Adds basic sequence diagrams behind a layout flag

cmd/d2/main.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
@@ -13,6 +14,7 @@ import (
 	"github.com/spf13/pflag"
 
 	"oss.terrastruct.com/d2"
+	"oss.terrastruct.com/d2/d2layouts/d2sequence"
 	"oss.terrastruct.com/d2/d2plugin"
 	"oss.terrastruct.com/d2/d2renderers/d2svg"
 	"oss.terrastruct.com/d2/d2renderers/textmeasure"
@@ -187,8 +189,13 @@ func compile(ctx context.Context, ms *xmain.State, plugin d2plugin.Plugin, theme
 		return nil, err
 	}
 
+	layout := plugin.Layout
+	// TODO: remove, this is just a feature flag to test sequence diagrams as we work on them
+	if os.Getenv("D2_SEQUENCE") == "1" {
+		layout = d2sequence.Layout
+	}
 	d, err := d2.Compile(ctx, string(input), &d2.CompileOptions{
-		Layout:  plugin.Layout,
+		Layout:  layout,
 		Ruler:   ruler,
 		ThemeID: themeID,
 	})

d2layouts/d2sequence/constants.go

@@ -0,0 +1,9 @@
+package d2sequence
+
+// min horizontal pad for actors, or edge labels, to consider the min distance between them
+const MIN_HORIZONTAL_PAD = 50.
+
+const MIN_ACTOR_DISTANCE = 200.
+
+// min vertical distance between edges
+const MIN_EDGE_DISTANCE = 100.

d2layouts/d2sequence/layout.go

@@ -0,0 +1,100 @@
+package d2sequence
+
+import (
+	"context"
+	"fmt"
+	"math"
+
+	"oss.terrastruct.com/d2/d2graph"
+	"oss.terrastruct.com/d2/lib/geo"
+	"oss.terrastruct.com/d2/lib/go2"
+	"oss.terrastruct.com/d2/lib/label"
+)
+
+func Layout(ctx context.Context, g *d2graph.Graph) (err error) {
+	pad := MIN_HORIZONTAL_PAD
+	edgeYStep := MIN_EDGE_DISTANCE
+	actorXStep := MIN_ACTOR_DISTANCE
+	maxActorHeight := 0.
+
+	for _, edge := range g.Edges {
+		edgeYStep = math.Max(edgeYStep, float64(edge.LabelDimensions.Height)+pad)
+		actorXStep = math.Max(actorXStep, float64(edge.LabelDimensions.Width)+pad)
+		maxActorHeight = math.Max(maxActorHeight, edge.Src.Height+pad)
+		maxActorHeight = math.Max(maxActorHeight, edge.Dst.Height+pad)
+	}
+
+	placeActors(g.Objects, maxActorHeight, actorXStep)
+	routeEdges(g.Edges, maxActorHeight, edgeYStep)
+	addLifelineEdges(g, g.Objects, edgeYStep)
+
+	return nil
+}
+
+// placeActors places actors bottom aligned, side by side
+func placeActors(actors []*d2graph.Object, maxHeight, xStep float64) {
+	x := 0.
+	for _, actors := range actors {
+		yOffset := maxHeight - actors.Height
+		actors.TopLeft = geo.NewPoint(x, yOffset)
+		x += actors.Width + xStep
+		actors.LabelPosition = go2.Pointer(string(label.InsideMiddleCenter))
+	}
+}
+
+// routeEdges routes horizontal edges from Src to Dst
+func routeEdges(edgesInOrder []*d2graph.Edge, startY, yStep float64) {
+	edgeY := startY + yStep // in case the first edge has a tall label
+	for _, edge := range edgesInOrder {
+		start := edge.Src.Center()
+		start.Y = edgeY
+		end := edge.Dst.Center()
+		end.Y = edgeY
+		edge.Route = []*geo.Point{start, end}
+		edgeY += yStep
+
+		if edge.Attributes.Label.Value != "" {
+			isLeftToRight := edge.Src.TopLeft.X < edge.Dst.TopLeft.X
+			if isLeftToRight {
+				edge.LabelPosition = go2.Pointer(string(label.OutsideTopCenter))
+			} else {
+				edge.LabelPosition = go2.Pointer(string(label.OutsideBottomCenter))
+			}
+		}
+	}
+}
+
+// addLifelineEdges adds a new edge for each actor in the graph that represents the
+// edge below the actor showing its lifespan
+// ┌──────────────┐
+// │     actor    │
+// └──────┬───────┘
+//        │
+//        │ lifeline
+//        │
+//        │
+func addLifelineEdges(g *d2graph.Graph, actors []*d2graph.Object, yStep float64) {
+	endY := g.Edges[len(g.Edges)-1].Route[0].Y + yStep
+	for _, actor := range actors {
+		actorBottom := actor.Center()
+		actorBottom.Y = actor.TopLeft.Y + actor.Height
+		actorLifelineEnd := actor.Center()
+		actorLifelineEnd.Y = endY
+		g.Edges = append(g.Edges, &d2graph.Edge{
+			Attributes: d2graph.Attributes{
+				Style: d2graph.Style{
+					StrokeDash:  &d2graph.Scalar{Value: "10"},
+					Stroke:      actor.Attributes.Style.Stroke,
+					StrokeWidth: actor.Attributes.Style.StrokeWidth,
+				},
+			},
+			Src:      actor,
+			SrcArrow: false,
+			Dst: &d2graph.Object{
+				ID: actor.ID + fmt.Sprintf("-lifeline-end-%d", go2.StringToIntHash(actor.ID+"-lifeline-end")),
+			},
+			DstArrow: false,
+			Route:    []*geo.Point{actorBottom, actorLifelineEnd},
+		})
+	}
+}

d2layouts/d2sequence/layout_test.go

@@ -0,0 +1,112 @@
+package d2sequence
+
+import (
+	"context"
+	"testing"
+
+	"oss.terrastruct.com/d2/d2graph"
+	"oss.terrastruct.com/d2/lib/geo"
+	"oss.terrastruct.com/d2/lib/log"
+)
+
+func TestLayout(t *testing.T) {
+	g := d2graph.NewGraph(nil)
+	g.Objects = []*d2graph.Object{
+		{
+			ID:  "Alice",
+			Box: geo.NewBox(nil, 100, 100),
+		},
+		{
+			ID:  "Bob",
+			Box: geo.NewBox(nil, 30, 30),
+		},
+	}
+
+	g.Edges = []*d2graph.Edge{
+		{
+			Src: g.Objects[0],
+			Dst: g.Objects[1],
+		},
+		{
+			Src: g.Objects[1],
+			Dst: g.Objects[0],
+		},
+		{
+			Src: g.Objects[0],
+			Dst: g.Objects[1],
+		},
+		{
+			Src: g.Objects[1],
+			Dst: g.Objects[0],
+		},
+	}
+	nEdges := len(g.Edges)
+
+	ctx := log.WithTB(context.Background(), t, nil)
+	Layout(ctx, g)
+
+	// asserts that actors were placed in the expected x order and at y=0
+	actors := []*d2graph.Object{
+		g.Objects[0],
+		g.Objects[1],
+	}
+	for i := 1; i < len(actors); i++ {
+		if actors[i].TopLeft.X < actors[i-1].TopLeft.X {
+			t.Fatalf("expected actor[%d].TopLeft.X > actor[%d].TopLeft.X", i, i-1)
+		}
+		actorBottom := actors[i].TopLeft.Y + actors[i].Height
+		prevActorBottom := actors[i-1].TopLeft.Y + actors[i-1].Height
+		if actorBottom != prevActorBottom {
+			t.Fatalf("expected actor[%d] and actor[%d] to be at the same bottom y", i, i-1)
+		}
+	}
+
+	nExpectedEdges := nEdges + len(actors)
+	if len(g.Edges) != nExpectedEdges {
+		t.Fatalf("expected %d edges, got %d", nExpectedEdges, len(g.Edges))
+	}
+
+	// assert that edges were placed in y order and have the endpoints at their actors
+	// uses `nEdges` because Layout creates some vertical edges to represent the actor lifeline
+	for i := 0; i < nEdges; i++ {
+		edge := g.Edges[i]
+		if len(edge.Route) != 2 {
+			t.Fatalf("expected edge[%d] to have only 2 points", i)
+		}
+		if edge.Route[0].Y != edge.Route[1].Y {
+			t.Fatalf("expected edge[%d] to be a horizontal line", i)
+		}
+		if edge.Route[0].X != edge.Src.Center().X {
+			t.Fatalf("expected edge[%d] source endpoint to be at the middle of the source actor", i)
+		}
+		if edge.Route[1].X != edge.Dst.Center().X {
+			t.Fatalf("expected edge[%d] target endpoint to be at the middle of the target actor", i)
+		}
+		if i > 0 {
+			prevEdge := g.Edges[i-1]
+			if edge.Route[0].Y < prevEdge.Route[0].Y {
+				t.Fatalf("expected edge[%d].TopLeft.Y > edge[%d].TopLeft.Y", i, i-1)
+			}
+		}
+	}
+
+	lastSequenceEdge := g.Edges[nEdges-1]
+	for i := nEdges; i < nExpectedEdges; i++ {
+		edge := g.Edges[i]
+		if len(edge.Route) != 2 {
+			t.Fatalf("expected edge[%d] to have only 2 points", i)
+		}
+		if edge.Route[0].X != edge.Route[1].X {
+			t.Fatalf("expected edge[%d] to be a vertical line", i)
+		}
+		if edge.Route[0].X != edge.Src.Center().X {
+			t.Fatalf("expected edge[%d] x to be at the actor center", i)
+		}
+		if edge.Route[0].Y != edge.Src.Height+edge.Src.TopLeft.Y {
+			t.Fatalf("expected edge[%d] to start at the bottom of the source actor", i)
+		}
+		if edge.Route[1].Y < lastSequenceEdge.Route[0].Y {
+			t.Fatalf("expected edge[%d] to end after the last sequence edge", i)
+		}
+	}
+}