#69 : Split fns

src/utils/lazy.js

@@ -0,0 +1,61 @@
+import { getValueInRange } from './math'
+
+export function getIndexesOfPagesWithoutClonesInScroll({
+  scrollIndex,
+  pagesToShow,
+  pagesToScroll,
+  pagesCount,
+}) {
+  const overlap = scrollIndex === 0 ? 0 : pagesToShow - pagesToScroll
+  const from = scrollIndex * pagesToShow - scrollIndex * overlap
+  const to = from + Math.max(pagesToShow, pagesToScroll) - 1
+  const indexes = []
+  for (let i=from; i<=Math.min(pagesCount - 1, to); i++) {
+    indexes.push(i)
+  }
+  return indexes
+}
+
+export function getAdjacentIndexes({
+  infinite,
+  scrollIndex,
+  scrollsCount,
+  pagesCount,
+  pagesToShow,
+  pagesToScroll,
+}) {
+  const _scrollIndex = getValueInRange(0, scrollIndex, scrollsCount - 1)
+
+  let rangeStart = _scrollIndex - 1
+  let rangeEnd = _scrollIndex + 1
+
+  rangeStart = infinite
+    ? rangeStart < 0 ? scrollsCount - 1 : rangeStart
+    : Math.max(0, rangeStart)
+
+  rangeEnd = infinite
+    ? rangeEnd > scrollsCount - 1 ? 0 : rangeEnd
+    : Math.min(scrollsCount - 1, rangeEnd)
+
+  const scrollIndexes = [...new Set([
+    rangeStart,
+    _scrollIndex,
+    rangeEnd,
+
+    // because of these values outputs for infinite/non-infinites are the same
+    0, // needed to clone first scroll pages
+    scrollsCount - 1, // needed to clone last scroll pages
+  ])].sort((a, b) => a - b)
+  const pageIndexes = scrollIndexes.flatMap(
+    scrollIndex => getIndexesOfPagesWithoutClonesInScroll({
+      scrollIndex,
+      pagesToShow,
+      pagesToScroll,
+      pagesCount,
+    })
+  )
+  return {
+    scrollIndexes,
+    pageIndexes: [...new Set(pageIndexes)].sort((a, b) => a - b),
+  }
+}