import {
  type AnyNode,
  type AnyNodeId,
  PipeSegmentNode,
  type PortConnection,
} from '@pascal-app/core'
import { getPipeFittingPorts, pipeFittingLegLength } from '../pipe-fitting/ports'
import type { PipeFittingNode } from '../pipe-fitting/schema'
import { type PipeElbowPlan, planPipeElbowAtPort, planPipeElbowRealign } from './auto-fitting'
import type { ScenePort } from './ports'

/**
 * Center-cube vertical-move auto-routing for pipe runs.
 *
 * When a run is lifted / lowered with the run-center cube's ±Y arrows, a
 * RUN-connected end should stay welded to its (stationary) partner by way of
 * an offset: an elbow on the lifted run, a plumb riser down to the partner's
 * height, and a second elbow that meets the partner — the classic pipe S/Z
 * offset. Without it, plain connectivity-follow would translate the collinear
 * partner run straight up too (it has no turn to absorb the lift), dragging
 * the whole network along.
 *
 * RUN-connected end (partner stays at its old height):
 *   - top elbow at the lifted endpoint, turning the run axis → vertical;
 *   - a plumb riser straight down (same X/Z) to one leg above the partner;
 *   - bottom elbow at the partner joint, turning vertical → the partner's
 *     axis; the partner run is trimmed back one leg so the elbow replaces
 *     that stretch.
 * The lifted run is trimmed back one leg at the offset end so it meets the
 * top elbow's collar instead of overlapping it.
 *
 * ELBOW-connected end (a clean L): the existing elbow STAYS PUT and re-aims so
 * its mated collar swings vertical (flattening toward a straight coupling); a
 * plumb riser rises from that collar to a single new TOP elbow that turns back
 * along the run axis onto the lifted endpoint. One new elbow + one riser — no
 * horizontal jog. Non-elbow fittings (and elbows whose re-aim is out of the
 * buildable 15–90° range) ride up via plain connectivity-follow instead. Open
 * ends likewise just ride up.
 */

type Point = [number, number, number]
type PipeProfile = {
  diameter: number
  pipeMaterial: PipeFittingNode['pipeMaterial']
}

/** Joint-coincidence epsilon (m), matching core's port connectivity. */
const COINCIDENT_EPS_M = 0.05
/** Shortest riser worth minting — below this there's no room to offset, so
 *  the caller keeps the plain vertical translate. */
const MIN_RISER_M = 0.05
const MAX_TRANSITION_STEPS = 8

/**
 * Three-state outcome of a connected vertical lift:
 *  - `null` — the run has NO connected ends, so the caller plain-translates it
 *    (nothing to keep welded; everyone is free to ride along or there's no one).
 *  - `{ status: 'invalid' }` — at least one connected end CANNOT form a clean
 *    offset at this height (no room for the elbows + riser, a non-elbow fitting,
 *    or a re-aim out of the buildable 15–90° range). The caller lifts ONLY the
 *    dragged run as a red preview, freezes every partner, and commits nothing on
 *    release. We never silently drag the network up to "absorb" the lift.
 *  - `{ status: 'valid', plan }` — every connected end welds back to its
 *    stationary partner via the planned offset; the caller lifts + trims, ghosts
 *    the new fittings green, and mints them on release.
 */
export type VerticalOffsetResult =
  | { status: 'valid'; plan: VerticalOffsetPlan }
  | { status: 'invalid' }
  | null

export type VerticalOffsetPlan = {
  /** Actual vertical offset used by the route. This can differ from the raw
   *  cursor delta when the route snaps through a topology transition. */
  dy: number
  /** The lifted run's new path: every point raised by `dy`, each RUN-offset
   *  end trimmed back one elbow-leg to meet its top elbow (fitting / open ends
   *  keep their lifted endpoint). */
  pipePath: Point[]
  /** The path to seed the caller's connectivity-follow from: identical to the
   *  lifted run except each RUN-offset end is reset to its ORIGINAL height, so
   *  its trimmed partner shows zero delta (we trim it via `updates` instead)
   *  while a FITTING / open end shows `+dy` — lifting its elbow rigidly and
   *  lengthening that elbow's riser into a clean L. */
  followPath: Point[]
  /** Two elbows per RUN-offset end (top + bottom). */
  fittings: PipeFittingNode[]
  /** One plumb riser per RUN-offset end. */
  risers: PipeSegmentNode[]
  /** Partner-run trims (the run mated at each RUN-offset end pulled back one
   *  leg). Fitting partners are rigid and never updated. */
  updates: { id: AnyNodeId; data: Partial<AnyNode> }[]
  /** Existing generated/absorbed parts to remove when an offset collapses into a direct L. */
  delete?: AnyNodeId[]
}

function distSq(a: Point | readonly number[], b: Point | readonly number[]): number {
  const dx = a[0]! - b[0]!
  const dy = a[1]! - b[1]!
  const dz = a[2]! - b[2]!
  return dx * dx + dy * dy + dz * dz
}

function addPoint(a: Point, b: Point): Point {
  return [a[0] + b[0], a[1] + b[1], a[2] + b[2]]
}

function subPoint(a: Point, b: Point): Point {
  return [a[0] - b[0], a[1] - b[1], a[2] - b[2]]
}

function lenSq(v: Point): number {
  return v[0] * v[0] + v[1] * v[1] + v[2] * v[2]
}

function decompose(delta: Point, axis: Point): { parallel: Point; perp: Point } {
  const dot = delta[0] * axis[0] + delta[1] * axis[1] + delta[2] * axis[2]
  const parallel: Point = [axis[0] * dot, axis[1] * dot, axis[2] * dot]
  return { parallel, perp: subPoint(delta, parallel) }
}

function distToAxisSq(point: Point, origin: Point, axis: Point): number {
  const dx = point[0] - origin[0]
  const dy = point[1] - origin[1]
  const dz = point[2] - origin[2]
  const along = dx * axis[0] + dy * axis[1] + dz * axis[2]
  const px = dx - axis[0] * along
  const py = dy - axis[1] * along
  const pz = dz - axis[2] * along
  return px * px + py * py + pz * pz
}

function crossesVerticalTarget(startY: number, endY: number, targetY: number): boolean {
  return Math.abs(endY - targetY) <= COINCIDENT_EPS_M || (startY - targetY) * (endY - targetY) < 0
}

/** Outward unit direction at the run endpoint `idx` (0 = start, last = end). */
function endpointOutwardDir(path: ReadonlyArray<readonly number[]>, idx: number): Point {
  const last = path.length - 1
  const [a, b] = idx === 0 ? [path[0]!, path[1]!] : [path[last]!, path[last - 1]!]
  const d: Point = [a[0]! - b[0]!, a[1]! - b[1]!, a[2]! - b[2]!]
  const len = Math.hypot(d[0], d[1], d[2])
  return len < 1e-9 ? [1, 0, 0] : [d[0] / len, d[1] / len, d[2] / len]
}

function endpointAxis(path: Point[], portId: string): Point {
  const n = path.length
  const [a, b] = portId === 'start' ? [path[1]!, path[0]!] : [path[n - 2]!, path[n - 1]!]
  const dir = subPoint(b, a)
  const l2 = lenSq(dir)
  if (l2 < 1e-12) return [0, 0, 0]
  const l = Math.sqrt(l2)
  return [dir[0] / l, dir[1] / l, dir[2] / l]
}

function collectPipePortsFromNodes(
  nodes: Record<string, AnyNode>,
  excludeNodeId: AnyNodeId,
): ScenePort[] {
  const ports: ScenePort[] = []
  for (const node of Object.values(nodes)) {
    if (!node || node.id === excludeNodeId) continue
    if (node.type === 'pipe-fitting') {
      ports.push(
        ...getPipeFittingPorts(node as PipeFittingNode).map((port) => ({
          ...port,
          nodeId: node.id as AnyNodeId,
        })),
      )
    } else if (node.type === 'pipe-segment') {
      const pipe = node as PipeSegmentNode
      if (pipe.path.length < 2) continue
      ports.push(
        {
          id: 'start',
          nodeId: pipe.id as AnyNodeId,
          position: pipe.path[0]!,
          direction: endpointOutwardDir(pipe.path, 0),
          diameter: 0,
          system: pipe.system,
        },
        {
          id: 'end',
          nodeId: pipe.id as AnyNodeId,
          position: pipe.path[pipe.path.length - 1]!,
          direction: endpointOutwardDir(pipe.path, pipe.path.length - 1),
          diameter: 0,
          system: pipe.system,
        },
      )
    }
  }
  return ports
}

function collectPipeConnections(
  pipe: PipeSegmentNode,
  nodes: Record<string, AnyNode>,
  scenePorts: ScenePort[],
): PortConnection[] {
  const eps2 = COINCIDENT_EPS_M * COINCIDENT_EPS_M
  const portsByNode = new Map<AnyNodeId, ScenePort[]>()
  for (const port of scenePorts) {
    const list = portsByNode.get(port.nodeId) ?? []
    list.push(port)
    portsByNode.set(port.nodeId, list)
  }

  const connections = new Map<AnyNodeId, PortConnection>()
  const queue: ScenePort[] = []
  for (const endIdx of pipe.path.length > 1 ? [0, pipe.path.length - 1] : [0]) {
    const end = pipe.path[endIdx]!
    queue.push({
      id: endIdx === 0 ? 'start' : 'end',
      nodeId: pipe.id as AnyNodeId,
      position: end,
      direction: endpointOutwardDir(pipe.path, endIdx),
      diameter: 0,
      system: pipe.system,
    })
  }

  while (queue.length > 0) {
    const source = queue.shift()!
    for (const mate of scenePorts) {
      if (mate.nodeId === source.nodeId) continue
      if (source.system && mate.system && source.system !== mate.system) continue
      if (distSq(mate.position, source.position) > eps2) continue
      if (connections.has(mate.nodeId)) continue
      const node = nodes[mate.nodeId]
      if (!node) continue
      if (node.type === 'pipe-segment') {
        connections.set(mate.nodeId, {
          kind: 'run',
          nodeId: mate.nodeId,
          startPath: (node as PipeSegmentNode).path.map((p) => [...p] as Point),
        })
      } else if (node.type === 'pipe-fitting') {
        connections.set(mate.nodeId, {
          kind: 'rigid-node',
          nodeId: mate.nodeId,
          startPosition: [...(node as PipeFittingNode).position] as Point,
        })
      } else {
        continue
      }
      for (const port of portsByNode.get(mate.nodeId) ?? []) {
        if (port.id !== mate.id) queue.push(port)
      }
    }
  }
  return [...connections.values()]
}

function mergeUpdates(
  updates: { id: AnyNodeId; data: Partial<AnyNode> }[],
): { id: AnyNodeId; data: Partial<AnyNode> }[] {
  const byId = new Map<AnyNodeId, { id: AnyNodeId; data: Partial<AnyNode> }>()
  for (const update of updates) {
    byId.set(update.id, {
      id: update.id,
      data: { ...(byId.get(update.id)?.data ?? {}), ...update.data } as Partial<AnyNode>,
    })
  }
  return [...byId.values()]
}

function uniqueNodeIds(ids: AnyNodeId[]): AnyNodeId[] {
  return [...new Set(ids)]
}

function resolveExplicitFollowUpdates(args: {
  pipe: PipeSegmentNode
  followPath: Point[]
  scenePorts: ScenePort[]
  nodesById: Record<string, AnyNode>
}): { id: AnyNodeId; data: Partial<AnyNode> }[] {
  const eps2 = COINCIDENT_EPS_M * COINCIDENT_EPS_M
  const movedId = args.pipe.id as AnyNodeId
  type LocalPort = { id: string; nodeId: AnyNodeId; position: Point; system?: string }
  type LocalNode = {
    id: AnyNodeId
    role: 'run' | 'fitting'
    ports: LocalPort[]
    path?: Point[]
    position?: Point
  }
  const portsByNode = new Map<AnyNodeId, LocalPort[]>()
  const addPort = (port: LocalPort) => {
    const list = portsByNode.get(port.nodeId) ?? []
    list.push(port)
    portsByNode.set(port.nodeId, list)
  }

  if (args.pipe.path.length >= 2) {
    addPort({
      id: 'start',
      nodeId: movedId,
      position: [...args.pipe.path[0]!] as Point,
      system: args.pipe.system,
    })
    addPort({
      id: 'end',
      nodeId: movedId,
      position: [...args.pipe.path[args.pipe.path.length - 1]!] as Point,
      system: args.pipe.system,
    })
  }

  for (const port of args.scenePorts) {
    const node = args.nodesById[port.nodeId]
    if (!node) continue
    let id = port.id
    if (node.type === 'pipe-segment') {
      const path = (node as PipeSegmentNode).path
      if (path.length < 2) continue
      if (distSq(port.position, path[0]!) <= eps2) id = 'start'
      else if (distSq(port.position, path[path.length - 1]!) <= eps2) id = 'end'
    }
    addPort({
      id,
      nodeId: port.nodeId,
      position: [...port.position] as Point,
      system: port.system,
    })
  }

  const nodes = new Map<AnyNodeId, LocalNode>()
  for (const [id, ports] of portsByNode) {
    if (id === movedId) {
      nodes.set(id, { id, role: 'run', ports, path: args.pipe.path.map((p) => [...p] as Point) })
      continue
    }
    const node = args.nodesById[id]
    if (!node) continue
    if (node.type === 'pipe-segment') {
      nodes.set(id, {
        id,
        role: 'run',
        ports,
        path: (node as PipeSegmentNode).path.map((p) => [...p] as Point),
      })
    } else if (node.type === 'pipe-fitting') {
      nodes.set(id, {
        id,
        role: 'fitting',
        ports,
        position: [...(node as PipeFittingNode).position] as Point,
      })
    }
  }

  const adjacency = new Map<string, { nodeId: AnyNodeId; portId: string }[]>()
  const key = (nodeId: AnyNodeId, portId: string) => `${nodeId}:${portId}`
  const allPorts = [...nodes.values()].flatMap((node) => node.ports)
  for (let i = 0; i < allPorts.length; i++) {
    const a = allPorts[i]!
    for (let j = i + 1; j < allPorts.length; j++) {
      const b = allPorts[j]!
      if (a.nodeId === b.nodeId) continue
      if (a.system && b.system && a.system !== b.system) continue
      if (distSq(a.position, b.position) > eps2) continue
      const ak = key(a.nodeId, a.id)
      const bk = key(b.nodeId, b.id)
      adjacency.set(ak, [...(adjacency.get(ak) ?? []), { nodeId: b.nodeId, portId: b.id }])
      adjacency.set(bk, [...(adjacency.get(bk) ?? []), { nodeId: a.nodeId, portId: a.id }])
    }
  }

  const queue: { nodeId: AnyNodeId; portId: string; delta: Point }[] = []
  const visited = new Set<AnyNodeId>([movedId])
  const enqueueMates = (nodeId: AnyNodeId, portId: string, delta: Point) => {
    for (const mate of adjacency.get(key(nodeId, portId)) ?? []) {
      if (visited.has(mate.nodeId)) continue
      queue.push({ ...mate, delta })
    }
  }

  const start = args.pipe.path[0]
  const nextStart = args.followPath[0]
  if (start && nextStart) {
    const delta = subPoint(nextStart as Point, start as Point)
    if (lenSq(delta) > 1e-8) enqueueMates(movedId, 'start', delta)
  }
  const last = args.pipe.path.length - 1
  const end = args.pipe.path[last]
  const nextEnd = args.followPath[last]
  if (end && nextEnd) {
    const delta = subPoint(nextEnd as Point, end as Point)
    if (lenSq(delta) > 1e-8) enqueueMates(movedId, 'end', delta)
  }

  const results: { id: AnyNodeId; data: Partial<AnyNode> }[] = []
  while (queue.length > 0) {
    const { nodeId, portId, delta } = queue.shift()!
    if (visited.has(nodeId)) continue
    visited.add(nodeId)
    const node = nodes.get(nodeId)
    if (!node) continue
    if (node.role === 'fitting') {
      const position = addPoint(node.position!, delta)
      results.push({ id: nodeId, data: { position } as Partial<AnyNode> })
      for (const port of node.ports) {
        if (port.id !== portId) enqueueMates(nodeId, port.id, delta)
      }
      continue
    }
    const path = node.path!.map((p) => [...p] as Point)
    const nearIdx = portId === 'start' ? 0 : path.length - 1
    const farPortId = portId === 'start' ? 'end' : 'start'
    const axis = endpointAxis(path, portId)
    const { parallel, perp } = decompose(delta, axis)
    const nextPath = path.map((p) => addPoint(p, perp))
    nextPath[nearIdx] = addPoint(nextPath[nearIdx]!, parallel)
    results.push({ id: nodeId, data: { path: nextPath } as Partial<AnyNode> })
    if (lenSq(perp) > 1e-8) enqueueMates(nodeId, farPortId, perp)
  }
  return results
}

/** Classifies whether the partner run's segment ADJACENT to its tip at
 *  `endPos` is a plumb existing riser that can safely stretch to `liftedEnd`.
 *  The lifted joint must stay on the same side of the fixed adjacent vertex;
 *  otherwise connectivity-follow would collapse the riser to zero or invert it
 *  through the upper/lower L, which makes connected pipework disappear. */
function alignedVerticalRiserStretch(
  path: ReadonlyArray<readonly number[]> | undefined,
  endPos: Point,
  liftedEnd: Point,
  eps2: number,
): 'stretch' | 'invalid' | null {
  if (!path || path.length < 2) return null
  const tip = path.findIndex((p) => distSq(p, endPos) <= eps2)
  if (tip !== 0 && tip !== path.length - 1) return null // not an endpoint joint
  const adj = tip === 0 ? path[1]! : path[path.length - 2]!
  const dxz = Math.hypot(adj[0]! - endPos[0], adj[2]! - endPos[2])
  if (dxz > COINCIDENT_EPS_M) return null
  const originalSpan = adj[1]! - endPos[1]
  if (Math.abs(originalSpan) <= MIN_RISER_M) return null
  const nextSpan = adj[1]! - liftedEnd[1]
  if (Math.sign(nextSpan) !== Math.sign(originalSpan) || Math.abs(nextSpan) <= MIN_RISER_M) {
    return 'invalid'
  }
  return 'stretch'
}

/** Minimal ScenePort the elbow planner needs (position + direction + system). */
function portLike(position: Point, direction: Point, system: string): ScenePort {
  return {
    id: 'x',
    nodeId: 'x' as AnyNodeId,
    position,
    direction,
    diameter: 0,
    system,
  } as unknown as ScenePort
}

/** A plumb riser pipe-segment between two points, carrying the run's profile. */
function makePipeRiser(from: Point, to: Point, pipe: PipeSegmentNode): PipeSegmentNode {
  return PipeSegmentNode.parse({
    object: 'node',
    parentId: null,
    visible: true,
    metadata: {},
    name: pipe.name ?? 'Pipe run',
    path: [from, to],
    diameter: pipe.diameter,
    pipeMaterial: pipe.pipeMaterial,
    system: pipe.system,
  })
}

function planElbowFromCollar(args: {
  collarPoint: Point
  elbowPortOutDir: Point
  awayDir: Point
  pipe: PipeSegmentNode
  profile: PipeProfile
}): PipeElbowPlan | null {
  const { collarPoint, elbowPortOutDir, awayDir, pipe, profile } = args
  const leg = pipeFittingLegLength(profile.diameter)
  const junction: Point = [
    collarPoint[0] - elbowPortOutDir[0] * leg,
    collarPoint[1] - elbowPortOutDir[1] * leg,
    collarPoint[2] - elbowPortOutDir[2] * leg,
  ]
  const portDir: Point = [-elbowPortOutDir[0], -elbowPortOutDir[1], -elbowPortOutDir[2]]
  return planPipeElbowAtPort(
    portLike(junction, portDir, pipe.system),
    awayDir,
    profile.diameter,
    profile.pipeMaterial,
  )
}

function pipeElbowProfilePatch(profile: PipeProfile): Partial<PipeFittingNode> {
  return {
    diameter: profile.diameter,
    diameter2: profile.diameter,
    pipeMaterial: profile.pipeMaterial,
  }
}

type FittingRiserAction =
  | { status: 'stretch' }
  | {
      status: 'collapse'
      delete: AnyNodeId[]
      updates: { id: AnyNodeId; data: Partial<AnyNode> }[]
      pipePoint: Point
    }
  | { status: 'snap'; dy: number }
  | null

type RiserCollapseAction =
  | {
      status: 'collapse'
      delete: AnyNodeId[]
      updates: { id: AnyNodeId; data: Partial<AnyNode> }[]
      pipePoint: Point
    }
  | { status: 'snap'; dy: number }
  | null

type TransitionOptions = {
  allowTransitionSnap: boolean
  allowEarlySnap: boolean
  depth: number
}

function directRiserCollapseAction(args: {
  riserId: AnyNodeId
  riserPath: ReadonlyArray<readonly number[]>
  riserTopPoint: Point
  liftedPipePoint: Point
  pipeEndPoint: Point
  pipePortDir: Point
  profilePatch: Partial<PipeFittingNode>
  connections: PortConnection[]
  scenePorts: ScenePort[]
  nodesById: Record<string, AnyNode>
  eps2: number
  allowEarlySnap: boolean
}): RiserCollapseAction {
  const {
    riserId,
    riserPath,
    riserTopPoint,
    liftedPipePoint,
    pipeEndPoint,
    pipePortDir,
    profilePatch,
    connections,
    scenePorts,
    nodesById,
    eps2,
    allowEarlySnap,
  } = args
  const tip = riserPath.findIndex((p) => distSq(p, riserTopPoint) <= eps2)
  if (tip !== 0 && tip !== riserPath.length - 1) return null
  const farPoint = tip === 0 ? riserPath[1]! : riserPath[riserPath.length - 2]!
  const farPort = scenePorts.find(
    (sp) =>
      sp.nodeId !== riserId &&
      distSq(sp.position, farPoint) <= eps2 &&
      connections.some((c) => c.nodeId === sp.nodeId && c.kind !== 'run'),
  )
  if (!farPort) return null
  const lower = nodesById[farPort.nodeId]
  if (!lower || lower.type !== 'pipe-fitting') return null
  const lowerElbow = { ...(lower as PipeFittingNode), ...profilePatch } as PipeFittingNode
  if (lowerElbow.fittingType !== 'elbow') return null

  const directDir: Point = [-pipePortDir[0], -pipePortDir[1], -pipePortDir[2]]
  const realign = planPipeElbowRealign(lowerElbow, farPort.id, directDir)
  if (!realign) return null
  const collarOnLiftedLevel: Point = [
    realign.collarPoint[0],
    liftedPipePoint[1],
    realign.collarPoint[2],
  ]
  if (distToAxisSq(collarOnLiftedLevel, liftedPipePoint, pipePortDir) > eps2) return null
  if (Math.abs(realign.collarPoint[1] - liftedPipePoint[1]) > COINCIDENT_EPS_M) {
    if (
      allowEarlySnap ||
      crossesVerticalTarget(pipeEndPoint[1], liftedPipePoint[1], realign.collarPoint[1])
    ) {
      return { status: 'snap', dy: realign.collarPoint[1] - pipeEndPoint[1] }
    }
    return null
  }
  return {
    status: 'collapse',
    delete: [riserId],
    updates: [
      {
        id: lowerElbow.id,
        data: { ...profilePatch, ...realign.update.data } as Partial<AnyNode>,
      },
    ],
    pipePoint: realign.collarPoint,
  }
}

function fittingRiserAction(args: {
  fitting: PipeFittingNode
  matedPortId: string
  pipePortDir: Point
  pipeEndPoint: Point
  liftedPipePoint: Point
  profilePatch: Partial<PipeFittingNode>
  dy: number
  connections: PortConnection[]
  scenePorts: ScenePort[]
  nodesById: Record<string, AnyNode>
  eps2: number
  allowEarlySnap: boolean
}): FittingRiserAction {
  const {
    fitting,
    matedPortId,
    pipePortDir,
    pipeEndPoint,
    liftedPipePoint,
    profilePatch,
    dy,
    connections,
    scenePorts,
    nodesById,
    eps2,
    allowEarlySnap,
  } = args
  const ports = getPipeFittingPorts(fitting)
  for (const port of ports) {
    if (port.id === matedPortId) continue
    const otherPort = scenePorts.find(
      (sp) =>
        sp.nodeId !== fitting.id &&
        distSq(sp.position, port.position) <= eps2 &&
        connections.some((c) => c.nodeId === sp.nodeId && c.kind === 'run'),
    )
    if (!otherPort) continue
    const partner = nodesById[otherPort.nodeId]
    const partnerPath = (partner as unknown as { path?: Point[] } | undefined)?.path
    const portPosition: Point = [port.position[0], port.position[1], port.position[2]]
    const liftedPort: Point = [port.position[0], port.position[1] + dy, port.position[2]]
    const stretch = alignedVerticalRiserStretch(partnerPath, portPosition, liftedPort, eps2)
    if ((stretch === 'stretch' || stretch === 'invalid') && partnerPath) {
      const tip = partnerPath.findIndex((p) => distSq(p, portPosition) <= eps2)
      if (tip !== 0 && tip !== partnerPath.length - 1) {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      const farPoint = tip === 0 ? partnerPath[1]! : partnerPath[partnerPath.length - 2]!
      const farPort = scenePorts.find(
        (sp) =>
          sp.nodeId !== fitting.id &&
          sp.nodeId !== otherPort.nodeId &&
          distSq(sp.position, farPoint) <= eps2 &&
          connections.some((c) => c.nodeId === sp.nodeId && c.kind !== 'run'),
      )
      if (!farPort) {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      const lower = nodesById[farPort.nodeId]
      if (!lower || lower.type !== 'pipe-fitting') {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      const lowerElbow = { ...(lower as PipeFittingNode), ...profilePatch } as PipeFittingNode
      if (lowerElbow.fittingType !== 'elbow') {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      const directDir: Point = [-pipePortDir[0], -pipePortDir[1], -pipePortDir[2]]
      const realign = planPipeElbowRealign(lowerElbow, farPort.id, directDir)
      if (!realign) {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      const collarOnLiftedLevel: Point = [
        realign.collarPoint[0],
        liftedPipePoint[1],
        realign.collarPoint[2],
      ]
      if (distToAxisSq(collarOnLiftedLevel, liftedPipePoint, pipePortDir) > eps2) {
        if (stretch === 'stretch') return { status: 'stretch' }
        continue
      }
      if (Math.abs(realign.collarPoint[1] - liftedPipePoint[1]) > COINCIDENT_EPS_M) {
        if (
          crossesVerticalTarget(pipeEndPoint[1], liftedPipePoint[1], realign.collarPoint[1]) ||
          (allowEarlySnap && stretch === 'invalid')
        ) {
          return { status: 'snap', dy: realign.collarPoint[1] - pipeEndPoint[1] }
        }
        if (stretch === 'stretch' || stretch === 'invalid') return { status: 'stretch' }
        continue
      }
      return {
        status: 'collapse',
        delete: [fitting.id as AnyNodeId, otherPort.nodeId],
        updates: [
          {
            id: lowerElbow.id,
            data: { ...profilePatch, ...realign.update.data } as Partial<AnyNode>,
          },
        ],
        pipePoint: realign.collarPoint,
      }
    }
    if (stretch === 'stretch') {
      return { status: 'stretch' }
    }
    if (stretch !== 'invalid' || !partnerPath) continue
  }
  return null
}

export function planVerticalOffsets(args: {
  pipe: PipeSegmentNode
  /** Signed vertical move (meters); +up / -down. */
  dy: number
  profile: PipeProfile
  /** The drag-start connectivity snapshot's connections. */
  connections: PortConnection[]
  /** Scene ports (excluding the lifted run) for partner direction lookup. */
  scenePorts: ScenePort[]
  /** Drag-start node snapshots keyed by id, so a connected elbow's ORIGINAL
   *  pose can be re-aimed each frame (the live store carries the last frame's
   *  re-aim). */
  nodesById: Record<string, AnyNode>
}): VerticalOffsetResult {
  return planVerticalOffsetsAtDy(args, args.dy, {
    allowTransitionSnap: true,
    allowEarlySnap: true,
    depth: 0,
  })
}

function planVerticalOffsetsAtDy(
  args: {
    pipe: PipeSegmentNode
    dy: number
    profile: PipeProfile
    connections: PortConnection[]
    scenePorts: ScenePort[]
    nodesById: Record<string, AnyNode>
  },
  routeDy: number,
  options: TransitionOptions,
): VerticalOffsetResult {
  const { pipe, profile, connections, scenePorts, nodesById } = args
  const dy = routeDy
  // No connected ends: nothing to weld, so the caller plain-translates the run.
  if (connections.length === 0) return null
  const leg = pipeFittingLegLength(profile.diameter)
  // A connected end that must MINT an offset (two elbows + a real riser) needs
  // room for both legs plus a non-degenerate riser. Checked per-end below, not
  // globally — a partner that's already a vertical riser just STRETCHES and
  // needs no such room, so a tiny lift on a riser-connected run is still valid.
  const hasOffsetRoom = Math.abs(dy) - 2 * leg >= MIN_RISER_M

  const startPath = pipe.path.map((p) => [...p] as Point)
  const last = startPath.length - 1
  const vSign = Math.sign(dy)
  const up: Point = [0, vSign, 0]
  const down: Point = [0, -vSign, 0]
  const eps2 = COINCIDENT_EPS_M * COINCIDENT_EPS_M

  // Lift the whole run; connected ends get adjusted below.
  const pipePath = startPath.map((p) => [p[0], p[1] + dy, p[2]] as Point)
  // Seed for the caller's connectivity-follow: starts as the lifted path, but
  // each RUN-offset end is reset to its original point below so that end shows
  // zero delta (its partner is trimmed via `updates`, not dragged), while any
  // FITTING / open end stays lifted so its partner follows.
  const followPath = startPath.map((p) => [p[0], p[1] + dy, p[2]] as Point)

  const fittings: PipeFittingNode[] = []
  const risers: PipeSegmentNode[] = []
  const updates: { id: AnyNodeId; data: Partial<AnyNode> }[] = []
  const deletes: AnyNodeId[] = []
  let offsetAny = false

  for (const endIdx of last > 0 ? [0, last] : [0]) {
    const endPos = startPath[endIdx]!
    // Partner port sitting on this end, owned by a snapshotted connection.
    const partnerPort = scenePorts.find(
      (sp) =>
        distSq(sp.position, endPos) <= eps2 && connections.some((c) => c.nodeId === sp.nodeId),
    )
    if (!partnerPort) continue // open end → just rides up
    const conn = connections.find((c) => c.nodeId === partnerPort.nodeId)!

    const pipePortDir = endpointOutwardDir(startPath, endIdx)
    const liftedEnd: Point = [endPos[0], endPos[1] + dy, endPos[2]]

    // FITTING partner: a clean L. The existing ELBOW stays put and re-aims so
    // its mated collar swings vertical (flattening toward a straight coupling);
    // a plumb riser rises from that collar to a single new TOP elbow that turns
    // back along the run axis onto the lifted endpoint. A non-elbow fitting, or
    // an elbow whose re-aim falls out of the buildable range, can't form a clean
    // offset here → the whole lift is invalid (the caller freezes everything and
    // shows a red preview rather than dragging the network up to absorb it).
    if (conn.kind !== 'run') {
      const partner = nodesById[conn.nodeId]
      if (!partner || partner.type !== 'pipe-fitting') return { status: 'invalid' }
      const elbow = partner as PipeFittingNode
      const profilePatch = pipeElbowProfilePatch(profile)
      const profiledElbow = { ...elbow, ...profilePatch } as PipeFittingNode
      if (profiledElbow.fittingType !== 'elbow') {
        if (!hasOffsetRoom) return { status: 'invalid' }
        const partnerDir: Point = [
          partnerPort.direction[0],
          partnerPort.direction[1],
          partnerPort.direction[2],
        ]
        const leg = pipeFittingLegLength(profile.diameter)
        const pipeCollar: Point = [
          liftedEnd[0] - pipePortDir[0] * leg * 2,
          liftedEnd[1] - pipePortDir[1] * leg * 2,
          liftedEnd[2] - pipePortDir[2] * leg * 2,
        ]
        const bottom = planElbowFromCollar({
          collarPoint: endPos,
          elbowPortOutDir: [-partnerDir[0], -partnerDir[1], -partnerDir[2]],
          awayDir: up,
          pipe,
          profile,
        })
        const top = planElbowFromCollar({
          collarPoint: pipeCollar,
          elbowPortOutDir: [-pipePortDir[0], -pipePortDir[1], -pipePortDir[2]],
          awayDir: down,
          pipe,
          profile,
        })
        if (!bottom || !top) return { status: 'invalid' }

        fittings.push(bottom.fitting, top.fitting)
        risers.push(makePipeRiser(bottom.collarPoint, top.collarPoint, pipe))
        pipePath[endIdx] = top.trimmedPortPoint
        followPath[endIdx] = [...startPath[endIdx]!] as Point
        offsetAny = true
        continue
      }
      const riserAction = fittingRiserAction({
        fitting: profiledElbow,
        matedPortId: partnerPort.id,
        pipePortDir,
        pipeEndPoint: endPos,
        liftedPipePoint: liftedEnd,
        profilePatch,
        dy,
        connections,
        scenePorts,
        nodesById,
        eps2,
        allowEarlySnap: options.allowEarlySnap,
      })
      if (riserAction?.status === 'stretch') {
        offsetAny = true
        continue
      }
      if (riserAction?.status === 'collapse') {
        pipePath[endIdx] = riserAction.pipePoint
        followPath[endIdx] = [...startPath[endIdx]!] as Point
        updates.push(...riserAction.updates)
        deletes.push(...riserAction.delete)
        offsetAny = true
        continue
      }
      if (riserAction?.status === 'snap' && options.allowTransitionSnap) {
        return continueAfterTransitionSnap(args, routeDy, riserAction.dy, options)
      }
      if (riserAction?.status === 'snap') return { status: 'invalid' }
      // Minting the top elbow + riser needs elbow-leg + riser room.
      if (!hasOffsetRoom) return { status: 'invalid' }
      // Re-aim the existing elbow's mated collar to vertical; its other collar
      // (mated to the rest of the run) stays fixed.
      const realign = planPipeElbowRealign(profiledElbow, partnerPort.id, up)
      if (!realign) return { status: 'invalid' }
      // Top elbow: junction plumb above the elbow at the lifted height. Its
      // "existing run" is the riser, whose top port faces UP; the new run
      // leaves back along the run axis (awayBack) onto the lifted endpoint.
      const topJunction: Point = [elbow.position[0], liftedEnd[1], elbow.position[2]]
      const awayBack: Point = [-pipePortDir[0], -pipePortDir[1], -pipePortDir[2]]
      const top = planPipeElbowAtPort(
        portLike(topJunction, up, pipe.system),
        awayBack,
        profile.diameter,
        profile.pipeMaterial,
      )
      if (!top) return { status: 'invalid' }

      fittings.push(top.fitting)
      // Plumb riser: the re-aimed elbow's vertical collar up to the top elbow's
      // riser collar (its trimmedPortPoint) — both at the elbow's XZ.
      risers.push(makePipeRiser(realign.collarPoint, top.trimmedPortPoint, pipe))
      // Re-aim patch for the existing elbow.
      updates.push({
        id: elbow.id,
        data: { ...profilePatch, ...realign.update.data } as Partial<AnyNode>,
      })
      // Lifted run ends on the top elbow's outlet collar (= the lifted
      // endpoint); zero this end's connectivity-follow delta — the re-aim
      // already reconnects it.
      pipePath[endIdx] = top.collarPoint
      followPath[endIdx] = [...startPath[endIdx]!] as Point
      offsetAny = true
      continue
    }

    const partnerDir: Point = [
      partnerPort.direction[0],
      partnerPort.direction[1],
      partnerPort.direction[2],
    ]
    const profilePatch = pipeElbowProfilePatch(profile)

    // Aligned vertical riser: the partner run is already plumb and the lift runs
    // along its axis, so there's nothing to turn — just STRETCH it. We leave this
    // end of `followPath` lifted; connectivity-follow decomposes the Y-lift into
    // the riser's (parallel) axis and slides the shared joint up while the far
    // end stays put. No elbows, no new riser, no partner trim. Works at any lift
    // height (a stretch needs no elbow-leg room).
    const partner = nodesById[conn.nodeId]
    const partnerPath = (partner as unknown as { path?: Point[] } | undefined)?.path
    const riserStretch =
      partner?.type === 'pipe-segment'
        ? alignedVerticalRiserStretch(partnerPath, endPos, liftedEnd, eps2)
        : null
    if (riserStretch === 'invalid' && partner?.type === 'pipe-segment' && partnerPath) {
      const collapse = directRiserCollapseAction({
        riserId: conn.nodeId,
        riserPath: partnerPath,
        riserTopPoint: endPos,
        liftedPipePoint: liftedEnd,
        pipeEndPoint: endPos,
        pipePortDir,
        profilePatch,
        connections,
        scenePorts,
        nodesById,
        eps2,
        allowEarlySnap: options.allowEarlySnap,
      })
      if (collapse?.status === 'snap' && options.allowTransitionSnap) {
        return continueAfterTransitionSnap(args, routeDy, collapse.dy, options)
      }
      if (collapse?.status === 'collapse') {
        pipePath[endIdx] = collapse.pipePoint
        followPath[endIdx] = [...startPath[endIdx]!] as Point
        updates.push(...collapse.updates)
        deletes.push(...collapse.delete)
        offsetAny = true
        continue
      }
      return { status: 'invalid' }
    }
    if (riserStretch === 'invalid') return { status: 'invalid' }
    if (riserStretch === 'stretch') {
      // followPath[endIdx] stays at the lifted point (set above); pipePath too.
      offsetAny = true
      continue
    }

    // Minting an offset here needs elbow-leg + riser room. Without it this end
    // can't form a clean offset at this height → the whole lift is invalid.
    if (!hasOffsetRoom) return { status: 'invalid' }

    // Bottom elbow at the partner joint: turn from the partner's axis up the
    // riser. Partner run trims to its inlet collar.
    const bottom = planPipeElbowAtPort(
      portLike(endPos, partnerDir, pipe.system),
      up,
      profile.diameter,
      profile.pipeMaterial,
    )
    // Top elbow at the lifted endpoint: turn from the run axis down the
    // riser. Lifted run trims to its inlet collar.
    const top = planPipeElbowAtPort(
      portLike(liftedEnd, pipePortDir, pipe.system),
      down,
      profile.diameter,
      profile.pipeMaterial,
    )
    if (!bottom || !top) return { status: 'invalid' }

    fittings.push(bottom.fitting, top.fitting)
    risers.push(makePipeRiser(bottom.collarPoint, top.collarPoint, pipe))
    pipePath[endIdx] = top.trimmedPortPoint
    // This end's partner is trimmed (below), not dragged — keep its follow-seed
    // at the original height so connectivity-follow sees zero delta here.
    followPath[endIdx] = [...startPath[endIdx]!] as Point

    // Trim the partner run's mated end back one leg.
    const path = conn.startPath.map((p) => [...p] as Point)
    const tip = path.findIndex((p) => distSq(p, endPos) <= eps2)
    if (tip !== -1) {
      path[tip] = bottom.trimmedPortPoint
      updates.push({ id: conn.nodeId, data: { path } as Partial<AnyNode> })
    }
    offsetAny = true
  }

  // Every connected end either offset cleanly or there were none to offset.
  // `offsetAny` false here means all ends were open — nothing to weld, plain
  // translate. Otherwise a valid offset plan welds each connected end.
  if (!offsetAny) return null
  return {
    status: 'valid',
    plan: { dy, pipePath, followPath, fittings, risers, updates, delete: deletes },
  }
}

function continueAfterTransitionSnap(
  args: {
    pipe: PipeSegmentNode
    dy: number
    profile: PipeProfile
    connections: PortConnection[]
    scenePorts: ScenePort[]
    nodesById: Record<string, AnyNode>
  },
  requestedDy: number,
  snappedDy: number,
  options: TransitionOptions,
): VerticalOffsetResult {
  const snapped = planVerticalOffsetsAtDy(args, snappedDy, {
    allowTransitionSnap: false,
    allowEarlySnap: false,
    depth: options.depth,
  })
  if (snapped?.status !== 'valid') return snapped

  const remainingDy = requestedDy - snapped.plan.dy
  if (Math.abs(remainingDy) <= MIN_RISER_M || Math.sign(remainingDy) !== Math.sign(requestedDy)) {
    return snapped
  }
  if (options.depth >= MAX_TRANSITION_STEPS) return snapped

  const nodesById: Record<string, AnyNode> = { ...args.nodesById }
  for (const id of snapped.plan.delete ?? []) {
    delete nodesById[id]
  }
  for (const update of snapped.plan.updates) {
    const current = nodesById[update.id]
    if (current) nodesById[update.id] = { ...current, ...update.data } as AnyNode
  }
  for (const update of resolveExplicitFollowUpdates({
    pipe: args.pipe,
    followPath: snapped.plan.followPath,
    scenePorts: args.scenePorts,
    nodesById: args.nodesById,
  })) {
    const current = nodesById[update.id]
    if (current) nodesById[update.id] = { ...current, ...update.data } as AnyNode
  }
  for (const node of [...snapped.plan.fittings, ...snapped.plan.risers]) {
    nodesById[node.id as AnyNodeId] = node as AnyNode
  }

  const collapsedPipe = PipeSegmentNode.parse({
    ...args.pipe,
    path: snapped.plan.pipePath,
  })
  nodesById[collapsedPipe.id as AnyNodeId] = collapsedPipe as AnyNode
  const scenePorts = collectPipePortsFromNodes(nodesById, collapsedPipe.id as AnyNodeId)
  const connections = collectPipeConnections(collapsedPipe, nodesById, scenePorts)
  const continued = planVerticalOffsetsAtDy(
    {
      ...args,
      pipe: collapsedPipe,
      dy: remainingDy,
      connections,
      scenePorts,
      nodesById,
    },
    remainingDy,
    {
      allowTransitionSnap: true,
      allowEarlySnap: false,
      depth: options.depth + 1,
    },
  )

  if (continued?.status !== 'valid') return snapped
  return {
    status: 'valid',
    plan: {
      dy: snapped.plan.dy + continued.plan.dy,
      pipePath: continued.plan.pipePath,
      followPath: continued.plan.followPath,
      fittings: [...snapped.plan.fittings, ...continued.plan.fittings],
      risers: [...snapped.plan.risers, ...continued.plan.risers],
      updates: mergeUpdates([...snapped.plan.updates, ...continued.plan.updates]),
      delete: uniqueNodeIds([...(snapped.plan.delete ?? []), ...(continued.plan.delete ?? [])]),
    },
  }
}