score: experimental extension novelty in sorting

Right now we boost a file extension that hasn't been seen to the 3rd
position. This is gated by an environment variable. I want to explore if
there are ways we can turn on this behaviour with the query language.

Test Plan: ZOEKT_NOVELTY=1 go run ./cmd/zoekt foo

contentprovider.go

@@ -18,9 +18,13 @@ import (
 	"bytes"
 	"fmt"
 	"log"
+	"os"
+	"path"
 	"sort"
 	"strings"
 	"unicode/utf8"
+
+	"golang.org/x/exp/slices"
 )
 
 var _ = log.Println
@@ -913,9 +917,53 @@ func sortChunkMatchesByScore(ms []ChunkMatch) {
 	sort.Sort(chunkMatchScoreSlice(ms))
 }
 
+var doNovelty = os.Getenv("ZOEKT_NOVELTY") != ""
+
 // SortFiles sorts files matches. The order depends on the match score, which includes both
 // query-dependent signals like word overlap, and file-only signals like the file ranks (if
 // file ranks are enabled).
 func SortFiles(ms []FileMatch) {
 	sort.Sort(fileMatchesByScore(ms))
+
+	if doNovelty {
+		// Experimentally boost something into the third filematch
+		boostNovelExtension(ms, 2, 0.9)
+	}
+}
+
+func boostNovelExtension(ms []FileMatch, boostOffset int, minScoreRatio float64) {
+	if len(ms) <= boostOffset+1 {
+		return
+	}
+
+	top := ms[:boostOffset]
+	candidates := ms[boostOffset:]
+
+	// Don't bother boosting something which is significantly different to the
+	// result it replaces.
+	minScoreForNovelty := candidates[0].Score * minScoreRatio
+
+	// We want to look for an ext that isn't in the top exts
+	exts := make([]string, len(top))
+	for i := range top {
+		exts[i] = path.Ext(top[i].FileName)
+	}
+
+	for i := range candidates {
+		// Do not assume sorted due to boostNovelExtension being called on subsets
+		if candidates[i].Score < minScoreForNovelty {
+			continue
+		}
+
+		if slices.Contains(exts, path.Ext(candidates[i].FileName)) {
+			continue
+		}
+
+		// Found what we are looking for, now boost to front of candidates (which
+		// is ms[boostOffset])
+		for ; i > 0; i-- {
+			candidates[i], candidates[i-1] = candidates[i-1], candidates[i]
+		}
+		return
+	}
 }