flow like the river

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

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import type { ASTVisitor } from '../../language/visitor';
+
+import type { ASTValidationContext } from '../ValidationContext';
+
+/**
+ * Unique input field names
+ *
+ * A GraphQL input object value is only valid if all supplied fields are
+ * uniquely named.
+ */
+export function UniqueInputFieldNamesRule(
+  context: ASTValidationContext,
+): ASTVisitor {
+  const knownNameStack = [];
+  let knownNames = Object.create(null);
+
+  return {
+    ObjectValue: {
+      enter() {
+        knownNameStack.push(knownNames);
+        knownNames = Object.create(null);
+      },
+      leave() {
+        knownNames = knownNameStack.pop();
+      },
+    },
+    ObjectField(node) {
+      const fieldName = node.name.value;
+      if (knownNames[fieldName]) {
+        context.reportError(
+          new GraphQLError(
+            `There can be only one input field named "${fieldName}".`,
+            [knownNames[fieldName], node.name],
+          ),
+        );
+      } else {
+        knownNames[fieldName] = node.name;
+      }
+    },
+  };
+}