#import "@preview/cetz:0.2.2": canvas, draw

#let X-SPACE = 2
#let Y-SPACE = 20

#let get-participants-i(participants) = {
  let pars-i = (:)
  for (i, p) in participants.enumerate() {
    pars-i.insert(p.name, i)
  }
  return pars-i
}

#let get-columns-width(participants, elements) = {
  let pars-i = get-participants-i(participants)
  let cells = ()
  for elmt in elements {
    if elmt.type == "seq" {
      let com = if elmt.comment == none {""} else {elmt.comment}
      let i1 = pars-i.at(elmt.p1)
      let i2 = pars-i.at(elmt.p2)
      cells.push(
        (
          elmt: elmt,
          i1: calc.min(i1, i2),
          i2: calc.max(i1, i2),
          cell: box(com, inset: 3pt)
        )
      )
    }
  }

  let widths = participants.slice(0, -1).map(_ => 0)
  for cell in cells.filter(c => c.i2 - c.i1 == 1) {
    let m = measure(cell.cell)
    widths.at(cell.i1) = calc.max(
      widths.at(cell.i1),
      m.width / 1pt
    )
  }
  return widths
}

#let render(participants, elements) = context canvas(length: 1pt, {
  let pars-i = get-participants-i(participants)

  let widths = get-columns-width(participants, elements)

  let x-pos = (0,)
  for width in widths {
    x-pos.push(x-pos.last() + width)
  }
  
  // Draw participants
  for (i, p) in participants.enumerate() {
    draw.content(
      (x-pos.at(i), 0),
      p.display-name,
      name: p.name,
      frame: "rect",
      padding: (5pt, 3pt),
      anchor: "south"
    )
  }

  let y = -Y-SPACE
  // Draw sequences
  for elmt in elements {
    if elmt.type == "seq" {
      let x1 = x-pos.at(pars-i.at(elmt.p1))
      let x2 = x-pos.at(pars-i.at(elmt.p2))
      let style = (
        mark: (end: "straight"),
        stroke: (
          dash: if elmt.dashed {"dashed"} else {"solid"},
          paint: elmt.color
        )
      )

      draw.line(
        (x1, y),
        (x2, y),
        ..style
      )
      if elmt.comment != none {
        draw.content(
          (calc.min(x1, x2), y),
          elmt.comment,
          anchor: "south-west",
          padding: 3pt
        )
      }
      y -= Y-SPACE
    }
  }

  // Draw vertical lines + end participants
  draw.on-layer(-1, {
    for (i, p) in participants.enumerate() {
      let x = x-pos.at(i)
      draw.line(
        (x, 0),
        (x, y),
        stroke: (dash: "dashed", paint: gray.darken(40%))
      )
      draw.content(
        (x, y),
        p.display-name,
        name: p.name,
        frame: "rect",
        padding: (5pt, 3pt),
        anchor: "north"
      )
    }
  })
})