Algorithm for computing inverse polynomials in NTRUEncrypt - cryptography

Question

I'm implementing in code an algorithm for computing inverse polynomials in the NTRU cryptosystem, and I'm using the paper "Almost Inverses and Fast NTRU Key Creation" by Joseph H. Silverman. I implemented the second pseudo-code as:
int inverse_mod_p(polynomial *r, polynomial *a)
{
int k;
int16_t b[NTRU_N + 1], c[NTRU_N + 1], f[NTRU_N + 1], g[NTRU_N + 1];
int i;
int16_t aux;
int zero_f;
int constant_f;
int deg_fg;
memset(b, 0, (NTRU_N + 1) * sizeof(int16_t));
b[0] = 1;
memset(c, 0, (NTRU_N + 1) * sizeof(int16_t));
memcpy(f, a->coeffs, NTRU_N * sizeof(int16_t));
f[NTRU_N] = 0;
memset(g, 0, (NTRU_N + 1) * sizeof(int16_t));
g[0] = -1;
g[NTRU_N] = 1;
while (1)
{
zero_f = 1;
for (i = 0; i < NTRU_N + 1; i++)
{
if (f[i] != 0)
{
zero_f = 0;
break;
}
}
if (zero_f)
return 1;
while (f[0] == 0)
{
for (i = 0; i < NTRU_N; i++)
{
f[i] = f[i + 1];
c[NTRU_N - i] = c[NTRU_N - i - 1];
}
f[NTRU_N] = 0;
c[0] = 0;
k++;
}
constant_f = 1;
for (i = 1; i < NTRU_N + 1; i++)
{
if (f[i] != 0)
{
constant_f = 0;
break;
}
}
if (constant_f)
break;
deg_fg = 0;
for (i = NTRU_N; i >= 0; i--)
{
if (f[i] == 0 && g[i] != 0)
{
deg_fg = 1;
break;
}
else if (f[i] != 0 && g[i] == 0)
{
break;
}
}
if (deg_fg)
{
for (i = 0; i < NTRU_N + 1; i++)
{
aux = f[i];
f[i] = g[i];
g[i] = aux;
aux = b[i];
b[i] = c[i];
c[i] = aux;
}
}
if (f[0] == g[0])
{
for (i = 0; i < NTRU_N + 1; i++)
{
f[i] = (f[i] - g[i] + 3) % 3;
b[i] = (b[i] - c[i] + 3) % 3;
}
}
else
{
for (i = 0; i < NTRU_N + 1; i++)
{
f[i] = (f[i] + g[i] + 3) % 3;
b[i] = (b[i] + c[i] + 3) % 3;
}
}
}
k = k % NTRU_N;
for (i = NTRU_N - 1; i >= 0; i--)
{
if (i - k < 0)
r->coeffs[i - k + NTRU_N] = b[i] * f[0];
else
r->coeffs[i - k] = b[i] * f[0];
}
for (i = 0; i < NTRU_N; i++)
r->coeffs[i] = (r->coeffs[i] + 3) % 3;
return 0;
}
But this seems to be wrong. I tested it using the example giveng in Wikipedia: https://en.wikipedia.org/wiki/NTRUEncrypt . The polynomial -1 + x + x^2 - x^4 + x^6 + x^9 - x^10 should have as inverse the polynomial 1 + 2x + 2x^3 + 2x^4 + x^5 + 2x^7 + x^8 - x^10 , but I got the following result:
Polinomial:
-1 1 1 0 -1 0 1 0 0 1 -1
Inverse polinomial:
0 2 2 1 0 2 1 2 0 1 2
Where is the error in the implementation?