feat(app): better diff/code comments (#14621)

Co-authored-by: adamelmore <2363879+adamdottv@users.noreply.github.com>
Co-authored-by: David Hill <iamdavidhill@gmail.com>

packages/ui/src/pierre/diff-selection.ts

@@ -0,0 +1,71 @@
+import { type SelectedLineRange } from "@pierre/diffs"
+
+export type DiffSelectionSide = "additions" | "deletions"
+
+export function findDiffSide(node: HTMLElement): DiffSelectionSide {
+  const line = node.closest("[data-line], [data-alt-line]")
+  if (line instanceof HTMLElement) {
+    const type = line.dataset.lineType
+    if (type === "change-deletion") return "deletions"
+    if (type === "change-addition" || type === "change-additions") return "additions"
+  }
+
+  const code = node.closest("[data-code]")
+  if (!(code instanceof HTMLElement)) return "additions"
+  return code.hasAttribute("data-deletions") ? "deletions" : "additions"
+}
+
+export function diffLineIndex(split: boolean, node: HTMLElement) {
+  const raw = node.dataset.lineIndex
+  if (!raw) return
+
+  const values = raw
+    .split(",")
+    .map((x) => parseInt(x, 10))
+    .filter((x) => !Number.isNaN(x))
+  if (values.length === 0) return
+  if (!split) return values[0]
+  if (values.length === 2) return values[1]
+  return values[0]
+}
+
+export function diffRowIndex(root: ShadowRoot, split: boolean, line: number, side: DiffSelectionSide | undefined) {
+  const rows = Array.from(root.querySelectorAll(`[data-line="${line}"], [data-alt-line="${line}"]`)).filter(
+    (node): node is HTMLElement => node instanceof HTMLElement,
+  )
+  if (rows.length === 0) return
+
+  const target = side ?? "additions"
+  for (const row of rows) {
+    if (findDiffSide(row) === target) return diffLineIndex(split, row)
+    if (parseInt(row.dataset.altLine ?? "", 10) === line) return diffLineIndex(split, row)
+  }
+}
+
+export function fixDiffSelection(root: ShadowRoot | undefined, range: SelectedLineRange | null) {
+  if (!range) return range
+  if (!root) return
+
+  const diffs = root.querySelector("[data-diff]")
+  if (!(diffs instanceof HTMLElement)) return
+
+  const split = diffs.dataset.diffType === "split"
+  const start = diffRowIndex(root, split, range.start, range.side)
+  const end = diffRowIndex(root, split, range.end, range.endSide ?? range.side)
+
+  if (start === undefined || end === undefined) {
+    if (root.querySelector("[data-line], [data-alt-line]") == null) return
+    return null
+  }
+  if (start <= end) return range
+
+  const side = range.endSide ?? range.side
+  const swapped: SelectedLineRange = {
+    start: range.end,
+    end: range.start,
+  }
+
+  if (side) swapped.side = side
+  if (range.endSide && range.side) swapped.endSide = range.side
+  return swapped
+}