// Lazy materialization for the lanes===1 fast path. Backed by a
// Float64Array (stride 2: start, size, …); VirtualItems are constructed on
// first indexed read, replacing the stored key. Saves the per-item object
// allocation at large list counts where most items are never visible.

import type { VirtualItem } from './index'

export function getMeasurementKey(
  item: VirtualItem | VirtualItem['key'],
): VirtualItem['key'] {
  return typeof item === 'object' ? item.key : item
}

export function createLazyMeasurementsView(
  cache: Array<VirtualItem | VirtualItem['key']>,
  flat: Float64Array,
): Array<VirtualItem> {
  const count = cache.length
  return new Proxy(cache, {
    get(target, prop, receiver) {
      if (typeof prop === 'string') {
        // Cheap digit-prefix sniff before number coerce.
        const c = prop.charCodeAt(0)
        if (c >= 48 && c <= 57) {
          const i = +prop
          if (Number.isInteger(i) && i >= 0 && i < count) {
            let v = target[i]!
            if (typeof v !== 'object') {
              const s = flat[i * 2]!
              v = target[i] = {
                index: i,
                key: v,
                start: s,
                size: flat[i * 2 + 1]!,
                end: s + flat[i * 2 + 1]!,
                lane: 0,
              }
            }
            return v
          }
        }
        if (prop === 'length') return count
      }
      return Reflect.get(target, prop, receiver)
    },
  }) as Array<VirtualItem>
}