Mumps, 91, 86, 82,79
r t i t?1A.1"." s a=$A(t),p=a>90*32+65 f i=1:1:25 { w *(a+i-p#26+p) w:t["." !}
Not such a modern language ;) I'm sure there's a bit of optimization space left..
Explanation:
r t
read input
i t?1A.1"."
check if t matches the required input
s a=$A(t),p=a>90*32+65 f i=1:1:25 { w *(a+i-p#26+p) w:t["." !}
basic for loop through the alphabet. Note that mumps is strictly evaluating left to right. True=1, so you get 65 or 97 as a result for p, # is the modulo operator
tests:
USER>d ^golf
d.e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
a
b
c
USER>d ^golf
tuvwxyzabcdefghijklmnopqrs
USER>d ^golf
h.i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
a
b
c
d
e
f
g
USER>d ^golf
hallo
USER>
(you'll need a mumps runtime env, ie Caché to run this this)
edit: bold heading
edit: had a wrong solution, fixed now. Thanks to rtfs and Averroees for pointing this out