private static final String FILE = "./measurements.txt";

/*

public static void main(String[] args) throws IOException, ExecutionException, InterruptedException {

long start = System.currentTimeMillis();

var filename = args.length == 0 ? FILE : args[0];

var file = new File(filename);

var resultsMap = getFileSegments(file).stream().map(segment -> {

var resultMap = new ByteArrayToResultMap();

long segmentEnd = segment.end();

try (var fileChannel = (FileChannel) Files.newByteChannel(Path.of(filename), StandardOpenOption.READ)) {

var bb = fileChannel.map(FileChannel.MapMode.READ_ONLY, segment.start(), segmentEnd - segment.start());

// Up to 100 characters for a city name

var buffer = new byte[100];

int startLine;

int limit = bb.limit();

while ((startLine = bb.position()) < limit) {

int currentPosition = startLine;

byte b;

int offset = 0;

int hash = 0;

while (currentPosition != segmentEnd && (b = bb.get(currentPosition++)) != ';') {

buffer[offset++] = b;

hash = 31 * hash + b;

}

int temp;

int negative = 1;

// Inspired by @yemreinci to unroll this even further

if (bb.get(currentPosition) == '-') {

negative = -1;

currentPosition++;

}

if (bb.get(currentPosition + 1) == '.') {

temp = negative * ((bb.get(currentPosition) - '0') * 10 + (bb.get(currentPosition + 2) - '0'));

currentPosition += 3;

}

else {

temp = negative * ((bb.get(currentPosition) - '0') * 100 + ((bb.get(currentPosition + 1) - '0') * 10 + (bb.get(currentPosition + 3) - '0')));

currentPosition += 4;

}

if (bb.get(currentPosition) == '\r') {

currentPosition++;

}

currentPosition++;

resultMap.putOrMerge(buffer, 0, offset, temp / 10.0, hash);

bb.position(currentPosition);

}

return resultMap;

}

catch (IOException e) {

throw new RuntimeException(e);

}

}).parallel().flatMap(partition -> partition.getAll().stream())

.collect(Collectors.toMap(e -> new String(e.key()), Entry::value, CalculateAverage_spullara::merge, TreeMap::new));

System.out.println(resultsMap);

}

private static List<FileSegment> getFileSegments(File file) throws IOException {

int numberOfSegments = Runtime.getRuntime().availableProcessors();

long fileSize = file.length();

long segmentSize = fileSize / numberOfSegments;

List<FileSegment> segments = new ArrayList<>(numberOfSegments);

// Pointless to split small files

if (segmentSize < 1_000_000) {

segments.add(new FileSegment(0, fileSize));

return segments;

}

try (RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r")) {

for (int i = 0; i < numberOfSegments; i++) {

long segStart = i * segmentSize;

long segEnd = (i == numberOfSegments - 1) ? fileSize : segStart + segmentSize;

segStart = findSegment(i, 0, randomAccessFile, segStart, segEnd);

segEnd = findSegment(i, numberOfSegments - 1, randomAccessFile, segEnd, fileSize);

segments.add(new FileSegment(segStart, segEnd));

}

}

return segments;

}

private static Result merge(Result v, Result value) {

return merge(v, value.min, value.max, value.sum, value.count);

}

private static Result merge(Result v, double value, double value1, double value2, long value3) {

v.min = Math.min(v.min, value);

v.max = Math.max(v.max, value1);

v.sum += value2;

v.count += value3;

return v;

}

private static long findSegment(int i, int skipSegment, RandomAccessFile raf, long location, long fileSize) throws IOException {

if (i != skipSegment) {

raf.seek(location);

while (location < fileSize) {

location++;

if (raf.read() == '\n')

break;

}

}

return location;

}

double min, max, sum;

long count;

Result(double value) {

min = max = sum = value;

this.count = 1;

}

@Override

public String toString() {

return round(min) + "/" + round(sum / count) + "/" + round(max);

}

double round(double v) {

return Math.round(v * 10.0) / 10.0;

}

public static final int MAPSIZE = 1024 * 128;

Result[] slots = new Result[MAPSIZE];

byte[][] keys = new byte[MAPSIZE][];

public void putOrMerge(byte[] key, int offset, int size, double temp, int hash) {

int slot = hash & (slots.length - 1);

var slotValue = slots[slot];

// Linear probe for open slot

while (slotValue != null && (keys[slot].length != size || !Arrays.equals(keys[slot], 0, size, key, offset, size))) {

slot = (slot + 1) & (slots.length - 1);

slotValue = slots[slot];

}

Result value = slotValue;

if (value == null) {

slots[slot] = new Result(temp);

byte[] bytes = new byte[size];

System.arraycopy(key, offset, bytes, 0, size);

keys[slot] = bytes;

} else {

value.min = Math.min(value.min, temp);

value.max = Math.max(value.max, temp);

value.sum += temp;

value.count += 1;

}

}

// Get all pairs

public List<Entry> getAll() {

List<Entry> result = new ArrayList<>(slots.length);

for (int i = 0; i < slots.length; i++) {

Result slotValue = slots[i];

if (slotValue != null) {

result.add(new Entry(keys[i], slotValue));

}

}

return result;

}

}