src/lib/sizes.js

// Output sizes: any side 128…1024 in steps of 16; these are the presets on the menu. The app
// shows only the ones inside the engine's limits (max side and area budget, App.svelte), so the
// list can run ahead of what a given build accepts. The examples' desktop and mobile sizes
// (examples.js) are all listed here.
export const SIZES = [
  [128, 128],
  [256, 256],
  [384, 384],
  [512, 512],
  [640, 640],
  [768, 768],
  [1024, 1024],
  [320, 192],
  [192, 320],
  [480, 288],
  [288, 480],
  [384, 256],
  [256, 384],
  [480, 320],
  [320, 480],
  [512, 256],
  [256, 512],
  [448, 256],
  [256, 448],
  [768, 512],
  [512, 768],
  [960, 640],
  [640, 960],
  [1024, 576],
  [576, 1024],
  [1024, 768],
  [768, 1024],
  [1024, 512],
  [512, 1024],
]
export const sizeKey = ([w, h]) => `${w}x${h}`
export const parseSize = (k) => k.split('x').map(Number)

// The preset closest to an image's aspect (then closest in area to the current one), within budget.
export function fitToAspect(w0, h0, sizes, current) {
  const want = Math.log(w0 / h0),
    cur = current[0] * current[1]
  let best = null
  for (const s of sizes) {
    const err = Math.abs(Math.log(s[0] / s[1]) - want),
      area = Math.abs(s[0] * s[1] - cur)
    if (!best || err < best.err - 1e-9 || (Math.abs(err - best.err) < 1e-9 && area < best.area))
      best = { s, err, area }
  }
  return best?.s ?? current
}

const gcd = (a, b) => (b ? gcd(b, a % b) : a)
export const ratioLabel = ([w, h]) => {
  const g = gcd(w, h)
  return `${w / g}:${h / g}`
}
export const orientation = ([w, h]) => (w === h ? 'Square' : w < h ? 'Portrait' : 'Landscape')

// Menu order: tallest first, widest last, smallest before largest within one shape. Listed that
// way the presets read as a fan, so "taller / wider / bigger" is answered by position alone.
export const byShape = (a, b) => a[0] * b[1] - b[0] * a[1] || a[0] * a[1] - b[0] * b[1]

// Longest side among a list of presets — the scale the menu's little rectangles are drawn
// against, so a glyph shows the real proportions *and* the real size next to its neighbours.
export const longestSide = (sizes) => Math.max(...sizes.flat(), 1)