movegen.gno

  1package minichess
  2
  3func lastRank(c Color) int {
  4	if c == White {
  5		return Size - 1
  6	}
  7	return 0
  8}
  9
 10// Attacked reports whether square sq is attacked by any piece of colour by.
 11func (p Position) Attacked(sq int, by Color) bool {
 12	return newEngine(&p).attacked(sq, by)
 13}
 14
 15// InCheck reports whether the side to move is in check.
 16func (p Position) InCheck() bool {
 17	return newEngine(&p).inCheck()
 18}
 19
 20// LegalMoves returns all legal moves for the side to move.
 21func (p Position) LegalMoves() []Move {
 22	enc := newEngine(&p).legal()
 23	out := make([]Move, len(enc))
 24	for i, m := range enc {
 25		out[i] = decode(m)
 26	}
 27	return out
 28}
 29
 30// Play applies a move without validating it and returns the new position.
 31// Callers must only pass moves obtained from LegalMoves or FindLegal.
 32func (p Position) Play(m Move) Position {
 33	e := newEngine(&p)
 34	e.make(m.encode())
 35	full := p.Fullmove
 36	if p.Turn == Black {
 37		full++
 38	}
 39	return e.position(full)
 40}
 41
 42// IsIrreversible reports whether m is a capture or pawn move (resets the
 43// repetition history and the 50-move counter).
 44func (p Position) IsIrreversible(m Move) bool {
 45	return p.Board[m.From].Type() == Pawn || p.Board[m.To] != Empty
 46}
 47
 48// FindLegal matches m against the legal moves. A promotion move given
 49// without a piece defaults to a queen. Returns false if m is illegal.
 50func (p Position) FindLegal(m Move) (Move, bool) {
 51	if m.Promo == Empty {
 52		pc := p.Board[m.From]
 53		if pc.Type() == Pawn && pc.Color() == p.Turn && m.To/Size == lastRank(p.Turn) {
 54			m.Promo = Queen
 55		}
 56	}
 57	want := m.encode()
 58	for _, lm := range newEngine(&p).legal() {
 59		if lm == want {
 60			return m, true
 61		}
 62	}
 63	return m, false
 64}
 65
 66// Status describes whether the game is still going and how it ended.
 67type Status int
 68
 69const (
 70	Ongoing Status = iota
 71	Checkmate
 72	Stalemate
 73	FiftyMoveDraw
 74	InsufficientMaterial
 75)
 76
 77func (s Status) String() string {
 78	switch s {
 79	case Ongoing:
 80		return "ongoing"
 81	case Checkmate:
 82		return "checkmate"
 83	case Stalemate:
 84		return "stalemate"
 85	case FiftyMoveDraw:
 86		return "50-move rule"
 87	case InsufficientMaterial:
 88		return "insufficient material"
 89	}
 90	return "unknown"
 91}
 92
 93// Status computes the game status from the position alone. Threefold
 94// repetition needs move history and is handled by the caller.
 95func (p Position) Status() Status {
 96	e := newEngine(&p)
 97	if len(e.legal()) == 0 {
 98		if e.inCheck() {
 99			return Checkmate
100		}
101		return Stalemate
102	}
103	if p.insufficientMaterial() {
104		return InsufficientMaterial
105	}
106	if p.Halfmove >= 100 {
107		return FiftyMoveDraw
108	}
109	return Ongoing
110}
111
112// insufficientMaterial: only kings, or kings plus a single minor piece.
113func (p Position) insufficientMaterial() bool {
114	minors := 0
115	for _, pc := range p.Board {
116		switch pc.Type() {
117		case Pawn, Rook, Queen:
118			return false
119		case Knight, Bishop:
120			minors++
121		}
122	}
123	return minors <= 1
124}