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package graphql.execution;

import graphql.ExecutionResult;
import graphql.execution.instrumentation.Instrumentation;
import graphql.execution.instrumentation.InstrumentationContext;
import graphql.execution.instrumentation.parameters.InstrumentationExecutionStrategyParameters;
import graphql.language.Field;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CompletableFuture;
import java.util.function.BiConsumer;

/**
 * Async non-blocking execution, but serial: only one field at the the time will be resolved.
 * See {@link AsyncExecutionStrategy} for a non serial (parallel) execution of every field.
 */
public class AsyncSerialExecutionStrategy extends AbstractAsyncExecutionStrategy {

    public AsyncSerialExecutionStrategy() {
        super(new SimpleDataFetcherExceptionHandler());
    }

    public AsyncSerialExecutionStrategy(DataFetcherExceptionHandler exceptionHandler) {
        super(exceptionHandler);
    }

    @Override
    public CompletableFuture execute(ExecutionContext executionContext, ExecutionStrategyParameters parameters) throws NonNullableFieldWasNullException {

        Instrumentation instrumentation = executionContext.getInstrumentation();
        InstrumentationExecutionStrategyParameters instrumentationParameters = new InstrumentationExecutionStrategyParameters(executionContext);
        InstrumentationContext> executionStrategyCtx = instrumentation.beginExecutionStrategy(instrumentationParameters);
        Map> fields = parameters.fields();
        List fieldNames = new ArrayList<>(fields.keySet());

        InstrumentationContext>> beginFieldsCtx = instrumentation.beginFields(instrumentationParameters);

        CompletableFuture> resultsFuture = Async.eachSequentially(fieldNames, (fieldName, index, prevResults) -> {
            List currentField = fields.get(fieldName);
            ExecutionPath fieldPath = parameters.path().segment(fieldName);
            ExecutionStrategyParameters newParameters = parameters
                    .transform(builder -> builder.field(currentField).path(fieldPath));
            return resolveField(executionContext, newParameters);
        });

        beginFieldsCtx.onEnd(fields, null);

        CompletableFuture overallResult = new CompletableFuture<>();
        BiConsumer, Throwable> listThrowableBiConsumer = handleResults(executionContext, fieldNames, overallResult);
        resultsFuture.whenComplete(listThrowableBiConsumer);

        executionStrategyCtx.onEnd(overallResult, null);
        return overallResult;
    }

}