#include <stdio.h>
/*
Example: 57 * 18
57 18
Halving the first column:
28 18
14
7
3
1
Doubling the second column:
28 36
14 72
7 144
3 288
1 576
Remove rows whose first cell is even:
28 36 x
14 72 x
7 144
3 288
1 576
Sum the remaining numbers in the right-hand column:
28 -
14 -
7 162
3 288
1 576
====
1026
*/
int ethiopianMultiply(int a, int b) {
int sum = 0;
while (a > 0) {
if (a % 2 == 1) { // keep only odd rows
sum += b;
printf("sum = %d\n", sum);
}
a /= 2; // halve left side
b *= 2; // double right side
printf("a = %d b = %d\n", a, b);
}
return sum;
}
int main(void) {
int a = 57;
int b = 18;
printf("%d\n", ethiopianMultiply(a, b));
return 0;
}
/*
run:
sum = 18
a = 28 b = 36
a = 14 b = 72
a = 7 b = 144
sum = 162
a = 3 b = 288
sum = 450
a = 1 b = 576
sum = 1026
a = 0 b = 1152
1026
*/
