fix(merman): avoid infinite loop in side parallel transitions with equal endpoints

packages/merman/src/state/diagram.test.ts

@@ -1383,4 +1383,26 @@ stateDiagram-v2
       }
     }
   })
+
+  test("renders transitions between nested composite states sharing horizontal rows without looping", () => {
+    const source = `stateDiagram-v2
+direction TB
+A --> B
+A --> C
+state D {
+  state E {
+    B --> F
+  }
+  B --> C
+}`
+
+    const output = renderStateDiagram(source)
+    expect(output).toContain("A")
+    expect(output).toContain("B")
+    expect(output).toContain("C")
+    expect(output).toContain("D")
+    expect(output).toContain("E")
+    expect(output).toContain("F")
+  })
 })
+

packages/merman/src/state/routing.test.ts

@@ -690,4 +690,30 @@ describe("reconverging vertical elbows", () => {
     expect(entering.label!.y).toBeLessThan(enteringHorizontalY)
     expect(leaving.label!.y).toBeLessThan(leavingHorizontalY)
   })
+
+  test("routes side-parallel transitions between states sharing the same vertical row without infinite allocation", () => {
+    const diagram: StateVisibleDiagram = {
+      direction: "TB",
+      states: [
+        { id: "A", label: "A", kind: "state", parentId: "P1" },
+        { id: "B", label: "B", kind: "state", parentId: "P2" },
+      ],
+      transitions: [{ from: "A", to: "B", label: "" }],
+      composites: [
+        { id: "P1", label: "P1" },
+        { id: "P2", label: "P2" },
+      ],
+      notes: [],
+    }
+    const placements = new Map([
+      ["A", bounds("A", 4, 8)],
+      ["B", bounds("B", 22, 8)],
+    ])
+
+    const plans = createStateTransitionRenderPlans(diagram, placements, 29)
+    expect(plans).toHaveLength(1)
+    expect(plans[0]!.cells.length).toBeGreaterThan(0)
+    expect(plans[0]!.cells.length).toBeLessThan(100)
+  })
 })
+

packages/merman/src/state/routing.ts

@@ -327,6 +327,7 @@ function sideParallelTargetApproach(
     spatialPathClaim(`side-target:${transition.from}:${transition.to}`, "side-target", "route", points)
 
   if (
+    from.centerY !== to.centerY &&
     space.isFree(
       claim([
         { x: railX, y: to.centerY },
@@ -486,6 +487,17 @@ export function createStateTransitionRoutePlans(
       const fromParent = statesById.get(transition.from)?.parentId
       const toParent = statesById.get(transition.to)?.parentId
       if (fromParent && toParent && fromParent !== toParent) {
+        if (from.centerY === to.centerY) {
+          const railY = allocateBottomRail()
+          return [
+            {
+              ...base,
+              kind: "bottom-parallel",
+              railY,
+              approachX: bottomApproachX(diagram, transition, from, to, bounds, railY),
+            },
+          ]
+        }
         return [sideParallel()]
       }
       if (verticalCorridorCrossesUnrelatedState(diagram, transition, from, to, bounds)) {
@@ -783,12 +795,14 @@ function addSideParallelTransition(builder: StateTransitionRenderBuilder): void
   const startX = from.left + from.width
   const startY = from.centerY
   const endY = targetApproach === "top" ? to.top - 2 : targetApproach === "bottom" ? to.top + to.height + 1 : to.centerY
-  const verticalStep: 1 | -1 = startY <= endY ? 1 : -1
   addRightDeparture(builder, from)
   addHorizontalLine(builder, startX, railX - 1, startY, 1)
-  addCell(builder, { x: railX, y: startY, char: verticalStep === 1 ? "╮" : "╯" })
-  for (let y = startY + verticalStep; y !== endY; y += verticalStep) addCell(builder, { x: railX, y, char: "│" })
-  addCell(builder, { x: railX, y: endY, char: verticalStep === 1 ? "╯" : "╮" })
+  if (startY !== endY) {
+    const verticalStep: 1 | -1 = startY <= endY ? 1 : -1
+    addCell(builder, { x: railX, y: startY, char: verticalStep === 1 ? "╮" : "╯" })
+    for (let y = startY + verticalStep; y !== endY; y += verticalStep) addCell(builder, { x: railX, y, char: "│" })
+    addCell(builder, { x: railX, y: endY, char: verticalStep === 1 ? "╯" : "╮" })
+  }
   if (targetApproach) {
     const targetX = innerConnectorX(to, from.centerX)
     for (let x = railX - 1; x > targetX; x--) addCell(builder, { x, y: endY, char: "─" })