Reducing the cost of autodecoding

[Patrick Schluter via Digitalmars-d]

At least with that lookup table below, you can detect isolated 
continuation bytes (192 and 193) and invalid codes (above 244).

__gshared static immutable ubyte[] charWidthTab = [
             1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
             2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
             3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
             4, 4, 4, 4, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
];

length 5 and 6 need not to be tested specifically for your goto.