Autotag optimisation (#2368)

* Add duration to autotag finish message
* No sorting scene/image/gallery where not specified
* Use an LRU cache for sqlite regexp function
* Compile path separator regex once
* Cache objects with single letter first names
* Move finished auto-tag log
* Add more verbose logging
* Add new changelog

pkg/database/regex.go

@@ -0,0 +1,42 @@
+package database
+
+import (
+	"regexp"
+
+	lru "github.com/hashicorp/golang-lru"
+)
+
+// size of the regex LRU cache in elements.
+// A small number number was chosen because it's most likely use is for a
+// single query - this function gets called for every row in the (filtered)
+// results. It's likely to only need no more than 1 or 2 in any given query.
+// After that point, it's just sitting in the cache and is unlikely to be used
+// again.
+const regexCacheSize = 10
+
+var regexCache *lru.Cache
+
+func init() {
+	regexCache, _ = lru.New(regexCacheSize)
+}
+
+// regexFn is registered as an SQLite function as "regexp"
+// It uses an LRU cache to cache recent regex patterns to reduce CPU load over
+// identical patterns.
+func regexFn(re, s string) (bool, error) {
+	entry, ok := regexCache.Get(re)
+	var compiled *regexp.Regexp
+
+	if !ok {
+		var err error
+		compiled, err = regexp.Compile(re)
+		if err != nil {
+			return false, err
+		}
+		regexCache.Add(re, compiled)
+	} else {
+		compiled = entry.(*regexp.Regexp)
+	}
+
+	return compiled.MatchString(s), nil
+}