compression interview questions
Top compression frequently asked interview questions
What is the the best JavaScript compressor available? I'm looking for a tool that:
- is easy to use
- has a high compression rate
- Produce reliable end results (doesn't mess up the code)
Source: (StackOverflow)
I have a webapp on a NGinx server. I set gzip on in the conf file and now I'm trying to see if it works. YSlow says it's not, but 5 out of 6 websites that do the test say it is. How can I get a definite answer on this and why is there a difference in the results?
Source: (StackOverflow)
Am I wrong in finding that Node.js does no gzip compression and there are no modules out there to perform gzip compression? How can a anyone use a web server that has no compression? What am I missing here? Should I try to—gasp—port the algorithm to JavaScript for server-side use?
Source: (StackOverflow)
Is it possible to compress (create a compressed archive) data while reading from stdin on Linux.
Source: (StackOverflow)
I am trying to create a .tar.xz compressed archive in one command. What is the specific syntax for that?
I have tried tar cf - file | xz file.tar.xz, but that does not work.
Source: (StackOverflow)
I use pdftk to compress a pdf using the following command line
pdftk file1.pdf output file2.pdf compress
It works as the weight of my file decreased.
Are there [options] to change the compression???
Or maybe other solutions to compress my file? It is heavy because some graphics have a lot of points. Is there a way to convert these graphs to jpg for instance and adapt the compression?
Source: (StackOverflow)
I need an example for compressing a string using GZip in android. I want to send a string like "hello" to the method and get the following zipped string:
BQAAAB+LCAAAAAAABADtvQdgHEmWJSYvbcp7f0r1StfgdKEIgGATJNiQQBDswYjN5pLsHWlHIymrKoHKZVZlXWYWQMztnbz33nvvvffee++997o7nU4n99//P1xmZAFs9s5K2smeIYCqyB8/fnwfPyLmeVlW/w+GphA2BQAAAA==
Then I need to decompress it. Can anybody give me an example and complete the following methods?
private String compressString(String input) {
//...
}
private String decompressString(String input) {
//...
}
Thanks,
update
According to scessor's answer, Now I have the following 4 methods. Android and .net compress and decompress methods. These methods are compatible with each other except in one case. I mean they are compatible in the first 3 states but incompatible in the 4th state:
- state 1) Android.compress <-> Android.decompress: (OK)
- state 2) Net.compress <-> Net.decompress: (OK)
- state 3) Net.compress -> Android.decompress: (OK)
- state 4) Android.compress -> .Net.decompress: (NOT OK)
can anybody solve it?
Android methods:
public static String compress(String str) throws IOException {
byte[] blockcopy = ByteBuffer
.allocate(4)
.order(java.nio.ByteOrder.LITTLE_ENDIAN)
.putInt(str.length())
.array();
ByteArrayOutputStream os = new ByteArrayOutputStream(str.length());
GZIPOutputStream gos = new GZIPOutputStream(os);
gos.write(str.getBytes());
gos.close();
os.close();
byte[] compressed = new byte[4 + os.toByteArray().length];
System.arraycopy(blockcopy, 0, compressed, 0, 4);
System.arraycopy(os.toByteArray(), 0, compressed, 4,
os.toByteArray().length);
return Base64.encode(compressed);
}
public static String decompress(String zipText) throws IOException {
byte[] compressed = Base64.decode(zipText);
if (compressed.length > 4)
{
GZIPInputStream gzipInputStream = new GZIPInputStream(
new ByteArrayInputStream(compressed, 4,
compressed.length - 4));
ByteArrayOutputStream baos = new ByteArrayOutputStream();
for (int value = 0; value != -1;) {
value = gzipInputStream.read();
if (value != -1) {
baos.write(value);
}
}
gzipInputStream.close();
baos.close();
String sReturn = new String(baos.toByteArray(), "UTF-8");
return sReturn;
}
else
{
return "";
}
}
.Net methods:
public static string compress(string text)
{
byte[] buffer = Encoding.UTF8.GetBytes(text);
MemoryStream ms = new MemoryStream();
using (GZipStream zip = new GZipStream(ms, CompressionMode.Compress, true))
{
zip.Write(buffer, 0, buffer.Length);
}
ms.Position = 0;
MemoryStream outStream = new MemoryStream();
byte[] compressed = new byte[ms.Length];
ms.Read(compressed, 0, compressed.Length);
byte[] gzBuffer = new byte[compressed.Length + 4];
System.Buffer.BlockCopy(compressed, 0, gzBuffer, 4, compressed.Length);
System.Buffer.BlockCopy(BitConverter.GetBytes(buffer.Length), 0, gzBuffer, 0, 4);
return Convert.ToBase64String(gzBuffer);
}
public static string decompress(string compressedText)
{
byte[] gzBuffer = Convert.FromBase64String(compressedText);
using (MemoryStream ms = new MemoryStream())
{
int msgLength = BitConverter.ToInt32(gzBuffer, 0);
ms.Write(gzBuffer, 4, gzBuffer.Length - 4);
byte[] buffer = new byte[msgLength];
ms.Position = 0;
using (GZipStream zip = new GZipStream(ms, CompressionMode.Decompress))
{
zip.Read(buffer, 0, buffer.Length);
}
return Encoding.UTF8.GetString(buffer);
}
}
Source: (StackOverflow)
I am newbie in .net. I am doing compression and decompression string in C#. There is a XML and I am converting in string and after that I am doing compression and decompression.There is no compilation error in my code except when I decompression my code and return my string, its returning only half of the XML.
Below is my code, please correct me where I am wrong.
Code:
class Program
{
public static string Zip(string value)
{
//Transform string into byte[]
byte[] byteArray = new byte[value.Length];
int indexBA = 0;
foreach (char item in value.ToCharArray())
{
byteArray[indexBA++] = (byte)item;
}
//Prepare for compress
System.IO.MemoryStream ms = new System.IO.MemoryStream();
System.IO.Compression.GZipStream sw = new System.IO.Compression.GZipStream(ms, System.IO.Compression.CompressionMode.Compress);
//Compress
sw.Write(byteArray, 0, byteArray.Length);
//Close, DO NOT FLUSH cause bytes will go missing...
sw.Close();
//Transform byte[] zip data to string
byteArray = ms.ToArray();
System.Text.StringBuilder sB = new System.Text.StringBuilder(byteArray.Length);
foreach (byte item in byteArray)
{
sB.Append((char)item);
}
ms.Close();
sw.Dispose();
ms.Dispose();
return sB.ToString();
}
public static string UnZip(string value)
{
//Transform string into byte[]
byte[] byteArray = new byte[value.Length];
int indexBA = 0;
foreach (char item in value.ToCharArray())
{
byteArray[indexBA++] = (byte)item;
}
//Prepare for decompress
System.IO.MemoryStream ms = new System.IO.MemoryStream(byteArray);
System.IO.Compression.GZipStream sr = new System.IO.Compression.GZipStream(ms,
System.IO.Compression.CompressionMode.Decompress);
//Reset variable to collect uncompressed result
byteArray = new byte[byteArray.Length];
//Decompress
int rByte = sr.Read(byteArray, 0, byteArray.Length);
//Transform byte[] unzip data to string
System.Text.StringBuilder sB = new System.Text.StringBuilder(rByte);
//Read the number of bytes GZipStream red and do not a for each bytes in
//resultByteArray;
for (int i = 0; i < rByte; i++)
{
sB.Append((char)byteArray[i]);
}
sr.Close();
ms.Close();
sr.Dispose();
ms.Dispose();
return sB.ToString();
}
static void Main(string[] args)
{
XDocument doc = XDocument.Load(@"D:\RSP.xml");
string val = doc.ToString(SaveOptions.DisableFormatting);
val = Zip(val);
val = UnZip(val);
}
}
My XML size is 63KB.
Source: (StackOverflow)
there are some js files in static/js/
1. a.js
2. b.js
3. c.js
how to config grunt.js to get below files:
1. a.min.js
2. b.min.js
3. c.min.js
as far, I have to type specific file name:
min: {
dist: {
src: 'js/**/*.js',
dest: 'js/min/xxx.min.js'
}
}
Source: (StackOverflow)
I have installed Static and dynamic compression for IIS7, as well as setting the two web.config values at my application Virtual Folder level. As I understand it, I don't need to enable compression at the server, or site level anymore, and I can manage it on a per folder basis using my web.config file.
I have two settings in my .config file that I have set to customize gzip for my app:
<httpCompression dynamicCompressionDisableCpuUsage="90"
dynamicCompressionEnableCpuUsage="0">
<scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
<dynamicTypes>
<remove mimeType="*/*"/>
<add mimeType="*/*" enabled="true" />
</dynamicTypes>
</httpCompression>
<urlCompression doDynamicCompression="true"
dynamicCompressionBeforeCache="true" />
However, when I run the application, I can clearly see that gzip is not used, because my page sizes are the same. I am also using YSlow for FireFox, which also confirms that my pages are not being gziped.
What am I missing here? In IIS6 it was a simple matter of specifying the file types, and setting the compression level between 0-10. I don't see the need documented to specify the file types or compression level, since the defaults seem to cover the file types, and I'm not seeing the level anywhere.
Source: (StackOverflow)
I am trying to implement GZip compression for my asp.net page (including my CSS and JS files). I tried the following code, but it only compresses my .aspx page (found it from YSlow)
HttpContext context = HttpContext.Current;
context.Response.Filter = new GZipStream(context.Response.Filter, CompressionMode.Compress);
HttpContext.Current.Response.AppendHeader("Content-encoding", "gzip");
HttpContext.Current.Response.Cache.VaryByHeaders["Accept-encoding"] = true;
The above code is only compressing my .aspx page code (markup) not the CSS and JS files which is included as external files.
Please tell me how can I implement GZip compression in ASP.NET using code (because I am on shared hosting server where I don't have access to IIS Server configurations). And also in the above code I am not getting the last two lines, why they are used and what's the purpose of these lines. Please explain!
Thanks
Source: (StackOverflow)
How can I create 7-Zip archives from my C# console application? I need to be able to extract the archives using the regular, widely available 7-Zip program.
Here are my results with the examples provided as answers to this question
- "Shelling out" to 7z.exe - this is the simplest and most effective approach, and I can confirm that it works nicely. As workmad3 mentions, I just need to guarantee that 7z.exe is installed on all target machines, which is something I can guarantee.
- 7Zip in memory compression - this refers to compressing cookies "in-memory" before sending to the client; this method seems somewhat promising. The wrapper methods (wrapping the LZMA SDK) return type
byte[]. When I write the byte[] array to a file, I can't extract it using 7-Zip (File.7z is not supported archive).
- 7zSharp Wrapper (found on CodePlex) - this wraps the 7z exe/LZMA SDK. I referenced the project from my app, and it successfully created some archive files, but I was unable to extract the files using the regular 7-Zip program (
File.7z is not supported archive).
- 7Zip SDK aka LZMA SDK - I guess I'm not smart enough to figure out how to use this (which is why I posted here)... Any working code examples that demonstrate creating a 7zip archive that is able to be extracted by the regular 7zip program?
- CodeProject C# (.NET) Interface for 7-Zip Archive DLLs - only supports extracting from 7zip archives... I need to create them!
- SharpZipLib - According to their FAQ, SharpZipLib doesn't support 7zip.
Source: (StackOverflow)
UPDATE Feb 10 2012:
zOompf has completed some very thorough research on this very topic here. It trumps any findings below.
UPDATE Sept 11 2010:
A testing platform has been created for this here
HTTP 1.1 definitions of GZIP and DEFLATE (zlib) for some background information:
" 'Gzip' is the gzip format, and 'deflate' is the zlib format. They
should probably have called the second one 'zlib' instead to avoid
confusion with the raw deflate compressed data format. While the HTTP
1.1 RFC 2616 correctly points to the zlib specification in RFC 1950
for the 'deflate' transfer encoding, there have been reports of
servers and browsers that incorrectly produce or expect raw deflate
data per the deflate specification in RFC 1951, most notably
Microsoft products. So even though the 'deflate' transfer encoding using the
zlib format would be the more efficient approach (and in fact exactly
what the zlib format was designed for), using the 'gzip' transfer
encoding is probably more reliable due to an unfortunate choice of
name on the part of the HTTP 1.1 authors." (source: http://www.gzip.org/zlib/zlib_faq.html)
So, my question: if I send RAW deflate data with NO zlib wrapper (or gzip,
for that matter) are there any modern browsers (e.g., IE6 and up, FF,
Chrome, Safari, etc) that can NOT understand the raw deflate
compressed data (assuming HTTP request header "Accept-Encoding" contains "deflate")?
Deflate data will ALWAYS be a few bytes smaller than GZIP.
If all these browsers can successfully decode the data, what
downsides are there to sending RAW deflate instead of zlib?
UPDATE Sept 11 2010:
A testing platform has been created for this here
Source: (StackOverflow)
I have a directory that I want to compress to send it by e-mail, I've tried this:
tar -cvf filename.tar.gz directory_to_compress/
But when I try send it by e-mail, Google says:
filename.tar.gz contains an executable file. For security reasons, Gmail does not allow you to send this type of file.
How to compress a directory into a tar.gz file from command line?
Source: (StackOverflow)
I'm searching for an algorithm to compress small text strings: 50-1000 bytes (i.e. URLs). Which algorithm works best for this?
Source: (StackOverflow)