feat: -threshold filters diff findings by bytes saved

README.md

@@ -103,6 +103,8 @@ structalign [flags] [packages]
   -summary        in diff mode, print a one-line summary after the diffs
   -sort           present results largest-first (diff: by bytes saved;
                   inspect: by struct size)
+  -threshold int  in diff mode, only show structs that save at least N bytes
+                  (default 0; negatives treated as 0)
 
   -type string    only consider named structs matching these comma-separated
                   glob patterns (e.g. "*Request,Config"); empty means all

internal/app/app.go

@@ -72,6 +72,7 @@ type options struct {
 	skipCachePadded bool
 	summary         bool
 	sort            bool
+	threshold       int
 }
 
 // savings is the absolute bytes a finding saves, or 0 when sizes are unknown or
@@ -108,6 +109,7 @@ func (a *App) Run(args []string) int {
 	fs.BoolVar(&opt.skipCachePadded, "skip-cache-padded", false, "skip structs containing a golang.org/x/sys/cpu.CacheLinePad field")
 	fs.BoolVar(&opt.summary, "summary", false, "in diff mode, print a one-line summary after the diffs")
 	fs.BoolVar(&opt.sort, "sort", false, "present results largest-first (diff: by bytes saved; inspect: by struct size)")
+	fs.IntVar(&opt.threshold, "threshold", 0, "in diff mode, only show structs that save at least this many bytes")
 	fs.Usage = func() {
 		fmt.Fprintf(a.Stderr, "structalign: print field-aligned struct reorderings (no file changes)\n\n")
 		fmt.Fprintf(a.Stderr, "usage: structalign [flags] [packages]\n\n")
@@ -197,6 +199,17 @@ func (a *App) Run(args []string) int {
 		}
 	}
 
+	if !opt.inspect && opt.threshold > 0 {
+		min := int64(opt.threshold)
+		kept := allFindings[:0]
+		for _, f := range allFindings {
+			if savings(f) >= min {
+				kept = append(kept, f)
+			}
+		}
+		allFindings = kept
+	}
+
 	if opt.sort {
 		if opt.inspect {
 			sort.SliceStable(allLayouts, func(i, j int) bool {

internal/app/threshold_test.go

@@ -0,0 +1,55 @@
+package app_test
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// These reuse sortApp / sortSrc from sort_test.go, where Small saves 8 bytes
+// (24->16) and Big saves 16 (40->24).
+
+func TestRunThresholdFiltersSmallSavings(t *testing.T) {
+	var out, errb bytes.Buffer
+	a := sortApp(t, &out, &errb)
+	// 16 is inclusive: keeps Big (saves 16), drops Small (saves 8).
+	code := a.Run([]string{"-threshold=16", "pkg"})
+	assert.Equal(t, 1, code)
+	s := out.String()
+	assert.Contains(t, s, "Big")
+	assert.NotContains(t, s, "Small")
+}
+
+func TestRunThresholdZeroShowsAll(t *testing.T) {
+	var out, errb bytes.Buffer
+	a := sortApp(t, &out, &errb)
+	_ = a.Run([]string{"-threshold=0", "pkg"})
+	s := out.String()
+	assert.Contains(t, s, "Big")
+	assert.Contains(t, s, "Small")
+}
+
+func TestRunThresholdNegativeIsZero(t *testing.T) {
+	var out, errb bytes.Buffer
+	a := sortApp(t, &out, &errb)
+	_ = a.Run([]string{"-threshold=-5", "pkg"})
+	assert.Contains(t, out.String(), "Small", "a negative threshold behaves like 0 (no filtering)")
+}
+
+func TestRunThresholdExitsZeroWhenAllFiltered(t *testing.T) {
+	var out, errb bytes.Buffer
+	a := sortApp(t, &out, &errb)
+	code := a.Run([]string{"-threshold=10000", "pkg"})
+	assert.Equal(t, 0, code, "all filtered out => no findings => exit 0")
+	assert.Equal(t, "", strings.TrimSpace(out.String()))
+}
+
+func TestRunThresholdSummaryReflectsFiltered(t *testing.T) {
+	var out, errb bytes.Buffer
+	a := sortApp(t, &out, &errb)
+	_ = a.Run([]string{"-threshold=16", "-summary", "pkg"})
+	assert.Contains(t, out.String(), "Summary: 1 struct affected, 16 bytes saved",
+		"summary counts only the structs that pass the threshold")
+}