Add in_double_check()

[fitztrev]

I was looking for a way to be able to detect "double checkmates" (example here)

I'll submit this as a draft PR for discussion, knowing that #319 will have a new development branch. If this gets approved, I will re-do against the FIX_ME branch.

[jhlywa]

Hey Trevor, could you do the same thing with something a little more generic? Maybe simplify the concept into an attackers() API call? In the end it's a few more lines of code for your specific use case, but I think it results in greater flexibility for the library. Something like:

const kingSquare = somehow get the king square for the side to move 
const attackers = chess.attackers(kingSquare) /// [{piece: 'n', square: 'f3', color: 'b'}, ...]
const doubleAttack = attackers.length === 2

So we'd need a way to retrieve the square for the king, or more generically any piece of either color. Maybe return a list of squares for the piece/color in question.

This approach also gives us defenders() for free too (something like function defenders(square) { return attackers(square, swap(this.turn)) }), right?

Thoughts?

[fitztrev]

I like that approach.

In this example, would the bishop, queen, and e1 rook count as attackers of e5? And if so, would that work with the way square/attack mappings are currently setup? I'll have to dig into that part of the code more to understand how it works.

[jhlywa]

Maybe something like attackers('e4', { xray: true }) for something like that?

Xray gets a little complicated if some of the attackers are pinned. I guess the pinned piece counts as an attacker but nothing "behind" it does? I'm ok punting on xray support for the time being.

[fitztrev]

I pushed some updates. Below would be a proposed sample use. Let me know if this is the right idea.

Given this position:

// get white attackers of d5
chess.attackers('w', 'd5')

// result
[
  { square: 'f3', color: 'w', type: 'q' },
  { square: 'd1', color: 'w', type: 'r' },
]

[JacobEvelyn]

A project I'm working on would really benefit from this attackers()/defenders() API. (The xray feature would be nice but is not mandatory for me.) Let me know if there's anything I can do to help get this polished and merged.