/** @category Validation Rules */

import { GraphQLError } from '../../error/GraphQLError';

import type { VariableDefinitionNode } from '../../language/ast';
import type { ASTVisitor } from '../../language/visitor';

import type { ValidationContext } from '../ValidationContext';

/**
 * No unused variables
 *
 * A GraphQL operation is only valid if all variables defined by an operation
 * are used, either directly or within a spread fragment.
 *
 * See https://spec.graphql.org/draft/#sec-All-Variables-Used
 * @param context - The validation context used while checking the document.
 * @returns A visitor that reports validation errors for this rule.
 * @example
 * ```ts
 * import { buildSchema, parse, validate } from 'graphql';
 * import { NoUnusedVariablesRule } from 'graphql/validation';
 *
 * const schema = buildSchema(`
 *   type Query {
 *     field(arg: ID): String
 *     name: String
 *   }
 * `);
 *
 * const invalidDocument = parse(`
 *   query ($id: ID) { name }
 * `);
 * const invalidErrors = validate(schema, invalidDocument, [NoUnusedVariablesRule]);
 *
 * invalidErrors.length; // => 1
 *
 * const validDocument = parse(`
 *   query ($id: ID) { field(arg: $id) }
 * `);
 * const validErrors = validate(schema, validDocument, [NoUnusedVariablesRule]);
 *
 * validErrors; // => []
 * ```
 */
export function NoUnusedVariablesRule(context: ValidationContext): ASTVisitor {
  let variableDefs: Array<VariableDefinitionNode> = [];

  return {
    OperationDefinition: {
      enter() {
        variableDefs = [];
      },
      leave(operation) {
        const variableNameUsed = Object.create(null);
        const usages = context.getRecursiveVariableUsages(operation);

        for (const { node } of usages) {
          variableNameUsed[node.name.value] = true;
        }

        for (const variableDef of variableDefs) {
          const variableName = variableDef.variable.name.value;
          if (variableNameUsed[variableName] !== true) {
            context.reportError(
              new GraphQLError(
                operation.name
                  ? `Variable "$${variableName}" is never used in operation "${operation.name.value}".`
                  : `Variable "$${variableName}" is never used.`,
                { nodes: variableDef },
              ),
            );
          }
        }
      },
    },
    VariableDefinition(def) {
      variableDefs.push(def);
    },
  };
}