export function getFieldFromDomain(domain: VgDomain): string {
  if (isDataRefDomain(domain) && isString(domain.field)) {
    return domain.field;
  } else if (isDataRefUnionedDomain(domain)) {
    let field;
    for (const nonUnionDomain of domain.fields) {
      if (isDataRefDomain(nonUnionDomain) && isString(nonUnionDomain.field)) {
        if (!field) {
          field = nonUnionDomain.field;
        } else if (field !== nonUnionDomain.field) {
          log.warn('Detected faceted independent scales that union domain of multiple fields from different data sources.  We will use the first field.  The result view size may be incorrect.');
          return field;
        }
      }
    }
    log.warn('Detected faceted independent scales that union domain of identical fields from different source detected.  We will assume that this is the same field from a different fork of the same data source.  However, if this is not case, the result view size maybe incorrect.');
    return field;
  } else if (isFieldRefUnionDomain(domain)) {
    log.warn('Detected faceted independent scales that union domain of multiple fields from the same data source.  We will use the first field.  The result view size may be incorrect.');
    const field = domain.fields[0];
    return isString(field) ? field : undefined;
  }

  return undefined;
}

export function canUseUnaggregatedDomain(
  fieldDef: TypedFieldDef<string>,
  scaleType: ScaleType
): {valid: boolean; reason?: string} {
  const {aggregate, type} = fieldDef;

  if (!aggregate) {
    return {
      valid: false,
      reason: log.message.unaggregateDomainHasNoEffectForRawField(fieldDef)
    };
  }

  if (isString(aggregate) && !SHARED_DOMAIN_OP_INDEX[aggregate]) {
    return {
      valid: false,
      reason: log.message.unaggregateDomainWithNonSharedDomainOp(aggregate)
    };
  }

  if (type === 'quantitative') {
    if (scaleType === 'log') {
      return {
        valid: false,
        reason: log.message.unaggregatedDomainWithLogScale(fieldDef)
      };
    }
  }

  return {valid: true};
}

    forEachLeaf(transform.filter, filter => {
      if (isFieldPredicate(filter)) {
        // Automatically add a parse node for filters with filter objects
        let val: string | number | boolean | DateTime = null;

        // For EqualFilter, just use the equal property.
        // For RangeFilter and OneOfFilter, all array members should have
        // the same type, so we only use the first one.
        if (isFieldEqualPredicate(filter)) {
          val = filter.equal;
        } else if (isFieldRangePredicate(filter)) {
          val = filter.range[0];
        } else if (isFieldOneOfPredicate(filter)) {
          val = (filter.oneOf || filter['in'])[0];
        } // else -- for filter expression, we can't infer anything
        if (val) {
          if (isDateTime(val)) {
            parse[filter.field] = 'date';
          } else if (isNumber(val)) {
            parse[filter.field] = 'number';
          } else if (isString(val)) {
            parse[filter.field] = 'string';
          }
        }

        if (filter.timeUnit) {
          parse[filter.field] = 'date';
        }
      }
    });

  public assemble(): VgLookupTransform {
    let foreign: Partial<VgLookupTransform>;

    if (this.transform.from.fields) {
      // lookup a few fields and add create a flat output
      foreign = {
        values: this.transform.from.fields,
        ... this.transform.as ? {as: ((this.transform.as instanceof Array) ? this.transform.as : [this.transform.as])} : {}
      };
    } else {
      // lookup full record and nest it
      let asName = this.transform.as;
      if (!isString(asName)) {
        log.warn(log.message.NO_FIELDS_NEEDS_AS);
        asName = '_lookup';
      }

      foreign = {
        as: [asName]
      };
    }

    return {
      type: 'lookup',
      from: this.secondary,
      key: this.transform.from.key,
      fields: [this.transform.lookup],
      ...foreign,
      ...(this.transform.default ? {default: this.transform.default} : {})
    };
  }

  public static makeFromTransform(parent: DataFlowNode, stackTransform: StackTransform) {
    const {stack, groupby, as, offset = 'zero'} = stackTransform;

    const sortFields: string[] = [];
    const sortOrder: VgComparatorOrder[] = [];
    if (stackTransform.sort !== undefined) {
      for (const sortField of stackTransform.sort) {
        sortFields.push(sortField.field);
        sortOrder.push(getFirstDefined(sortField.order, 'ascending'));
      }
    }
    const sort: VgCompare = {
      field: sortFields,
      order: sortOrder
    };
    let normalizedAs: string[];
    if (isValidAsArray(as)) {
      normalizedAs = as;
    } else if (isString(as)) {
      normalizedAs = [as, as + '_end'];
    } else {
      normalizedAs = [stackTransform.stack + '_start', stackTransform.stack + '_end'];
    }

    return new StackNode(parent, {
      stackField: stack,
      groupby,
      offset,
      sort,
      facetby: [],
      as: normalizedAs
    });
  }

    return wrapCondition(model, channelDef, 'tooltip', cDef => {
      // use valueRef based on channelDef first
      const tooltipRefFromChannelDef = ref.text(cDef, model.config, opt.reactiveGeom ? 'datum.datum' : 'datum');
      if (tooltipRefFromChannelDef) {
        return tooltipRefFromChannelDef;
      }

      if (cDef === null) {
        // Allow using encoding.tooltip = null to disable tooltip
        return undefined;
      }

      // If tooltipDef does not exist, then use value from markDef or config
      const markTooltip = getFirstDefined(markDef.tooltip, getMarkConfig('tooltip', markDef, config));
      if (isString(markTooltip)) {
        return {value: markTooltip};
      } else if (isObject(markTooltip)) {
        // `tooltip` is `{fields: 'encodings' | 'fields'}`
        if (markTooltip.content === 'encoding') {
          return ref.tooltipForEncoding(encoding, config, opt);
        } else {
          return {signal: 'datum'};
        }
      }

      return undefined;
    });

 return logicalExpr(filterOp, (predicate: Predicate) => {
   if (isString(predicate)) {
     return predicate;
   } else if (isSelectionPredicate(predicate)) {
     return selectionPredicate(model, predicate.selection, node);
   } else { // Filter Object
     return fieldFilterExpression(predicate);
   }
 });

export function normalize(channelDef: ChannelDef, channel: Channel): ChannelDef<any> {
  if (isString(channelDef) || isNumber(channelDef) || isBoolean(channelDef)) {
    const primitiveType = isString(channelDef) ? 'string' : isNumber(channelDef) ? 'number' : 'boolean';
    log.warn(log.message.primitiveChannelDef(channel, primitiveType, channelDef));
    return {value: channelDef};
  }

  // If a fieldDef contains a field, we need type.
  if (isFieldDef(channelDef)) {
    return normalizeFieldDef(channelDef, channel);
  } else if (hasConditionalFieldDef(channelDef)) {
    return {
      ...channelDef,
      // Need to cast as normalizeFieldDef normally return FieldDef, but here we know that it is definitely Condition<FieldDef>
      condition: normalizeFieldDef(channelDef.condition, channel) as Conditional<TypedFieldDef<string>>
    };
  }
  return channelDef;
}

export function parseUnitSelection(model: UnitModel, selDefs: Dict<SelectionDef>) {
  const selCmpts: Dict<SelectionComponent<any /* this has to be "any" so typing won't fail in test files*/>> = {};
  const selectionConfig = model.config.selection;

  if (selDefs) {
    selDefs = duplicate(selDefs); // duplicate to avoid side effects to original spec
  }

  for (let name in selDefs) {
    if (!selDefs.hasOwnProperty(name)) {
      continue;
    }

    const selDef = selDefs[name];
    const {fields, encodings, ...cfg} = selectionConfig[selDef.type]; // Project transform applies its defaults.

    // Set default values from config if a property hasn't been specified,
    // or if it is true. E.g., "translate": true should use the default
    // event handlers for translate. However, true may be a valid value for
    // a property (e.g., "nearest": true).
    for (const key in cfg) {
      // A selection should contain either `encodings` or `fields`, only use
      // default values for these two values if neither of them is specified.
      if ((key === 'encodings' && selDef.fields) || (key === 'fields' && selDef.encodings)) {
        continue;
      }

      if (key === 'mark') {
        selDef[key] = {...cfg[key], ...selDef[key]};
      }

      if (selDef[key] === undefined || selDef[key] === true) {
        selDef[key] = cfg[key] || selDef[key];
      }
    }

    name = varName(name);
    const selCmpt = (selCmpts[name] = {
      ...selDef,
      name: name,
      events: isString(selDef.on) ? parseSelector(selDef.on, 'scope') : selDef.on
    } as any);

    forEachTransform(selCmpt, txCompiler => {
      if (txCompiler.parse) {
        txCompiler.parse(model, selDef, selCmpt);
      }
    });
  }

  return selCmpts;
}

  return ({
    partName,
    mark,
    positionPrefix,
    endPositionPrefix = undefined,
    extraEncoding = {}
  }: {
    partName: keyof P;
    mark: Mark | MarkDef;
    positionPrefix: string;
    endPositionPrefix?: string;
    extraEncoding?: Encoding<string>;
  }) => {
    const title =
      axis && axis.title !== undefined
        ? undefined
        : continuousAxisChannelDef.title !== undefined
        ? continuousAxisChannelDef.title
        : continuousAxisChannelDef.field;

    return partLayerMixins<P>(compositeMarkDef, partName, compositeMarkConfig, {
      mark, // TODO better remove this method and just have mark as a parameter of the method
      encoding: {
        [continuousAxis]: {
          field: positionPrefix + '_' + continuousAxisChannelDef.field,
          type: continuousAxisChannelDef.type,
          ...(title ? {title} : {}),
          ...(scale ? {scale} : {}),
          ...(axis ? {axis} : {})
        },
        ...(isString(endPositionPrefix)
          ? {
              [continuousAxis + '2']: {
                field: endPositionPrefix + '_' + continuousAxisChannelDef.field,
                type: continuousAxisChannelDef.type
              }
            }
          : {}),
        ...sharedEncoding,
        ...extraEncoding
      }
    });
  };