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 {

android/paths_test.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -1255,3 +1256,51 @@ func ExampleOutputPath_FileInSameDir() {
 	// out/system/framework/boot.art out/system/framework/oat/arm/boot.vdex
 	// boot.art oat/arm/boot.vdex
 }
+
+func BenchmarkFirstUniquePaths(b *testing.B) {
+	implementations := []struct {
+		name string
+		f    func(Paths) Paths
+	}{
+		{
+			name: "list",
+			f:    firstUniquePathsList,
+		},
+		{
+			name: "map",
+			f:    firstUniquePathsMap,
+		},
+	}
+	const maxSize = 1024
+	uniquePaths := make(Paths, maxSize)
+	for i := range uniquePaths {
+		uniquePaths[i] = PathForTesting(strconv.Itoa(i))
+	}
+	samePath := make(Paths, maxSize)
+	for i := range samePath {
+		samePath[i] = uniquePaths[0]
+	}
+
+	f := func(b *testing.B, imp func(Paths) Paths, paths Paths) {
+		for i := 0; i < b.N; i++ {
+			b.ReportAllocs()
+			paths = append(Paths(nil), paths...)
+			imp(paths)
+		}
+	}
+
+	for n := 1; n <= maxSize; n <<= 1 {
+		b.Run(strconv.Itoa(n), func(b *testing.B) {
+			for _, implementation := range implementations {
+				b.Run(implementation.name, func(b *testing.B) {
+					b.Run("same", func(b *testing.B) {
+						f(b, implementation.f, samePath[:n])
+					})
+					b.Run("unique", func(b *testing.B) {
+						f(b, implementation.f, uniquePaths[:n])
+					})
+				})
+			}
+		})
+	}
+}

android/util.go

@@ -193,6 +193,14 @@ func RemoveFromList(s string, list []string) (bool, []string) {
 // FirstUniqueStrings returns all unique elements of a slice of strings, keeping the first copy of
 // each.  It modifies the slice contents in place, and returns a subslice of the original slice.
 func FirstUniqueStrings(list []string) []string {
+	// 128 was chosen based on BenchmarkFirstUniqueStrings results.
+	if len(list) > 128 {
+		return firstUniqueStringsMap(list)
+	}
+	return firstUniqueStringsList(list)
+}
+
+func firstUniqueStringsList(list []string) []string {
 	k := 0
 outer:
 	for i := 0; i < len(list); i++ {
@@ -207,6 +215,20 @@ outer:
 	return list[:k]
 }
 
+func firstUniqueStringsMap(list []string) []string {
+	k := 0
+	seen := make(map[string]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]
+}
+
 // LastUniqueStrings returns all unique elements of a slice of strings, keeping the last copy of
 // each.  It modifies the slice contents in place, and returns a subslice of the original slice.
 func LastUniqueStrings(list []string) []string {

android/util_test.go

@@ -17,6 +17,7 @@ package android
 import (
 	"fmt"
 	"reflect"
+	"strconv"
 	"testing"
 )
 
@@ -59,15 +60,25 @@ var firstUniqueStringsTestCases = []struct {
 }
 
 func TestFirstUniqueStrings(t *testing.T) {
-	for _, testCase := range firstUniqueStringsTestCases {
+	f := func(t *testing.T, imp func([]string) []string, in, want []string) {
-		out := FirstUniqueStrings(testCase.in)
+		t.Helper()
-		if !reflect.DeepEqual(out, testCase.out) {
+		out := imp(in)
+		if !reflect.DeepEqual(out, want) {
 			t.Errorf("incorrect output:")
-			t.Errorf("     input: %#v", testCase.in)
+			t.Errorf("     input: %#v", in)
-			t.Errorf("  expected: %#v", testCase.out)
+			t.Errorf("  expected: %#v", want)
 			t.Errorf("       got: %#v", out)
 		}
 	}
+
+	for _, testCase := range firstUniqueStringsTestCases {
+		t.Run("list", func(t *testing.T) {
+			f(t, firstUniqueStringsList, testCase.in, testCase.out)
+		})
+		t.Run("map", func(t *testing.T) {
+			f(t, firstUniqueStringsMap, testCase.in, testCase.out)
+		})
+	}
 }
 
 var lastUniqueStringsTestCases = []struct {
@@ -568,3 +579,51 @@ func Test_Shard(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkFirstUniqueStrings(b *testing.B) {
+	implementations := []struct {
+		name string
+		f    func([]string) []string
+	}{
+		{
+			name: "list",
+			f:    firstUniqueStringsList,
+		},
+		{
+			name: "map",
+			f:    firstUniqueStringsMap,
+		},
+	}
+	const maxSize = 1024
+	uniqueStrings := make([]string, maxSize)
+	for i := range uniqueStrings {
+		uniqueStrings[i] = strconv.Itoa(i)
+	}
+	sameString := make([]string, maxSize)
+	for i := range sameString {
+		sameString[i] = uniqueStrings[0]
+	}
+
+	f := func(b *testing.B, imp func([]string) []string, s []string) {
+		for i := 0; i < b.N; i++ {
+			b.ReportAllocs()
+			s = append([]string(nil), s...)
+			imp(s)
+		}
+	}
+
+	for n := 1; n <= maxSize; n <<= 1 {
+		b.Run(strconv.Itoa(n), func(b *testing.B) {
+			for _, implementation := range implementations {
+				b.Run(implementation.name, func(b *testing.B) {
+					b.Run("same", func(b *testing.B) {
+						f(b, implementation.f, sameString[:n])
+					})
+					b.Run("unique", func(b *testing.B) {
+						f(b, implementation.f, uniqueStrings[:n])
+					})
+				})
+			}
+		})
+	}
+}