.Dd November 9, 2018
.Dt ADLER32 3
.Os
.
.Sh NAME
.Nm adler32 ,
.Nm adler32_z ,
.Nm adler32_combine
.Nd update Adler-32 checksum
.
.Sh LIBRARY
.Lb libz
.
.Sh SYNOPSIS
.In zlib.h
.Ft uLong
.Fn adler32 "uLong adler" "const Bytef *buf" "uInt len"
.Ft uLong
.Fn adler32_z "uLong adler" "const Bytef *buf" "z_size_t len"
.Ft uLong
.Fn adler32_combine "uLong adler1" "uLong adler2" "z_off_t len2"
.
.Sh DESCRIPTION
Update a running Adler-32 checksum with the bytes
.Fa "buf[0..len-1]"
and return the updated checksum.
If
.Fa buf
is
.Dv Z_NULL ,
this function returns
the required initial value for the checksum.
.
.Pp
An Adler-32 checksum is almost as reliable as a CRC-32
but can be computed much faster.
.
.Pp
.Fn adler32_z
is the same as
.Fn adler32 ,
but with a
.Vt size_t
length.
.
.Pp
.Fn adler32_combine
combines two Adler-32 checksums into one.
For two sequences of bytes,
.Va seq1
and
.Va seq2
with lengths
.Va len1
and
.Va len2 ,
Adler-32 checksums were calculated for each,
.Va adler1
and
.Va adler2 .
.Fn adler32_combine
returns the Adler-32 checksum of
.Va seq1
and
.Va seq2
concatenated,
requiring only
.Fa adler1 ,
.Fa adler2 ,
and
.Fa len2 .
Note that the
.Vt z_off_t
type
(like
.Vt off_t )
is a signed integer.
If
.Fa len2
is negative,
the result has no meaning or utility.
.
.Sh EXAMPLES
.Bd -literal -offset indent
uLong adler = adler32(0L, Z_NULL, 0);

while (read_buffer(buffer, length) != EOF) {
	adler = adler32(adler, buffer, length);
}
if (adler != original_adler) error();
.Ed