flow like the river

BACK_BACK/node_modules/graphql/validation/rules/UniqueDirectiveNamesRule.js.flow

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { SDLValidationContext } from '../ValidationContext';
+
+/**
+ * Unique directive names
+ *
+ * A GraphQL document is only valid if all defined directives have unique names.
+ */
+export function UniqueDirectiveNamesRule(
+  context: SDLValidationContext,
+): ASTVisitor {
+  const knownDirectiveNames = Object.create(null);
+  const schema = context.getSchema();
+
+  return {
+    DirectiveDefinition(node) {
+      const directiveName = node.name.value;
+
+      if (schema?.getDirective(directiveName)) {
+        context.reportError(
+          new GraphQLError(
+            `Directive "@${directiveName}" already exists in the schema. It cannot be redefined.`,
+            node.name,
+          ),
+        );
+        return;
+      }
+
+      if (knownDirectiveNames[directiveName]) {
+        context.reportError(
+          new GraphQLError(
+            `There can be only one directive named "@${directiveName}".`,
+            [knownDirectiveNames[directiveName], node.name],
+          ),
+        );
+      } else {
+        knownDirectiveNames[directiveName] = node.name;
+      }
+
+      return false;
+    },
+  };
+}