// isDecomposable checks if an aggregate function is decomposable. An aggregation function $F$ is decomposable
// if there exist aggregation functions F_1 and F_2 such that F(S_1 union all S_2) = F_2(F_1(S_1),F_1(S_2)),
// where S_1 and S_2 are two sets of values. We call S_1 and S_2 partial groups.
// It's easy to see that max, min, first row is decomposable, no matter whether it's distinct, but sum(distinct) and
// count(distinct) is not.
// Currently we don't support avg and concat.
func (a *aggPushDownSolver) isDecomposable(fun expression.AggregationFunction) bool {
	switch fun.GetName() {
	case ast.AggFuncAvg, ast.AggFuncGroupConcat:
		// TODO: Support avg push down.
		return false
	case ast.AggFuncMax, ast.AggFuncMin, ast.AggFuncFirstRow:
		return true
	case ast.AggFuncSum, ast.AggFuncCount:
		return !fun.IsDistinct()
	default:
		return false
	}
}

// getAggFuncChildIdx gets which children it belongs to, 0 stands for left, 1 stands for right, -1 stands for both.
func (a *aggPushDownSolver) getAggFuncChildIdx(aggFunc expression.AggregationFunction, schema expression.Schema) int {
	fromLeft, fromRight := false, false
	var cols []*expression.Column
	for _, arg := range aggFunc.GetArgs() {
		cols = append(cols, expression.ExtractColumns(arg)...)
	}
	for _, col := range cols {
		if schema.GetIndex(col) != -1 {
			fromLeft = true
		} else {
			fromRight = true
		}
	}
	if fromLeft && fromRight {
		return -1
	} else if fromLeft {
		return 0
	}
	return 1
}

func (b *executorBuilder) newAggFuncToPBExpr(client kv.Client, aggFunc expression.AggregationFunction,
	tbl *model.TableInfo) *tipb.Expr {
	var tp tipb.ExprType
	switch aggFunc.GetName() {
	case ast.AggFuncCount:
		tp = tipb.ExprType_Count
	case ast.AggFuncFirstRow:
		tp = tipb.ExprType_First
	case ast.AggFuncGroupConcat:
		tp = tipb.ExprType_GroupConcat
	case ast.AggFuncMax:
		tp = tipb.ExprType_Max
	case ast.AggFuncMin:
		tp = tipb.ExprType_Min
	case ast.AggFuncSum:
		tp = tipb.ExprType_Sum
	case ast.AggFuncAvg:
		tp = tipb.ExprType_Avg
	}
	if !client.SupportRequestType(kv.ReqTypeSelect, int64(tp)) {
		return nil
	}

	children := make([]*tipb.Expr, 0, len(aggFunc.GetArgs()))
	for _, arg := range aggFunc.GetArgs() {
		pbArg := b.newExprToPBExpr(client, arg, tbl)
		if pbArg == nil {
			return nil
		}
		children = append(children, pbArg)
	}
	return &tipb.Expr{Tp: tp.Enum(), Children: children}
}

func aggFuncToPBExpr(sc *variable.StatementContext, client kv.Client, aggFunc expression.AggregationFunction) *tipb.Expr {
	pc := pbConverter{client: client, sc: sc}
	var tp tipb.ExprType
	switch aggFunc.GetName() {
	case ast.AggFuncCount:
		tp = tipb.ExprType_Count
	case ast.AggFuncFirstRow:
		tp = tipb.ExprType_First
	case ast.AggFuncGroupConcat:
		tp = tipb.ExprType_GroupConcat
	case ast.AggFuncMax:
		tp = tipb.ExprType_Max
	case ast.AggFuncMin:
		tp = tipb.ExprType_Min
	case ast.AggFuncSum:
		tp = tipb.ExprType_Sum
	case ast.AggFuncAvg:
		tp = tipb.ExprType_Avg
	}
	if !client.SupportRequestType(kv.ReqTypeSelect, int64(tp)) {
		return nil
	}

	children := make([]*tipb.Expr, 0, len(aggFunc.GetArgs()))
	for _, arg := range aggFunc.GetArgs() {
		pbArg := pc.exprToPB(arg)
		if pbArg == nil {
			return nil
		}
		children = append(children, pbArg)
	}
	return &tipb.Expr{Tp: tp, Children: children}
}

// decompose splits an aggregate function to two parts: a final mode function and a partial mode function. Currently
// there are no differences between partial mode and complete mode, so we can confuse them.
func (a *aggPushDownSolver) decompose(aggFunc expression.AggregationFunction, schema expression.Schema, id string) ([]expression.AggregationFunction, expression.Schema) {
	// Result is a slice because avg should be decomposed to sum and count. Currently we don't process this case.
	result := []expression.AggregationFunction{aggFunc.Clone()}
	for _, aggFunc := range result {
		schema = append(schema, &expression.Column{
			ColName:  model.NewCIStr(fmt.Sprintf("join_agg_%d", len(schema))), // useless but for debug
			FromID:   id,
			Position: len(schema),
			RetType:  aggFunc.GetType(),
		})
	}
	aggFunc.SetArgs(expression.Schema2Exprs(schema[len(schema)-len(result):]))
	aggFunc.SetMode(expression.FinalMode)
	return result, schema
}

func needValue(af expression.AggregationFunction) bool {
	return af.GetName() == ast.AggFuncSum || af.GetName() == ast.AggFuncAvg || af.GetName() == ast.AggFuncFirstRow ||
		af.GetName() == ast.AggFuncMax || af.GetName() == ast.AggFuncMin || af.GetName() == ast.AggFuncGroupConcat
}

func needCount(af expression.AggregationFunction) bool {
	return af.GetName() == ast.AggFuncCount || af.GetName() == ast.AggFuncAvg
}