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On 30/09/2006, at 4:48 AM, David Jones wrote:

Actually I think NG was suggesting an approach where the parser, having eaten an 'x' say, changes the lexer state so that the next '+' is tokenised as part of a number, not as the token '+'.

An interesting side-effect of the modified parser/lexer is that it is slightly faster. Why?

Consider the case of this statement: a = -2

In standard Lua the lexer processes 4 tokens: a, =, -, 2

In the modified version we only have 3 tokens: a, =, -2

Thus we save a trip through the lexer, and probably a bit of code in the parser as well.

My timings:

Original Lua:

$ echo 'for i = 1,1000000 do loadstring "a = -2" end' | time lua
18.63user 0.00system 0:18.63elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (189major+30minor)pagefaults 0swaps

Modified Lua:

$ echo 'for i = 1,1000000 do loadstring "a = -2" end' | time
18.06user 0.00system 0:18.07elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (185major+30minor)pagefaults 0swaps

The modified version runs in 96% of the time taken by the original version.

- Nick