1 files changed, 87 insertions, 0 deletions

diff --git a/subprojects/frontend/src/language/indentation.ts b/subprojects/frontend/src/language/indentation.ts
new file mode 100644
index 00000000..6d36ed3b
--- /dev/null
+++ b/subprojects/frontend/src/language/indentation.ts
@@ -0,0 +1,87 @@
+import { TreeIndentContext } from '@codemirror/language';
+
+/**
+ * Finds the `from` of first non-skipped token, if any,
+ * after the opening keyword in the first line of the declaration.
+ *
+ * Based on
+ * https://github.com/codemirror/language/blob/cd7f7e66fa51ddbce96cf9396b1b6127d0ca4c94/src/indent.ts#L246
+ *
+ * @param context the indentation context
+ * @returns the alignment or `null` if there is no token after the opening keyword
+ */
+function findAlignmentAfterOpening(context: TreeIndentContext): number | null {
+  const {
+    node: tree,
+    simulatedBreak,
+  } = context;
+  const openingToken = tree.childAfter(tree.from);
+  if (openingToken === null) {
+    return null;
+  }
+  const openingLine = context.state.doc.lineAt(openingToken.from);
+  const lineEnd = simulatedBreak == null || simulatedBreak <= openingLine.from
+    ? openingLine.to
+    : Math.min(openingLine.to, simulatedBreak);
+  const { cursor } = openingToken;
+  while (cursor.next() && cursor.from < lineEnd) {
+    if (!cursor.type.isSkipped) {
+      return cursor.from;
+    }
+  }
+  return null;
+}
+
+/**
+ * Indents text after declarations by a single unit if it begins on a new line,
+ * otherwise it aligns with the text after the declaration.
+ *
+ * Based on
+ * https://github.com/codemirror/language/blob/cd7f7e66fa51ddbce96cf9396b1b6127d0ca4c94/src/indent.ts#L275
+ *
+ * @example
+ * Result with no hanging indent (indent unit = 2 spaces, units = 1):
+ * ```
+ * scope
+ *   Family = 1,
+ *   Person += 5..10.
+ * ```
+ *
+ * @example
+ * Result with hanging indent:
+ * ```
+ * scope Family = 1,
+ *       Person += 5..10.
+ * ```
+ *
+ * @param context the indentation context
+ * @param units the number of units to indent
+ * @returns the desired indentation level
+ */
+function indentDeclarationStrategy(context: TreeIndentContext, units: number): number {
+  const alignment = findAlignmentAfterOpening(context);
+  if (alignment !== null) {
+    return context.column(alignment);
+  }
+  return context.baseIndent + units * context.unit;
+}
+
+export function indentBlockComment(): number {
+  // Do not indent.
+  return -1;
+}
+
+export function indentDeclaration(context: TreeIndentContext): number {
+  return indentDeclarationStrategy(context, 1);
+}
+
+export function indentPredicateOrRule(context: TreeIndentContext): number {
+  const clauseIndent = indentDeclarationStrategy(context, 1);
+  if (/^\s+[;.]/.exec(context.textAfter) !== null) {
+    return clauseIndent - 2;
+  }
+  if (/^\s+(~>)/.exec(context.textAfter) !== null) {
+    return clauseIndent - 3;
+  }
+  return clauseIndent;
+}