fix, cleanup, test grid layout cuts

d2layouts/d2grid/layout.go

@@ -590,7 +590,7 @@ func (gd *gridDiagram) getBestLayout(targetSize float64, columns bool) [][]*d2gr
 		nCuts = gd.rows - 1
 	}
 	if nCuts == 0 {
-		return genLayout(gd.objects, nil)
+		return GenLayout(gd.objects, nil)
 	}
 
 	var bestLayout [][]*d2graph.Object
@@ -692,7 +692,7 @@ func (gd *gridDiagram) getBestLayout(targetSize float64, columns bool) [][]*d2gr
 	// .       A │ B │ C   D   E                                  └────────────┘
 	// of these divisions, find the layout with rows closest to the targetSize
 	tryDivision := func(division []int) bool {
-		layout := genLayout(gd.objects, division)
+		layout := GenLayout(gd.objects, division)
 		dist := getDistToTarget(layout, targetSize, float64(gd.horizontalGap), float64(gd.verticalGap), columns)
 		if dist < bestDist {
 			bestLayout = layout
@@ -786,7 +786,9 @@ func (gd *gridDiagram) fastLayout(targetSize float64, nCuts int, columns bool) (
 			size = o.Width
 		}
 		if rowSize == 0 {
+			// if a single object meets the target size, end the row here
 			if size > targetSize-debt {
+				// cut row with just this object
 				fastDivision = append(fastDivision, i)
 				// we build up a debt of distance past the target size across rows
 				newDebt := size - targetSize
@@ -797,7 +799,11 @@ func (gd *gridDiagram) fastLayout(targetSize float64, nCuts int, columns bool) (
 			continue
 		}
 		// debt is paid by decreasing threshold to start new row and ending below targetSize
-		if rowSize+(gap+size)/2. > targetSize-debt {
+		if rowSize+gap+(size)/2. > targetSize-debt {
+			// start a new row before this object since it is mostly past the target size
+			// .              size
+			// ├...row─┼gap┼───┼───┤
+			// ├──targetSize──┤ (debt=0)
 			fastDivision = append(fastDivision, i-1)
 			newDebt := rowSize - targetSize
 			debt += newDebt
@@ -807,7 +813,7 @@ func (gd *gridDiagram) fastLayout(targetSize float64, nCuts int, columns bool) (
 		}
 	}
 	if len(fastDivision) == nCuts {
-		layout = genLayout(gd.objects, fastDivision)
+		layout = GenLayout(gd.objects, fastDivision)
 	}
 
 	return layout
@@ -885,7 +891,9 @@ func iterDivisions(objects []*d2graph.Object, nCuts int, f iterDivision, check c
 }
 
 // generate a grid of objects from the given cut indices
-func genLayout(objects []*d2graph.Object, cutIndices []int) [][]*d2graph.Object {
+// each cut index applies after the object at that index
+// e.g. [0 1 2 3 4 5 6 7] with cutIndices [0, 2, 6] => [[0], [1, 2], [3,4,5,6], [7]]
+func GenLayout(objects []*d2graph.Object, cutIndices []int) [][]*d2graph.Object {
 	layout := make([][]*d2graph.Object, len(cutIndices)+1)
 	objIndex := 0
 	for i := 0; i <= len(cutIndices); i++ {

d2layouts/d2grid/layout_test.go

@@ -0,0 +1,72 @@
+package d2grid_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"oss.terrastruct.com/d2/d2graph"
+	"oss.terrastruct.com/d2/d2layouts/d2grid"
+)
+
+func TestGenLayout(t *testing.T) {
+	objects := []*d2graph.Object{
+		{ID: "1"},
+		{ID: "2"},
+		{ID: "3"},
+		{ID: "4"},
+		{ID: "5"},
+		{ID: "6"},
+		{ID: "7"},
+		{ID: "8"},
+	}
+	var cutIndices []int
+	var layout [][]*d2graph.Object
+	cutIndices = []int{0}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	fmt.Printf("layout %v\n", len(layout))
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 2 rows from 1 cut")
+	assert.Equalf(t, 1, len(layout[0]), "expected first row to be 1 object")
+	assert.Equalf(t, 7, len(layout[1]), "expected second row to be 7 objects")
+	assert.Equalf(t, objects[0].ID, layout[0][0].ID, "expected first object to be 1")
+
+	cutIndices = []int{6}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 2 rows from 1 cut")
+	assert.Equalf(t, 7, len(layout[0]), "expected first row to be 7 objects")
+	assert.Equalf(t, 1, len(layout[1]), "expected second row to be 1 object")
+
+	cutIndices = []int{0, 6}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 3 rows from 2 cuts")
+	assert.Equalf(t, 1, len(layout[0]), "expected first row to be 1 objects")
+	assert.Equalf(t, 6, len(layout[1]), "expected second row to be 6 objects")
+	assert.Equalf(t, 1, len(layout[2]), "expected second row to be 1 object")
+
+	cutIndices = []int{1, 5}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 3 rows from 2 cuts")
+	assert.Equalf(t, 2, len(layout[0]), "expected first row to be 2 objects")
+	assert.Equalf(t, 4, len(layout[1]), "expected second row to be 6 objects")
+	assert.Equalf(t, 2, len(layout[2]), "expected second row to be 2 object")
+
+	cutIndices = []int{5}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 2 rows from 1 cut")
+	assert.Equalf(t, 6, len(layout[0]), "expected first row to be 6 objects")
+	assert.Equalf(t, 2, len(layout[1]), "expected second row to be 2 object")
+
+	cutIndices = []int{1}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 2 rows from 1 cut")
+	assert.Equalf(t, 2, len(layout[0]), "expected first row to be 2 object")
+	assert.Equalf(t, 6, len(layout[1]), "expected second row to be 6 objects")
+
+	cutIndices = []int{0, 1, 2, 3, 4, 5, 6}
+	layout = d2grid.GenLayout(objects, cutIndices)
+	assert.Equalf(t, len(cutIndices)+1, len(layout), "expected 3 rows from 2 cuts")
+	for i := range layout {
+		assert.Equalf(t, 1, len(layout[i]), "expected row %d to be 1 object", i)
+	}
+}