如何理解xyz的判断点在凸包内模板

int n,m,tot;
struct point 
{
 double x,y;
}p[100000],a[100000],ss;
bool cmp(point A,point B)
{
 if(A.x!=B.x)
 return A.x<B.x;
 return A.y<B.y;
}
point operator -(point A,point B)
{
 point c;
 c.x=A.x-B.x;
 c.y=A.y-B.y;
 return c;
}
double cross(point A,point B)
{
 return A.x*B.y-B.x*A.y;
}
void dopack()
{
 tot=0;
 for(int i=1;i<=n;i++)
 {
 while(tot>1&&cross(p[tot-1]-p[tot-2],a[i]-p[tot-2])<=0)tot--;
 p[tot++]=a[i];
 }
 int k=tot;
 for(int i=n-1;i>0;i--)
 {
 while(tot>k&&cross(p[tot-1]-p[tot-2],a[i]-p[tot-2])<=0)tot--;
 p[tot++]=a[i];
 }
 if(n>1)tot--;
}
bool check(point A)
{
 int l=1,r=tot-2,mid;
 while(l<=r)
 {
 mid=(l+r)>>1;
 double a1=cross(p[mid]-p[0],A-p[0]);
 double a2=cross(p[mid+1]-p[0],A-p[0]);
 if(a1>=0&&a2<=0)
 {
 if(cross(p[mid+1]-p[mid],A-p[mid])>=0)return true;
 return false;
 }
 else if(a1<0)
 {
 r=mid-1;
 }
 else 
 {
 l=mid+1;
 }
 }
 return false;
}