w1102/bigint

C library for operations with integers of arbitrary size

BigInt

C library for operations with signed integers of arbitrary size.

Introduction

BigInt supports various basic mathematical operations: addition, subtraction, multiplication, and comparison. The primary use case is when very large numbers are needed; BigInts can contain up to the number of digits as the maximum value of an unsigned long (typically 2^32 - 1 on a 32-bit machine).

A BigInt is a struct with the following fields:

• digits -- An array of digits 0-9. The lowest index is the least significant digit.
• num_digits -- The number of digits in the digits array.
• is_negative -- Nonzero if the BigInt is negative, zero if the BigInt is positive.
• num_allocated_digits -- The amount of space allocated for digits; caller doesn't need to care about this.

Usage

Obtain a pointer to a new BigInt through a call to BigInt_construct:

BigInt* new_big_int = BigInt_construct(42); // Obtains a BigInt initialized to 42

The caller is responsible for freeing a BigInt allocated with BigInt_construct with a call to BigInt_free:

BigInt_free(new_big_int);

The contents of a BigInt can be printed to stdout with a call to BigInt_print.

BigInt_print(big_int);

Get a normal int back from a BigInt with using BigInt_to_int; but be careful; the BigInt must fit in an int type!

int a = BigInt_to_int(big_int);

BigInt operations take two BigInt parameters and place the result in the first parameter:

BigInt* a = BigInt_construct(15);
BigInt* b = BigInt_construct(-20);
BigInt_add(a, b);
BigInt_print(a); // Prints -5

The exception is BigInt_compare; this takes two BigInt parameters, changes neither, and returns the value of the comparison:

BigInt a = BigInt_construct(15);
BigInt b = BigInt_construct(-20);
printf("%i\n", BigInt_compare(a, b)); // prints 1
printf("%i\n", BigInt_compare(b, a)); // prints -1
printf("%i\n", BigInt_compare(a, a)); // prints 0