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Desafio 07 em Go by MatMercer (#965)
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# Solução desafio 7 unix tac em Go | ||
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Executar: | ||
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```bash | ||
go build tac.go | ||
./tac 1GB.txt > out.txt | ||
md5sum out.txt | ||
2b4fd25f11d75c285ec69ecac420bd07 out.txt | ||
``` | ||
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Para testar os 512MB de memória com control groups v1: | ||
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```bash | ||
# install cgroup | ||
sudo apt-get install cgroup-tools | ||
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# setup cgroup | ||
export CGROUP=osProgramadoresD7 | ||
export CGROUPP="memory/$CGROUP" | ||
sudo cgcreate -t $USER:$USER -a $USER:$USER -g memory:"$CGROUP" | ||
echo 512M > "/sys/fs/cgroup/$CGROUPP/memory.limit_in_bytes" | ||
echo 0 > "/sys/fs/cgroup/$CGROUPP/memory.swappiness" | ||
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# run in cgroup | ||
cgexec -g memory:$CGROUP ./tac 1GB.txt > out.txt | ||
md5sum out.txt | ||
2b4fd25f11d75c285ec69ecac420bd07 out.txt | ||
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# check max mem used | ||
cat "/sys/fs/cgroup/$CGROUPP/memory.max_usage_in_bytes" | numfmt --to=iec | ||
512M | ||
``` |
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package main | ||
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import ( | ||
"bufio" | ||
"bytes" | ||
"io" | ||
"log" | ||
"os" | ||
) | ||
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// in MB size, currently the program uses a max of 2*bufSize iff file size > bufSize | ||
const maxBufSize = int64(250 << (10 * 2)) | ||
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var stdout *bufio.Writer | ||
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// min standard math.Min supports only float64 | ||
func min(x int64, y int64) int64 { | ||
if x > y { | ||
return y | ||
} | ||
return x | ||
} | ||
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type ReverseReader struct { | ||
f *os.File | ||
offset int64 | ||
} | ||
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func (r *ReverseReader) Read(b []byte) (n int, err error) { | ||
r.offset -= int64(len(b)) | ||
_, err = r.f.Seek(r.offset, io.SeekStart) | ||
if err != nil { | ||
return 0, err | ||
} | ||
read, err := r.f.Read(b) | ||
if err != nil { | ||
return 0, err | ||
} | ||
return read, nil | ||
} | ||
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func tac(fileName string) error { | ||
f, err := os.Open(fileName) | ||
if err != nil { | ||
return err | ||
} | ||
fi, err := f.Stat() | ||
if err != nil { | ||
return err | ||
} | ||
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// buffers stdout by 128kb | ||
stdout = bufio.NewWriterSize(os.Stdout, 128<<(10)) | ||
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fileSize := fi.Size() | ||
bufSize := min(fileSize, maxBufSize) | ||
readBuf := make([]byte, bufSize) | ||
maxRead := bufSize | ||
start := fileSize | ||
lineAcc := bytes.NewBuffer([]byte{}) | ||
r := &ReverseReader{f, fileSize} | ||
for start != 0 { | ||
start -= bufSize | ||
if start < 0 { | ||
// prevent over reading if result is less than buf size | ||
maxRead += start | ||
start = 0 | ||
} | ||
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_, err = r.Read(readBuf[:maxRead]) | ||
if err != nil { | ||
return err | ||
} | ||
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// search until backwards \n and prints it | ||
lastEnd := maxRead | ||
for i := maxRead - 1; i >= 0; i-- { | ||
if readBuf[i] == '\n' { | ||
// write everything but '\n', since readBuf[i] == '\n' | ||
_, err = stdout.Write(readBuf[i+1 : lastEnd]) | ||
if err != nil { | ||
return err | ||
} | ||
// writes accumulated value | ||
_, err = stdout.Write(lineAcc.Bytes()) | ||
if err != nil { | ||
return err | ||
} | ||
// reset the accumulator to receive next line | ||
lineAcc.Reset() | ||
// makes '\n' be printed in next iteration | ||
lastEnd = i + 1 | ||
} | ||
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// on last iteration, create a new accumulator with [max-read][current-accumulator] | ||
if i == 0 { | ||
newAcc := bytes.NewBuffer(nil) | ||
_, err = newAcc.Write(readBuf[i:lastEnd]) | ||
if err != nil { | ||
return err | ||
} | ||
_, err = newAcc.Write(lineAcc.Bytes()) | ||
if err != nil { | ||
return err | ||
} | ||
lineAcc = newAcc | ||
} | ||
} | ||
} | ||
// closes the file | ||
_ = f.Close() | ||
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// prints last chunk of data | ||
// fmt.Println is unbuffered https://github.com/golang/go/issues/36619 | ||
_, err = stdout.Write(lineAcc.Bytes()) | ||
if err != nil { | ||
return err | ||
} | ||
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err = stdout.Flush() | ||
if err != nil { | ||
return err | ||
} | ||
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return nil | ||
} | ||
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func main() { | ||
logger := log.New(os.Stderr, "", 0) | ||
if len(os.Args) != 2 { | ||
logger.Fatalln("usage: tac [file]") | ||
} | ||
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fileName := os.Args[1] | ||
err := tac(fileName) | ||
if err != nil { | ||
logger.Fatalf("tac error: %v", err) | ||
} | ||
} |