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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
define MAX_HASHCHARS 18
# STHASH -- Compute the hash index of a key, i.e., the index of a thread in
# the symbol table index. Multiple keys may hash to the same thread. The
# ideal hash function will uniformly map keys into index space, both when the
# keys are selected randomly and when the keys form patterns, e.g., when keys
# share a common prefix. The SYMTAB package uses a simple hash function which
# is computed inline. The STHASH function is NOT used at present, but is
# included in the library anyway for use in other packages and because this
# is a slightly better (more uniform) hashing function than the simple inline
# version used in SYMTAB.
int procedure sthash (key, modulus)
char key[ARB] # character string serving as a key
int modulus # number of possible output values
int i
long sum
int primes[MAX_HASHCHARS]
data (primes(i),i=1,9) /101,103,107,109,113,127,131,137,139/
data (primes(i),i=10,18) /149,151,157,163,167,173,179,181,191/
begin
sum = 0
# Hash up to len(primes)=18 characters from the key.
do i = 1, MAX_HASHCHARS {
if (key[i] == EOS)
break
sum = sum + (key[i] * primes[i])
}
return (mod (sum, modulus) + 1)
end
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