/**
 * Sequence diagram layout.
 *
 * Participants get one column each, with lifelines running the full height and
 * a box repeated at top and bottom. Column gaps are solved from the widest
 * thing that has to fit between any two columns — a message label, a note, a
 * self-message stub — then items stack down the canvas in source order.
 */

import { Canvas, D, drawTextOverEdges, L, R, U } from './canvas.ts'
import { fitLabel, WRAP_WIDTH } from './labels.ts'
import { type CanvasResult, drawBox, type Placed } from './layout.ts'
import type { NoteAnchor, SeqItem, Sequence } from './parse.ts'
import { stringWidth } from './width.ts'

const PAD = 1
/** Minimum columns between adjacent lifelines. */
const SEQ_GAP = 5
const MAX_CANVAS_CELLS = 1 << 21

const sat = (a: number, b: number): number => Math.max(0, a - b)
const half = (n: number): number => Math.floor(n / 2)

/** Where a note box sits, given the lifeline positions. */
function noteGeometry(xs: number[], anchor: NoteAnchor, textW: number): { x: number; w: number } {
  if (anchor.kind === 'over') {
    const center = half(xs[anchor.from] + xs[anchor.to])
    const w = Math.max(xs[anchor.to] - xs[anchor.from] + 5, textW + 2 * PAD + 2)
    return { x: sat(center, half(w)), w }
  }
  const w = textW + 2 * PAD + 2
  if (anchor.kind === 'left') return { x: sat(xs[anchor.at], 2 + w - 1), w }
  return { x: xs[anchor.at] + 2, w }
}

const itemTextW = (text: string | null): number => (text === null ? 0 : stringWidth(text))

export function layoutSequence(seq: Sequence): CanvasResult {
  const n = seq.labels.length
  const labels = seq.labels.map((l) => fitLabel(l, WRAP_WIDTH))
  const boxW = labels.map((l) => Math.max(1, stringWidth(l)) + 2 * PAD + 2)
  const boxH = 3

  const gaps = Array.from({ length: sat(n, 1) }, (_, i) =>
    Math.max(SEQ_GAP, Math.ceil(boxW[i] / 2) + Math.ceil(boxW[i + 1] / 2) + 1),
  )

  // Each requirement is "columns l..r together need at least `need` cells".
  const reqs: [number, number, number][] = []
  for (const item of seq.items) {
    if (item.kind === 'message') {
      const tw = itemTextW(item.text)
      if (item.from !== item.to) {
        reqs.push([Math.min(item.from, item.to), Math.max(item.from, item.to), Math.max(tw + 2, 4)])
      } else if (item.from + 1 < n) {
        reqs.push([item.from, item.from + 1, 5 + tw + 2])
      }
    } else if (item.kind === 'note') {
      const tw = stringWidth(item.text)
      const a = item.anchor
      if (a.kind === 'over' && a.from < a.to) {
        reqs.push([a.from, a.to, sat(tw, 1)])
      } else if (a.kind === 'over') {
        const need = Math.ceil((tw + 4) / 2) + 2
        if (a.from > 0) reqs.push([a.from - 1, a.from, need])
        if (a.from + 1 < n) reqs.push([a.from, a.from + 1, need])
      } else if (a.kind === 'left' && a.at > 0) {
        reqs.push([a.at - 1, a.at, tw + 7])
      } else if (a.kind === 'right' && a.at + 1 < n) {
        reqs.push([a.at, a.at + 1, tw + 7])
      }
    }
  }
  // Narrowest spans first, so a wide requirement absorbs what they already gave.
  reqs.sort((a, b) => a[1] - a[0] - (b[1] - b[0]))
  for (const [l, r, need] of reqs) {
    let cur = 0
    for (let i = l; i < r; i++) cur += gaps[i]
    if (cur < need) gaps[r - 1] += need - cur
  }

  const xs = new Array<number>(n)
  xs[0] = half(boxW[0])
  for (let i = 1; i < n; i++) xs[i] = xs[i - 1] + gaps[i - 1]

  let canvasW = xs[n - 1] + Math.ceil(boxW[n - 1] / 2) + 1
  for (const item of seq.items) {
    if (item.kind === 'message' && item.from === item.to) {
      canvasW = Math.max(canvasW, xs[item.from] + 5 + itemTextW(item.text) + 1)
    } else if (item.kind === 'note') {
      const g = noteGeometry(xs, item.anchor, stringWidth(item.text))
      canvasW = Math.max(canvasW, g.x + g.w + 1)
    } else if (item.kind === 'divider') {
      canvasW = Math.max(canvasW, stringWidth(item.text) + 4)
    }
  }

  const rows: number[] = []
  let y = boxH + 1
  for (const item of seq.items) {
    rows.push(y)
    y += rowHeight(item)
  }
  const bottomTop = y
  const canvasH = bottomTop + boxH

  if (canvasW * canvasH > MAX_CANVAS_CELLS) return null

  const canvas = new Canvas(canvasW, canvasH)

  for (let i = 0; i < n; i++) {
    for (const by of [0, bottomTop]) {
      drawBox(canvas, box(sat(xs[i], half(boxW[i])), by, boxW[i], boxH), [labels[i]], 'rect')
    }
  }
  seq.items.forEach((item, k) => {
    if (item.kind !== 'note') return
    const g = noteGeometry(xs, item.anchor, stringWidth(item.text))
    drawBox(canvas, box(g.x, rows[k], g.w, 3), [item.text], 'rect')
  })

  for (const x of xs) {
    canvas.junction(x, boxH - 1, D)
    canvas.segV(x, boxH, bottomTop - 1)
    canvas.junction(x, bottomTop, U)
  }

  seq.items.forEach((item, k) => {
    const r = rows[k]
    if (item.kind === 'message') drawMessage(canvas, item, xs, r)
    else if (item.kind === 'divider') drawDivider(canvas, item.text, r, canvasW)
  })

  canvas.finalizeMask()
  return canvas
}

function rowHeight(item: SeqItem): number {
  if (item.kind === 'note') return 4
  if (item.kind === 'divider') return 2
  if (item.from === item.to) return 4
  return item.text !== null ? 3 : 2
}

/** Geometry for a box drawn by position and size; ranks are irrelevant here. */
const box = (x: number, y: number, w: number, h: number): Placed => ({
  x,
  y,
  w,
  h,
  cx: x + half(w),
  cy: y + 1,
  rank: 0,
})

function drawMessage(
  canvas: Canvas,
  item: Extract<SeqItem, { kind: 'message' }>,
  xs: number[],
  r: number,
): void {
  const lineCh = item.dashed ? '╌' : '─'

  if (item.from === item.to) {
    // A stub that leaves the lifeline and returns two rows down.
    const x = xs[item.from]
    canvas.junction(x, r, R)
    canvas.set(x + 1, r, lineCh, 'edge')
    canvas.set(x + 2, r, lineCh, 'edge')
    canvas.set(x + 3, r, '╮', 'edge')
    canvas.set(x + 3, r + 1, '│', 'edge')
    canvas.set(x + 1, r + 2, item.head === 'cross' ? '×' : '◄', 'edge')
    canvas.set(x + 2, r + 2, lineCh, 'edge')
    canvas.set(x + 3, r + 2, '╯', 'edge')
    if (item.text !== null) drawTextOverEdges(canvas, item.text, x + 5, r + 1, 'text')
    return
  }

  const x0 = xs[item.from]
  const x1 = xs[item.to]
  const rightward = x1 > x0
  // A labelled message writes its text on `r` and draws the arrow below it.
  const arrowRow = item.text !== null ? r + 1 : r
  const lo = Math.min(x0, x1)
  const hi = Math.max(x0, x1)

  canvas.junction(x0, arrowRow, rightward ? R : L)
  for (let x = lo + 1; x < hi; x++) canvas.set(x, arrowRow, lineCh, 'edge')
  const headCh = item.head === 'cross' ? '×' : rightward ? '▶' : '◄'
  canvas.set(rightward ? x1 - 1 : x1 + 1, arrowRow, headCh, 'edge')

  if (item.text !== null) {
    const span = hi - lo - 1
    const t = fitLabel(item.text, Math.max(1, span))
    drawTextOverEdges(canvas, t, lo + 1 + half(sat(span, stringWidth(t))), r, 'text')
  }
}

/** A full-width rule labelling a `loop` / `alt` / `opt` block boundary. */
function drawDivider(canvas: Canvas, text: string, r: number, canvasW: number): void {
  for (let x = 0; x < canvasW; x++) canvas.set(x, r, '─', 'edge')
  drawTextOverEdges(canvas, ` ${fitLabel(text, sat(canvasW, 4))} `, 2, r, 'edgeLabel')
}