Can't do what I did, as the rolling function is not the same as the
non-rolling one. Instead this uses an improved version of the rolling
adler32 to accomplish the same thing. (PR filed on upstream, so should
be able to use that directly in the future.)
The rolling version of adler32 is just a wrapper around the standard
hash/adler32 when used in a non-rolling fashion, but it's inefficient as
it allocates a new hash instance for every Write(). This uses the
default version instead in the block hasher, and adds a test to verify
the result is the same as they were before. It reduces allocations by
88% and increases speed about 5%.
benchmark old ns/op new ns/op delta
BenchmarkHashFile-8 64434698 61303647 -4.86%
benchmark old MB/s new MB/s speedup
BenchmarkHashFile-8 276.65 290.78 1.05x
benchmark old allocs new allocs delta
BenchmarkHashFile-8 1238 150 -87.88%
benchmark old bytes new bytes delta
BenchmarkHashFile-8 17877363 49292 -99.72%
