| 1 | This directory contains some examples illustrating techniques for extracting
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| 2 | high-performance from flex scanners. Each program implements a simplified
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| 3 | version of the Unix "wc" tool: read text from stdin and print the number of
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| 4 | characters, words, and lines present in the text. All programs were compiled
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| 5 | using gcc (version unavailable, sorry) with the -O flag, and run on a
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| 6 | SPARCstation 1+. The input used was a PostScript file, mainly containing
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| 7 | figures, with the following "wc" counts:
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| 8 |
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| 9 | lines words characters
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| 10 | 214217 635954 2592172
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| 11 |
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| 12 |
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| 13 | The basic principles illustrated by these programs are:
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| 14 |
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| 15 | - match as much text with each rule as possible
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| 16 | - adding rules does not slow you down!
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| 17 | - avoid backing up
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| 18 |
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| 19 | and the big caveat that comes with them is:
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| 20 |
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| 21 | - you buy performance with decreased maintainability; make
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| 22 | sure you really need it before applying the above techniques.
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| 23 |
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| 24 | See the "Performance Considerations" section of flexdoc for more
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| 25 | details regarding these principles.
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| 26 |
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| 27 |
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| 28 | The different versions of "wc":
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| 29 |
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| 30 | mywc.c
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| 31 | a simple but fairly efficient C version
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| 32 |
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| 33 | wc1.l a naive flex "wc" implementation
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| 34 |
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| 35 | wc2.l somewhat faster; adds rules to match multiple tokens at once
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| 36 |
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| 37 | wc3.l faster still; adds more rules to match longer runs of tokens
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| 38 |
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| 39 | wc4.l fastest; still more rules added; hard to do much better
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| 40 | using flex (or, I suspect, hand-coding)
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| 41 |
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| 42 | wc5.l identical to wc3.l except one rule has been slightly
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| 43 | shortened, introducing backing-up
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| 44 |
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| 45 | Timing results (all times in user CPU seconds):
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| 46 |
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| 47 | program time notes
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| 48 | ------- ---- -----
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| 49 | wc1 16.4 default flex table compression (= -Cem)
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| 50 | wc1 6.7 -Cf compression option
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| 51 | /bin/wc 5.8 Sun's standard "wc" tool
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| 52 | mywc 4.6 simple but better C implementation!
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| 53 | wc2 4.6 as good as C implementation; built using -Cf
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| 54 | wc3 3.8 -Cf
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| 55 | wc4 3.3 -Cf
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| 56 | wc5 5.7 -Cf; ouch, backing up is expensive
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