Small and fast Big Integer library in C

Made this for one of my own projects. malloc() is not used in this library. Everything is done with variables and alloca, so you may want to be careful if your numbers are bigger than a megabyte 😜

Based on the amazing kokke/tiny-bignum-c, but with the following changes:

• Karatsuba multiplication algorithm: shocking speed improvement for bigger numbers
• Added function to convert denary strings to integers
• No use of sscanf and sprintf for converting to and from hex
• Size of big ints is not set at compile time, but rather passed as an argument to all functions
• Optional use of 64 bit words: nice speed up if your CPU has 64→128 multiplications (it probably does)
• The add, subtract and multiply functions can take differently sized inputs

If what you want is the fastest library for big numbers, look at GNU GMP.

Usage example:

// integers in this demo will be 100 words in size
int words = 100;

// allocate 3 numbers, all 100 words in length
BigInt_t * number1 = malloc(BigIntWordSize*words);
BigInt_t * number2 = malloc(BigIntWordSize*words);
BigInt_t * number3 = malloc(BigIntWordSize*words);

// set number1 and number2 values to some random digits
BigInt_from_string(words, number1, "1234567890123456789012345678902187617826");
BigInt_from_string(words, number1, "12134");

// multiply number1 by number2, outputting to 3
BigInt_mul(words, number1, words, number2, words, number3);

... also I have not tested everything yet 😕