grisupowgen is a command-line tool that generates the lookup table of cached powers of 10 used by the Grisu algorithms.

Each power of 10 is stored as a normalized 64-bit mantissa rounded to nearest and a binary exponent, so that 10^k ≈ mantissa × 2^exponent.

The tool outputs a fully self-contained C++ header containing:

MIN_EXPONENT, MAX_EXPONENT
a CachedPower { uint64_t mantissa; int exponent; } structure
a constexpr CachedPower powers[] lookup table

Usage

Option	Description
`-h`	Show help message
`-l <lower>`	Set lower exponent (default: `-343`)
`-u <upper>`	Set upper exponent (default: `343`)
`-v`	Show program version

Example

Generate powers for exponents −200 through +200 and write them to cached_power.hpp:

bash

grisupowgen -l -200 -u 200 > cached_power.hpp

The generated header will contain definitions like:

 cpp
#ifndef __CACHED_POWER_HPP__
#define __CACHED_POWER_HPP__
 
#include <cstdint>
 
#define MIN_EXPONENT -200
#define MAX_EXPONENT 200
 
struct CachedPower {
    uint64_t mantissa;
    int exponent;
};
 
constexpr CachedPower powers[] = {
    {0xc3f490aa77bd60fd, -728},
    ...
    ...
    ...
};
 
#endif

License

MIT