Olaoluwa Osuntokun 3b39edcaa1 txscript: optimize sigcache lookup (#598) ...

Profiles discovered that lookups into the signature cache included an
expensive comparison to the stored `sigInfo` struct. This lookup had the
potential to be more expensive than directly verifying the signature
itself!

In addition, evictions were rather expensive because they involved
reading from /dev/urandom, or equivalent, for each eviction once the
signature cache was full as well as potentially iterating over every
item in the cache in the worst-case.

To remedy this poor performance several changes have been made:
* Change the lookup key to the fixed sized 32-byte signature hash
* Perform a full equality check only if there is a cache hit which
    results in a significant  speed up for both insertions and existence
checks
* Override entries in the case of a colliding hash on insert Add an
* .IsEqual() method to the Signature and PublicKey types in the
  btcec package to facilitate easy equivalence testing
* Allocate the signature cache map with the max number of entries in
  order to avoid unnecessary map re-sizes/allocations
* Optimize evictions from the signature cache Delete the first entry
* seen which is safe from manipulation due to
    the pre image resistance of the hash function
* Double the default maximum number of entries within the signature
  cache due to the reduction in the size of a cache entry
  * With this eviction scheme, removals are effectively O(1)

Fixes #575.

2016-04-13 21:56:10 -05:00

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