Post History

# [BQN](https://mlochbaum.github.io/BQN/), 33 bytes<sup>[SBCS](https://github.com/mlochbaum/BQN/blob/master/commentary/sbcs.bqn)</sup>

[Run online!](https://mlochbaum.github.io/BQN/try.html#code=Q0Mg4oaQIHviiKjCtOKImOKIvjgo4qWKKOKKkOKMvuKlivCdlakp4o23wrfiiaDijJzLnDJ8L+KImOKliinCqDEr4oaV4omg4qWK8J2VqX0KQ0PCqCDin6gKICBbImFhIiwiYWEiXQogIFsiYXFxYWFxIiwicWFhcXFhIl0KICBbImNjY2NjY2NjIl0KCiAgWyJhYmJhYWEiXQogIFsiYyIsImMiLCJkIiwiYyJdCiAjWyJiYmFhYmJhYWJiYWFiYmFhYiJdICAjIFRha2VzIGEgZmV3IHNlY29uZHMKICBbImFiYyJdCiAgWyJhYWJiIiwiYmJjYyJdCuKfqQ==)

```
{∨´∘∾8(⥊(⊐⌾⥊𝕩)⍷·≠⌜˜2|/∘⥊)¨1+↕≠⥊𝕩}
```

Brute force: tests against all boards up to the number of characters in `𝕩` using [Find](https://mlochbaum.github.io/BQN/doc/find.html) (`⍷`). Using the longest dimension is faster but would cost a byte.

Not explained below, `⊐⌾⥊𝕩` applies [Classify](https://mlochbaum.github.io/BQN/doc/selfcmp.html#classify) [under](https://mlochbaum.github.io/BQN/doc/under.html) [Deshape](https://mlochbaum.github.io/BQN/doc/reshape.html) to convert the argument from characters to numbers. If there are only two unique characters, they'll be 0 and 1, and a third would be 2 and so on. The top-left character is 0, like the top left of the checkerboard, so an 8-square checkerboard will always fit if the argument is valid.

Also used: [Table](https://mlochbaum.github.io/BQN/doc/map.html#table), [Indices](https://mlochbaum.github.io/BQN/doc/replicate.html#indices).

```
{∨´∘∾8(⥊(⊐⌾⥊𝕩)⍷·≠⌜˜2|/∘⥊)¨1+↕≠⥊𝕩}
                             ≠⥊𝕩   # Length of flattened argument
                          1+↕      # Range from 1 to that length
     8(                 )¨         # On each size:        n
     8                 ⥊           #   Take 8 of them     n n n…
                     /∘            #   Each index n times 0 0…0 1 1…1 2 2…2… …7
                   2|              #   Mod 2              0 0…0 1 1…1 0 0…0… …1
               ·≠⌜                 #   Not-equals table
                  ˜                #     with itself, AKA checkerboard
        (⊐⌾⥊𝕩)⍷                    #   Find the numberized argument
       ⥊                           #   Convert to list
 ∨´                                # Any true in
   ∘∾                              #   the join of that?
```