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categories: Jan,2010,MartinC

Very, Very, Very Long Strings in Gawk

In this discussion from comp.lang.awk, Martin Cohen builds a really, really, really long string in Gawk (300 million characters). He writes....

I had to extract 25-bit fields from a 90MB binary file, with frames of 10,000 fields indicated by a 33-bit sync value. The words I was interested in were indicated by being preceded by a special tag word.

My first step was to convert the binary file to hex text using od. I then wrote some gawk code to read the text file and extract the (32- bit) words preceded by the tag word. There were 9 million of them.

I concatenated them into a single string of 72 million hex characters (had to do byte-swapping along the way), and then, one character at a time, converted that into a string of 0's and 1's 300 million characters long. I could then easily (using index) search for the sync pattern (independent of any word boundaries) and find the data I wanted.

The total run time was just under 7 minutes (under Red Hat 5.1).

Some optimizations I had to do:

  • To build up the string of 9 million hex words, I had to group them 256 words at a time before concatenating them to the big string. When I just did one word at a time, I took forever - I had to stop it.
  • Similarly, When converting the hex to binary, I converted groups of 256 characters at a time before appending them to the big binary string.
  • Thinking about it now, I could probably combine the gathering of the hex words with the conversion to binary - my program was a revision of one where that combining wasn't done.

Anyway, it's nice that gawk can handle really long strings.

Author

Martin Cohen
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