Just when you thought it couldn't get any faster...
Here's a comparison of the revised roboticus version (with ikegami's speed suggestion):

source: horse:cow:dog:cat
              Rate      ruzam  roboticus roboticus2
ruzam       4695/s         --       -80%       -86%
roboticus  23049/s       391%         --       -33%
roboticus2 34607/s       637%        50%         --

source: horse
               Rate      ruzam roboticus2  roboticus
ruzam       64954/s         --       -71%       -76%
roboticus2 223392/s       244%         --       -16%
roboticus  266354/s       310%        19%         --

source: horse:cat
               Rate      ruzam  roboticus roboticus2
ruzam       26625/s         --       -72%       -76%
roboticus   96711/s       263%         --       -13%
roboticus2 111706/s       320%        16%         --

source: horse:cow:cat
              Rate      ruzam  roboticus roboticus2
ruzam      11023/s         --       -77%       -82%
roboticus  46987/s       326%         --       -24%
roboticus2 62225/s       465%        32%         --

source: horse:cow:dog:cat:mouse
              Rate      ruzam  roboticus roboticus2
ruzam       1913/s         --       -82%       -89%
roboticus  10826/s       466%         --       -39%
roboticus2 17652/s       823%        63%         --

source: horse:cow:dog:cat:rat:mouse
             Rate      ruzam  roboticus roboticus2
ruzam       798/s         --       -85%       -91%
roboticus  5253/s       559%         --       -43%
roboticus2 9221/s      1056%        76%         --
[download]

The new function is now an magnitude faster than my original attempt. That's no small change. Is this the limit of optimizing or can yet more juice be squeezed out of this function?

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