I've created a type of alphabet cipher in which each character is defined based on how distant it is (in the alphabet) from the preceding character.
So, assuming that $A=0$, $B=1$, $C=2$, etc., and you have a text input of "GOOSE," you would construct the output like this:
G $(6)$ is $2$ letters in front of E $(4)$, so output is C $(2)$;
O $(14)$ is $8$ letters in front of G $(6)$, so output is I $(8)$;
O $(14)$ is $0$ letters in front of O $(14)$, so output is A $(0)$;
S $(18)$ is $4$ letters in front of O $(14)$, so output is E $(4)$;
E $(4)$ is $12$ letters in front of S $(18)$, so output is M $(12)$.
One of the benefits of this type of cipher is that, unlike a Caesar cipher, it can't be broken using normal letter-frequency analysis, since the cipher-letter that represents each actual letter is not constant throughout.
I know very little about ciphers, so I'm assuming that this type of thing has been done before.
I'm curious: Does this type of cipher have a name -- and how easy is it to decipher, compared with other techniques?