Упражнение 5. Translate paying attention to the underlined suffixes. Переведите, обращая внимание на подчеркнутые части слов.

Это суффиксы разных причастий одного и того же глагола: суффикс причастия прошедшего времени (-анный), да еще и пассивный, и суффикс причастия настоящего времени (-ющий), активный:

Model: when expanded «когда распакованный», будучи распакованным, when expending «когда распаковывающий», при распаковке;

when compressed, when compression;

the degree of compression achieved, when achieving the goal;

reduced redundancy;

a compressed file, a compression program; when determine the set, when determined.

Упражнение 6. Translate, memorize the difference between the words: Переведите, запоминая разницу между словами:

into the same space; examine some methods; in the same shift group; application of some techique; to send the same code; to represent the same number; for some types of file; the same mapping is used; with the same values; some algorithms are acceptable; the same re­sult; with the same initial tree.

Упражнение 7. Find in the text I or translate: Найдите в Тексте I или переведите: _v

I) стоимость байта памяти; 2) технологии сжатия данных; 3) может быть выгодно; 4) нечастое обращение; 5) требоваться быстро; 6) сберечь время (деньги); 7) исходный текст; 8) распространять программное обеспечение; 9) уменьшить количество; 10) информационный обзор;

II) методы сжатия текста; 12) избыточность на входе; 13) произвольные строки символов; 14) текст на естественном языке; 15) встречаться с равной частотой; 16) удалить избыточность; 17) стоящий; 18) точно восстанавливаемый; 19) техника восстановления; 20) неточная распа­ковка; 21) запись в произвольном виде; 22) точно такое же располо­жение; 23) при распаковке; 24) из-за накладных расходов; 25) установ­ление соответствий путем наложения ... на; 26) заменяемый входной объект; 27) не без недостатков; 28) ошибки в хранении и передаче; 29) восприимчивость к ошибкам; 30) распространять через.

Упражнение 8. Give synonyms for the following: Подберите синонимы к следующим словам:

to decline; rapidly; cost; to compress; frequent; expansion; occur; to remove; inexact; layout; error; normally.

Упражнение 9. Be sure that you remember what the words mean: Убедитесь, что вы помните, что означают эти служебные слова:

although; almost; always; still; therefore; thus; usually; that is; then; oth­ers; however; with; without.

Упражнение 10. Read the text I very carefully. Прочтите Текст I очень внимательно. Подчеркните все наречия в тексте и переведите. Обратите внимание на их место в предложении.

Text I

COMPRESSION

Although the cost of a byte of storage has declined rapidly, and is still declining, use of data compression techniques can almost always reduce the effective cost still further by squeezing more data into the same space. Consider a text archive or collection of documents. It may be advantageous to hold it in compressed form to save

space if access to a particular document is infrequent (therefore expansion of it is performed rarely) but the document may be required quickly(thus it needs to be on­line) . Compression could also save time (and money) when data is transmitted; for example, compression of source code might reduce the number of diskettes needed to distribute software. In this chapter we examine some methods for com­pressing text. Lelewer and Hirschberg cover most of them, and others, in their survey.

Data compression relies on there being redundancy in the input. Random strings of characters are not compressible to any great degree, neither are object files. Natural language text is redundant in that not all text units (characters, character pairs, words) occur with equal fre­quency. Compression tends to remove the redundancy, thus compression of a compressed file is normally not worth­while. Usually compression without loss of information is required, that is, the input file should be exactly recov­erable by application of some corresponding expansion tech­nique. In some cases, an inexact reversal may be accepta­ble. For example, when expanded, a source program in a free format language may not need to have exactly the same layout as the original.

There are many ways of measuring the degree of compression achieved, the following is a useful one:

length(input) - length (output) - size(X) length (input)

X is any information that we need in addition to the compressed text in order to be able to recreate the origi­nal. For example, if the original file is 2000 byte long and is compressed to 1000 bytes, and a 100-byte table is re­quired to expand the file back again, then the degree of compression is 45%. Because of overheads, the "compressed" version of a short file might be larger than the original. In general, compression techniques operate by mapping sections of the input file onto (smaller) sections of the output file. We can classify techniques by the type of the input object replaced (fixed or variable length) and the type of out-put object (fixed or variable length). In the following sections we look at the four categories of methods this leads to. Additional characteristics of a compression method are whether the mapping is adaptive (varies as the input is

processed) or static and whether the compression requires one pass or more than one pass over the input file.

Compression is not without disadvantages: reduced redun­dancy makes a file more vulnerable to storage and tran­smission errors. Lelewer and Hirschberg discuss the sus­ceptibility to error of the output of various algorithms. They note, for example, that the output from adaptive algorithms is more vulnerable than the output from stat­ic algorithms but even when a static algorithm is used, an error can propagate through an entire file.