algorithm - What's the fastest way to obtain the highest decimal digit of an integer? -

this question has answer here:

• how retrieve first decimal digit of number efficiently 10 answers

what's fastest way implement

template <typename t> unsigned highest_decimal_digit(t x); 

(which returns e.g. 3 356431, 7 71 , 9 9)?

the best can think of is:

• constexpr-calculate "middle-size" power of 10 fits t.
• perform binary search (over powers of 10, possibly using constexpr-constructed lookup table) find p, highest power-of-10 lower x.
• return x divided p

... maybe there's approach.

notes:

• i expressed question , approach in c++14ish terms, , solution in code nice, abstract solution (or solution in x86_64 assembly) fine. want work (unsigned) integer types, though.
• you may ignore signed integral types.
• i didn't specify "fast" is, hardware-conscious please.

the best option seems to combine clz approach , divide precalculated power of 10. so, in pseudocode:

powers10=[1,1,1,1,10,10,10,10,100,100...]; // contains powers of 10 map clz values int firstdigit(unsigned int_type a) {     if (a<10) return a; // 1 digit, screw     int j=typesize(a)-clz(a);     if (j==3) return 1; // 10-16 hit this, return 1     int k=a/powers10[j];     if (k>9) return 1; else return k; } 

typesize() returns 64 long long, 32 int , 16 short int.