X =
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
Y =
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
X ↔ Y
Ext-GCD(0, 15) :
i
GCD
Bezout
AOdd :
AEven :
A
↔
B
ax
ay
bx
by
|A-B|
A↔B
Sx
Sy
ax ←
Sx' ay ←
Sy' A ←
A/2 ax ←
ax + Y ay ←
ay + X ax ←
ax/2 ay ←
ay/2
0
0
15
1
0
0
1
1
0
15
1
0
0
1
15
2
0
15
1
0
0
1
15
3
0
15
1
0
0
1
15
4
0
15
1
0
0
1
15
5
0
15
1
0
0
1
15
6
0
15
1
0
0
1
15
7
15
↔
0
0
1
1
0
15
GCD = 15; P, Q Prohibited (X = 0, so Q is negative)