The Talavera border

Problem

Only a stretch of the Talavera tile border survived on the wall of a chapel in Puebla, and Crispín, the tile maker, wants to make the stencil that repeats it. The shorter the stencil, the fewer tiles he has to paint by hand.

Write a function that takes border, a string of lowercase letters with at least one letter: each letter is a tile. A stencil of length k works if every tile, from position k (counting from 0) to the end, is equal to the one k places before it; the stretch may end halfway through a copy. Return the smallest k that works, a whole number between 1 and the length of the string. The length always works, since no tile is left to check.

With "abcabcab": 1 does not work (the b is not equal to the a), 2 does not either (the c is not equal to the a), and 3 does, because the border reads abc, abc and ab, the last copy cut short. Return 3. With "aaaa" return 1, and with "abcd" return 4.

Examples

• The example

  "abcabcab" → 3

• All the tiles are the same

  "aaaa" → 1

• No short stencil works

  "abcd" → 4

• A single tile

  "a" → 1

• The last copy is cut short

  "aabaa" → 3

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def shortest_stencil(border):
    pass

function shortestStencil(border) {
}