// The return type of a CASE expression is the compatible aggregated type of all return values,
// but also depends on the context in which it is used.
// If used in a string context, the result is returned as a string.
// If used in a numeric context, the result is returned as a decimal, real, or integer value.
func (v *typeInferrer) handleCaseExpr(x *ast.CaseExpr) {
	var currType *types.FieldType
	for _, w := range x.WhenClauses {
		t := w.Result.GetType()
		if currType == nil {
			currType = t
			continue
		}
		mtp := types.MergeFieldType(currType.Tp, t.Tp)
		if mtp == t.Tp && mtp != currType.Tp {
			currType.Charset = t.Charset
			currType.Collate = t.Collate
		}
		currType.Tp = mtp

	}
	if x.ElseClause != nil {
		t := x.ElseClause.GetType()
		if currType == nil {
			currType = t
		} else {
			mtp := types.MergeFieldType(currType.Tp, t.Tp)
			if mtp == t.Tp && mtp != currType.Tp {
				currType.Charset = t.Charset
				currType.Collate = t.Collate
			}
			currType.Tp = mtp
		}
	}
	x.SetType(currType)
	// TODO: We need a better way to set charset/collation
	x.Type.Charset, x.Type.Collate = types.DefaultCharsetForType(x.Type.Tp)
}

func (e *Evaluator) caseExpr(v *ast.CaseExpr) bool {
	tmp := types.NewDatum(boolToInt64(true))
	target := &tmp
	if v.Value != nil {
		target = v.Value.GetDatum()
	}
	if !target.IsNull() {
		for _, val := range v.WhenClauses {
			cmp, err := target.CompareDatum(*val.Expr.GetDatum())
			if err != nil {
				e.err = errors.Trace(err)
				return false
			}
			if cmp == 0 {
				v.SetDatum(*val.Result.GetDatum())
				return true
			}
		}
	}
	if v.ElseClause != nil {
		v.SetDatum(*v.ElseClause.GetDatum())
	} else {
		v.SetNull()
	}
	return true
}

func (e *Evaluator) caseExpr(v *ast.CaseExpr) bool {
	var target interface{} = true
	if v.Value != nil {
		target = v.Value.GetValue()
	}
	if target != nil {
		for _, val := range v.WhenClauses {
			cmp, err := types.Compare(target, val.Expr.GetValue())
			if err != nil {
				e.err = errors.Trace(err)
				return false
			}
			if cmp == 0 {
				v.SetValue(val.Result.GetValue())
				return true
			}
		}
	}
	if v.ElseClause != nil {
		v.SetValue(v.ElseClause.GetValue())
	} else {
		v.SetValue(nil)
	}
	return true
}