Compression, Encryption and Hashing

why is compression used?

• reduce storage space of files on disk

• enable web pages to load quicker

• reduce buffering on video/audio streaming

• obey bandwidth restrictions by ISPs, mobile phone networks, email providers

what is lossy compression?

• removes non-essential data from the original file

• the resulting file can have lower quality as a result

• used in phone calls and in sound and video (eg. MP3 files remove frequencies higher than human hearing and quieter sounds)

what is lossless compression?

• no data is lost from the original file

• lossless compression records patterns in data and a set of instructions on how to use them

• computer can reverse procedure and reassemble the original file with exact accuracy

• useful for compression of program files, text files where lost characters would result in an error

• results in a larger file than lossy compression, but still smaller than original file

what is run length encoding?

• instead of recording every pixel in a sequence, it records the pixel’s value and the number of times it repeats

• reduces file size because 1 byte used to store colour (etc.) of pixel and 1 byte used to store number of times repeated

• unsuitable for text files without repeated strings

what are dictionary-based compression techniques?