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Fix Reed-Solomon decoder.

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Calculate the correct number of syndromes, and fix calculation of the
error locator polynomial. Many QR codes with logos in the middle should
now work.
This commit is contained in:
Daniel Beer 2016-09-23 14:34:07 +12:00
parent ab341691c9
commit 87e79e769b
1 changed files with 36 additions and 29 deletions
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65
lib/decode.c
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@ -127,30 +127,6 @@ const static struct galois_field gf256 = {
* Polynomial operations * Polynomial operations
*/ */
static void poly_mult(uint8_t *r, const uint8_t *a, const uint8_t *b,
const struct galois_field *gf)
{
int i;
memset(r, 0, MAX_POLY);
for (i = 0; i < MAX_POLY; i++) {
int j;
for (j = 0; j + i < MAX_POLY; j++) {
uint8_t ca = a[i];
uint8_t cb = b[j];
if (!(ca && cb))
continue;
r[i + j] ^= gf->exp[(gf->log[ca] +
gf->log[cb]) %
gf->p];
}
}
}
static void poly_add(uint8_t *dst, const uint8_t *src, uint8_t c, static void poly_add(uint8_t *dst, const uint8_t *src, uint8_t c,
int shift, const struct galois_field *gf) int shift, const struct galois_field *gf)
{ {
@ -274,7 +250,7 @@ static int block_syndromes(const uint8_t *data, int bs, int npar, uint8_t *s)
continue; continue;
s[i] ^= gf256_exp[((int)gf256_log[c] + s[i] ^= gf256_exp[((int)gf256_log[c] +
(i + 1) * j) % 255]; i * j) % 255];
} }
if (s[i]) if (s[i])
@ -284,9 +260,41 @@ static int block_syndromes(const uint8_t *data, int bs, int npar, uint8_t *s)
return nonzero; return nonzero;
} }
static quirc_decode_error_t correct_block(uint8_t *data, const struct quirc_rs_params *ecc) static void eloc_poly(uint8_t *omega,
const uint8_t *s, const uint8_t *sigma,
int npar)
{ {
int npar = ecc->ce; int i;
memset(omega, 0, MAX_POLY);
for (i = 0; i < npar; i++) {
const uint8_t a = sigma[i];
const uint8_t log_a = gf256_log[a];
int j;
if (!a)
continue;
for (j = 0; j + 1 < MAX_POLY; j++) {
const uint8_t b = s[j + 1];
if (i + j >= npar)
break;
if (!b)
continue;
omega[i + j] ^=
gf256_exp[(log_a + gf256_log[b]) % 255];
}
}
}
static quirc_decode_error_t correct_block(uint8_t *data,
const struct quirc_rs_params *ecc)
{
int npar = ecc->bs - ecc->dw;
uint8_t s[MAX_POLY]; uint8_t s[MAX_POLY];
uint8_t sigma[MAX_POLY]; uint8_t sigma[MAX_POLY];
uint8_t sigma_deriv[MAX_POLY]; uint8_t sigma_deriv[MAX_POLY];
@ -305,8 +313,7 @@ static quirc_decode_error_t correct_block(uint8_t *data, const struct quirc_rs_p
sigma_deriv[i] = sigma[i + 1]; sigma_deriv[i] = sigma[i + 1];
/* Compute error evaluator polynomial */ /* Compute error evaluator polynomial */
poly_mult(omega, sigma, s, &gf256); eloc_poly(omega, s, sigma, npar - 1);
memset(omega + npar, 0, MAX_POLY - npar);
/* Find error locations and magnitudes */ /* Find error locations and magnitudes */
for (i = 0; i < ecc->bs; i++) { for (i = 0; i < ecc->bs; i++) {