The answer, like so much else in computation, relates to a power of 2:
impute.ado:
/* vm is coded m.v. pattern for x */
gen long `vm' = 0 if `touse'
scalar `bit' = 1
foreach var of local rhs {
replace `vm' = `vm' +
`bit'*(!missing(`var'))
scalar `bit' = `bit' * 2
}
The missing value pattern stores the pattern for as many variables as
there are bits in a 4-byte word, namely 32. Stata does not support a
'long long' integer type, and it would not be feasible to store as a
floating point value (e.g. double). If I wanted to rewrite this code to
deal with the limitation, I'd have to keep track of more than one
longword. That is certainly feasible, but Nick Cox's question applies.
