/**
* Layout: tree → coordinates.
*
* Two passes, the same shape as a flexbox implementation:
*
* measure — bottom-up. A leaf reports its intrinsic size; a container sums its
* children along the flow axis, takes the maximum across it, and adds
* padding and its title strip. A container therefore always ends up big
* enough to hold what is inside it, which is why "child spills out of its
* frame" cannot happen by construction.
*
* place — top-down. Each container distributes its now-known interior among its
* children.
*
* The model never supplies a coordinate. It declares nesting, direction and gap; every
* x/y/width/height comes from here.
*
* Five container kinds share those two passes, because they differ only in how a parent
* distributes its interior:
*
* group — children stacked along one axis. Cloud architecture, nested frames.
* grid — children packed into a fixed number of columns.
* pool — a sparse (lane × column) grid. Swimlane and BPMN diagrams.
* sequence — participants across the top, lifelines below. Sequence diagrams.
* radial — a centre with branches fanning out, or hanging below. Mind maps, org charts.
*
* Ported from drawio-ai-kit (MIT) — see NOTICE. The pool geometry follows that project's
* `pool()` primitive; sequence and radial are original to this repository.
*/
import { resolveShape } from "./shapes"
import { type Role, roleMetrics } from "./theme"
import type {
Align,
ContainerNode,
DiagramNode,
Justify,
PoolNode,
RadialNode,
Rect,
SequenceNode,
} from "./types"
import { isContainer } from "./types"
/** Default glyph size for a catalog icon. */
export const ICON_SIZE = 48
/** Interior padding of a container. */
const PAD = 24
/** A group's interior padding: its own `pad` when declared, the default otherwise. */
function padOf(n: ContainerNode): number {
return n.kind === "group" && n.pad != null ? Math.max(0, n.pad) : PAD
}
/** The flex-grow weight a node declared, 0 when none. */
function growOf(n: DiagramNode): number {
const g = (n.kind === "box" || n.kind === "group") && n.grow
return typeof g === "number" && g > 0 ? g : 0
}
/**
* The cross-axis alignment a node ends up with: its own `align`, else the parent's
* `alignItems`, else centred. Same cascade as CSS, where align-self overrides the
* container's align-items.
*/
function alignOf(n: DiagramNode, parent?: ContainerNode): Align {
const own = (n.kind === "box" || n.kind === "group") && n.align
if (
own === "start" ||
own === "end" ||
own === "stretch" ||
own === "center"
)
return own
const inherited = parent?.kind === "group" ? parent.alignItems : undefined
if (
inherited === "start" ||
inherited === "end" ||
inherited === "stretch" ||
inherited === "center"
)
return inherited
return "center"
}
/**
* The main-axis distribution a container declared, or null when it declared none.
*
* Null matters: it selects the engine's original per-axis defaults rather than any value
* in this vocabulary. A row centred its children and padded their gaps, a column packed to
* the top — neither is expressible as one `Justify`, and both are what every diagram built
* before this existed relies on. Declaring `justify` opts out of them.
*/
function justifyOf(n: ContainerNode): Justify | null {
const j = n.kind === "group" ? n.justify : undefined
return j === "start" ||
j === "center" ||
j === "end" ||
j === "between" ||
j === "around" ||
j === "evenly"
? j
: null
}
/** The width cap a node declared, or Infinity. */
function maxWOf(n: DiagramNode): number {
const m = (n.kind === "box" || n.kind === "group") && n.maxW
return typeof m === "number" && m > 0 ? m : Number.POSITIVE_INFINITY
}
/**
* Divide `room` among weighted children, honouring each one's floor and ceiling.
*
* The naive version — give each child `room * weight / total` and never go below its
* content width — overflows: a child whose content is wider than its share keeps the
* wider figure, and the total then exceeds what there was to divide, so the last child
* hangs out of the frame.
*
* CSS resolves this by FREEZING any item that cannot take its share and re-dividing the
* rest among those that still can, repeating until nothing changes. That is what this
* does. It terminates because every round either freezes at least one child or stops.
*
* Returns the width for each child, in order; a child with no weight keeps its size.
*/
function shareOut(
sizes: number[],
weights: number[],
caps: number[],
floors: number[],
room: number,
): number[] {
const out = [...sizes]
const frozen = sizes.map((_, i) => weights[i] <= 0)
for (;;) {
const liveTotal = weights.reduce(
(s, w, i) => s + (frozen[i] ? 0 : w),
0,
)
if (liveTotal <= 0) return out
// What is left once everything already settled has taken its width.
const rest = room - out.reduce((s, v, i) => s + (frozen[i] ? v : 0), 0)
let changed = false
for (let i = 0; i < out.length; i++) {
if (frozen[i]) continue
const share = (rest * weights[i]) / liveTotal
// A child cannot go below its own content, and cannot pass a declared cap.
// Either way it settles here and the others divide what is left.
if (share < floors[i]) {
out[i] = floors[i]
frozen[i] = true
changed = true
} else if (share > caps[i]) {
out[i] = caps[i]
frozen[i] = true
changed = true
}
}
if (changed) continue
for (let i = 0; i < out.length; i++)
if (!frozen[i]) out[i] = (rest * weights[i]) / liveTotal
return out
}
}
/**
* The narrowest a node may be squeezed to when weights divide a row.
*
* Its own content width, unless it opted out with `minW0` (CSS's `min-width: 0`), in which
* case a weight may take it below that. Matching CSS here is deliberate: `min-width`
* defaults to `auto`, so in a browser too a `flex: 2` column stops shrinking at its text
* and a declared 2:1 comes out closer to 1.4:1 — surprising, but it is what everyone
* writing flexbox already works with.
*/
function floorOf(n: DiagramNode, contentW: number): number {
const opted = (n.kind === "box" || n.kind === "group") && n.minW0
return opted ? 0 : contentW
}
/**
* Does this node need the full width of its parent to mean what it says?
*
* A row whose children carry `grow` weights does: the weights are shares of the row's
* width, so if the row is only as wide as its own content there is nothing to share and
* every declared proportion silently comes out 1:1.
*
* This is where CSS and this engine disagree, and the disagreement is why it has to be
* inferred. CSS and Yoga default `align-items` to `stretch`, so a row inside a column
* fills that column's width for free. This engine defaults to `center`, which is the
* better default for diagrams — a lone icon in a wide frame should sit in the middle, not
* be smeared across it — but it means a row of weighted columns gets no width unless
* something asks. Declaring weights IS the ask.
*/
function needsFullWidth(n: DiagramNode): boolean {
return (
n.kind === "group" &&
n.dir === "row" &&
n.children.some((c) => growOf(c) > 0)
)
}
/**
* Where the children start, and how much goes between them — CSS's justify-content.
*
* `slack` is what is left after the children and their base gaps. Returning both the
* leading offset and the per-gap addition covers all six values in one place, so the
* row and column branches no longer need their own contradictory policies.
*/
function distribute(
justify: Justify,
slack: number,
count: number,
): { lead: number; extraGap: number } {
if (slack <= 0 || count === 0) return { lead: 0, extraGap: 0 }
switch (justify) {
case "center":
return { lead: slack / 2, extraGap: 0 }
case "end":
return { lead: slack, extraGap: 0 }
case "between":
return count > 1
? { lead: 0, extraGap: slack / (count - 1) }
: { lead: 0, extraGap: 0 }
case "around": {
// Half a share before the first child and after the last, a full share between.
const share = slack / count
return { lead: share / 2, extraGap: share }
}
case "evenly": {
const share = slack / (count + 1)
return { lead: share, extraGap: share }
}
default:
return { lead: 0, extraGap: 0 }
}
}
/** Height of a container's title strip. Zero when it has no label — an empty strip
* reads as a dead band at the top of the frame. */
const HEADER = 36
/** Approximate width of one label character at the engine's font size. */
const CHAR_W = 6.6
// ---- pool geometry, shared with render.ts so the bands land under the nodes ----
/** Interior padding of a pool. Tighter than a group's: lane bands sit flush. */
export const POOL_PAD = 16
/** Width of the lane-name column (height, when the pool is vertical). */
export const LANE_LABEL = 110
/** Height of the milestone band (width, when the pool is vertical). */
const PHASE_LABEL = 26
/** A pool's own title strip. */
const POOL_HEADER = 34
/** Vertical padding inside a lane band, so nodes do not touch the band's edges. */
const LANE_PAD = 14
// ---- sequence geometry ----
/** Height of a participant head. */
const HEAD_H = 44
/** Vertical distance from the participant heads to the first message. */
const LIFELINE_TOP = 28
/** How far the lifeline runs past the last message. */
const LIFELINE_TAIL = 36
/**
* The geometry a pool needs, derived once and used by both layout and rendering.
*
* Rendering has to paint the lane bands and label columns at exactly the positions layout
* used, or the nodes sit off their bands. Computing it in one place is what keeps the two
* from drifting.
*/
export interface PoolMetrics {
horizontal: boolean
lanes: number
cols: number
/** Cell size along the flow axis. */
cellW: number
/** Cell size across the lane axis. */
cellH: number
header: number
phaseLabel: number
/** Where cell (0,0) starts. */
contentX: number
contentY: number
/** Total extent of the cell area. */
contentW: number
contentH: number
}
export function poolMetrics(
n: PoolNode,
rect: Rect,
kids: { rect: Rect }[],
): PoolMetrics {
const horizontal = n.orientation !== "vertical"
const lanes = Math.max(1, n.lanes.length)
const cols = Math.max(1, ...n.children.map((c) => poolCellOf(c).col + 1))
const cellW = Math.max(80, ...kids.map((k) => k.rect.w))
const cellH = Math.max(40, ...kids.map((k) => k.rect.h)) + LANE_PAD
const header = n.label ? POOL_HEADER : 0
const phaseLabel = n.phases.length ? PHASE_LABEL : 0
const contentW = horizontal
? cols * cellW + n.gap * (cols - 1)
: lanes * cellW
const contentH = horizontal
? lanes * cellH
: cols * cellH + n.gap * (cols - 1)
return {
horizontal,
lanes,
cols,
cellW,
cellH,
header,
phaseLabel,
contentX: horizontal
? rect.x + POOL_PAD + LANE_LABEL
: rect.x + POOL_PAD,
contentY: horizontal
? rect.y + header + phaseLabel + POOL_PAD
: rect.y + header + POOL_PAD + LANE_LABEL,
contentW,
contentH,
}
}
/** How far a sequence diagram's lifelines run, and where each message sits. */
export interface SequenceMetrics {
/** Bottom of the lifeline. */
bottom: number
/** Vertical position of message N, for N starting at 1. */
messageY: (step: number) => number
}
export function sequenceMetrics(
n: SequenceNode,
rect: Rect,
messages: number,
): SequenceMetrics {
const head = n.label ? HEADER : 0
// The first message hangs a fixed distance below the participant heads.
const first = rect.y + head + PAD + HEAD_H + LIFELINE_TOP
return {
bottom: first + Math.max(0, messages - 1) * n.step + LIFELINE_TAIL,
messageY: (step) => first + Math.max(0, step - 1) * n.step,
}
}
/** A node with its computed box. Layout works on this, leaving the tree untouched. */
export interface Placed {
node: DiagramNode
rect: Rect
children: Placed[]
}
/** One line of a wrapped row: which children sit on it, and how big it is. */
interface WrapLine {
items: Placed[]
width: number
height: number
}
/**
* Break a row of children into lines that each fit `room`.
*
* Greedy, the same rule as a text line-breaker and as CSS flex-wrap: keep adding to the
* current line while it fits, otherwise start a new one. A single child wider than the
* whole row still gets its own line rather than being dropped.
*
* Shared by measure and place so both agree on where the breaks fall — computing them
* twice from the same input is cheap, keeping two copies in sync is not.
*/
function wrapLines(kids: Placed[], room: number, gap: number): WrapLine[] {
const lines: WrapLine[] = []
let cur: WrapLine | null = null
for (const k of kids) {
const next = cur ? cur.width + gap + k.rect.w : k.rect.w
if (cur && next > room && cur.items.length > 0) {
lines.push(cur)
cur = null
}
if (!cur) cur = { items: [k], width: k.rect.w, height: k.rect.h }
else {
cur.items.push(k)
cur.width = next
cur.height = Math.max(cur.height, k.rect.h)
}
}
if (cur) lines.push(cur)
return lines
}
/**
* The arrows layout needs, which the node tree alone does not carry.
*
* Three of the five container kinds are laid out from the diagram's arrows, not from
* nesting: a sequence diagram's messages set how tall the lifelines have to be, and a mind
* map's hierarchy IS its arrows. Links live on the tree, not on the node, so they are
* passed down rather than read from a parent pointer.
*/
export type LayoutLinks = { source: string; target: string; step?: number }[]
/**
* Reduce a label to the text draw.io will actually lay out.
*
* Labels may carry inline HTML (every style has `html=1`): a
is a line break, any
* other tag is invisible markup around visible text. Measuring the raw string counted
* `` as thirty characters of text, making rich boxes twice as
* wide as their content.
*/
function visibleText(label: string): string {
return String(label ?? "")
.replace(//gi, "\n")
.replace(/<\/?(?:b|i|u|s|sub|sup|font|span|div)(?:\s[^<>]*)?>/gi, "")
}
/**
* Intrinsic size of a text box: widest wrapped line by line count.
*
* The role scales the estimate: a banner sets 20px type and a footnote 9px, and layout has
* to reserve what render will draw or the text overflows its cell.
*
* `atWidth` is the width the box will ACTUALLY be drawn at, when that is already known
* (see the reflow pass in `layoutForest`). Height is then counted for that width while
* the reported width stays intrinsic — which is what a browser does, and what this was
* missing: a paragraph measured at its 260px natural width needs eight lines, the same
* paragraph stretched to 750px needs three, and reserving the eight-line height left
* every panel with a slab of dead space under its text.
*/
export function autoBoxSize(
label: string,
role?: Role,
shape?: string,
atWidth?: number,
maxW?: number,
): { w: number; h: number } {
const spec = shape ? resolveShape(shape)?.spec : undefined
// A glyph shape (umlActor…) has a fixed figure with the label below it: the slot is
// the figure plus a line of text, and the text length does not scale the figure.
if (spec?.labelOutside && spec.glyph) {
const text = visibleText(label)
return {
w: Math.max(spec.glyph.w + 20, Math.min(160, text.length * 7 + 16)),
h: spec.glyph.h + 22,
}
}
const r = roleMetrics(role)
// Two caps: the role's own default, and whatever the model declared. The declared one
// is allowed to go BELOW the floor of 120 — capping a box at 90px has to mean 90px,
// or the cap silently does nothing on short labels.
const roleCap = Math.round(260 * Math.max(1, r.charScale))
const declared = maxW != null && maxW > 0 ? maxW : Number.POSITIVE_INFINITY
const explicit = visibleText(label).split("\n")
const longest = Math.max(1, ...explicit.map((l) => l.length))
const natural = Math.max(
120,
Math.round(longest * CHAR_W * r.charScale + 28),
)
const w = Math.min(declared, roleCap, natural)
const s = spec?.textScale ?? 1
// Count the lines the text ACTUALLY occupies: draw.io wraps at the box width, so a
// long line becomes several. Estimating by explicit newlines alone left the box one
// line tall while the text wrapped to six — and overflowed straight out of it.
//
// Wrapping happens at the DRAWN width, which for a stretched box is wider than the
// intrinsic one. `atWidth` carries it; the shape factor is divided back out because
// it is applied to the final height below.
// A declared cap also bounds the reflow hint: a box capped at 200 never gets to count
// its lines as if it had been drawn at 600, however wide its parent turned out.
const textW = Math.min(
declared,
Math.max(w, atWidth != null ? atWidth / s : 0),
)
const charsPerLine = Math.max(
8,
Math.floor((textW - 28) / (CHAR_W * r.charScale)),
)
const lines = explicit.reduce(
(sum, l) => sum + Math.max(1, Math.ceil(l.length / charsPerLine)),
0,
)
const lineH = Math.round(r.fontSize * 1.6)
const h = Math.max(r.minH, lines * lineH + 26)
// A non-rectangular outline inscribes a smaller text area than its bounding box —
// a rhombus exactly half — so the box grows by the shape's measured factor.
// Verified in the real editor: the same sentence overflows a 1.0× rhombus and fits
// a 1.5× one.
return { w: Math.round(w * s), h: Math.round(h * s) }
}
/**
* Intrinsic size of an icon cell: the glyph, plus room for the label underneath, and
* wide enough that a long label does not overflow the cell it is centred in.
*/
function iconSize(label: string, glyph: number): { w: number; h: number } {
return {
w: Math.max(96, glyph + 20, Math.min(200, label.length * 7 + 24)),
h: glyph + 34,
}
}
/** A container is never narrower than its own title. */
function titleFloor(label: string, pad: number): number {
return label ? Math.ceil(label.length * CHAR_W) + pad * 2 : 0
}
function headerFor(n: ContainerNode): number {
if (n.kind === "pool") return n.label ? POOL_HEADER : 0
return n.label ? HEADER : 0
}
/**
* The cell a node occupies inside a pool. Absent means (0,0).
*
* No clamping needed: `add_icon`/`add_box` clamp at the boundary where the model's numbers
* arrive, and the only other way a cell gets set is the parser, whose `dai_cell` pattern
* matches digits only. So by here it is already non-negative.
*/
export function poolCellOf(n: DiagramNode): { lane: number; col: number } {
if ((n.kind === "icon" || n.kind === "box") && n.cell) return n.cell
return { lane: 0, col: 0 }
}
/**
* How many messages a sequence container has: the highest step number among the links
* between its participants, or the link count when the model numbered nothing.
*
* Steps are what order the messages vertically, so a diagram whose links carry no step
* still needs one row per message — otherwise every arrow lands on the same y.
*/
export function messageCount(n: SequenceNode, links: LayoutLinks): number {
const own = new Set(n.children.map((c) => c.id))
const mine = links.filter((l) => own.has(l.source) && own.has(l.target))
const steps = mine
.map((l) => l.step)
.filter((s): s is number => s != null && s > 0)
return Math.max(mine.length, ...(steps.length ? steps : [0]))
}
/**
* One node of a radial tree: a placed box plus the branches hanging off it.
*
* Separate from `Placed` because the tree is derived from the LINKS, not from nesting, so it
* exists only during a radial container's layout.
*/
interface RadialTree {
p: Placed
kids: RadialTree[]
/** How much room this whole subtree needs across the branching axis. */
extent: number
}
/**
* Build the branch hierarchy of a radial container from the diagram's arrows.
*
* The root is the node nothing points at. Every other node hangs off whichever node points
* at it — the FIRST one, if several do, since a mind map is a tree and a second parent has
* to be drawn as a plain cross-link instead.
*
* A node no arrow reaches at all becomes a branch of the root, so it is still drawn. Dropping
* it would silently lose a box the model asked for.
*/
function radialHierarchy(
kids: Placed[],
links: LayoutLinks,
across: "w" | "h",
gap: number,
): { root: RadialTree; branches: RadialTree[] } | null {
if (kids.length === 0) return null
const own = new Map(kids.map((k) => [k.node.id, k]))
const parent = new Map()
for (const l of links) {
if (!own.has(l.source) || !own.has(l.target)) continue
if (l.source === l.target) continue
if (!parent.has(l.target)) parent.set(l.target, l.source)
}
// Guard against a cycle in the arrows: walking up must terminate.
const rootOf = (id: string): string => {
const seen = new Set([id])
let cur = id
for (;;) {
const up = parent.get(cur)
if (up === undefined || seen.has(up)) return cur
seen.add(up)
cur = up
}
}
// The first declared node that is nobody's child is the centre. Falling back to the first
// child keeps a cycle-only graph drawable.
const rootId =
kids.find((k) => !parent.has(k.node.id))?.node.id ??
rootOf(kids[0].node.id)
const childrenOf = new Map()
for (const k of kids) {
if (k.node.id === rootId) continue
const up = parent.get(k.node.id)
// An orphan, or a node whose parent chain loops back on itself, attaches to the root.
// `up === k.node.id` cannot happen: self-links are skipped when `parent` is built.
const attach =
up !== undefined && rootOf(k.node.id) === rootId ? up : rootId
const list = childrenOf.get(attach)
if (list) list.push(k)
else childrenOf.set(attach, [k])
}
// No visited-set needed: `parent` records at most one parent per node, so `childrenOf`
// is a forest by construction, and `rootOf` above already reattached anything whose
// parent chain looped. The recursion cannot revisit a node.
const build = (p: Placed): RadialTree => {
const kidTrees = (childrenOf.get(p.node.id) ?? []).map(build)
const total =
kidTrees.reduce((s, t) => s + t.extent, 0) +
gap * Math.max(0, kidTrees.length - 1)
return {
p,
kids: kidTrees,
extent: Math.max(p.rect[across], total),
}
}
const root = build(own.get(rootId) as Placed)
return { root, branches: root.kids }
}
/** Widest node at each generation, for laying a radial tree out in even rings. */
function widestPerLevel(trees: RadialTree[], along: "w" | "h"): number[] {
const out: number[] = []
const visit = (t: RadialTree, level: number) => {
out[level] = Math.max(out[level] ?? 0, t.p.rect[along])
for (const k of t.kids) visit(k, level + 1)
}
for (const t of trees) visit(t, 0)
return out
}
/**
* How far one side of a radial map reaches from the centre.
*
* Each generation contributes one gap plus the width of the widest node in it. This has to be
* computed per SIDE, not once for the whole map: a mind map whose left branches go three
* generations deep and whose right branches go one needs an asymmetric frame, and reserving
* the same room on both sides would push the deeper side off the page.
*/
function radialReach(
side: RadialTree[],
along: "w" | "h",
gap: number,
): number {
// widestPerLevel writes one entry per generation that exists, so its length IS the depth
// of the deepest branch on this side. An empty side yields an empty list, and reducing
// that from 0 already gives 0.
return widestPerLevel(side, along).reduce((s, v) => s + v + gap, 0)
}
/**
* Split a radial map's branches into the two sides they will be drawn on.
*
* The same split has to be used by measure and by place, or the frame is sized for one
* arrangement and the branches are drawn in another.
*/
function radialSides(branches: RadialTree[]): {
right: RadialTree[]
left: RadialTree[]
} {
const half = Math.ceil(branches.length / 2)
return { right: branches.slice(0, half), left: branches.slice(half) }
}
/**
* measure: give every node a size, bottom-up.
*
* Siblings are equalised across the cross axis — frames in a row share a bottom edge,
* frames in a column share left and right edges. Only containers stretch; a leaf keeps
* its natural size, because stretching an icon would distort the glyph.
*/
function measure(
n: DiagramNode,
defaultGlyph: number,
links: LayoutLinks,
widthHints?: Map,
): Placed {
if (n.kind === "icon") {
const glyph = n.size ?? defaultGlyph
const s = iconSize(n.label, glyph)
return { node: n, rect: { x: 0, y: 0, ...s }, children: [] }
}
if (n.kind === "box") {
// The hint is the width this box was drawn at last pass; its text rewraps to
// that width, so its height has to be counted there.
const auto = autoBoxSize(
n.label,
n.role,
n.shape,
widthHints?.get(n.id),
n.maxW,
)
return {
node: n,
rect: { x: 0, y: 0, w: n.w ?? auto.w, h: n.h ?? auto.h },
children: [],
}
}
if (n.kind === "title") {
return { node: n, rect: { x: 0, y: 0, w: 0, h: 30 }, children: [] }
}
const kids = n.children.map((c) =>
measure(c, defaultGlyph, links, widthHints),
)
const head = headerFor(n)
const gap = n.gap
if (n.kind === "pool") {
const m = poolMetrics(n, { x: 0, y: 0, w: 0, h: 0 }, kids)
const w = m.horizontal
? POOL_PAD * 2 + LANE_LABEL + m.contentW
: POOL_PAD * 2 + m.contentW + m.phaseLabel
const h = m.horizontal
? m.header + m.phaseLabel + POOL_PAD * 2 + m.contentH
: m.header + POOL_PAD * 2 + LANE_LABEL + m.contentH
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(w, titleFloor(n.label, POOL_PAD)),
h,
},
children: kids,
}
}
if (n.kind === "sequence") {
// Participants sit side by side; the lifelines below them set the height.
const w =
PAD * 2 +
kids.reduce((s, k) => s + k.rect.w, 0) +
gap * Math.max(0, kids.length - 1)
const m = sequenceMetrics(
n,
{ x: 0, y: 0, w: 0, h: 0 },
messageCount(n, links),
)
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(w, titleFloor(n.label, PAD)),
h: m.bottom + PAD,
},
children: kids,
}
}
if (n.kind === "radial") {
const down = n.spread === "down"
const across = down ? "w" : "h"
const tree = radialHierarchy(kids, links, across, n.gap)
if (!tree)
return {
node: n,
rect: { x: 0, y: 0, w: PAD * 2, h: head + PAD * 2 },
children: kids,
}
const { root, branches } = tree
const spanOf = (bs: RadialTree[]) =>
bs.length === 0
? 0
: bs.reduce((s, b) => s + b.extent, 0) + n.gap * (bs.length - 1)
if (down) {
// Everything hangs below the centre: one direction, so one reach.
const h = root.p.rect.h + radialReach(branches, "h", n.gap)
const w = Math.max(root.p.rect.w, spanOf(branches))
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(PAD * 2 + w, titleFloor(n.label, PAD)),
h: head + PAD * 2 + h,
},
children: kids,
}
}
// Radial: the two sides reach different distances, so each is measured on its own.
// Using one figure for both would leave the deeper side hanging outside the frame.
const { right, left } = radialSides(branches)
const w =
radialReach(left, "w", n.gap) +
root.p.rect.w +
radialReach(right, "w", n.gap)
const h = Math.max(root.p.rect.h, spanOf(right), spanOf(left))
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(PAD * 2 + w, titleFloor(n.label, PAD)),
h: head + PAD * 2 + h,
},
children: kids,
}
}
if (n.kind === "grid") {
const cols = Math.max(1, n.cols)
const rows = Math.ceil(kids.length / cols) || 1
const cellW = Math.max(0, ...kids.map((k) => k.rect.w))
const cellH = Math.max(0, ...kids.map((k) => k.rect.h))
const w = PAD * 2 + cols * cellW + gap * (cols - 1)
const h = head + PAD * 2 + rows * cellH + gap * (rows - 1)
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(w, titleFloor(n.label, PAD)),
h,
},
children: kids,
}
}
// group: row or col
const pad = padOf(n)
// A declared cap wins over the measured content, so a row of six cards capped at 900
// reports 900 and the place pass below has real negative slack to shrink into.
const cap = maxWOf(n)
if (n.dir === "row") {
const tallest = Math.max(0, ...kids.map((k) => k.rect.h))
// Only a group stretches to match its siblings. A leaf keeps its natural size,
// because stretching an icon distorts the glyph; and a grid, pool, sequence or
// radial computes its interior from its own rule, so forcing one bigger leaves dead
// space inside rather than filling anything — and for a pool it would detach the
// lane bands from the nodes sitting on them.
for (const k of kids)
if (k.node.kind === "group") k.rect.h = Math.max(k.rect.h, tallest)
// A capped row wraps into as many lines as it takes, so the cap is a real limit
// rather than something the content silently overflows. Sized here and positioned
// by the same line-breaking in `place`, so measure and place cannot disagree.
if (cap < Number.POSITIVE_INFINITY) {
const lines = wrapLines(kids, cap - pad * 2, gap)
const h =
head +
pad * 2 +
lines.reduce((s, l) => s + l.height, 0) +
gap * Math.max(0, lines.length - 1)
const widestLine = Math.max(0, ...lines.map((l) => l.width))
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(
Math.min(cap, pad * 2 + widestLine),
titleFloor(n.label, pad),
),
h,
},
children: kids,
}
}
const w =
pad * 2 +
kids.reduce((s, k) => s + k.rect.w, 0) +
gap * Math.max(0, kids.length - 1)
const h = head + pad * 2 + Math.max(0, ...kids.map((k) => k.rect.h))
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.max(w, titleFloor(n.label, pad)),
h,
},
children: kids,
}
}
const widest = Math.max(0, ...kids.map((k) => k.rect.w))
// Only a group stretches — same reasoning as the row branch above.
for (const k of kids)
if (k.node.kind === "group") k.rect.w = Math.max(k.rect.w, widest)
// A row of weighted columns divides a width it does not have yet (see needsFullWidth).
// Its share of the extra has to be handed out during MEASURE: `place` runs top-down, so
// a child widened there leaves this container already sized for the narrow version, and
// the child then sticks out of the frame that is supposed to contain it.
//
// Distributed by weight rather than to the full interior, because that is the answer
// `place` will independently arrive at — the two passes have to agree or the frame is
// sized for one arrangement and drawn as another.
for (const k of kids) {
if (!needsFullWidth(k.node) || k.node.kind !== "group") continue
const kp = padOf(k.node)
const room =
widest - kp * 2 - k.node.gap * Math.max(0, k.children.length - 1)
const weights = k.children.map((c) => growOf(c.node))
// Unweighted children keep their size and take their width off the top; the rest is
// what the weights divide.
const fixed = k.children.reduce(
(s, c, i) => s + (weights[i] ? 0 : c.rect.w),
0,
)
const widths = shareOut(
k.children.map((c) => c.rect.w),
weights,
k.children.map((c) => maxWOf(c.node)),
k.children.map((c) => floorOf(c.node, c.rect.w)),
room - fixed,
)
k.children.forEach((c, i) => {
if (weights[i]) c.rect.w = widths[i]
})
k.rect.w = Math.max(
k.rect.w,
kp * 2 +
k.children.reduce((s, c) => s + c.rect.w, 0) +
k.node.gap * Math.max(0, k.children.length - 1),
)
}
const w = pad * 2 + Math.max(0, ...kids.map((k) => k.rect.w))
const h =
head +
pad * 2 +
kids.reduce((s, k) => s + k.rect.h, 0) +
gap * Math.max(0, kids.length - 1)
return {
node: n,
rect: {
x: 0,
y: 0,
w: Math.min(cap, Math.max(w, titleFloor(n.label, pad))),
h,
},
children: kids,
}
}
/**
* place: assign absolute positions, top-down.
*
* When a container ended up larger than its content — because a sibling forced it
* wider, or its own title did — the slack is shared between the children rather than
* left as dead margin on one side. The extra spacing is capped at one base gap so a
* stretched frame reads as deliberately spaced instead of sparse, and the resulting
* cluster is centred.
*/
function place(
p: Placed,
x: number,
y: number,
links: LayoutLinks,
/**
* True when this container's width was decided from outside its own content — the page
* aspect gave the top level a width, or an ancestor stretched it. Only then do `grow`
* weights read as absolute proportions ("3:1"), because only then is there a total to
* take a share OF. It passes down through stretched children: a full-width column that
* inherited its width hands that same certainty to the row inside it.
*/
definiteWidth = false,
): void {
p.rect.x = Math.round(x)
p.rect.y = Math.round(y)
const n = p.node
if (!isContainer(n)) return
const head = headerFor(n)
const pad = n.kind === "group" ? padOf(n) : PAD
const innerX = p.rect.x + pad
const innerTop = p.rect.y + head + pad
const innerW = p.rect.w - pad * 2
const innerH = p.rect.h - head - pad * 2
const kids = p.children
if (n.kind === "grid") {
const cols = Math.max(1, n.cols)
const cellW = Math.max(0, ...kids.map((k) => k.rect.w))
const cellH = Math.max(0, ...kids.map((k) => k.rect.h))
kids.forEach((k, i) => {
const r = Math.floor(i / cols)
const c = i % cols
const cx = innerX + c * (cellW + n.gap)
const cy = innerTop + r * (cellH + n.gap)
// centre each child in its cell so a short label does not sit off-axis
place(
k,
cx + (cellW - k.rect.w) / 2,
cy + (cellH - k.rect.h) / 2,
links,
)
})
return
}
if (n.kind === "pool") {
// Each child goes to the (lane, column) cell it declared. Empty cells stay empty:
// in a swimlane diagram, "this role does nothing at this step" is information.
const m = poolMetrics(n, p.rect, kids)
for (const k of kids) {
const { lane, col } = poolCellOf(k.node)
const cx = m.horizontal
? m.contentX + col * (m.cellW + n.gap)
: m.contentX + Math.min(lane, m.lanes - 1) * m.cellW
const cy = m.horizontal
? m.contentY + Math.min(lane, m.lanes - 1) * m.cellH
: m.contentY + col * (m.cellH + n.gap)
place(
k,
cx + (m.cellW - k.rect.w) / 2,
cy + (m.cellH - k.rect.h) / 2,
links,
)
}
return
}
if (n.kind === "sequence") {
// Participants in a row across the top. Their lifelines hang below, emitted by the
// renderer, so nothing else has to be placed here.
let cur = innerX
for (const k of kids) {
place(k, cur, innerTop, links)
cur += k.rect.w + n.gap
}
return
}
if (n.kind === "radial") {
placeRadial(p, n, innerX, innerTop, innerW, innerH, links)
return
}
const alongRow = n.dir === "row"
// A capped row that had to WRAP lays each line out like its own row, so the two passes
// cannot disagree about where the breaks fall. A capped row that still fits on one line
// falls through to the ordinary path below — it is an ordinary row, and skipping that
// path would skip `grow`, leaving declared proportions unapplied.
const wrapped =
alongRow && maxWOf(n) < Number.POSITIVE_INFINITY && kids.length > 0
? wrapLines(kids, innerW, n.gap)
: null
if (wrapped && wrapped.length > 1) {
const lines = wrapped
let lineTop = innerTop
for (const line of lines) {
const used = line.items.reduce((s, k) => s + k.rect.w, 0)
const room = Math.max(
0,
innerW - used - n.gap * (line.items.length - 1),
)
// A wrapped line follows the same row default as an unwrapped one: centred,
// with its gaps padded by up to one extra gap.
const declared = justifyOf(n)
const { lead, extraGap } = declared
? distribute(declared, room, line.items.length)
: line.items.length > 1
? (() => {
const e = Math.min(
n.gap,
room / (line.items.length - 1),
)
return {
lead: Math.max(
0,
(room - e * (line.items.length - 1)) / 2,
),
extraGap: e,
}
})()
: { lead: room / 2, extraGap: 0 }
let x = innerX + lead
for (const kid of line.items) {
const a = alignOf(kid.node, n)
const off =
a === "start"
? 0
: a === "end"
? line.height - kid.rect.h
: a === "stretch"
? 0
: (line.height - kid.rect.h) / 2
if (a === "stretch") kid.rect.h = line.height
place(kid, x, lineTop + Math.max(0, off), links)
x += kid.rect.w + n.gap + extraGap
}
lineTop += line.height + n.gap
}
return
}
const sizes = kids.map((k) => (alongRow ? k.rect.w : k.rect.h))
const content = sizes.reduce((s, v) => s + v, 0)
const extent = alongRow ? innerW : innerH
const k = kids.length
let slack = Math.max(0, extent - content - n.gap * (k - 1))
// flex-grow: children with a weight split the leftover space between them, TeX's
// glue. This runs before the justify distribution below — declared weights are a
// statement about where the slack should go, and spreading it into the gaps instead
// would silently override that statement.
//
// A LEAF box never grows along a column: growing its height just inflates a text
// box around its own text — the giant hollow panels of an early poster. Along a row
// it stays legal (two bars splitting a card's width is real layout), and containers
// grow on either axis, since they distribute the space onwards.
const weights = kids.map((kid) =>
!alongRow && kid.node.kind === "box" ? 0 : growOf(kid.node),
)
const totalWeight = weights.reduce((s, v) => s + v, 0)
if (totalWeight > 0 && slack > 0) {
// Along a ROW, when the container's width was set from OUTSIDE (a declared page
// aspect, or its own cap), the weights divide the whole track: `grow: 3` beside
// `grow: 1` then really is three times as wide, which is what writing those numbers
// means and what CSS's `flex: 3` shorthand does by zeroing flex-basis.
//
// Otherwise only the slack is divided, which is the older contract: the width came
// from the content itself, so treating the weights as absolute proportions would
// shrink a column below the text already in it.
const proportional =
alongRow &&
(definiteWidth ||
needsFullWidth(n) ||
maxWOf(n) < Number.POSITIVE_INFINITY)
if (proportional) {
// The weights divide the whole track. shareOut settles anyone who cannot take
// their share — too wide already, or capped — and re-divides among the rest, so
// the total never exceeds the room available and no child spills out.
const fixed = kids.reduce(
(s, kid, i) => s + (weights[i] ? 0 : kid.rect.w),
0,
)
const widths = shareOut(
kids.map((kid) => kid.rect.w),
weights,
kids.map((kid) => maxWOf(kid.node)),
kids.map((kid) => floorOf(kid.node, kid.rect.w)),
extent - n.gap * (k - 1) - fixed,
)
kids.forEach((kid, i) => {
if (weights[i]) kid.rect.w = widths[i]
})
} else {
kids.forEach((kid, i) => {
if (!weights[i]) return
const share = (slack * weights[i]) / totalWeight
// Never past a declared cap: min/max outranks grow, Yoga's rule too.
if (alongRow)
kid.rect.w = Math.min(maxWOf(kid.node), kid.rect.w + share)
else kid.rect.h += share
})
}
// Recompute: a child clamped by its cap or its content refused part of its share,
// and that remainder is still free space the distribution below has to place.
const used = kids.reduce(
(s, kid) => s + (alongRow ? kid.rect.w : kid.rect.h),
0,
)
slack = Math.max(0, extent - used - n.gap * (k - 1))
}
// How the remaining slack is spread. A declared `justify` decides it; otherwise the
// engine's original per-axis defaults stand, because they are what every diagram built
// before `justify` existed was laid out with:
//
// ROW — spread the gaps by up to one extra gap, then centre the result. A flowchart
// layer reads as a pyramid, and dead space at the right edge of a row looks like a
// mistake.
// COLUMN — pack to the top and leave the slack at the bottom. A column is usually
// tall because a SIBLING made it tall, and stretching its gaps turns every panel into
// a big frame with three lines floating in the middle.
const declared = justifyOf(n)
let lead: number
let extraGap: number
if (declared) {
;({ lead, extraGap } = distribute(declared, slack, k))
} else if (alongRow && k > 1) {
extraGap = Math.min(n.gap, slack / (k - 1))
lead = Math.max(0, (slack - extraGap * (k - 1)) / 2)
} else {
lead = 0
extraGap = 0
}
const gap = n.gap + extraGap
let cur = (alongRow ? innerX : innerTop) + lead
for (const kid of kids) {
// A stretching role fills the cross axis: a masthead spans its page, a section
// heading spans its column. Measured at its text width, then widened here — the
// container's size still comes from the widest ordinary child.
const kn = kid.node
const a = alignOf(kn, n)
const stretches =
a === "stretch" ||
(kn.kind === "box" && kn.role && roleMetrics(kn.role).stretch) ||
// A row of weighted columns fills this column, whatever the alignment default
// says — see needsFullWidth. Only along a column: across a row the cross axis
// is height, and a row does not hand its height out proportionally.
(!alongRow && needsFullWidth(kn))
// Cross-axis position: centred unless the child asked for an edge.
const cross = (room: number, size: number): number => {
if (a === "start") return 0
if (a === "end") return Math.max(0, room - size)
return (room - size) / 2
}
if (alongRow) {
if (stretches) kid.rect.h = innerH
// A row's child got its width from the weights above, so if this row's own
// width was definite the child's is too.
place(
kid,
cur,
innerTop + cross(innerH, kid.rect.h),
links,
definiteWidth && growOf(kn) > 0,
)
cur += kid.rect.w + gap
} else {
// Stretching along a column still respects a declared cap.
if (stretches) kid.rect.w = Math.min(maxWOf(kn), innerW)
// A stretched child fills a width this column already knew, so it inherits
// that certainty; an unstretched one is still sized by its own content.
place(
kid,
innerX + cross(innerW, kid.rect.w),
cur,
links,
definiteWidth && stretches,
)
cur += kid.rect.h + gap
}
}
}
/**
* Place a radial container: a centre with its branches fanning out.
*
* Two shapes, because a mind map and an org chart want opposite things. A mind map reads
* best with branches on both sides of the centre, which keeps it compact and balanced. An
* org chart must hang everything downwards — a reporting line drawn upwards or sideways
* reads as the wrong relationship, no matter how much space it saves.
*
* Each generation sits in its own ring, the ring's depth set by the widest node in it, so
* siblings line up instead of stepping raggedly outwards.
*/
function placeRadial(
p: Placed,
n: RadialNode,
innerX: number,
innerTop: number,
innerW: number,
innerH: number,
links: LayoutLinks,
): void {
const down = n.spread === "down"
const along = down ? "h" : "w"
const across = down ? "w" : "h"
const tree = radialHierarchy(p.children, links, across, n.gap)
if (!tree) return
const { root, branches } = tree
const centre = root.p
/**
* Lay one generation out along the cross axis, then recurse.
*
* `start` is the middle of the band this generation has to fill; each branch gets a slice
* of it as wide as its own subtree needs, and is centred in that slice. Sizing slices by
* subtree extent — not by branch count — is what keeps a bushy branch from being drawn
* over a bare sibling.
*/
const spread = (
items: RadialTree[],
start: number,
alongPos: number,
sign: 1 | -1,
) => {
const total =
items.reduce((s, b) => s + b.extent, 0) +
n.gap * Math.max(0, items.length - 1)
let cur = start - total / 2
for (const b of items) {
const mid = cur + b.extent / 2
// On the left side the ring position is the branch's FAR edge, so its own size has
// to come off to get its origin.
const a = sign > 0 ? alongPos : alongPos - b.p.rect[along]
if (down) place(b.p, mid - b.p.rect.w / 2, a, links)
else place(b.p, a, mid - b.p.rect.h / 2, links)
if (b.kids.length) {
// `alongPos` means the NEAR edge going outwards and the FAR edge coming back,
// which is why the two directions are not symmetric here: on the left the
// recursion subtracts the child's own width, so subtracting the ring width as
// well would place it a full ring too far out — off the frame.
const next = sign > 0 ? a + b.p.rect[along] + n.gap : a - n.gap
spread(b.kids, mid, next, sign)
}
cur += b.extent + n.gap
}
}
if (down) {
place(centre, innerX + (innerW - centre.rect.w) / 2, innerTop, links)
spread(
branches,
centre.rect.x + centre.rect.w / 2,
centre.rect.y + centre.rect.h + n.gap,
1,
)
return
}
// Radial: split the branches between the two sides, keeping declaration order within each
// side so the model can predict where a branch lands.
//
// The centre goes at the LEFT side's reach, not at the frame's middle. Those are the same
// only when both sides are equally deep; centring a lopsided map would push the deeper
// side out past the frame's edge and off the page.
const { right, left } = radialSides(branches)
place(
centre,
innerX + radialReach(left, "w", n.gap),
innerTop + (innerH - centre.rect.h) / 2,
links,
)
const midY = centre.rect.y + centre.rect.h / 2
spread(right, midY, centre.rect.x + centre.rect.w + n.gap, 1)
spread(left, midY, centre.rect.x - n.gap, -1)
}
export interface LayoutResult {
/** Placed roots, in the order given. */
roots: Placed[]
/** Page size that fits everything, with a margin. */
page: { w: number; h: number }
}
/** Where the tree starts on the page. Leaves room for a title above it. */
const ORIGIN = { x: 40, y: 90 }
const MARGIN = { right: 40, bottom: 50 }
/**
* Area of draw.io's default page (A4 at 850x1100), the yardstick a declared aspect ratio
* is measured against. Using the editor's own page size means aspect 1 lands on a square
* about one page in area, rather than on some number invented here.
*/
const PAGE_AREA = 850 * 1100
/**
* Lay out a forest of roots side by side and report the page size that fits them.
*
* A pinned node keeps the position it already had: the user moved it deliberately, and
* the whole point of the pin is that a re-layout does not undo that.
*/
export function layoutForest(
roots: DiagramNode[],
opts: {
iconSize?: number
gap?: number
/** The diagram's links. Needed by sequence containers, which size themselves from
* the number of messages between their participants. */
links?: LayoutLinks
/**
* Target width : height for the page. When set, the top level is given a width
* that lands near it, which is the only way a proportional rule has anything to
* divide: without a definite width there is no leftover space, so every `grow`
* weight resolves to zero. Yoga's own docs say the same — a container distributes
* "any remaining space" among its children, so some space has to remain.
*/
aspect?: number
} = {},
): LayoutResult {
const glyph = opts.iconSize ?? ICON_SIZE
const gap = opts.gap ?? 70
const links: LayoutLinks = opts.links ?? []
const run = (hints?: Map, target?: number): Placed[] => {
// A target page width narrower than the content is a request to WRAP, and wrapping
// has to happen during measure — the line breaks change every height. So the cap is
// pushed onto the root group before measuring, unless it declared its own.
const rootCap = new Map()
if (target) {
for (const r of roots)
if (
r.kind === "group" &&
r.dir === "row" &&
!r.pinned &&
r.maxW == null
) {
rootCap.set(r.id, target)
r.maxW = target
}
}
const placed = roots.map((r) => measure(r, glyph, links, hints))
// Widen the roots to the target width when they came out narrower. Only a group can
// absorb it — a grid, pool, sequence or radial computes its interior from its own
// rule, so forcing one wider just adds dead space inside it.
if (target) {
const own = placed.filter(
(p) => p.node.kind !== "title" && !p.node.pinned,
)
const spread = gap * Math.max(0, own.length - 1)
const share = (target - spread) / Math.max(1, own.length)
for (const p of own)
if (p.node.kind === "group")
p.rect.w = Math.min(
maxWOf(p.node),
Math.max(p.rect.w, share),
)
}
let cur = ORIGIN.x
for (const p of placed) {
const n = p.node
const held =
n.kind === "title" ? null : n.pinned ? (n.rect ?? null) : null
if (held) {
place(p, held.x, held.y, links)
} else {
// A target width makes the top level's width definite, which is what lets
// grow weights inside it read as proportions of a whole.
place(p, cur, ORIGIN.y, links, Boolean(target))
cur += p.rect.w + gap
}
}
// Undo only now: `place` needs the cap to break lines in the same places `measure`
// did, but `roots` is the caller's tree and must come back exactly as it went in.
for (const r of roots)
if (rootCap.has(r.id) && r.kind === "group") r.maxW = undefined
return placed
}
const extent = (placed: Placed[]) => {
let maxX = 0
let maxY = 0
const visit = (p: Placed) => {
maxX = Math.max(maxX, p.rect.x + p.rect.w)
maxY = Math.max(maxY, p.rect.y + p.rect.h)
p.children.forEach(visit)
}
placed.forEach(visit)
return { maxX, maxY }
}
let placed = run()
if (opts.aspect && opts.aspect > 0) {
// The target width has to come from OUTSIDE the content, or it cannot create the
// spare space that proportional rules divide: deriving it from the area the content
// already occupies just returns that content's own width back, leaving nothing over.
// draw.io's page is the natural external reference — one A4 at 850x1100 — so
// width = sqrt(pageArea x aspect) is the first guess.
let want = Math.round(Math.sqrt(PAGE_AREA * opts.aspect))
// Then iterate, because width and height are not independent: widening the page
// makes every paragraph rewrap to fewer lines, which SHORTENS it, which changes the
// ratio that was being aimed at. One pass therefore lands wide of the mark — asking
// for 0.8 gave 1.13. Each round measures what the last width actually produced and
// corrects toward the target; three is enough to get inside a few percent, and the
// loop stops early once the correction is negligible.
//
// The hint map is what makes the correction real: it carries the width each box was
// drawn at, so its text is re-counted at that width instead of at its intrinsic one.
for (let pass = 0; pass < 4; pass++) {
placed = run(undefined, want)
const hints = new Map()
const collect = (p: Placed) => {
if (p.node.kind === "box") hints.set(p.node.id, p.rect.w)
p.children.forEach(collect)
}
placed.forEach(collect)
placed = run(hints, want)
const { maxX, maxY } = extent(placed)
const w = maxX + MARGIN.right
const h = maxY + MARGIN.bottom
const err = w / h / opts.aspect
if (Math.abs(err - 1) < 0.04) break
// Geometric correction: to move the ratio by a factor, move the width by its
// square root, since shrinking the width lengthens the page and vice versa.
const next = Math.round(want / Math.sqrt(err))
if (next === want) break
want = next
}
}
const { maxX, maxY } = extent(placed)
return {
roots: placed,
page: {
w: Math.round(maxX + MARGIN.right),
h: Math.round(maxY + MARGIN.bottom),
},
}
}
/** Flatten a placed forest into (node, rect, parentId) triples in document order. */
export function flatten(
roots: Placed[],
): { node: DiagramNode; rect: Rect; parent: string }[] {
const out: { node: DiagramNode; rect: Rect; parent: string }[] = []
const walk = (p: Placed, parent: string) => {
out.push({ node: p.node, rect: p.rect, parent })
for (const c of p.children) walk(c, p.node.id)
}
for (const r of roots) walk(r, "1")
return out
}