Optimize FirstUniqueStrings and FirstUniquePaths

FirstUniquePaths is called on some long lists where the O(n^2)
behavior is problematic.  Use a map-based implementation for
longer lists.

Test: TestFirstUniqueStrings
Change-Id: I7181aba869e5ccc0f99c2fa7b8f03839f06e4307

android/paths.go

@@ -470,6 +470,14 @@ func (p Paths) Strings() []string {
 // FirstUniquePaths returns all unique elements of a Paths, keeping the first copy of each.  It
 // modifies the Paths slice contents in place, and returns a subslice of the original slice.
 func FirstUniquePaths(list Paths) Paths {
+	// 128 was chosen based on BenchmarkFirstUniquePaths results.
+	if len(list) > 128 {
+		return firstUniquePathsMap(list)
+	}
+	return firstUniquePathsList(list)
+}
+
+func firstUniquePathsList(list Paths) Paths {
 	k := 0
 outer:
 	for i := 0; i < len(list); i++ {
@@ -484,6 +492,20 @@ outer:
 	return list[:k]
 }
 
+func firstUniquePathsMap(list Paths) Paths {
+	k := 0
+	seen := make(map[Path]bool, len(list))
+	for i := 0; i < len(list); i++ {
+		if seen[list[i]] {
+			continue
+		}
+		seen[list[i]] = true
+		list[k] = list[i]
+		k++
+	}
+	return list[:k]
+}
+
 // LastUniquePaths returns all unique elements of a Paths, keeping the last copy of each.  It
 // modifies the Paths slice contents in place, and returns a subslice of the original slice.
 func LastUniquePaths(list Paths) Paths {