Convert to and from RS addresses in C

rs.c

/*
    NXT address converter,
    Ported from original javascript (nxtchg) 
	To C by Jones	
*/

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>


int gexp[] = {1, 2, 4, 8, 16, 5, 10, 20, 13, 26, 17, 7, 14, 28, 29, 31, 27, 19, 3, 6, 12, 24, 21, 15, 30, 25, 23, 11, 22, 9, 18, 1};
int glog[] = {0, 0, 1, 18, 2, 5, 19, 11, 3, 29, 6, 27, 20, 8, 12, 23, 4, 10, 30, 17, 7, 22, 28, 26, 21, 25, 9, 16, 13, 14, 24, 15};

int cwmap[] = {3, 2, 1, 0, 7, 6, 5, 4, 13, 14, 15, 16, 12, 8, 9, 10, 11};

char alphabet[] = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ";

	int gmult(int a, int b) 
	{
		if (a == 0 || b == 0) return 0;

		int idx = (glog[a] + glog[b]) % 31;

		return gexp[idx];
	}

	int letterval(char letter)
	{
		int ret = 0;
		if(letter < '9') ret = letter - '2';
		else
		{
			ret = letter - 'A' + 8;
			if(letter > 'I') ret --;
			if(letter > 'O') ret --;
		}
		
		return ret;
	}

	long RS_decode(const char * rs)
	{
		int code[] = {1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
		int p=4;
		for(int i=0;i<17;i++) {
			code[cwmap[i]] = letterval(rs[p]);
			p++;

			if(rs[p] == '-') p++;
		}	

		long out = 0;
		for(int i=12;i>=0;i--)
		{
			out = out * 32 + code[i];
		}

		return out;

	}

	char * RS_encode(long id)
	{
		int code[] = {1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

		char * acc = malloc(21);
		sprintf(acc, "%ld", id);

		int inp[32];
		int out[32];
		int pos = 0;
		int len = 0;
		for(int a=0;*(acc+a) != '\0'; a++)
			len ++;

		if (len == 20 && *acc != '1') return "error";

		for (int i = 0; i < len; i++) {
			inp[i] = (int) *(acc+i) - (int) '0';
		}

		free(acc);

		int divide = 0;
		int newlen = 0;

		do // base 10 to base 32 conversion
		{
			divide = 0;
			newlen = 0;
			for (int i = 0; i < len; i++) {
				divide = divide * 10 + inp[i];

				if (divide >= 32) {
					inp[newlen++] = divide >> 5;
					divide &= 31;
				} else if (newlen > 0) {
					inp[newlen++] = 0;
				}
			}

			len = newlen;
			out[pos++] = divide;
		} while (newlen);

		for (int i = 0; i < 13; i++) // copy to code in reverse, pad with 0's
		{
			code[i] = (--pos >= 0 ? out[i] : 0);
		}

		int p[] = {0, 0, 0, 0};

		for (int i = 12; i >= 0; i--) {
			int fb = code[i] ^ p[3];

			p[3] = p[2] ^ gmult(30, fb);
			p[2] = p[1] ^ gmult(6, fb);
			p[1] = p[0] ^ gmult(9, fb);
			p[0] = gmult(17, fb);
		}

		code[13] = p[0];
		code[14] = p[1];
		code[15] = p[2];
		code[16] = p[3];


		char * rs = malloc(35);
		strcpy(rs, "NXT-");
		int j=4;
		for (int i = 0; i < 17; i++) {
			rs[j] = alphabet[code[cwmap[i]]];
			j++;

			if ((j % 5) == 3 && j < 20) 
			{
				rs[j] = '-';
				j++;

			}
		}

		return rs;
	}


	int main(int argc, const char* argv[])
	{
		long id = atol(argv[1]);

		char * rs = RS_encode(id);

		int i=0; // print out the 
		while(rs[i] != '\0')
		{
			printf("%c", rs[i]);
			i++;
		}
		printf("\n");

		printf("%ld \n", RS_decode(rs));

	}