How to sensibly split an array around an object

arrays, ruby

Solution

a = [1, 2, 3, 4, 5]
n = a.index(3) || -1
a.map.with_index{|e, i| e + (i <=> n)}
# => [0, 1, 3, 5, 6]

Problem

I have an array of uncertain length, let's say `[1,2,3,4,5]`. I want to subtract `1` from everything before `3`, and add `1` to everything after `3`, making the example `[0,1,3,5,6]`. If there is no `3`, add `1` to everything: `[1,2,4,5]` => `[2,3,5,6]`. What is the most graceful way of doing this?

Original source