const ATTACHED_FLAG = "svgDownloadAttached"
const SVG_NS = "http://www.w3.org/2000/svg"

const TABLER_DOWNLOAD_ICON = `<svg xmlns="${SVG_NS}" class="icon icon-tabler icon-tabler-download" width="14" height="14" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"/><path d="M4 17v2a2 2 0 0 0 2 2h12a2 2 0 0 0 2 -2v-2"/><path d="M7 11l5 5l5 -5"/><path d="M12 4l0 12"/></svg>`

const getSvgPixelSize = (svg: SVGSVGElement): { width: number; height: number } => {
  const viewBox = svg.viewBox?.baseVal
  if (viewBox && viewBox.width > 0 && viewBox.height > 0) {
    return { width: viewBox.width, height: viewBox.height }
  }
  const rect = svg.getBoundingClientRect()
  if (rect.width > 0 && rect.height > 0) {
    return { width: rect.width, height: rect.height }
  }
  return { width: 800, height: 600 }
}

const buildExportableSvgString = (svg: SVGSVGElement): string => {
  const clone = svg.cloneNode(true) as SVGSVGElement
  if (!clone.getAttribute("xmlns")) clone.setAttribute("xmlns", SVG_NS)
  if (!clone.getAttribute("xmlns:xlink"))
    clone.setAttribute("xmlns:xlink", "http://www.w3.org/1999/xlink")

  const { width, height } = getSvgPixelSize(svg)
  if (!clone.getAttribute("viewBox")) {
    clone.setAttribute("viewBox", `0 0 ${width} ${height}`)
  }
  clone.setAttribute("width", String(width))
  clone.setAttribute("height", String(height))

  const bg = document.createElementNS(SVG_NS, "rect")
  bg.setAttribute("x", "0")
  bg.setAttribute("y", "0")
  bg.setAttribute("width", "100%")
  bg.setAttribute("height", "100%")
  bg.setAttribute("fill", "#ffffff")
  clone.insertBefore(bg, clone.firstChild)

  return new XMLSerializer().serializeToString(clone)
}

const triggerBlobDownload = (blob: Blob, filename: string): void => {
  const url = URL.createObjectURL(blob)
  const a = document.createElement("a")
  a.href = url
  a.download = filename
  document.body.appendChild(a)
  a.click()
  a.remove()
  setTimeout(() => URL.revokeObjectURL(url), 1000)
}

const downloadAsSvg = (svg: SVGSVGElement, filename: string): void => {
  const xml = `<?xml version="1.0" encoding="UTF-8"?>\n${buildExportableSvgString(svg)}`
  const blob = new Blob([xml], { type: "image/svg+xml;charset=utf-8" })
  triggerBlobDownload(blob, filename)
}

const downloadAsPng = async (
  svg: SVGSVGElement,
  filename: string,
  scale = 2
): Promise<void> => {
  const { width, height } = getSvgPixelSize(svg)
  const svgString = buildExportableSvgString(svg)
  const svgBlob = new Blob([svgString], { type: "image/svg+xml;charset=utf-8" })
  const svgUrl = URL.createObjectURL(svgBlob)

  try {
    const img = new Image()
    img.crossOrigin = "anonymous"
    await new Promise<void>((resolve, reject) => {
      img.onload = () => resolve()
      img.onerror = () => reject(new Error("Failed to load SVG for PNG export"))
      img.src = svgUrl
    })

    const canvas = document.createElement("canvas")
    canvas.width = Math.max(1, Math.round(width * scale))
    canvas.height = Math.max(1, Math.round(height * scale))
    const ctx = canvas.getContext("2d")
    if (!ctx) throw new Error("Canvas 2D context unavailable")
    ctx.fillStyle = "#ffffff"
    ctx.fillRect(0, 0, canvas.width, canvas.height)
    ctx.drawImage(img, 0, 0, canvas.width, canvas.height)

    const pngBlob = await new Promise<Blob | null>((resolve) =>
      canvas.toBlob(resolve, "image/png")
    )
    if (!pngBlob) throw new Error("Failed to encode PNG")
    triggerBlobDownload(pngBlob, filename)
  } finally {
    URL.revokeObjectURL(svgUrl)
  }
}

const makeButton = (label: string, title: string, onClick: () => void): HTMLButtonElement => {
  const btn = document.createElement("button")
  btn.type = "button"
  btn.className = "btn btn-xs btn-neutral svg-download-button"
  btn.title = title
  btn.setAttribute("aria-label", title)
  btn.innerHTML = `${TABLER_DOWNLOAD_ICON}<span>${label}</span>`
  btn.addEventListener("click", (e) => {
    e.preventDefault()
    e.stopPropagation()
    onClick()
  })
  return btn
}

export const attachSvgDownloads = (scope: ParentNode | null | undefined): void => {
  if (!scope) return
  const svgs = scope.querySelectorAll<SVGSVGElement>("svg")
  let index = 0
  svgs.forEach((svg) => {
    if (svg.closest(".svg-download-button")) return
    if (svg.closest(".svg-download-host")) return

    const host = document.createElement("span")
    host.className = "svg-download-host"
    if (svg.dataset) host.dataset[ATTACHED_FLAG] = "true"

    const parent = svg.parentNode
    if (!parent) return
    parent.insertBefore(host, svg)
    host.appendChild(svg)

    const toolbar = document.createElement("div")
    toolbar.className = "svg-download-buttons"

    const n = index++
    const baseName = `diagram-${n + 1}`
    toolbar.appendChild(
      makeButton("SVG", "Download SVG", () =>
        downloadAsSvg(svg, `${baseName}.svg`)
      )
    )
    toolbar.appendChild(
      makeButton("PNG ×2", "Download PNG (2×)", () => {
        void downloadAsPng(svg, `${baseName}.png`, 2)
      })
    )
    host.appendChild(toolbar)
  })
}