The table generator for the NES emulator, written in PHP, consists
of two parts. opdata.php lists all the instructions supported by the
emulator.

You can see here a C++ version of the opdata.php file:
   http://bisqwit.iki.fi/src/arch/nescom-1.1.5.2.tar.gz

See files insdata.cc and insdata.hh .

Indeed, the PHP file was in fact a direct translation of the table
in this C++ code that I wrote years ago.
Then I extended the table to support the various unofficial opcodes.

Then the other part in the PHP version, mktables.php , is a lot more
obfuscated, because it went through a lot of revisions and testing.
Basically it uses a combination of the addressing mode and the
instruction name to decide which micro-components ($plan elements)
are required to synthesize that particular opcode.

For example, if the instruction is indicated as requiring the X register
as a source in all cases, this line adds the corresponding micro-operation
into the plan for that particular instruction:

  if($source_always_X)         $plan[1101] = "t &= X;";

And the code below checks that if the instruction is a DEC, a DEX, a DEY,
or a DCP -- all of which decrement an operand by one -- the corresponding
micro-operation is added into the plan for that particular instruction.

  if(preg_match('/dec|dex|dey|dcp/', $op))
                               $plan[1240] = "t = u8(t - 1);";