1 files changed, 115 insertions, 0 deletions

diff --git a/subprojects/frontend/src/language/folding.ts b/subprojects/frontend/src/language/folding.ts
new file mode 100644
index 00000000..5d51f796
--- /dev/null
+++ b/subprojects/frontend/src/language/folding.ts
@@ -0,0 +1,115 @@
+import { EditorState } from '@codemirror/state';
+import type { SyntaxNode } from '@lezer/common';
+
+export type FoldRange = { from: number, to: number };
+
+/**
+ * Folds a block comment between its delimiters.
+ *
+ * @param node the node to fold
+ * @returns the folding range or `null` is there is nothing to fold
+ */
+export function foldBlockComment(node: SyntaxNode): FoldRange {
+  return {
+    from: node.from + 2,
+    to: node.to - 2,
+  };
+}
+
+/**
+ * Folds a declaration after the first element if it appears on the opening line,
+ * otherwise folds after the opening keyword.
+ *
+ * @example
+ * First element on the opening line:
+ * ```
+ * scope Family = 1,
+ *       Person += 5..10.
+ * ```
+ * becomes
+ * ```
+ * scope Family = 1,[...].
+ * ```
+ *
+ * @example
+ * First element not on the opening line:
+ * ```
+ * scope Family
+ *       = 1,
+ *       Person += 5..10.
+ * ```
+ * becomes
+ * ```
+ * scope [...].
+ * ```
+ *
+ * @param node the node to fold
+ * @param state the editor state
+ * @returns the folding range or `null` is there is nothing to fold
+ */
+export function foldDeclaration(node: SyntaxNode, state: EditorState): FoldRange | null {
+  const { firstChild: open, lastChild: close } = node;
+  if (open === null || close === null) {
+    return null;
+  }
+  const { cursor } = open;
+  const lineEnd = state.doc.lineAt(open.from).to;
+  let foldFrom = open.to;
+  while (cursor.next() && cursor.from < lineEnd) {
+    if (cursor.type.name === ',') {
+      foldFrom = cursor.to;
+      break;
+    }
+  }
+  return {
+    from: foldFrom,
+    to: close.from,
+  };
+}
+
+/**
+ * Folds a node only if it has at least one sibling of the same type.
+ *
+ * The folding range will be the entire `node`.
+ *
+ * @param node the node to fold
+ * @returns the folding range or `null` is there is nothing to fold
+ */
+function foldWithSibling(node: SyntaxNode): FoldRange | null {
+  const { parent } = node;
+  if (parent === null) {
+    return null;
+  }
+  const { firstChild } = parent;
+  if (firstChild === null) {
+    return null;
+  }
+  const { cursor } = firstChild;
+  let nSiblings = 0;
+  while (cursor.nextSibling()) {
+    if (cursor.type === node.type) {
+      nSiblings += 1;
+    }
+    if (nSiblings >= 2) {
+      return {
+        from: node.from,
+        to: node.to,
+      };
+    }
+  }
+  return null;
+}
+
+export function foldWholeNode(node: SyntaxNode): FoldRange {
+  return {
+    from: node.from,
+    to: node.to,
+  };
+}
+
+export function foldConjunction(node: SyntaxNode): FoldRange | null {
+  if (node.parent?.type?.name === 'PredicateBody') {
+    return foldWithSibling(node);
+  }
+  return foldWholeNode(node);
+}