Initial commit with files

Still need to fix the imports

src/lib/utils/calculate-active-index.ts

@@ -0,0 +1,84 @@
+function assertNever(x: never): never {
+    throw new Error('Unexpected object: ' + x)
+}
+
+export enum Focus {
+    /** Focus the first non-disabled item. */
+    First,
+
+    /** Focus the previous non-disabled item. */
+    Previous,
+
+    /** Focus the next non-disabled item. */
+    Next,
+
+    /** Focus the last non-disabled item. */
+    Last,
+
+    /** Focus a specific item based on the `id` of the item. */
+    Specific,
+
+    /** Focus no items at all. */
+    Nothing,
+}
+
+export function calculateActiveIndex<TItem>(
+    action: { focus: Focus.Specific; id: string } | { focus: Exclude<Focus, Focus.Specific> },
+    resolvers: {
+        resolveItems(): TItem[]
+        resolveActiveIndex(): number | null
+        resolveId(item: TItem): string
+        resolveDisabled(item: TItem): boolean
+    }
+) {
+    let items = resolvers.resolveItems()
+    if (items.length <= 0) return null
+
+    let currentActiveIndex = resolvers.resolveActiveIndex()
+    let activeIndex = currentActiveIndex ?? -1
+
+    let nextActiveIndex = (() => {
+        switch (action.focus) {
+            case Focus.First:
+                return items.findIndex(item => !resolvers.resolveDisabled(item))
+
+            case Focus.Previous: {
+                let idx = items
+                    .slice()
+                    .reverse()
+                    .findIndex((item, idx, all) => {
+                        if (activeIndex !== -1 && all.length - idx - 1 >= activeIndex) return false
+                        return !resolvers.resolveDisabled(item)
+                    })
+                if (idx === -1) return idx
+                return items.length - 1 - idx
+            }
+
+            case Focus.Next:
+                return items.findIndex((item, idx) => {
+                    if (idx <= activeIndex) return false
+                    return !resolvers.resolveDisabled(item)
+                })
+
+            case Focus.Last: {
+                let idx = items
+                    .slice()
+                    .reverse()
+                    .findIndex(item => !resolvers.resolveDisabled(item))
+                if (idx === -1) return idx
+                return items.length - 1 - idx
+            }
+
+            case Focus.Specific:
+                return items.findIndex(item => resolvers.resolveId(item) === action.id)
+
+            case Focus.Nothing:
+                return null
+
+            default:
+                assertNever(action)
+        }
+    })()
+
+    return nextActiveIndex === -1 ? currentActiveIndex : nextActiveIndex
+}