extract scroll code from renderer into hook

Signed-off-by: Tilman Vatteroth <tilman.vatteroth@tu-dortmund.de>

src/components/editor/scroll/hooks/use-user-scroll.ts

@@ -0,0 +1,41 @@
+import { RefObject, useCallback } from 'react'
+import { LineMarkerPosition } from '../../../markdown-renderer/types'
+import { ScrollState } from '../scroll-props'
+
+export const useUserScroll = (lineMarks: LineMarkerPosition[] | undefined, renderer: RefObject<HTMLElement>, onScroll: ((newScrollState: ScrollState) => void)|undefined): () => void =>
+  useCallback(() => {
+    if (!renderer.current || !lineMarks || lineMarks.length === 0 || !onScroll) {
+      return
+    }
+
+    const scrollTop = renderer.current.scrollTop
+
+    const lineMarksBeforeScrollTop = lineMarks.filter(lineMark => lineMark.position <= scrollTop)
+    if (lineMarksBeforeScrollTop.length === 0) {
+      return
+    }
+
+    const lineMarksAfterScrollTop = lineMarks.filter(lineMark => lineMark.position > scrollTop)
+    if (lineMarksAfterScrollTop.length === 0) {
+      return
+    }
+
+    const beforeLineMark = lineMarksBeforeScrollTop
+      .reduce((prevLineMark, currentLineMark) =>
+        prevLineMark.line >= currentLineMark.line ? prevLineMark : currentLineMark)
+
+    const afterLineMark = lineMarksAfterScrollTop
+      .reduce((prevLineMark, currentLineMark) =>
+        prevLineMark.line < currentLineMark.line ? prevLineMark : currentLineMark)
+
+    const componentHeight = afterLineMark.position - beforeLineMark.position
+    const distanceToBefore = scrollTop - beforeLineMark.position
+    const percentageRaw = (distanceToBefore / componentHeight)
+    const lineCount = afterLineMark.line - beforeLineMark.line
+    const line = Math.floor(lineCount * percentageRaw + beforeLineMark.line)
+    const lineHeight = componentHeight / lineCount
+    const innerScrolling = Math.floor((distanceToBefore % lineHeight) / lineHeight * 100)
+
+    const newScrollState: ScrollState = { firstLineInView: line, scrolledPercentage: innerScrolling }
+    onScroll(newScrollState)
+  }, [lineMarks, onScroll, renderer])