ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Working with byte arrays
The next part of the code reads the header byte at offset position 8 and stores the value in the variable compMethod.
This byte contains a value indicating the compression method that was used to compress this file. Several compression
methods are allowed, but in practice nearly all .zip files use the DEFLATE compression algorithm. If the current file is
compressed with DEFLATE compression, compMethod is 8; if the file is uncompressed, compMethod is 0.
bytes.position = 8;
compMethod = bytes.readByte(); // store compression method (8 == Deflate)
Following the first 30 bytes is a variable-length portion of the header that contains the file name and, possibly, an extra
field. The variable offset stores the size of this portion. The size is calculated by adding the file name length and extra
field length, read from the header at offsets 26 and 28.
offset = 0;// stores length of variable portion of metadata
bytes.position = 26; // offset to file name length
flNameLength = bytes.readShort();// store file name
offset += flNameLength; // add length of file name
bytes.position = 28;// offset to extra field length
xfldLength = bytes.readShort();
offset += xfldLength;// add length of extra field
Next the program reads the variable-length portion of the file header for the number of bytes stored in the offset
variable.
// read variable length bytes between fixed-length header and compressed file data
zStream.readBytes(bytes, 30, offset);
The program reads the file name from the variable length portion of the header and displays it in the text area along
with the compressed (zipped) and uncompressed (original) sizes of the file.
// Flash version
bytes.position = 30;
fileName = bytes.readUTFBytes(flNameLength); // read file name
taFiles.appendText(fileName + "\n"); // write file name to text area
bytes.position = 18;
compSize = bytes.readUnsignedInt(); // store size of compressed portion
taFiles.appendText("\tCompressed size is: " + compSize + '\n');
bytes.position = 22; // offset to uncompressed size
uncompSize = bytes.readUnsignedInt(); // store uncompressed size
taFiles.appendText("\tUncompressed size is: " + uncompSize + '\n');
// Flex version
bytes.position = 30;
fileName = bytes.readUTFBytes(flNameLength); // read file name
taFiles.text += fileName + "\n"; // write file name to text area
bytes.position = 18;
compSize = bytes.readUnsignedInt(); // store size of compressed portion
taFiles.text += "\tCompressed size is: " + compSize + '\n';
bytes.position = 22; // offset to uncompressed size
uncompSize = bytes.readUnsignedInt(); // store uncompressed size
taFiles.text += "\tUncompressed size is: " + uncompSize + '\n';
The example reads the rest of the file from the file stream into bytes for the length specified by the compressed size,
overwriting the file header in the first 30 bytes. The compressed size is accurate even if the file is not compressed
because in that case the compressed size is equal to the uncompressed size of the file.
Last updated 6/6/2012
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