1 2 3 4

up-sweep

log2(4)=2
d=0 <1
	k=0 to 3 k+=2^(d+1) in parallel do

d=0;(parallel)
k=0;
x[0+2-1]=x[0+1-1]+x[0+2-1]
=>x[1]=x[0]+x[1]//3

d=0;(parallel)
k=2;
x[2+2-1]=x[2+1-1]+x[2+2-1]
=>x[3]=x[2]+x[3]//7

1 3 3 7

down-sweep
x[3]=0;
for d=1 to 0
	for k=0 to 3 k+=2^(d+1) in parallel do

d=1;
k=0;
t=x[0+2-1]
t=x[1];//3
x[1]=x[3];//0
x[3]=t+x[3]//3+0=3;


d=0;(parallel)
k=0;
t=x[0+1-1]=x[0]//1
x[0]=x[1];//0
x[1]=1;

d=0;(parallel)
k=2;
t=x[2+1-1]=x[2];//3
x[2]=x[2+2-1]=x[3];//3
x[3]=t+x[2];//3+3=6;

0 1 3 6(exclusive)