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package dev.morling.onebrc;
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;
import java.util.*;
import java.util.stream.*;
public class CalculateAverage_bjhara {
private static final String FILE = "./measurements.txt";
private static class Measurement {
private double min = Double.POSITIVE_INFINITY;
private double max = Double.NEGATIVE_INFINITY;
private double sum;
private long count;
public String toString() {
return round(min) + "/" + round(sum / count) + "/" + round(max);
}
private double round(double value) {
return Math.round(value * 10.0) / 10.0;
}
public static Measurement combine(Measurement m1, Measurement m2) {
var mres = new Measurement();
mres.min = m1.min < m2.min ? m1.min : m2.min;
mres.max = m1.max > m2.max ? m1.max : m2.max;
mres.sum = m1.sum + m2.sum;
mres.count = m1.count + m2.count;
return mres;
}
}
public static void main(String[] args) throws IOException {
try (FileChannel fileChannel = (FileChannel) Files.newByteChannel(Path.of(FILE),
EnumSet.of(StandardOpenOption.READ))) {
var cities = splitFileChannel(fileChannel)
.parallel()
.map(CalculateAverage_bjhara::parseBuffer)
.collect(Collectors.reducing(CalculateAverage_bjhara::combineMaps));
var sortedCities = new TreeMap<>(cities.orElseThrow());
System.out.println(sortedCities);
}
}
private static Map<String, Measurement> combineMaps(Map<String, Measurement> map1,
Map<String, Measurement> map2) {
for (var entry : map2.entrySet()) {
map1.merge(entry.getKey(), entry.getValue(), Measurement::combine);
}
return map1;
}
private static Stream<ByteBuffer> splitFileChannel(final FileChannel fileChannel) throws IOException {
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(new Iterator<ByteBuffer>() {
private static final long CHUNK_SIZE = 1024 * 1024 * 10L;
private final long size = fileChannel.size();
private long start = 0;
@Override
public boolean hasNext() {
return start < size;
}
@Override
public ByteBuffer next() {
try {
MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, start,
Math.min(CHUNK_SIZE, size - start));
// don't split the data in the middle of lines
// find the closest previous newline
int realEnd = mappedByteBuffer.limit() - 1;
while (mappedByteBuffer.get(realEnd) != '\n')
realEnd--;
realEnd++;
mappedByteBuffer.limit(realEnd);
start += realEnd;
return mappedByteBuffer;
}
catch (IOException ex) {
throw new UncheckedIOException(ex);
}
}
}, Spliterator.IMMUTABLE), false);
}
private static Map<String, Measurement> parseBuffer(ByteBuffer bb) {
Map<String, Measurement> cities = new HashMap<>();
final int limit = bb.limit();
final byte[] buffer = new byte[128];
while (bb.position() < limit) {
final int currentPosition = bb.position();
// find the ; separator
int separator = currentPosition;
while (separator != limit && bb.get(separator) != ';')
separator++;
// find the end of the line
int end = separator + 1;
while (end != limit && !Character.isWhitespace((char) bb.get(end)))
end++;
// get the name as a string
int nameLength = separator - currentPosition;
bb.get(buffer, 0, nameLength);
String city = new String(buffer, 0, nameLength);
// get rid of the separator
bb.get();
// get the double value
int valueLength = end - separator - 1;
bb.get(buffer, 0, valueLength);
String valueStr = new String(buffer, 0, valueLength);
double value = Double.parseDouble(valueStr);
// and get rid of the new line (handle both kinds)
byte newline = bb.get();
if (newline == '\r')
bb.get();
// update the map with the new measurement
Measurement agg = cities.get(city);
if (agg == null) {
agg = new Measurement();
cities.put(city, agg);
}
agg.min = agg.min < value ? agg.min : value;
agg.max = agg.max > value ? agg.max : value;
agg.sum += value;
agg.count++;
}
return cities;
}
}
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