Add searchable android.DirectorySortedPaths

Add an android.DirectorySortedPaths that stores paths sorted such
that all paths in a directory including subdirectories are in a
contiguous subslice.  This will allow efficient O(log(N)) finding
of all paths in a directory using a binary search on the directory
prefix.

Test: TestDirectorySortedPaths in paths_test.go
Change-Id: I5a06a89351ae06e88c06526be54a6b79075361b7

android/paths.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"reflect"
+	"sort"
 	"strings"
 
 	"github.com/google/blueprint"
@@ -380,6 +381,38 @@ func (p Paths) FilterOutByExt(ext string) Paths {
 	return ret
 }
 
+// DirectorySortedPaths is a slice of paths that are sorted such that all files in a directory
+// (including subdirectories) are in a contiguous subslice of the list, and can be found in
+// O(log(N)) time using a binary search on the directory prefix.
+type DirectorySortedPaths Paths
+
+func PathsToDirectorySortedPaths(paths Paths) DirectorySortedPaths {
+	ret := append(DirectorySortedPaths(nil), paths...)
+	sort.Slice(ret, func(i, j int) bool {
+		return ret[i].String() < ret[j].String()
+	})
+	return ret
+}
+
+// PathsInDirectory returns a subslice of the DirectorySortedPaths as a Paths that contains all entries
+// that are in the specified directory and its subdirectories.
+func (p DirectorySortedPaths) PathsInDirectory(dir string) Paths {
+	prefix := filepath.Clean(dir) + "/"
+	start := sort.Search(len(p), func(i int) bool {
+		return prefix < p[i].String()
+	})
+
+	ret := p[start:]
+
+	end := sort.Search(len(ret), func(i int) bool {
+		return !strings.HasPrefix(ret[i].String(), prefix)
+	})
+
+	ret = ret[:end]
+
+	return Paths(ret)
+}
+
 // WritablePaths is a slice of WritablePaths, used for multiple outputs.
 type WritablePaths []WritablePath