/** @category Validation Rules */

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

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

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

/**
 * 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 {
  return {
    FragmentDefinition(fragment) {
      const usages = context.getVariableUsages(fragment);
      const argumentNameUsed = new Set<string>(
        usages.map(({ node }) => node.name.value),
      );
      const variableDefinitions = fragment.variableDefinitions ?? [];
      for (const varDef of variableDefinitions) {
        const argName = varDef.variable.name.value;
        if (!argumentNameUsed.has(argName)) {
          context.reportError(
            new GraphQLError(
              `Variable "$${argName}" is never used in fragment "${fragment.name.value}".`,
              { nodes: varDef },
            ),
          );
        }
      }
    },
    OperationDefinition(operation) {
      const usages = context.getRecursiveVariableUsages(operation);
      const operationVariableNameUsed = new Set<string>();
      for (const { node, fragmentVariableDefinition } of usages) {
        const varName = node.name.value;
        if (!fragmentVariableDefinition) {
          operationVariableNameUsed.add(varName);
        }
      }

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