Editor: Vega-Lite expression intelligence; single-home render errors

src/core/expr-validate.ts

@@ -85,6 +85,63 @@ export function referencedFields(expr: string): string[] {
   return fields;
 }
 
+/** The function call a cursor sits inside, and which argument it is on. */
+export interface ActiveCall {
+  /** The called function's name (the identifier before the open paren). */
+  name: string;
+  /** Zero-based index of the argument the cursor is in (commas seen so far). */
+  activeParam: number;
+}
+
+/**
+ * Given the expression text from its start up to the cursor, the innermost
+ * function call the cursor sits inside — its name and the argument index — or
+ * `null` when the cursor is not within a call. Used to drive editor signature
+ * help. A forward scan keeps a stack of bracket frames (parens and square
+ * brackets), skips string literals, and counts commas per frame; the nearest
+ * unclosed frame whose open paren follows an identifier is the active call, and
+ * that frame's comma count is the active argument.
+ */
+export function activeCall(prefix: string): ActiveCall | null {
+  interface Frame {
+    name: string | null;
+    commas: number;
+  }
+  const stack: Frame[] = [];
+  for (let i = 0; i < prefix.length; i++) {
+    const c = prefix[i];
+    if (c === '\\') {
+      i++; // an escape consumes the next character
+      continue;
+    }
+    if (c === '"' || c === "'") {
+      // Skip a string literal so its parens/commas don't disturb the scan.
+      const quote = c;
+      i++;
+      while (i < prefix.length && prefix[i] !== quote) {
+        if (prefix[i] === '\\') i++;
+        i++;
+      }
+      continue;
+    }
+    if (c === '(') {
+      const name = /([A-Za-z_$][A-Za-z0-9_$]*)\s*$/.exec(prefix.slice(0, i));
+      stack.push({ name: name ? name[1] : null, commas: 0 });
+    } else if (c === '[') {
+      stack.push({ name: null, commas: 0 });
+    } else if (c === ')' || c === ']') {
+      stack.pop();
+    } else if (c === ',' && stack.length > 0) {
+      stack[stack.length - 1].commas++;
+    }
+  }
+  for (let i = stack.length - 1; i >= 0; i--) {
+    const frame = stack[i];
+    if (frame.name !== null) return { name: frame.name, activeParam: frame.commas };
+  }
+  return null;
+}
+
 /**
  * The static field name of a `datum.<name>` / `datum['name']` member access, or
  * `null` when the node isn't such an access (a different object, computed-dynamic