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-This directory contains some examples illustrating techniques for extracting
-high-performance from flex scanners. Each program implements a simplified
-version of the Unix "wc" tool: read text from stdin and print the number of
-characters, words, and lines present in the text. All programs were compiled
-using gcc (version unavailable, sorry) with the -O flag, and run on a
-SPARCstation 1+. The input used was a PostScript file, mainly containing
-figures, with the following "wc" counts:
- lines words characters
- 214217 635954 2592172
-The basic principles illustrated by these programs are:
- - match as much text with each rule as possible
- - adding rules does not slow you down!
- - avoid backing up
-and the big caveat that comes with them is:
- - you buy performance with decreased maintainability; make
- sure you really need it before applying the above techniques.
-See the "Performance Considerations" section of flexdoc for more
-details regarding these principles.
-The different versions of "wc":
- mywc.c
- a simple but fairly efficient C version
- wc1.l a naive flex "wc" implementation
- wc2.l somewhat faster; adds rules to match multiple tokens at once
- wc3.l faster still; adds more rules to match longer runs of tokens
- wc4.l fastest; still more rules added; hard to do much better
- using flex (or, I suspect, hand-coding)
- wc5.l identical to wc3.l except one rule has been slightly
- shortened, introducing backing-up
-Timing results (all times in user CPU seconds):
- program time notes
- ------- ---- -----
- wc1 16.4 default flex table compression (= -Cem)
- wc1 6.7 -Cf compression option
- /bin/wc 5.8 Sun's standard "wc" tool
- mywc 4.6 simple but better C implementation!
- wc2 4.6 as good as C implementation; built using -Cf
- wc3 3.8 -Cf
- wc4 3.3 -Cf
- wc5 5.7 -Cf; ouch, backing up is expensive