用Go写一个中国象棋（九）| 象棋AI
我们已经让电脑学会了如何去获得每个棋的走法，但电脑并不知道每步棋是好棋还是坏棋，所以我们需要加入棋子力价值。
棋子力价值
我们给每个棋子不同的力价值，并且适当调整——棋子力价值是跟它的绝对位置相关的。
最明显的例子是中国象棋中的兵(卒)，过河前我们给它很低的分数，过河后分数大涨，越接近九宫格分数越高，九宫中心甚至接近一个马或炮的值。
打开define.go，增加下面的内容
const (
//LimitDepth 最大的搜索深度
LimitDepth = 32
//MateValue 最高分值，即将死的分值
MateValue = 10000
//WinValue 搜索出胜负的分值界限，超出此值就说明已经搜索出杀棋了
WinValue = MateValue - 100
//AdvancedValue 先行权分值
AdvancedValue = 3
)
// 子力位置价值表
var cucvlPiecePos = [7][256]int{
{ // 帅(将)
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 11, 15, 11, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 仕(士)
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 20, 0, 20, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 23, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 20, 0, 20, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 相(象)
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 20, 0, 0, 0, 20, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 18, 0, 0, 0, 23, 0, 0, 0, 18, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 20, 0, 0, 0, 20, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 马
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 90, 90, 90, 96, 90, 96, 90, 90, 90, 0, 0, 0, 0,
0, 0, 0, 90, 96, 103, 97, 94, 97, 103, 96, 90, 0, 0, 0, 0,
0, 0, 0, 92, 98, 99, 103, 99, 103, 99, 98, 92, 0, 0, 0, 0,
0, 0, 0, 93, 108, 100, 107, 100, 107, 100, 108, 93, 0, 0, 0, 0,
0, 0, 0, 90, 100, 99, 103, 104, 103, 99, 100, 90, 0, 0, 0, 0,
0, 0, 0, 90, 98, 101, 102, 103, 102, 101, 98, 90, 0, 0, 0, 0,
0, 0, 0, 92, 94, 98, 95, 98, 95, 98, 94, 92, 0, 0, 0, 0,
0, 0, 0, 93, 92, 94, 95, 92, 95, 94, 92, 93, 0, 0, 0, 0,
0, 0, 0, 85, 90, 92, 93, 78, 93, 92, 90, 85, 0, 0, 0, 0,
0, 0, 0, 88, 85, 90, 88, 90, 88, 90, 85, 88, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 车
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 206, 208, 207, 213, 214, 213, 207, 208, 206, 0, 0, 0, 0,
0, 0, 0, 206, 212, 209, 216, 233, 216, 209, 212, 206, 0, 0, 0, 0,
0, 0, 0, 206, 208, 207, 214, 216, 214, 207, 208, 206, 0, 0, 0, 0,
0, 0, 0, 206, 213, 213, 216, 216, 216, 213, 213, 206, 0, 0, 0, 0,
0, 0, 0, 208, 211, 211, 214, 215, 214, 211, 211, 208, 0, 0, 0, 0,
0, 0, 0, 208, 212, 212, 214, 215, 214, 212, 212, 208, 0, 0, 0, 0,
0, 0, 0, 204, 209, 204, 212, 214, 212, 204, 209, 204, 0, 0, 0, 0,
0, 0, 0, 198, 208, 204, 212, 212, 212, 204, 208, 198, 0, 0, 0, 0,
0, 0, 0, 200, 208, 206, 212, 200, 212, 206, 208, 200, 0, 0, 0, 0,
0, 0, 0, 194, 206, 204, 212, 200, 212, 204, 206, 194, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 炮
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 100, 100, 96, 91, 90, 91, 96, 100, 100, 0, 0, 0, 0,
0, 0, 0, 98, 98, 96, 92, 89, 92, 96, 98, 98, 0, 0, 0, 0,
0, 0, 0, 97, 97, 96, 91, 92, 91, 96, 97, 97, 0, 0, 0, 0,
0, 0, 0, 96, 99, 99, 98, 100, 98, 99, 99, 96, 0, 0, 0, 0,
0, 0, 0, 96, 96, 96, 96, 100, 96, 96, 96, 96, 0, 0, 0, 0,
0, 0, 0, 95, 96, 99, 96, 100, 96, 99, 96, 95, 0, 0, 0, 0,
0, 0, 0, 96, 96, 96, 96, 96, 96, 96, 96, 96, 0, 0, 0, 0,
0, 0, 0, 97, 96, 100, 99, 101, 99, 100, 96, 97, 0, 0, 0, 0,
0, 0, 0, 96, 97, 98, 98, 98, 98, 98, 97, 96, 0, 0, 0, 0,
0, 0, 0, 96, 96, 97, 99, 99, 99, 97, 96, 96, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
{ // 兵(卒)
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 9, 9, 9, 11, 13, 11, 9, 9, 9, 0, 0, 0, 0,
0, 0, 0, 19, 24, 34, 42, 44, 42, 34, 24, 19, 0, 0, 0, 0,
0, 0, 0, 19, 24, 32, 37, 37, 37, 32, 24, 19, 0, 0, 0, 0,
0, 0, 0, 19, 23, 27, 29, 30, 29, 27, 23, 19, 0, 0, 0, 0,
0, 0, 0, 14, 18, 20, 27, 29, 27, 20, 18, 14, 0, 0, 0, 0,
0, 0, 0, 7, 0, 13, 0, 16, 0, 13, 0, 7, 0, 0, 0, 0,
0, 0, 0, 7, 0, 7, 0, 15, 0, 7, 0, 7, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}}PositionStruct结构体
打开rule.go，修改PositionStruct结构体：
type PositionStruct struct {
sdPlayer int // 轮到谁走，0=红方，1=黑方
vlRed int //红方的子力价值
vlBlack int //黑方的子力价值
nDistance int // 距离根节点的步数
ucpcSquares [256]int // 棋盘上的棋子
}vlRed和vlBlack表示红黑双方局面棋子力价值。
nDistance表示搜索的深度。
PositionStruct方法
在放入棋子(addPiece)和拿走棋子(delPiece)的时候，计算红黑方的棋力值，有了棋力值就能用评价函数(evaluate)对当前局面进行评价：
//addPiece 在棋盘上放一枚棋子
func (p *PositionStruct) addPiece(sq, pc int) {
p.ucpcSquares[sq] = pc
// 红方加分，黑方(注意"cucvlPiecePos"取值要颠倒)减分
if pc < 16 {
p.vlRed += cucvlPiecePos[pc-8][sq]
} else {
p.vlBlack += cucvlPiecePos[pc-16][squareFlip(sq)]
}
}//delPiece 从棋盘上拿走一枚棋子
func (p *PositionStruct) delPiece(sq, pc int) {
p.ucpcSquares[sq] = 0
// 红方减分，黑方(注意"cucvlPiecePos"取值要颠倒)加分
if pc < 16 {
p.vlRed -= cucvlPiecePos[pc-8][sq]
} else {
p.vlBlack -= cucvlPiecePos[pc-16][squareFlip(sq)]
}
}//evaluate 局面评价函数
func (p *PositionStruct) evaluate() int {
if p.sdPlayer == 0 {
return p.vlRed - p.vlBlack + AdvancedValue
}
return p.vlBlack - p.vlRed + AdvancedValue
}在startup函数里不再直接赋值，而是使用addPiece用来计算子力价值：
//startup 初始化棋盘
func (p *PositionStruct) startup() {
pc := 0
p.sdPlayer, p.vlRed, p.vlBlack, p.nDistance = 0, 0, 0, 0
for i := 0; i < 256; i++ {
p.ucpcSquares[i] = 0
}
for sq := 0; sq < 256; sq++ {
pc = cucpcStartup[sq]
if pc != 0 {
p.addPiece(sq, pc)
}
}
}
在走子的时候，同步修改子力价值 ：
//movePiece 搬一步棋的棋子
func (p *PositionStruct) movePiece(mv int) int {
sqSrc := src(mv)
sqDst := dst(mv)
pcCaptured := p.ucpcSquares[sqDst]
if pcCaptured != 0 {
p.delPiece(sqDst, pcCaptured)
}
pc := p.ucpcSquares[sqSrc]
p.delPiece(sqSrc, pc)
p.addPiece(sqDst, pc)
return pcCaptured
}//undoMovePiece 撤消搬一步棋的棋子
func (p *PositionStruct) undoMovePiece(mv, pcCaptured int) {
sqSrc := src(mv)
sqDst := dst(mv)
pc := p.ucpcSquares[sqDst]
p.delPiece(sqDst, pc)
p.addPiece(sqSrc, pc)
if pcCaptured != 0 {
p.addPiece(sqDst, pcCaptured)
}
}
//makeMove 走一步棋
func (p *PositionStruct) makeMove(mv int) (bool, int) {
pcCaptured := p.movePiece(mv)
if p.checked() {
p.undoMovePiece(mv, pcCaptured)
return false, pcCaptured
}
p.changeSide()
p.nDistance++
return true, pcCaptured
}//undoMakeMove 撤消走一步棋
func (p *PositionStruct) undoMakeMove(mv, pcCaptured int) {
p.nDistance--
p.changeSide()
p.UndoMakeMove(mv, pcCaptured)
}有了对当前局面的评价，我们可以获得红黑双方的子力价值，就能找出最佳的局面，从而得出最佳的走法。
在下一节中，我们将学习如何使用Alpha-Beta算法搜索出最佳走法？
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