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limit.go
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limit.go
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// Copyright 2023 Sneller, 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.
package vm
import (
"io"
"sync/atomic"
)
// Limit is a QuerySink that
// limits the number of rows written
// to the next QuerySink.
//
// See NewLimit
type Limit struct {
remaining int64
dst QuerySink
}
type limiter struct {
parent *Limit
dst rowConsumer
done bool
}
// NewLimit constructs a Limit that will
// write no more than 'n' rows to 'dst'.
func NewLimit(n int64, dst QuerySink) *Limit {
return &Limit{
dst: dst,
remaining: n,
}
}
func (l *Limit) Open() (io.WriteCloser, error) {
w, err := l.dst.Open()
if err != nil {
return nil, err
}
return splitter(&limiter{
parent: l,
dst: asRowConsumer(w),
}), nil
}
func (l *Limit) Close() error {
return l.dst.Close()
}
func (l *limiter) Close() error {
if !l.done {
l.done = true
return l.dst.Close()
}
return nil
}
func (l *limiter) symbolize(st *symtab, aux *auxbindings) error {
return l.dst.symbolize(st, aux)
}
func (l *limiter) next() rowConsumer { return l.dst }
func (l *limiter) writeRows(rows []vmref, rp *rowParams) error {
if l.done {
return io.EOF
}
c := int64(len(rows))
avail := atomic.AddInt64(&l.parent.remaining, -c)
if avail < 0 {
// adjust c so that we only
// write the rows we are interested
// in writing
c += avail
if c <= 0 {
// close early so that the next
// sub-query can begin finalization
// as early as possible
l.done = true
err := l.dst.Close()
if err == nil {
err = io.EOF
}
return err
}
}
// limit aux rows as well
for j := range rp.auxbound {
rp.auxbound[j] = rp.auxbound[j][:c]
}
err := l.dst.writeRows(rows[:c], rp)
if avail == 0 && err == nil {
l.done = true
err = l.dst.Close()
if err == nil {
err = io.EOF
}
}
return err
}