feat(comment): order changed projects by absolute cost change

pkg/vcs/comment/costs.go

@@ -49,11 +49,37 @@ func (data *Data) buildCostTableEntries() []CostTableEntry {
 				project.PastTotalMonthlyCost, project.TotalMonthlyCost, data.Currency,
 				costChangeOpts{},
 			),
-			NewTotalCost: formatCost(project.TotalMonthlyCost, data.Currency),
+			NewTotalCost:       formatCost(project.TotalMonthlyCost, data.Currency),
+			absTotalCostChange: safeSub(project.TotalMonthlyCost, project.PastTotalMonthlyCost).Abs(),
 		})
 	}
 
+	// If at least one project changed cost, hide the projects whose cost did not
+	// change. When no project changed cost, keep them all so the table is not
+	// empty.
+	hasCostChange := false
+	for _, entry := range entries {
+		if !entry.absTotalCostChange.IsZero() {
+			hasCostChange = true
+			break
+		}
+	}
+	if hasCostChange {
+		changed := make([]CostTableEntry, 0, len(entries))
+		for _, entry := range entries {
+			if !entry.absTotalCostChange.IsZero() {
+				changed = append(changed, entry)
+			}
+		}
+		entries = changed
+	}
+
+	// Biggest changes first, falling back to alphabetical ordering for entries
+	// with an equal absolute cost change.
 	sort.Slice(entries, func(i, j int) bool {
+		if !entries[i].absTotalCostChange.Equals(entries[j].absTotalCostChange) {
+			return entries[i].absTotalCostChange.GreaterThan(entries[j].absTotalCostChange)
+		}
 		if entries[i].ProjectName != entries[j].ProjectName {
 			return entries[i].ProjectName < entries[j].ProjectName
 		}
@@ -130,8 +156,8 @@ func projectHasDiff(project ProjectResult) bool {
 
 // costChangeOpts controls formatting of cost change strings.
 type costChangeOpts struct {
-	skipPercent  bool
-	skipIfZero   bool
+	skipPercent   bool
+	skipIfZero    bool
 	skipPlusMinus bool
 }
 

pkg/vcs/comment/costs_test.go

@@ -3,8 +3,91 @@ package comment
 import (
 	"testing"
 	"unicode/utf8"
+
+	"github.com/infracost/go-proto/pkg/rat"
 )
 
+func TestBuildCostTableEntriesOrdering(t *testing.T) {
+	project := func(name string, past, current int64) ProjectResult {
+		return ProjectResult{
+			Name:                 name,
+			PastTotalMonthlyCost: rat.New(past),
+			TotalMonthlyCost:     rat.New(current),
+			DiffBreakdown: &CostBreakdown{
+				Resources: []BreakdownResource{{Name: "aws_instance.web"}},
+			},
+		}
+	}
+
+	data := &Data{
+		Projects: []ProjectResult{
+			project("b-same-change", 100, 110),
+			project("small-increase", 100, 150),
+			project("big-decrease", 1000, 100),
+			project("a-same-change", 100, 110),
+			project("big-increase", 100, 700),
+		},
+	}
+
+	entries := data.buildCostTableEntries()
+
+	want := []string{
+		"big-decrease",   // -$900
+		"big-increase",   // +$600
+		"small-increase", // +$50
+		"a-same-change",  // +$10, alphabetical tie-break
+		"b-same-change",  // +$10
+	}
+
+	if len(entries) != len(want) {
+		t.Fatalf("got %d entries, want %d", len(entries), len(want))
+	}
+	for i, name := range want {
+		if entries[i].ProjectName != name {
+			t.Errorf("entry %d = %q, want %q", i, entries[i].ProjectName, name)
+		}
+	}
+}
+
+func TestBuildCostTableEntriesHidesUnchangedProjects(t *testing.T) {
+	project := func(name string, past, current int64) ProjectResult {
+		return ProjectResult{
+			Name:                 name,
+			PastTotalMonthlyCost: rat.New(past),
+			TotalMonthlyCost:     rat.New(current),
+			DiffBreakdown: &CostBreakdown{
+				Resources: []BreakdownResource{{Name: "aws_instance.web"}},
+			},
+		}
+	}
+
+	t.Run("some projects changed", func(t *testing.T) {
+		data := &Data{Projects: []ProjectResult{
+			project("unchanged", 100, 100),
+			project("changed", 100, 150),
+		}}
+
+		entries := data.buildCostTableEntries()
+
+		if len(entries) != 1 || entries[0].ProjectName != "changed" {
+			t.Errorf("got %+v, want only the changed project", entries)
+		}
+	})
+
+	t.Run("no projects changed", func(t *testing.T) {
+		data := &Data{Projects: []ProjectResult{
+			project("a", 100, 100),
+			project("b", 50, 50),
+		}}
+
+		entries := data.buildCostTableEntries()
+
+		if len(entries) != 2 {
+			t.Errorf("got %d entries, want both projects kept", len(entries))
+		}
+	})
+}
+
 func TestTruncateMiddle(t *testing.T) {
 	tests := []struct {
 		name   string

pkg/vcs/comment/template.go

@@ -7,6 +7,8 @@ import (
 	"strings"
 	"text/template"
 	"unicode/utf8"
+
+	"github.com/infracost/go-proto/pkg/rat"
 )
 
 const (
@@ -541,6 +543,10 @@ type CostTableEntry struct {
 
 	// NewTotalCost is the formatted new monthly cost, e.g. "$560.00".
 	NewTotalCost string
+
+	// absTotalCostChange is the absolute total cost change, used to sort rows so
+	// the biggest changes show first. It is not rendered in the template.
+	absTotalCostChange *rat.Rat
 }
 
 type ProjectError struct {