R写的三子棋（井字棋）程序

我家小朋友最近迷上了井字棋，每天都要拉着大人玩两盘，这两天恰好有闲，写个井字棋程序练练手，同时看看能否提起小朋友对算法的兴趣。
时间有限，目前只完成了核心部分，即根据当前棋盘状态选择下一步最可能获胜的走法，用了基本的maxmin方法。只要用R的交互绘图功能给核心程序套上衣服就可以玩了。

rotate<- function(x, clockwise=T) {
  if (clockwise) { 
    t( apply(x, 2, rev))
  } else {
    apply( t(x),2, rev)
  } 
}


checkstate=function(state){
  cw=colSums(state)
  rw=rowSums(state)
  pdw=sum(diag(state))
  sdw=sum(diag(rotate(state)))
  w=c(cw,rw,pdw,sdw)
  
  if(any(w==3))
    return('x')#x:1 win
  else if(any(w==-3))
    return('o')#o:-1 win
  
  if(!any(state==0))
    return('e')#equal
  else
    return('u')#in progress
}

nextStates=function(state,player){
  ids=which(state==0)
  if(length(ids)==0)
    return(0)#leaf node
  
  post=array(dim=c(3,3,length(ids)))
  
  if(player=='x')
    v=1
  else 
    v=-1
  
  for (i in 1:length(ids)){
    ns=state
    ns[ids[i]]=v
    post[,,i]=ns
  }
  
  return(post)
}

ceval=function(state){
  cc=checkstate(state)
  if(cc!='u'){#leaf node
    return(c(switch(which(c('x','o','e')==cc),1,-1,0),1))
  }
  
  ns=nextStates(state,'x')
  rs=apply(X = ns,MARGIN = 3,FUN = peval)
  i=which.max(rs[1,])
  v=rs[1,i]
  
  return(c(v,i))
}

peval=function(state){
  cc=checkstate(state)
  if(cc!='u'){#leaf node
    return(c(switch(which(c('x','o','e')==cc),1,-1,0),1))
  }
  
  ns=nextStates(state,'o')
  rs=apply(X = ns,MARGIN = 3,FUN = ceval)
  i=which.min(rs[1,])
  v=rs[1,i]
  return(c(v,i))
}

#state=matrix(data=c(1,-1,0,-1,1,1,1,-1,-1),3,3)
#state=matrix(0,3,3)
state=matrix(data=c(1,-1,0,0,-1,0,0,1,0),3,3)

v=ceval(state)
v