// Roughly orders moves in order of most likely to be good to least.
// Examines all checks first, followed by captures, followed by good moves.
// "Good moves" are sorted by their board evaluation after they are played.
// If quiescence is set to true, then only checks and captures are returned.
func orderedMoves(b *engine.Board, quiescence bool) []*engine.Move {
checks := make([]*engine.Move, 0)
captures := make([]*engine.Move, 0)
rest := make([]*engine.Move, 0)
// parentscore := EvalBoard(b)
for _, move := range b.AllLegalMoves() {
b.ForceMove(move)
if b.IsCheck(b.Turn) {
checks = append(checks, move)
} else if move.Capture != 0 {
captures = append(captures, move)
} else if !quiescence {
childscore := EvalBoard(b) * float64(b.Turn*-1)
// if (b.Turn == -1 && childscore > parentscore) || (b.Turn == 1 && childscore < parentscore) {
move.Score = childscore
rest = append(rest, move)
// }
}
b.UndoMove(move)
}
if !quiescence {
sort.Sort(sort.Reverse(ByScore(rest)))
}
orderedmoves := make([]*engine.Move, len(checks)+len(captures)+len(rest))
index := 0
for _, l := range [][]*engine.Move{checks, captures, rest} {
for _, m := range l {
m.Score = 0
orderedmoves[index] = m
index++
}
}
return orderedmoves
}
// Child level returns an evaluation
func AlphaBetaChild(b *engine.Board, depth int, alpha, beta float64, volatile bool) float64 {
var movelist []*engine.Move
if b.IsOver() != 0 {
return EvalBoard(b)
} else if depth == 0 {
if !volatile {
return EvalBoard(b)
}
depth += 1
movelist = orderedMoves(b, true)
} else {
movelist = orderedMoves(b, false)
}
var score float64
if b.Turn == 1 {
for _, move := range movelist {
b.ForceMove(move)
if !volatile && (move.Capture != 0 || b.IsCheck(b.Turn)) {
score = AlphaBetaChild(b, depth-1, alpha, beta, true)
} else {
score = AlphaBetaChild(b, depth-1, alpha, beta, false)
}
b.UndoMove(move)
if score > alpha {
alpha = score
}
if alpha >= beta {
return alpha
}
}
return alpha
} else {
for _, move := range movelist {
b.ForceMove(move)
if !volatile && (move.Capture != 0 || b.IsCheck(b.Turn)) {
score = AlphaBetaChild(b, depth-1, alpha, beta, true)
} else {
score = AlphaBetaChild(b, depth-1, alpha, beta, false)
}
b.UndoMove(move)
if score < beta {
beta = score
}
if beta <= alpha {
return beta
}
}
return beta
}
return 0
}
// Standard minmax search with alpha beta pruning.
// Initial call: alpha set to lowest value, beta set to highest.
// Top level returns a move.
func AlphaBeta(b *engine.Board, depth int, alpha, beta float64) *engine.Move {
if b.IsOver() != 0 || depth == 0 {
return nil
}
var bestmove *engine.Move = nil
var result float64
movelist := orderedMoves(b, false)
if b.Turn == 1 {
for _, move := range movelist {
b.ForceMove(move)
if move.Capture != 0 || b.IsCheck(b.Turn) {
result = AlphaBetaChild(b, depth-1, alpha, beta, true)
} else {
result = AlphaBetaChild(b, depth-1, alpha, beta, false)
}
b.UndoMove(move)
if result > alpha {
alpha = result
bestmove = move
bestmove.Score = alpha
}
if alpha >= beta {
bestmove = move
bestmove.Score = alpha
return bestmove
}
}
if bestmove == nil {
return b.AllLegalMoves()[0]
}
return bestmove
} else {
for _, move := range movelist {
b.ForceMove(move)
if move.Capture != 0 || b.IsCheck(b.Turn) {
result = AlphaBetaChild(b, depth-1, alpha, beta, true)
} else {
result = AlphaBetaChild(b, depth-1, alpha, beta, false)
}
if LOG {
fmt.Println(move.ToString(), result)
}
b.UndoMove(move)
if result < beta {
beta = result
bestmove = move
bestmove.Score = beta
}
if beta <= alpha {
bestmove = move
bestmove.Score = beta
return bestmove
}
}
if bestmove == nil {
return b.AllLegalMoves()[0]
}
return bestmove
}
if bestmove == nil {
return b.AllLegalMoves()[0]
}
return bestmove
}