export const VISIBLE_ROWS = 5
export const CENTER_OFFSET = (VISIBLE_ROWS - 1) / 2 // 2

/** Index de valeur logique (0..length-1) centré pour un scrollTop donné. */
export function valueIndexFromScroll(scrollTop: number, itemHeight: number, length: number): number {
  const flat = Math.round(scrollTop / itemHeight) + CENTER_OFFSET
  return ((flat % length) + length) % length
}

/** scrollTop qui centre l'index donné dans la copie du milieu (buffer à 3 copies). */
export function scrollTopForValueIndex(valueIndex: number, itemHeight: number, length: number): number {
  const flat = length + valueIndex - CENTER_OFFSET
  return flat * itemHeight
}

/** Recentre le scrollTop dans la copie du milieu [length, 2*length) si on a dérivé. */
export function loopCorrection(scrollTop: number, itemHeight: number, length: number): number {
  const block = length * itemHeight
  const centeredFlat = Math.round(scrollTop / itemHeight) + CENTER_OFFSET
  if (centeredFlat < length) return scrollTop + block
  if (centeredFlat >= 2 * length) return scrollTop - block
  return scrollTop
}