flow like the river

VISUALIZACION/node_modules/graphql/validation/rules/UniqueDirectivesPerLocationRule.js.flow

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+// @flow strict
+import { GraphQLError } from '../../error/GraphQLError';
+
+import { Kind } from '../../language/kinds';
+import type { ASTVisitor } from '../../language/visitor';
+import {
+  isTypeDefinitionNode,
+  isTypeExtensionNode,
+} from '../../language/predicates';
+
+import { specifiedDirectives } from '../../type/directives';
+
+import type {
+  SDLValidationContext,
+  ValidationContext,
+} from '../ValidationContext';
+
+/**
+ * Unique directive names per location
+ *
+ * A GraphQL document is only valid if all non-repeatable directives at
+ * a given location are uniquely named.
+ */
+export function UniqueDirectivesPerLocationRule(
+  context: ValidationContext | SDLValidationContext,
+): ASTVisitor {
+  const uniqueDirectiveMap = Object.create(null);
+
+  const schema = context.getSchema();
+  const definedDirectives = schema
+    ? schema.getDirectives()
+    : specifiedDirectives;
+  for (const directive of definedDirectives) {
+    uniqueDirectiveMap[directive.name] = !directive.isRepeatable;
+  }
+
+  const astDefinitions = context.getDocument().definitions;
+  for (const def of astDefinitions) {
+    if (def.kind === Kind.DIRECTIVE_DEFINITION) {
+      uniqueDirectiveMap[def.name.value] = !def.repeatable;
+    }
+  }
+
+  const schemaDirectives = Object.create(null);
+  const typeDirectivesMap = Object.create(null);
+
+  return {
+    // Many different AST nodes may contain directives. Rather than listing
+    // them all, just listen for entering any node, and check to see if it
+    // defines any directives.
+    enter(node) {
+      if (node.directives == null) {
+        return;
+      }
+
+      let seenDirectives;
+      if (
+        node.kind === Kind.SCHEMA_DEFINITION ||
+        node.kind === Kind.SCHEMA_EXTENSION
+      ) {
+        seenDirectives = schemaDirectives;
+      } else if (isTypeDefinitionNode(node) || isTypeExtensionNode(node)) {
+        const typeName = node.name.value;
+        seenDirectives = typeDirectivesMap[typeName];
+        if (seenDirectives === undefined) {
+          typeDirectivesMap[typeName] = seenDirectives = Object.create(null);
+        }
+      } else {
+        seenDirectives = Object.create(null);
+      }
+
+      for (const directive of node.directives) {
+        const directiveName = directive.name.value;
+
+        if (uniqueDirectiveMap[directiveName]) {
+          if (seenDirectives[directiveName]) {
+            context.reportError(
+              new GraphQLError(
+                `The directive "@${directiveName}" can only be used once at this location.`,
+                [seenDirectives[directiveName], directive],
+              ),
+            );
+          } else {
+            seenDirectives[directiveName] = directive;
+          }
+        }
+      }
+    },
+  };
+}