src/lib/landing/scroll.ts

/**
 * Global scroll progress, kept OUTSIDE React state so the R3F render loop can
 * read it every frame (via getScroll()) without triggering re-renders. Lenis
 * writes to it; the 3D scene and any rAF consumer read from it.
 */

export interface ScrollState {
  /** 0–1 progress across the entire page. */
  progress: number
  /** Raw scroll offset in px. */
  scrollY: number
  /** Total scrollable height in px. */
  limit: number
  /** Viewport height in px. */
  viewport: number
}

const state: ScrollState = {
  progress: 0,
  scrollY: 0,
  limit: 1,
  viewport: typeof window !== 'undefined' ? window.innerHeight : 1,
}

export function setScroll(scrollY: number, limit: number) {
  state.scrollY = scrollY
  state.limit = Math.max(1, limit)
  state.progress = Math.min(1, Math.max(0, scrollY / state.limit))
  state.viewport = window.innerHeight
}

export function getScroll(): ScrollState {
  return state
}

/**
 * Progress (0–1) of a single section, given its index and the total number of
 * full-viewport sections. Lets the 3D scene know "how far through the hero / the
 * descent are we" independent of total page height.
 */
export function sectionProgress(
  sectionIndex: number,
  totalSections: number,
): number {
  const per = 1 / totalSections
  const start = sectionIndex * per
  const local = (state.progress - start) / per
  return Math.min(1, Math.max(0, local))
}