Desafio 07 em Go by MatMercer (#965)

desafio-07/MatMercer/go/README.md

@@ -0,0 +1,33 @@
+# Solução desafio 7 unix tac em Go
+
+Executar:
+
+```bash
+go build tac.go
+./tac 1GB.txt > out.txt
+md5sum out.txt
+2b4fd25f11d75c285ec69ecac420bd07  out.txt
+```
+
+Para testar os 512MB de memória com control groups v1:
+
+```bash
+# install cgroup
+sudo apt-get install cgroup-tools
+
+# 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/$CGROUPP/memory.swappiness"
+
+# run in cgroup
+cgexec -g memory:$CGROUP ./tac 1GB.txt > out.txt
+md5sum out.txt
+2b4fd25f11d75c285ec69ecac420bd07  out.txt
+
+# check max mem used
+cat "/sys/fs/cgroup/$CGROUPP/memory.max_usage_in_bytes" | numfmt --to=iec
+512M
+```

desafio-07/MatMercer/go/tac.go

@@ -0,0 +1,139 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"log"
+	"os"
+)
+
+// in MB size, currently the program uses a max of 2*bufSize iff file size > bufSize
+const maxBufSize = int64(250 << (10 * 2))
+
+var stdout *bufio.Writer
+
+// min standard math.Min supports only float64
+func min(x int64, y int64) int64 {
+	if x > y {
+		return y
+	}
+	return x
+}
+
+type ReverseReader struct {
+	f      *os.File
+	offset int64
+}
+
+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
+}
+
+func tac(fileName string) error {
+	f, err := os.Open(fileName)
+	if err != nil {
+		return err
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+
+	// buffers stdout by 128kb
+	stdout = bufio.NewWriterSize(os.Stdout, 128<<(10))
+
+	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
+		}
+
+		_, err = r.Read(readBuf[:maxRead])
+		if err != nil {
+			return err
+		}
+
+		// 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
+			}
+
+			// 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()
+
+	// 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
+	}
+
+	err = stdout.Flush()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func main() {
+	logger := log.New(os.Stderr, "", 0)
+	if len(os.Args) != 2 {
+		logger.Fatalln("usage: tac [file]")
+	}
+
+	fileName := os.Args[1]
+	err := tac(fileName)
+	if err != nil {
+		logger.Fatalf("tac error: %v", err)
+	}
+}