notes.txt

Mappings can only be state variables, not memory.  This means we can't
use a map to remember pushed rules.  Fortunately, we can know all rules
in advance and use an in-memory struct to keep a stack for each of them
(Tracery doesn't permit dynamic creation of rule names.)

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Strings don't have a way to append, or insert other than low-level memcpy's.
Seem better to use a bytes array to build up a string, and only return a
string at the last minute.  memcpy() from the solidity-stringutils package
is written partially in assembly.

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I tested three ways of picking strings, in testchoice.sol.  I then measured
their code size (if compiled one by one in a sample transaction) and their
gas usage in Remix online.

1) Nested if with a literal in each clause.

code size: 4460
gas cost (excluding transaction): 22710, 22548, 22602

2) Static array literal of strings

code size: 4198
gas cost: 24682, 24682, 24682

3) Nested if referencing portions of a packed string

code size: 3676

gas cost: 22798, 22879, 22744

The packed string seems a clear winner.

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animals.sol compiled with per-function packing: 12352 bytes
animals.sol compiles with global packing: 12264

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TODOs:

* Unicode characters aren't handled correctly, figure out where I need to
do .encode( "utf-8" ) before output
* Warn about contract length if too long
* Come up with a clever string-packing algorithm to save a little more space
   * "Longest common supersequence"?
* Look for ways to make the contract more efficient.
   * Idea: eliminate the loop when copying a literal less than 32 bytes long?
* Allow deletion of content.
   * I was thinking of making this a paid operation.
* Would running a tracery virtual machine inside Solidity be more
space-efficient?  Perhaps at higher gas cost.