull: false, type: 'array'}),
+    item: input({acceptsNull: false}),
+
+    offset: input({type: 'number'}),
+    wrap: input({type: 'boolean', defaultValue: false}),
+  },
+
+  outputs: ['#nearbyItem'],
+
+  steps: () => [
+    withIndexInList({
+      list: input('list'),
+      item: input('item'),
+    }),
+
+    raiseOutputWithoutDependency({
+      dependency: '#index',
+      mode: input.value('index'),
+
+      output: input.value({
+        ['#nearbyItem']:
+          null,
+      }),
+    }),
+
+    {
+      dependencies: [
+        input('list'),
+        input('offset'),
+        input('wrap'),
+        '#index',
+      ],
+
+      compute: (continuation, {
+        [input('list')]: list,
+        [input('offset')]: offset,
+        [input('wrap')]: wrap,
+        ['#index']: index,
+      }) => continuation({
+        ['#nearbyItem']:
+          atOffset(list, index, offset, {wrap}),
+      }),
+    },
+  ],
+});