diff --git a/src/algorithms/math/extended-euclidean-algorithm/README.md b/src/algorithms/math/extended-euclidean-algorithm/README.md index 4c60494e..62cce7a6 100644 --- a/src/algorithms/math/extended-euclidean-algorithm/README.md +++ b/src/algorithms/math/extended-euclidean-algorithm/README.md @@ -1,6 +1,19 @@

Extended Euclidean Algorithm

The standard Euclidean algorithm only calculates the gcd of a and b given 2 integers a and b. The extended Euclidean algorithm takes this one step further. For any 2 integers -a, b, the extended Euclidean algorithm calculates x and y such that ax + by = gcd(a, b). +a and b, the extended Euclidean algorithm calculates x and y such that ax + by = gcd(a, b). To perform this, the extended euclidean algorithm makes some slight modifications to the original euclidean algorithm. +The original euclidean algorithm arrived at a = gcd(a,b) and b = 0. Notice that if the original a and b equalled these values, then the solution would merely be x = 1 and y = 0. We can use these 2 values as a starting point for our algorithm. That way, all we would have to do is figure out how x and y change during the transition from (a, b) to (b, a mod b). +Let us say that we have x1 and y1 such that b * x1 + (a mod b) * y1 = gcd(a, b).
+ +Notice that we can substitute a mod b with a - floor(a/b) * b.
+ +Thus, after rearranging the terms, we get gcd(a, b) = a * y1 + b * (x1 - y1 * floor(a / b)).
+ +So after every step,
+ +x = y1
+y = x1 - y1 * floor(a/b).
+ +**We can use this to eventually backtrack and arrive at the desired x and y.**