Show percentage savings in report

Fixes orijtech#23

Author: Cuong Manh Le

structslop.go

@@ -69,12 +69,13 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		curPkgPath := pass.Pkg.Path()
 		optStyp := formatStruct(r.suggestedStruct, curPkgPath)
 		msg := fmt.Sprintf(
-			"struct has size %d (size class %d), could be %d (size class %d), rearrange to %s for optimal size",
+			"struct has size %d (size class %d), could be %d (size class %d), rearrange to %s for optimal size (%.2f%% savings)",
 			r.oldGcSize,
 			r.oldRuntimeSize,
 			r.newGcSize,
 			r.newRuntimeSize,
 			optStyp,
+			r.savings(),
 		)
 		pass.Report(analysis.Diagnostic{
 			Pos:            n.Pos(),
@@ -98,6 +99,10 @@ func (r result) sloppy() bool {
 	return r.oldRuntimeSize > r.newRuntimeSize
 }
 
+func (r result) savings() float64 {
+	return float64(r.oldRuntimeSize-r.newRuntimeSize) / float64(r.oldRuntimeSize) * 100
+}
+
 func checkSloppy(pass *analysis.Pass, origStruct *types.Struct) result {
 	optStruct := optimalStructArrangement(pass.TypesSizes, origStruct)
 	r := result{

testdata/src/struct/p.go

@@ -27,13 +27,13 @@ type s2 struct {
 	j int
 }
 
-type s3 struct { // want `struct has size 24 \(size class 32\), could be 16 \(size class 16\), rearrange to struct{y uint64; x uint32; z uint32} for optimal size`
+type s3 struct { // want `struct has size 24 \(size class 32\), could be 16 \(size class 16\), rearrange to struct{y uint64; x uint32; z uint32} for optimal size \(50.00% savings\)`
 	x uint32
 	y uint64
 	z uint32
 }
 
-type s4 struct { // want `struct has size 40 \(size class 48\), could be 24 \(size class 32\), rearrange to struct{_ \[0\]func\(\); i1 int; i2 int; a3 \[3\]bool; b bool} for optimal size`
+type s4 struct { // want `struct has size 40 \(size class 48\), could be 24 \(size class 32\), rearrange to struct{_ \[0\]func\(\); i1 int; i2 int; a3 \[3\]bool; b bool} for optimal size \(33.33% savings\)`
 	b  bool
 	i1 int
 	i2 int
@@ -56,15 +56,15 @@ type s6 struct { // should be good, see #16
 	index uintptr
 }
 
-type s7 struct { // want `struct has size 40 \(size class 48\), could be 32 \(size class 32\), rearrange to struct{y uint64; t \*httptest.Server; w uint64; x uint32; z uint32} for optimal size`
+type s7 struct { // want `struct has size 40 \(size class 48\), could be 32 \(size class 32\), rearrange to struct{y uint64; t \*httptest.Server; w uint64; x uint32; z uint32} for optimal size \(33.33% savings\)`
 	x uint32
 	y uint64
 	t *httptest.Server
 	z uint32
 	w uint64
 }
 
-type s8 struct { // want `struct has size 40 \(size class 48\), could be 32 \(size class 32\), rearrange to struct{y uint64; t \*s; w uint64; x uint32; z uint32} for optimal size`
+type s8 struct { // want `struct has size 40 \(size class 48\), could be 32 \(size class 32\), rearrange to struct{y uint64; t \*s; w uint64; x uint32; z uint32} for optimal size \(33.33% savings\)`
 	x uint32
 	y uint64
 	t *s