shortened iterations [code golf]

A codegolf lazy morning exercise towards finding the sequence of integers that starts with an arbitrary value n and gets updated by blocks of four as

until the last term is not an integer. While the update can be easily implemented with the appropriate stopping rule, a simple congruence analysis shows that, depending on n, the sequence is 4, 8 or 12 values long when

respectively. But sadly the more interesting fixed length solution

`~`=rep #redefine function
b=(scan()-1)*c(32~4,8,40~4,1,9~3)/32+c(1,1,3,0~3,6,-c(8,1,9,-71,17)/8)
b[!b%%1] #keep integers only

ends up being longer than the more basic one:

a=scan()
while(!a[T]%%1)a=c(a,d<-a[T]*T,d+T+1,e<-d-1,e/((T<-T+4)-1))
a[-T]

where Robin’s suggestion of using T rather than length is very cool as T has double meaning, first TRUE (and 1) then the length of a…