/*-
 * #%L
 * HAPI FHIR - Core Library
 * %%
 * Copyright (C) 2014 - 2024 Smile CDR, Inc.
 * %%
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * #L%
 */
package ca.uhn.fhir.util;

import ca.uhn.fhir.context.BaseRuntimeChildDefinition;
import ca.uhn.fhir.context.BaseRuntimeElementCompositeDefinition;
import ca.uhn.fhir.context.BaseRuntimeElementDefinition;
import ca.uhn.fhir.context.FhirContext;
import ca.uhn.fhir.context.RuntimeChildChoiceDefinition;
import ca.uhn.fhir.context.RuntimeResourceDefinition;
import ca.uhn.fhir.i18n.Msg;
import org.apache.commons.lang3.Validate;
import org.apache.commons.lang3.tuple.Triple;
import org.hl7.fhir.instance.model.api.IBase;
import org.hl7.fhir.instance.model.api.IBaseBackboneElement;
import org.hl7.fhir.instance.model.api.IBaseResource;
import org.hl7.fhir.instance.model.api.IPrimitiveType;
import org.slf4j.Logger;

import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static org.slf4j.LoggerFactory.getLogger;

public final class TerserUtil {

        public static final String FIELD_NAME_IDENTIFIER = "identifier";
        /**
         * Exclude for id, identifier and meta fields of a resource.
         */
        public static final Collection<String> IDS_AND_META_EXCLUDES =
                        Collections.unmodifiableSet(Stream.of("id", "identifier", "meta").collect(Collectors.toSet()));
        /**
         * Exclusion predicate for id, identifier, meta fields.
         */
        public static final Predicate<String> EXCLUDE_IDS_AND_META = new Predicate<String>() {
                @Override
                public boolean test(String s) {
                        return !IDS_AND_META_EXCLUDES.contains(s);
                }
        };
        /**
         * Exclusion predicate for id/identifier, meta and fields with empty values. This ensures that source / target resources,
         * empty source fields will not results in erasure of target fields.
         */
        public static final Predicate<Triple<BaseRuntimeChildDefinition, IBase, IBase>> EXCLUDE_IDS_META_AND_EMPTY =
                        new Predicate<Triple<BaseRuntimeChildDefinition, IBase, IBase>>() {
                                @Override
                                public boolean test(Triple<BaseRuntimeChildDefinition, IBase, IBase> theTriple) {
                                        if (!EXCLUDE_IDS_AND_META.test(theTriple.getLeft().getElementName())) {
                                                return false;
                                        }
                                        BaseRuntimeChildDefinition childDefinition = theTriple.getLeft();
                                        boolean isSourceFieldEmpty = childDefinition
                                                        .getAccessor()
                                                        .getValues(theTriple.getMiddle())
                                                        .isEmpty();
                                        return !isSourceFieldEmpty;
                                }
                        };
        /**
         * Exclusion predicate for keeping all fields.
         */
        public static final Predicate<String> INCLUDE_ALL = new Predicate<String>() {
                @Override
                public boolean test(String s) {
                        return true;
                }
        };

        private static final Logger ourLog = getLogger(TerserUtil.class);
        private static final String EQUALS_DEEP = "equalsDeep";

        private TerserUtil() {}

        /**
         * Given an Child Definition of `identifier`, a R4/DSTU3 Identifier, and a new resource, clone the identifier into that resources' identifier list if it is not already present.
         */
        public static void cloneIdentifierIntoResource(
                        FhirContext theFhirContext,
                        BaseRuntimeChildDefinition theIdentifierDefinition,
                        IBase theNewIdentifier,
                        IBaseResource theResourceToCloneInto) {
                // FHIR choice types - fields within fhir where we have a choice of ids
                BaseRuntimeElementCompositeDefinition<?> childIdentifierElementDefinition =
                                (BaseRuntimeElementCompositeDefinition<?>)
                                                theIdentifierDefinition.getChildByName(FIELD_NAME_IDENTIFIER);

                List<IBase> existingIdentifiers = getValues(theFhirContext, theResourceToCloneInto, FIELD_NAME_IDENTIFIER);
                if (existingIdentifiers != null) {
                        for (IBase existingIdentifier : existingIdentifiers) {
                                if (equals(existingIdentifier, theNewIdentifier)) {
                                        ourLog.trace(
                                                        "Identifier {} already exists in resource {}", theNewIdentifier, theResourceToCloneInto);
                                        return;
                                }
                        }
                }

                IBase newIdentifierBase = childIdentifierElementDefinition.newInstance();

                FhirTerser terser = theFhirContext.newTerser();
                terser.cloneInto(theNewIdentifier, newIdentifierBase, true);
                theIdentifierDefinition.getMutator().addValue(theResourceToCloneInto, newIdentifierBase);
        }

        /**
         * Checks if the specified fields has any values
         *
         * @param theFhirContext Context holding resource definition
         * @param theResource    Resource to check if the specified field is set
         * @param theFieldName   name of the field to check
         * @return Returns true if field exists and has any values set, and false otherwise
         */
        public static boolean hasValues(FhirContext theFhirContext, IBaseResource theResource, String theFieldName) {
                RuntimeResourceDefinition resourceDefinition = theFhirContext.getResourceDefinition(theResource);
                BaseRuntimeChildDefinition resourceIdentifier = resourceDefinition.getChildByName(theFieldName);
                if (resourceIdentifier == null) {
                        return false;
                }
                return !(resourceIdentifier.getAccessor().getValues(theResource).isEmpty());
        }

        /**
         * Gets all values of the specified field.
         *
         * @param theFhirContext Context holding resource definition
         * @param theResource    Resource to check if the specified field is set
         * @param theFieldName   name of the field to check
         * @return Returns all values for the specified field or null if field with the provided name doesn't exist
         */
        public static List<IBase> getValues(FhirContext theFhirContext, IBaseResource theResource, String theFieldName) {
                RuntimeResourceDefinition resourceDefinition = theFhirContext.getResourceDefinition(theResource);
                BaseRuntimeChildDefinition resourceIdentifier = resourceDefinition.getChildByName(theFieldName);
                if (resourceIdentifier == null) {
                        ourLog.info("There is no field named {} in Resource {}", theFieldName, resourceDefinition.getName());
                        return null;
                }
                return resourceIdentifier.getAccessor().getValues(theResource);
        }

        /**
         * Gets the first available value for the specified field.
         *
         * @param theFhirContext Context holding resource definition
         * @param theResource    Resource to check if the specified field is set
         * @param theFieldName   name of the field to check
         * @return Returns the first value for the specified field or null if field with the provided name doesn't exist or
         * has no values
         */
        public static IBase getValueFirstRep(FhirContext theFhirContext, IBaseResource theResource, String theFieldName) {
                List<IBase> values = getValues(theFhirContext, theResource, theFieldName);
                if (values == null || values.isEmpty()) {
                        return null;
                }
                return values.get(0);
        }

        /**
         * Clones specified composite field (collection). Composite field values must conform to the collections
         * contract.
         *
         * @param theFrom  Resource to clone the specified field from
         * @param theTo    Resource to clone the specified field to
         * @param theField Field name to be copied
         */
        public static void cloneCompositeField(
                        FhirContext theFhirContext, IBaseResource theFrom, IBaseResource theTo, String theField) {
                FhirTerser terser = theFhirContext.newTerser();

                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theFrom);
                BaseRuntimeChildDefinition childDefinition = definition.getChildByName(theField);
                Validate.notNull(childDefinition);

                List<IBase> theFromFieldValues = childDefinition.getAccessor().getValues(theFrom);
                List<IBase> theToFieldValues = childDefinition.getAccessor().getValues(theTo);

                for (IBase theFromFieldValue : theFromFieldValues) {
                        if (containsPrimitiveValue(theFromFieldValue, theToFieldValues)) {
                                continue;
                        }

                        IBase newFieldValue = newElement(terser, childDefinition, theFromFieldValue, null);
                        terser.cloneInto(theFromFieldValue, newFieldValue, true);

                        try {
                                theToFieldValues.add(newFieldValue);
                        } catch (Exception e) {
                                childDefinition.getMutator().setValue(theTo, newFieldValue);
                        }
                }
        }

        private static boolean containsPrimitiveValue(IBase theItem, List<IBase> theItems) {
                PrimitiveTypeEqualsPredicate predicate = new PrimitiveTypeEqualsPredicate();
                return theItems.stream().anyMatch(i -> {
                        return predicate.test(i, theItem);
                });
        }

        private static Method getMethod(IBase theBase, String theMethodName) {
                Method method = null;
                for (Method m : theBase.getClass().getDeclaredMethods()) {
                        if (m.getName().equals(theMethodName)) {
                                method = m;
                                break;
                        }
                }
                return method;
        }

        /**
         * Checks if two items are equal via {@link #EQUALS_DEEP} method
         *
         * @param theItem1 First item to compare
         * @param theItem2 Second item to compare
         * @return Returns true if they are equal and false otherwise
         */
        public static boolean equals(IBase theItem1, IBase theItem2) {
                if (theItem1 == null) {
                        return theItem2 == null;
                }

                final Method method = getMethod(theItem1, EQUALS_DEEP);
                Validate.notNull(method);
                return equals(theItem1, theItem2, method);
        }

        private static boolean equals(IBase theItem1, IBase theItem2, Method theMethod) {
                if (theMethod != null) {
                        try {
                                return (Boolean) theMethod.invoke(theItem1, theItem2);
                        } catch (Exception e) {
                                throw new RuntimeException(
                                                Msg.code(1746) + String.format("Unable to compare equality via %s", EQUALS_DEEP), e);
                        }
                }
                return theItem1.equals(theItem2);
        }

        private static boolean contains(IBase theItem, List<IBase> theItems) {
                final Method method = getMethod(theItem, EQUALS_DEEP);
                return theItems.stream().anyMatch(i -> equals(i, theItem, method));
        }

        /**
         * Merges all fields on the provided instance. <code>theTo</code> will contain a union of all values from <code>theFrom</code>
         * instance and <code>theTo</code> instance.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFrom        The resource to merge the fields from
         * @param theTo          The resource to merge the fields into
         */
        public static void mergeAllFields(FhirContext theFhirContext, IBaseResource theFrom, IBaseResource theTo) {
                mergeFields(theFhirContext, theFrom, theTo, INCLUDE_ALL);
        }

        /**
         * Replaces all fields that have matching field names by the given inclusion strategy. <code>theTo</code> will contain a copy of the
         * values from <code>theFrom</code> instance.
         *
         * @param theFhirContext        Context holding resource definition
         * @param theFrom               The resource to merge the fields from
         * @param theTo                 The resource to merge the fields into
         * @param theFieldNameInclusion Inclusion strategy that checks if a given field should be replaced
         */
        public static void replaceFields(
                        FhirContext theFhirContext,
                        IBaseResource theFrom,
                        IBaseResource theTo,
                        Predicate<String> theFieldNameInclusion) {
                Predicate<Triple<BaseRuntimeChildDefinition, IBase, IBase>> predicate =
                                (t) -> theFieldNameInclusion.test(t.getLeft().getElementName());
                replaceFieldsByPredicate(theFhirContext, theFrom, theTo, predicate);
        }

        /**
         * Replaces fields on theTo resource that test positive by the given predicate. <code>theTo</code> will contain a copy of the
         * values from <code>theFrom</code> for which predicate tests positive. Please note that composite fields will be replaced fully.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFrom        The resource to merge the fields from
         * @param theTo          The resource to merge the fields into
         * @param thePredicate   Predicate that checks if a given field should be replaced
         */
        public static void replaceFieldsByPredicate(
                        FhirContext theFhirContext,
                        IBaseResource theFrom,
                        IBaseResource theTo,
                        Predicate<Triple<BaseRuntimeChildDefinition, IBase, IBase>> thePredicate) {
                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theFrom);
                FhirTerser terser = theFhirContext.newTerser();
                for (BaseRuntimeChildDefinition childDefinition : definition.getChildrenAndExtension()) {
                        if (thePredicate.test(Triple.of(childDefinition, theFrom, theTo))) {
                                replaceField(terser, theFrom, theTo, childDefinition);
                        }
                }
        }

        /**
         * Checks if the field exists on the resource
         *
         * @param theFhirContext Context holding resource definition
         * @param theFieldName   Name of the field to check
         * @param theInstance    Resource instance to check
         * @return Returns true if resource definition has a child with the specified name and false otherwise
         */
        public static boolean fieldExists(FhirContext theFhirContext, String theFieldName, IBaseResource theInstance) {
                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theInstance);
                return definition.getChildByName(theFieldName) != null;
        }

        /**
         * Replaces the specified fields on <code>theTo</code> resource with the value from <code>theFrom</code> resource.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFrom        The resource to replace the field from
         * @param theTo          The resource to replace the field on
         */
        public static void replaceField(
                        FhirContext theFhirContext, String theFieldName, IBaseResource theFrom, IBaseResource theTo) {
                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theFrom);
                Validate.notNull(definition);
                replaceField(
                                theFhirContext.newTerser(),
                                theFrom,
                                theTo,
                                theFhirContext.getResourceDefinition(theFrom).getChildByName(theFieldName));
        }

        /**
         * Clears the specified field on the resource provided
         *
         * @param theFhirContext Context holding resource definition
         * @param theResource
         * @param theFieldName
         */
        public static void clearField(FhirContext theFhirContext, IBaseResource theResource, String theFieldName) {
                BaseRuntimeChildDefinition childDefinition =
                                getBaseRuntimeChildDefinition(theFhirContext, theFieldName, theResource);
                childDefinition.getMutator().setValue(theResource, null);
        }

        /**
         * Clears the specified field on the resource provided by the FHIRPath.  If more than one value matches
         * the FHIRPath, all values will be cleared.
         *
         * @param theFhirContext
         * @param theResource
         * @param theFhirPath
         */
        public static void clearFieldByFhirPath(FhirContext theFhirContext, IBaseResource theResource, String theFhirPath) {

                if (theFhirPath.contains(".")) {
                        String parentPath = theFhirPath.substring(0, theFhirPath.lastIndexOf("."));
                        String fieldName = theFhirPath.substring(theFhirPath.lastIndexOf(".") + 1);
                        FhirTerser terser = theFhirContext.newTerser();
                        List<IBase> parents = terser.getValues(theResource, parentPath);
                        for (IBase parent : parents) {
                                clearField(theFhirContext, fieldName, parent);
                        }
                } else {
                        clearField(theFhirContext, theResource, theFhirPath);
                }
        }

        /**
         * Clears the specified field on the element provided
         *
         * @param theFhirContext Context holding resource definition
         * @param theFieldName   Name of the field to clear values for
         * @param theBase        The element definition to clear values on
         */
        public static void clearField(FhirContext theFhirContext, String theFieldName, IBase theBase) {
                BaseRuntimeElementDefinition definition = theFhirContext.getElementDefinition(theBase.getClass());
                BaseRuntimeChildDefinition childDefinition = definition.getChildByName(theFieldName);
                Validate.notNull(childDefinition);
                BaseRuntimeChildDefinition.IAccessor accessor = childDefinition.getAccessor();
                clear(accessor.getValues(theBase));
                List<IBase> newValue = accessor.getValues(theBase);

                if (newValue != null && !newValue.isEmpty()) {
                        // Our clear failed, probably because it was an immutable SingletonList returned by a FieldPlainAccessor
                        // that cannot be cleared.
                        // Let's just null it out instead.
                        childDefinition.getMutator().setValue(theBase, null);
                }
        }

        /**
         * Sets the provided field with the given values. This method will add to the collection of existing field values
         * in case of multiple cardinality. Use {@link #clearField(FhirContext, IBaseResource, String)}
         * to remove values before setting
         *
         * @param theFhirContext Context holding resource definition
         * @param theFieldName   Child field name of the resource to set
         * @param theResource    The resource to set the values on
         * @param theValues      The values to set on the resource child field name
         */
        public static void setField(
                        FhirContext theFhirContext, String theFieldName, IBaseResource theResource, IBase... theValues) {
                setField(theFhirContext, theFhirContext.newTerser(), theFieldName, theResource, theValues);
        }

        /**
         * Sets the provided field with the given values. This method will add to the collection of existing field values
         * in case of multiple cardinality. Use {@link #clearField(FhirContext, IBaseResource, String)}
         * to remove values before setting
         *
         * @param theFhirContext Context holding resource definition
         * @param theTerser      Terser to be used when cloning field values
         * @param theFieldName   Child field name of the resource to set
         * @param theResource    The resource to set the values on
         * @param theValues      The values to set on the resource child field name
         */
        public static void setField(
                        FhirContext theFhirContext,
                        FhirTerser theTerser,
                        String theFieldName,
                        IBaseResource theResource,
                        IBase... theValues) {
                BaseRuntimeChildDefinition childDefinition =
                                getBaseRuntimeChildDefinition(theFhirContext, theFieldName, theResource);
                List<IBase> theFromFieldValues = childDefinition.getAccessor().getValues(theResource);
                if (theFromFieldValues.isEmpty()) {
                        for (IBase value : theValues) {
                                try {
                                        childDefinition.getMutator().addValue(theResource, value);
                                } catch (UnsupportedOperationException e) {
                                        ourLog.warn(
                                                        "Resource {} does not support multiple values, but an attempt to set {} was made. Setting the first item only",
                                                        theResource,
                                                        theValues);
                                        childDefinition.getMutator().setValue(theResource, value);
                                        break;
                                }
                        }
                        return;
                }
                List<IBase> theToFieldValues = Arrays.asList(theValues);
                mergeFields(theTerser, theResource, childDefinition, theFromFieldValues, theToFieldValues);
        }

        /**
         * Sets the specified value at the FHIR path provided.
         *
         * @param theTerser   The terser that should be used for cloning the field value.
         * @param theFhirPath The FHIR path to set the field at
         * @param theResource The resource on which the value should be set
         * @param theValue    The value to set
         */
        public static void setFieldByFhirPath(
                        FhirTerser theTerser, String theFhirPath, IBaseResource theResource, IBase theValue) {
                List<IBase> theFromFieldValues = theTerser.getValues(theResource, theFhirPath, true, false);
                for (IBase theFromFieldValue : theFromFieldValues) {
                        theTerser.cloneInto(theValue, theFromFieldValue, true);
                }
        }

        /**
         * Sets the specified value at the FHIR path provided.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFhirPath    The FHIR path to set the field at
         * @param theResource    The resource on which the value should be set
         * @param theValue       The value to set
         */
        public static void setFieldByFhirPath(
                        FhirContext theFhirContext, String theFhirPath, IBaseResource theResource, IBase theValue) {
                setFieldByFhirPath(theFhirContext.newTerser(), theFhirPath, theResource, theValue);
        }

        /**
         * Returns field values ant the specified FHIR path from the resource.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFhirPath    The FHIR path to get the field from
         * @param theResource    The resource from which the value should be retrieved
         * @return Returns the list of field values at the given FHIR path
         */
        public static List<IBase> getFieldByFhirPath(FhirContext theFhirContext, String theFhirPath, IBase theResource) {
                return theFhirContext.newTerser().getValues(theResource, theFhirPath, false, false);
        }

        /**
         * Returns the first available field value at the specified FHIR path from the resource.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFhirPath    The FHIR path to get the field from
         * @param theResource    The resource from which the value should be retrieved
         * @return Returns the first available value or null if no values can be retrieved
         */
        public static IBase getFirstFieldByFhirPath(FhirContext theFhirContext, String theFhirPath, IBase theResource) {
                List<IBase> values = getFieldByFhirPath(theFhirContext, theFhirPath, theResource);
                if (values == null || values.isEmpty()) {
                        return null;
                }
                return values.get(0);
        }

        private static void replaceField(
                        FhirTerser theTerser,
                        IBaseResource theFrom,
                        IBaseResource theTo,
                        BaseRuntimeChildDefinition childDefinition) {
                List<IBase> fromValues = childDefinition.getAccessor().getValues(theFrom);
                List<IBase> toValues = childDefinition.getAccessor().getValues(theTo);

                if (fromValues.isEmpty() && !toValues.isEmpty()) {
                        childDefinition.getMutator().setValue(theTo, null);
                } else if (fromValues != toValues) {
                        clear(toValues);

                        mergeFields(theTerser, theTo, childDefinition, fromValues, toValues);
                }
        }

        /**
         * Merges values of all fields except for "identifier" and "meta" from <code>theFrom</code> resource to
         * <code>theTo</code> resource. Fields values are compared via the equalsDeep method, or via object identity if this
         * method is not available.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFrom        Resource to merge the specified field from
         * @param theTo          Resource to merge the specified field into
         */
        public static void mergeFieldsExceptIdAndMeta(
                        FhirContext theFhirContext, IBaseResource theFrom, IBaseResource theTo) {
                mergeFields(theFhirContext, theFrom, theTo, EXCLUDE_IDS_AND_META);
        }

        /**
         * Merges values of all field from <code>theFrom</code> resource to <code>theTo</code> resource. Fields
         * values are compared via the equalsDeep method, or via object identity if this method is not available.
         *
         * @param theFhirContext    Context holding resource definition
         * @param theFrom           Resource to merge the specified field from
         * @param theTo             Resource to merge the specified field into
         * @param inclusionStrategy Predicate to test which fields should be merged
         */
        public static void mergeFields(
                        FhirContext theFhirContext,
                        IBaseResource theFrom,
                        IBaseResource theTo,
                        Predicate<String> inclusionStrategy) {
                FhirTerser terser = theFhirContext.newTerser();

                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theFrom);
                for (BaseRuntimeChildDefinition childDefinition : definition.getChildrenAndExtension()) {
                        if (!inclusionStrategy.test(childDefinition.getElementName())) {
                                continue;
                        }

                        List<IBase> theFromFieldValues = childDefinition.getAccessor().getValues(theFrom);
                        List<IBase> theToFieldValues = childDefinition.getAccessor().getValues(theTo);

                        mergeFields(terser, theTo, childDefinition, theFromFieldValues, theToFieldValues);
                }
        }

        /**
         * Merges value of the specified field from <code>theFrom</code> resource to <code>theTo</code> resource. Fields
         * values are compared via the equalsDeep method, or via object identity if this method is not available.
         *
         * @param theFhirContext Context holding resource definition
         * @param theFieldName   Name of the child filed to merge
         * @param theFrom        Resource to merge the specified field from
         * @param theTo          Resource to merge the specified field into
         */
        public static void mergeField(
                        FhirContext theFhirContext, String theFieldName, IBaseResource theFrom, IBaseResource theTo) {
                mergeField(theFhirContext, theFhirContext.newTerser(), theFieldName, theFrom, theTo);
        }

        /**
         * Merges value of the specified field from <code>theFrom</code> resource to <code>theTo</code> resource. Fields
         * values are compared via the equalsDeep method, or via object identity if this method is not available.
         *
         * @param theFhirContext Context holding resource definition
         * @param theTerser      Terser to be used when cloning the field values
         * @param theFieldName   Name of the child filed to merge
         * @param theFrom        Resource to merge the specified field from
         * @param theTo          Resource to merge the specified field into
         */
        public static void mergeField(
                        FhirContext theFhirContext,
                        FhirTerser theTerser,
                        String theFieldName,
                        IBaseResource theFrom,
                        IBaseResource theTo) {
                BaseRuntimeChildDefinition childDefinition =
                                getBaseRuntimeChildDefinition(theFhirContext, theFieldName, theFrom);

                List<IBase> theFromFieldValues = childDefinition.getAccessor().getValues(theFrom);
                List<IBase> theToFieldValues = childDefinition.getAccessor().getValues(theTo);

                mergeFields(theTerser, theTo, childDefinition, theFromFieldValues, theToFieldValues);
        }

        private static BaseRuntimeChildDefinition getBaseRuntimeChildDefinition(
                        FhirContext theFhirContext, String theFieldName, IBaseResource theFrom) {
                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theFrom);
                BaseRuntimeChildDefinition childDefinition = definition.getChildByName(theFieldName);
                Validate.notNull(childDefinition);
                return childDefinition;
        }

        /**
         * Creates a new element taking into consideration elements with choice that are not directly retrievable by element
         * name
         *
         * @param theFhirTerser
         * @param theChildDefinition  Child to create a new instance for
         * @param theFromFieldValue   The base parent field
         * @param theConstructorParam Optional constructor param
         * @return Returns the new element with the given value if configured
         */
        private static IBase newElement(
                        FhirTerser theFhirTerser,
                        BaseRuntimeChildDefinition theChildDefinition,
                        IBase theFromFieldValue,
                        Object theConstructorParam) {
                BaseRuntimeElementDefinition runtimeElementDefinition;
                if (theChildDefinition instanceof RuntimeChildChoiceDefinition) {
                        runtimeElementDefinition =
                                        theChildDefinition.getChildElementDefinitionByDatatype(theFromFieldValue.getClass());
                } else {
                        runtimeElementDefinition = theChildDefinition.getChildByName(theChildDefinition.getElementName());
                }
                if ("contained".equals(runtimeElementDefinition.getName())) {
                        IBaseResource sourceResource = (IBaseResource) theFromFieldValue;
                        return theFhirTerser.clone(sourceResource);
                } else if (theConstructorParam == null) {
                        return runtimeElementDefinition.newInstance();
                } else {
                        return runtimeElementDefinition.newInstance(theConstructorParam);
                }
        }

        private static void mergeFields(
                        FhirTerser theTerser,
                        IBaseResource theTo,
                        BaseRuntimeChildDefinition childDefinition,
                        List<IBase> theFromFieldValues,
                        List<IBase> theToFieldValues) {
                for (IBase theFromFieldValue : theFromFieldValues) {
                        if (contains(theFromFieldValue, theToFieldValues)) {
                                continue;
                        }

                        IBase newFieldValue = newElement(theTerser, childDefinition, theFromFieldValue, null);
                        if (theFromFieldValue instanceof IPrimitiveType) {
                                try {
                                        Method copyMethod = getMethod(theFromFieldValue, "copy");
                                        if (copyMethod != null) {
                                                newFieldValue = (IBase) copyMethod.invoke(theFromFieldValue, new Object[] {});
                                        }
                                } catch (Throwable t) {
                                        ((IPrimitiveType) newFieldValue)
                                                        .setValueAsString(((IPrimitiveType) theFromFieldValue).getValueAsString());
                                }
                        } else {
                                theTerser.cloneInto(theFromFieldValue, newFieldValue, true);
                        }

                        try {
                                theToFieldValues.add(newFieldValue);
                        } catch (UnsupportedOperationException e) {
                                childDefinition.getMutator().setValue(theTo, newFieldValue);
                                theToFieldValues = childDefinition.getAccessor().getValues(theTo);
                        }
                }
        }

        /**
         * Clones the specified resource.
         *
         * @param theFhirContext Context holding resource definition
         * @param theInstance    The instance to be cloned
         * @param <T>            Base resource type
         * @return Returns a cloned instance
         */
        public static <T extends IBaseResource> T clone(FhirContext theFhirContext, T theInstance) {
                RuntimeResourceDefinition definition = theFhirContext.getResourceDefinition(theInstance.getClass());
                T retVal = (T) definition.newInstance();

                FhirTerser terser = theFhirContext.newTerser();
                terser.cloneInto(theInstance, retVal, true);
                return retVal;
        }

        /**
         * Creates a new element instance
         *
         * @param theFhirContext Context holding resource definition
         * @param theElementType Element type name
         * @param <T>            Base element type
         * @return Returns a new instance of the element
         */
        public static <T extends IBase> T newElement(FhirContext theFhirContext, String theElementType) {
                BaseRuntimeElementDefinition def = theFhirContext.getElementDefinition(theElementType);
                return (T) def.newInstance();
        }

        /**
         * Creates a new element instance
         *
         * @param theFhirContext      Context holding resource definition
         * @param theElementType      Element type name
         * @param theConstructorParam Initialization parameter for the element
         * @param <T>                 Base element type
         * @return Returns a new instance of the element with the specified initial value
         */
        public static <T extends IBase> T newElement(
                        FhirContext theFhirContext, String theElementType, Object theConstructorParam) {
                BaseRuntimeElementDefinition def = theFhirContext.getElementDefinition(theElementType);
                Validate.notNull(def);
                return (T) def.newInstance(theConstructorParam);
        }

        /**
         * Creates a new resource definition.
         *
         * @param theFhirContext  Context holding resource definition
         * @param theResourceName Name of the resource in the context
         * @param <T>             Type of the resource
         * @return Returns a new instance of the resource
         */
        public static <T extends IBase> T newResource(FhirContext theFhirContext, String theResourceName) {
                RuntimeResourceDefinition def = theFhirContext.getResourceDefinition(theResourceName);
                return (T) def.newInstance();
        }

        /**
         * Creates a new resource definition.
         *
         * @param theFhirContext      Context holding resource definition
         * @param theResourceName     Name of the resource in the context
         * @param theConstructorParam Initialization parameter for the new instance
         * @param <T>                 Type of the resource
         * @return Returns a new instance of the resource
         */
        public static <T extends IBase> T newResource(
                        FhirContext theFhirContext, String theResourceName, Object theConstructorParam) {
                RuntimeResourceDefinition def = theFhirContext.getResourceDefinition(theResourceName);
                return (T) def.newInstance(theConstructorParam);
        }

        /**
         * Creates a new BackboneElement.
         *
         * @param theFhirContext        Context holding resource definition
         * @param theTargetResourceName Name of the resource in the context
         * @param theTargetFieldName    Name of the backbone element in the resource
         * @return Returns a new instance of the element
         */
        public static IBaseBackboneElement instantiateBackboneElement(
                        FhirContext theFhirContext, String theTargetResourceName, String theTargetFieldName) {
                BaseRuntimeElementDefinition<?> targetParentElementDefinition =
                                theFhirContext.getResourceDefinition(theTargetResourceName);
                BaseRuntimeChildDefinition childDefinition = targetParentElementDefinition.getChildByName(theTargetFieldName);
                return (IBaseBackboneElement)
                                childDefinition.getChildByName(theTargetFieldName).newInstance();
        }

        private static void clear(List<IBase> values) {
                if (values == null) {
                        return;
                }

                try {
                        values.clear();
                } catch (Throwable t) {
                        ourLog.debug("Unable to clear values " + String.valueOf(values), t);
                }
        }
}