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Chequer checker [FINALIZED]

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Now posted: Chequer checker

A chequer board (also spelt "checker board" in some places) is an 8 by 8 grid of squares alternating between 2 colours. Check whether a provided pattern could be a region of a chequer board.

The colours in the pattern will be represented by letters of the alphabet.

The squares of a chequer board all have the same side length. For this challenge, this length can be any positive integer.

Some of the squares may be cut by the boundary of the input region (the edges of the region shown in the input will not necessarily coincide with the edges of the squares). For example:

abba
baab
baab
abba

This shows a small region of a chequer board with squares of side length 2. Only the central square shows in its full 2 by 2 size. The squares adjacent to it have been sliced in half by the edges of the region.

Here is a diagram showing the adjacent squares being sliced in half by the region outlined in red:

A chequer board with a region outlined in red that slices some of the squares in half

Input

Text in a rectangular grid (newline separated lines of equal length)
Each character is a letter of the English alphabet (a to z, or A to Z - you can choose which case to accept)
No other characters will be present - just letters and newlines
You can choose whether the inputs have a trailing newline
The input will contain at least one letter (the input region will be non-empty)
You can choose to accept a grid of letters in a different format, such as lists/arrays of strings, or 2D arrays of characters
The input will not be more than 255 by 255 letters

Output

A truthy output if the input is a region of an 8 by 8 chequer board (axis aligned - not squares at any other angle) and a falsy output otherwise
Falsy if there are more than 8 squares (including partial squares) horizontally or vertically
Falsy if there are more than 2 colours (more than 2 distinct letters)

Test cases

Truthy test cases

aa
aa

b

cccccccc

aabba
aabba
bbaab

cddddddddd
dccccccccc
dccccccccc

abababab
babababa
abababab
babababa
abababab
babababa
abababab
babababa

aqqaaq
qaaqqa

ba
ab

zzzzaaaazzzzaaaazzzzaaaazzzzaaaa
zzzzaaaazzzzaaaazzzzaaaazzzzaaaa
zzzzaaaazzzzaaaazzzzaaaazzzzaaaa
zzzzaaaazzzzaaaazzzzaaaazzzzaaaa

Falsy test cases

abbaaa

c
c
d
c

aabb
abba

ccdcc
cdddc
ddddd
cdddc
ccdcc

aba
aba

cdcdcdcdc

c
d
c
d
c
d
c
d
c

bbaabbaabbaabba

abbaabbaabbaabb

abbaabbaabbaabba

abc

aabb
bbcc

Explanations in answers are optional, but I'm more likely to upvote answers that have one.

posted about 2 years ago

CC BY-SA 4.0

2y ago

trichoplax‭

1020 reputation 83 17 129 471

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2 comment threads

More than two different letters in the input? (2 comments)

On the sandbox questions (2 comments)

celtschk‭ wrote about 2 years ago

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My take on the sandbox questions:

I think empty input should be allowed, with truthy value.
I think allowing other forms of input is a good idea. In particular, I would allow lists/arrays of strings, and 2D arrays of characters.
An upper limit in the size the program needs to handle would be a good idea; of course programs able to handle larger sizes shouldn't disqualify.
There should be a truthy testcase with non-consecutive characters, possibly with equal lower bits in the ASCII encoding; for example
```
aqqaaq
qaaqqa
```
The format of the test cases is fine. The separation definitely is useful.
It is unclear to me if the code has to handle non-letter characters (leading to a falsy result), or simply may assume that no non-letters (other than newlines) occur.

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