Move an item to a new position

Drag a task from the bottom of a list to the top and something has to compute the new order. Doing it with splice on the state array directly is how you get a list that reorders in memory but never re-renders.

Implement move(items, from, to). Take the item at index from, remove it, and re-insert it so that it ends up at index to in the returned array. Both indices may be negative, counting back from the end, so -1 refers to the last item. Return a new array; the input is never modified. If either index, after resolving a negative value, falls outside the range of valid positions in the array, return an unchanged shallow copy of the input.

What it has to do

• Return a new array with the item relocated, leaving the other items in order.
• Treat negative indices as counting back from the end, so -1 is the last item.
• Return an unchanged shallow copy when either resolved index is out of range.
• Handle moving both forwards and backwards through the array.
• Do not mutate the input array.

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