AggregateFunction.cs

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// 
//  Copyright 2010-2015 Deveel
// 
//    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.
//

using System;

using Deveel.Data;
using Deveel.Data.Sql;
using Deveel.Data.Sql.Expressions;
using Deveel.Data.Types;

namespace Deveel.Data.Routines {
    public abstract class AggregateFunction : Function {
        protected AggregateFunction(ObjectName name, RoutineParameter[] parameters, SqlType returnType) 
            : base(name, parameters, returnType, FunctionType.Aggregate) {
        }

        protected abstract DataObject Evaluate(DataObject value1, DataObject value2, IRequest query, IGroupResolver group);

        protected virtual DataObject PostEvaluate(DataObject result, IRequest query, IGroupResolver group) {
            // By default, do nothing....
            return result;
        }

        public override InvokeResult Execute(InvokeContext context) {
            if (context == null)
                throw new ArgumentNullException("context");

            var group = context.GroupResolver;

            if (group == null)
                throw new Exception(String.Format("'{0}' can only be used as an aggregate function.", FunctionName));

            DataObject result = null;
            // All aggregates functions return 'null' if group size is 0
            int size = group.Count;
            if (size == 0) {
                // Return a NULL of the return type
                return context.Result(DataObject.Null(ReturnType(context)));
            }

            DataObject val;
            ObjectName v = context.Arguments[0].AsReferenceName();
            // If the aggregate parameter is a simple variable, then use optimal
            // routine,
            if (v != null) {
                for (int i = 0; i < size; ++i) {
                    val = group.Resolve(v, i);
                    result = Evaluate(result, val, context.Request, group);
                }
            } else {
                // Otherwise we must resolve the expression for each entry in group,
                // This allows for expressions such as 'sum(quantity * price)' to
                // work for a group.
                var exp = context.Arguments[0];
                for (int i = 0; i < size; ++i) {
                    val = exp.EvaluateToConstant(context.Request, group.GetVariableResolver(i));
                    result = Evaluate(result, val, context.Request, group);
                }
            }

            // Post method.
            result = PostEvaluate(result, context.Request, group);

            return context.Result(result);
        }
    }
}