I need optimized toString() method of BigInteger - java

I have a very large digit (the length number is 200000). And when i want to print this number
System.out.println(myBigInt);
a lot of time is spent on an implicit call method toString() of BigInteger number(about 5 sec)
Do you know any way how i can print biginteger on console? May be first need convert to byte array but i don't know what i do next.
I need print on console a very large biginteger number faster then
System.out.println(myBigInt);

By default, System.out.print() is only line-buffered and does a lot work related to Unicode handling. Because of its small buffer size, System.out.println() is not well suited to handle many repetitive outputs in a batch mode. Each line is flushed right away. If your output is mainly ASCII-based then by removing the Unicode-related activities, the overall execution time will be better.
you can use the following to increase the speed.
BufferedWriter out = new BufferedWriter(new OutputStreamWriter(new
FileOutputStream(java.io.FileDescriptor.out), "ASCII"), 512);
out.write(String.valueOf(myBigInt));

Related

Go back 'n' lines in file using Stream.lines

I need to build an application which scans through a large amount of files. These files contain blocks with some data about a sessions, in which each line has a different value. E.g.: "=ID: 39487".
At that point I have that line, but the problem I now face is that I need the value n lines above that ID. I was thinking about an Iterator but it only has forward methods. I also thought about saving the results in a List but that defies the reason to use Stream and some files are huge so that would cause memory problems.
I was wondering if something like this is possible using the Stream API (Files)? Or perhaps a better question, is there a better way to approach this?
Stream<String> lines = Files.lines(Paths.get(file.getName()));
Iterator<String> search = lines.iterator();
You can't arbitrarily read backwards and forwards through the file with the same reader (no matter if you're using streams, iterators, or a plain BufferedReader.)
If you need:
m lines before a given line
n lines after the given line
You don't know the value of m and n in advance, until you reach that line
...then you essentially have three options:
Read the whole file once, keep it in memory, and then your task is trivial (but this uses the most memory.)
Read the whole file once, mark the line numbers that you need, then do a second pass where you extract the lines you require.
Read the whole file once, storing some form of metadata about line lengths as you go, then use a RandomAccessFile to extract the specific bits you need without having to read the whole file again.
I'd suggest given the files are huge, the second option here is probably the most realistic. The third will probably give you better performance, but will require much more in the way of development effort.
As an alternative if you can guarantee that both n and m are below a certain value, and that value is a reasonable size - you could also just keep a certain number of lines in a buffer as you're processing the file, and read through that buffer when you need to read lines "backwards".
Try my library. abacus-util
try(Reader reader = new FileReader(yourFile)) {
StreamEx.of(reader)
.sliding(n, n, ArrayList::new)
.filter(l -> l.get(l.size() - 1).contains("=ID: 39487"))
./* then do your work */
}
No matter how big your file is. as long as n is small number, not millions

Like to know the time complexity of this code snippet in Java

I would like to know the time complexity of the following code snippet,
FileReader fr = new FileReader("myfile.txt");
BufferedReader br = new BufferedReader(fr);
for (long i = 0; i < n-1; i++ ) {
br.readLine();
}
System.out.println("Line content:" + br.readLine());
br.close();
fr.close();
Edit: I would like to say, n = a constant number, e.g. 100000
The complexity is O(n) but that doesn't tell you much because you don't know how much time each readLine() needs.
Calculating the complexity doesn't make much sense when individual operations have a very variable runtime behavior.
In this case, the loop is very cheap and will not contribute much to the runtime of the whole program. The loading from disk, on the other hand, will contribute very much to the runtime but it's hard to say without statistical information about the average number of lines per file and the average length of a line.
This is a very simple case, but here's how to find the time complexity. The same method can be applied for more complex algorithms.
For the following portion of code (and regardless of the complexity of readline())
for (long i = 0; i < n-1; i++ ) {
br.readLine();
}
i = 0 will be executed (n-1) times, i < n-1 will be executed n times, i++ will be executed n-1 times, and br.readline(); will be executed n-1 times.
this gives us n-1+n+n-1+n-1 = 4*n-3. This is proportional to n, so the complexity is O(n).
I'm not sure what you mean by "time complexity", but it would appear that it's performance is linear (AKA O(n)) with the size of the file it reads from.
The readLine() function has to scan every character of the input up to the next newline. This should be O(N), where N is the number of bytes in the first n lines (which you read). Using a buffered reader does not reduce algorithmic complexity, it just reduces the number of actual IO calls needed to read a given number of bytes (a good thing, since IO calls are expensive). In this case, the only way that would change things is if the buffer's read size was much larger than the total number of bytes you were going to read.
The time complexity of reading an entire file should be O(N) where N is the size of the file.
However, proving this would be difficult, given the amount of software that is involved. You have got the Java code in the main method, the Reader stack (including the Charset decoder) and the JVM. Then you have the code in the OS. Then you have to take into account file buffering in kernel memory, file system organizations, disk seek times, etcetera.
(It is not meaningful to just consider just the time taken by the application. We can safely predict that component of the total time taken will be dominated by the other components.)
And, as Aaron says the complexity measure is not going to be a reliable predictor of the actual file read time.

Algorithm to convert a double to a char array in Java (without using objects like Double.toString or StringBuilder)?

Does anyone know where I can find that algorithm? It takes a double and StringBuilder and appends the double to the StringBuilder without creating any objects or garbage. Of course I am not looking for:
sb.append(Double.toString(myDouble));
// or
sb.append(myDouble);
I tried poking around the Java source code (I am sure it does it somehow) but I could not see any block of code/logic clear enough to be re-used.
I have written this for ByteBuffer. You should be able to adapt it. Writing it to a direct ByteBuffer saves you having to convert it to bytes or copy it into "native" space.
See public ByteStringAppender append(double d)
If you are logging this to a file, you might use the whole library as it can write around 20 million doubles per second sustained. It can do this without system calls as it writes to a memory mapped file.

Extract first valid line of string from byte array

I am writing a utility in Java that reads a stream which may contain both text and binary data. I want to avoid having I/O wait. To do that I create a thread to keep reading the data (and wait for it) putting it into a buffer, so the clients can check avialability and terminate the waiting whenever they want (by closing the input stream which will generate IOException and stop waiting). This works every well as far as reading bytes out of it; as binary is concerned.
Now, I also want to make it easy for the client to read line out of it like '.hasNextLine()' and '.readLine()'. Without using an I/O-wait stream like buffered stream, (Q1) How can I check if a binary (byte[]) contain a valid unicode line (in the form of the length of the first line)? I look around the String/CharSet API but could not find it (or I miss it?). (NOTE: If possible I don't want to use non-build-in library).
Since I could not find one, I try to create one. Without being so complicated, here is my algorithm.
1). I look from the start of the byte array until I find '\n' or '\r' without '\n'.
2). Then, I cut the byte array from the start to that point and using it to create a string (with CharSet if specified) using 'new String(byte[])' or 'new String(byte[], CharSet)'.
3). If that success without exception, we found the first valid line and return it.
4). Otherwise, these bytes may not be a string, so I look further to another '\n' or '\r' w/o '\n'. and this process repeat.
5. If the search ends at the end of available bytes I stop and return null (no valid line found).
My question is (Q2)Is the following algorithm adequate?
Just when I was about to implement it, I searched on Google and found that there are many other codes for new line, for example U+2424, U+0085, U+000C, U+2028 and U+2029.
So my last question is (Q3), Do I really need to detect these code? If I do, Will it increase the chance of false alarm?
I am well aware that recognize something from binary is not absolute. I am just trying to find the best balance.
To sum up, I have an array of byte and I want to extract a first valid string line from it with/without specific CharSet. This must be done in Java and avoid using any non-build-in library.
Thanks you all in advance.
I am afraid your problem is not well-defined. You write that you want to extract the "first valid string line" from your data. But whether somet byte sequence is a "valid string" depends on the encoding. So you must decide which encoding(s) you want to use in testing.
Sensible choices would be:
the platform default encoding (Java property "file.encoding")
UTF-8 (as it is most common)
a list of encodings you know your clients will use (such as several Russian or Chinese encodings)
What makes sense will depend on the data, there's no general answer.
Once you have your encodings, the problem of line termination should follow, as most encodings have rules on what terminates a line. In ASCII or Latin-1, LF,CR-LF and LF-CR would suffice. On Unicode, you need all the ones you listed above.
But again, there's no general answer, as new line codes are not strictly regulated. Again, it would depend on your data.
First of all let me ask you a question, is the data you are trying to process a legacy data? In other words, are you responsible for the input stream format that you are trying to consume here?
If you are indeed controlling the input format, then you probably want to take a decision Binary vs. Text out of the Q1 algorithm. For me this algorithm has one troubling part.
`4). Otherwise, these bytes may not be a string, so I look further to
another '\n' or '\r' w/o '\n'. and this process repeat.`
Are you dismissing input prior to line terminator and take the bytes that start immediately after, or try to reevaluate the string with now 2 line terminators? If former, you may have broken binary data interface, if latter you may still not parse the text correctly.
I think having well defined markers for binary data and text data in your stream will simplify your algorithm a lot.
Couple of words on String constructor. new String(byte[], CharSet) will not generate any exception if the byte array is not in particular CharSet, instead it will create a string full of question marks ( probably not what you want ). If you want to generate an exception you should use CharsetDecoder.
Also note that in Java 6 there are 2 constructors that take charset
String(byte[] bytes, String charsetName) and String(byte[] bytes, Charset charset). I did some simple performance test a while ago, and constructor with String charsetName is magnitudes faster than the one that takes Charset object ( Question to Sun: bug, feature? ).
I would try this:
make the IO reader put strings/lines into a thread safe collection (for example some implementation of BlockingQueue)
the main code has only reference to the synced collection and checks for new data when needed, like queue.peek(). It doesn't need to know about the io thread nor the stream.
Some pseudo java code (missing exception & io handling, generics, imports++) :
class IORunner extends Thread {
IORunner(InputStream in, BlockingQueue outputQueue) {
this.reader = new BufferedReader(new InputStreamReader(in, "utf-8"));
this.outputQueue = outputQueue;
}
public void run() {
String line;
while((line=reader.readLine())!=null)
this.outputQueue.put(line);
}
}
class Main {
public static void main(String args[]) {
...
BlockingQueue dataQueue = new LinkedBlockingQueue();
new IORunner(myStreamFromSomewhere, dataQueue).start();
while(true) {
if(!dataQueue.isEmpty()) { // can also use .peek() != null
System.out.println(dataQueue.take());
}
Thread.sleep(1000);
}
}
}
The collection decouples the input(stream) more from the main code. You can also limit the number of lines stored/mem used by creating the queue with a limited capacity (see blockingqueue doc).
The BufferedReader handles the checking of new lines for you :) The InputStreamReader handles the charset (recommend setting one yourself since the default one changes depending on OS etc.).
The java.text namespace is designed for this sort of natural language operation. The BreakIterator.getLineInstance() static method returns an iterator that detects line breaks. You do need to know the locale and encoding for best results, though.
Q2: The method you use seems reasonable enough to work.
Q1: Can't think of something better than the algorithm that you are using
Q3: I believe it will be enough to test for \r and \n. The others are too exotic for usual text files.
I just solved this to get test stubb working for Datagram - I did byte[] varName= String.getBytes(); then final int len = varName.length; then send the int as DataOutputStream and then the byte array and just do readInt() on the rcv then read bytes(count) using the readInt.
Not a lib, not hard to do either. Just read up on readUTF and do what they did for the bytes.
The string should construct from the byte array recovered that way, if not you have other problems. If the string can be reconstructed, it can be buffered ... no?
May be able to just use read / write UTF() in DataStream - why not?
{ edit: per OP's request }
//Sending end
String data = new String("fdsfjal;sajssaafe8e88e88aa");// fingers pounding keyboard
DataOutputStream dataOutputStream = new DataOutputStream();//
final Integer length = new Integer(data.length());
dataOutputStream.writeInt(length.intValue());//
dataOutputStream.write(data.getBytes());//
dataOutputStream.flush();//
dataOutputStream.close();//
// rcv end
DataInputStream dataInputStream = new DataInputStream(source);
final int sizeToRead = dataInputStream.readInt();
byte[] datasink = new byte[sizeToRead.intValue()];
dataInputStream.read(datasink,sizeToRead);
dataInputStream.close;
try
{
// constructor
// String(byte[] bytes, int offset, int length)
final String result = new String(datasink,0x00000000,sizeToRead);//
// continue coding here
Do me a favor, keep the heat off of me. This is very fast right in the posting tool - code probably contains substantial errors - it's faster for me just to explain it writing Java ~ there will be others who can translate it to other code language ( s ) which you can too if you wish it in another codebase. You will need exception trapping an so on, just do a compile and start fixing errors. When you get a clean compile, start over from the beginnning and look for blunders. ( that's what a blunder is called in engineering - a blunder )

In Java, what's the fastest way to "build" and use a string, character by character?

I have a Java socket connection that is receiving data intermittently. The number of bytes of data received with each burst varies. The data may or may not be terminated by a well-known character (such as CR or LF). The length of each burst of data is variable.
I'm attempting to build a string out of each burst of data. What is the fastest way (speed, not memory), to build a string that would later need to be parsed?
I began by using a byte array to store the incoming bytes, then converting them to a String with each burst, like so:
byte[] message = new byte[1024];
...
message[i] = //byte from socket
i++;
...
String messageStr = new String(message);
...
//parse the string here
The obvious disadvantage of this is that some bursts may be longer than 1024. I don't want to arbitrarily create a larger byte array (what if my burst is larger?).
What is the best way of doing this? Should I create a StringBuilder object and append() to it? That way I don't have to convert from StringBuilder to String (since the former has most of the methods I need).
Again, speed of execution is my biggest concern.
TIA.
I would probably use an InputStreamReader wrapped around a BufferedInputStream, which in turn wraps the socket. And write code that processes a message at a time, potentially blocking for input. If the input is bursty, I might run on a background thread and use a concurrent queue to hold the messages.
Reading a buffer at a time and trying to convert it to characters is exactly what BufferedInputStream/InputStreamReader does. And it does so while paying attention to encoding, something that (as other people have noted) your solution does not.
I don't know why you're focused on speed, but you'll find that the time to process data coming off a socket is far less than the time it takes to transmit over that socket.
Note that as you're transmitting across network layers, your speed of conversion may not be the bottleneck. It would be worth measuring, if you believe this to be important.
Note (also) that you're not specifying a character encoding in your conversion from bytes to String (via characters). I would enforce that somehow, otherwise your client/server communication can become corrupted if/when your client/server run in different environments. You can enforce that via JVM runtime args, but it's not a particularly safe option.
Given the above, you may want to consider StringBuilder(int capacity) to configure it in advance with an appropriate size, such that it doesn't have to reallocate on the fly.
First of all, you are making a lot of assumptions about charachter encoding that you receive from your client. Is it US-ASCII, ISO-8859-1, UTF-8?
Because in Java string is not a sequence of bytes, when it comes to building portable String serialization code you should make explicit decisions about character encoding. For this reason you should NEVER use StringBuilder to convert bytes to String. If you look at StringBuilder interface you will notice that it does not even have an append( byte ) method, and that's not because designers just overlooked it.
In your case you should definetly use a ByteArrayOutputStream. The only drawback of using straight implementation of ByteArrayOutputStream is that its toByteArray() method returns a copy of the array held by the object internaly. For this reason you may create your own subclass of ByteArrayOutputStream and provide direct access to the protected buf member.
Note that if you don't use default implementation, remember to specify byte array bounds in your String constructor. Your code should look something like this:
MyByteArrayOutputStream message = new MyByteArrayOutputStream( 1024 );
...
message.write( //byte from socket );
...
String messageStr = new String(message.buf, 0, message.size(), "ISO-8859-1");
Substitute ISO-8859-1 for the character set that's suitable for your needs.
StringBuilder is your friend. Add as many characters as needed, then call toString() to obtain the String.
I would create a "small" array of characters and append characters to it.
When the array is full (or transmission ends), use the StringBuilder.append(char[] str) method to append the content of the array to your string.
Now for the "small" size of the array - you will need to try various sizes and see which one is fastest for your production environment (performance "may" depend on the JVM, OS, processor type and speed and so on)
EDIT: Other people mentioned ByteArrayOutputStream, I agree it is another option as well.
You may wish to look at ByteArrayOutputStream depending if you are dealing with Bytes instead of Characters.
I generally will use a ByteArrayOutputStream to assemble a message then use toString/toByteArray to retrive it when the message is finished.
Edit: ByteArrayOutputStream can handle various Character set encoding through the toString call.
Personally, independent of language, I would send all characters to an in-memory data stream and once I need the string, I would read all characters from this stream into a string.
As an alternative, you could use a dynamic array, making it bigger whenever you need to add more characters. Even better, keep track of the actual length and increase the array with additional blocks instead of single characters. Thus, you would start with 1 character in an array of 1000 chars. Once you get at 1001, the array needs to be resized to 2000, then 3000, 4000, etc...
Fortunately, several languages including Java have a special build-in class that specializes in this. These are the stringbuilder classes. Whatever technique they use isn't that important but they have been created to boost performance so they should be your fastest option.
Have a look at the Text class. It's faster (for the operations you perform) and more deterministic than StringBuilder.
Note: the project containing the class is aimed at RTSJ VMs. It is perfectly usable in standard SE/EE environments though.

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