#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#define MM 8 /* RS code over GF(2**MM) */
#define KK 239 /* KK = number of information symbols */
#define NN ((1 << MM) - 1)
#define TT 8 /* Error correction capability */
#define DEBUG 0 /* Disable debug output by default */
#if (MM < 8)
typedef unsigned char dtype;
#else
typedef unsigned int dtype;
#endif
#if (KK >= NN)
#error "KK must be less than 2**MM - 1"
#endif
/* Primitive polynomials */
#if(MM == 2)
int pp[MM + 1] = { 1, 1, 1 };
#elif(MM == 3)
int pp[MM + 1] = { 1, 1, 0, 1 };
#elif(MM == 4)
int pp[MM + 1] = { 1, 1, 0, 0, 1 };
#elif(MM == 5)
int pp[MM + 1] = { 1, 0, 1, 0, 0, 1 };
#elif(MM == 6)
int pp[MM + 1] = { 1, 1, 极0, 0, 0, 0, 1 };
#elif(MM == 7)
int pp[MM + 1] = { 1, 0, 0, 1, 0, 0, 0, 1 };
#elif(MM == 8)
int pp[MM + 1] = { 1, 0, 1, 1, 1, 0, 0, 0, 1 };
#elif(MM == 9)
int pp[MM + 1] = { 1, 0, 0, 0, 1, 0, 0, 0, 0, 1 };
#elif(MM == 10)
int pp[MM + 1] = { 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1 };
#elif(MM == 11)
int pp[MM + 1] = { 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 };
#elif(MM == 12)
int pp[MM + 1] = { 1, 1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1 };
#elif(MM == 13)
int pp[MM + 1] = { 1, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1 };
#elif(MM == 14)
int pp[MM + 1] = { 1, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1 };
#elif(MM == 15)
int pp[MM + 1] = { 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 };
#elif(MM == 16)
int pp[MM + 1] = { 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1 };
#else
#error "MM must be in range 2-16"
#endif
int alpha_to[NN + 1];
int index_of[NN + 1];
int gg[NN - KK + 1];
int recd[NN];
int err_flag;
dtype data[KK], bb[NN - KK];
void generate_gf() {
register int i, mask;
mask = 1;
alpha_to[MM] = 0;
for (i = 0; i < MM; i++) {
alpha_to[i] = mask;
index_of[alpha_to[i]] = i;
if (pp[i] != 0)
alpha_to[MM] ^= mask;
mask <<= 1;
}
index_of[alpha_to[MM]] = MM;
mask >>= 1;
for (i = MM + 1; i < NN; i++) {
if (alpha_to[i - 1] >= mask)
alpha_to[i] = alpha_to[MM] ^ ((alpha_to[i - 1] ^ mask) << 1);
else
alpha_to[i] = alpha_to[i - 1] << 1;
index_of[alpha_to[i]] = i;
}
index_of[0] = -1;
}
void gen_poly() {
register int i, j;
gg[0] = 2; /* primitive element alpha = 2 */
gg[1] = 1; /* g(x) = (X+alpha) initially */
for (i = 2; i <= NN - KK; i++) {
gg[i] = 1;
for (j = i - 1; j > 0; j--)
if (gg[j] != 0)
gg[j] = gg[j - 1] ^ alpha_to[(index_of[gg[j]] + i) % NN];
else
gg[j] = gg[j - 1];
gg[0] = alpha_to[(index_of[gg[0]] + i) % NN];
}
/* convert gg[] to index form */
for (i = 0; i <= NN - KK; i++)
gg[i] = index_of[gg[i]];
}
void encode_rs() {
register int i, j;
int feedback;
for (i = 0; i < NN - KK; i++)
bb[i] = 0;
for (i = KK - 1; i >= 极0; i--) {
feedback = index_of[data[i] ^ bb[NN - KK - 1]];
if (feedback != -1) {
for (j = NN - KK - 1; j > 0; j--)
if (gg[j] != -1)
bb[j] = bb[j - 1] ^ alpha_to[(gg[j] + feedback) % NN];
else
bb[j] = bb[j - 1];
bb[0] = alpha_to[(gg[0] + feedback) % NN];
}
else {
for (j = NN - KK - 1; j > 0; j--)
bb[j] = bb[j - 1];
bb[0] = 0;
}
}
}
void decode_rs() {
register int i, j, u, q;
int elp[NN - KK + 2][NN - KK], d[NN - KK + 2], l[NN - KK + 2], u_lu[NN - KK + 2], s[NN - KK + 1];
int count = 0, syn_error = 0, root[TT], loc[TT], z[TT + 1], err[NN], reg[TT + 1];
int erasures[TT * 2]; // Array to store erasure positions
int erasure_count = 0; // Number of erasures
err_flag = 0;
// Detect erasures (positions with -1)
for (i = 0; i < NN; i++) {
if (recd[i] == -1) {
if (erasure_count < 2 * TT) {
erasures[erasure_count++] = i;
}
}
}
/* Compute syndromes */
for (i = 1; i <= NN - KK; i++) {
s[i] = 0;
for (j = 0; j < NN; j++) {
// 修改:删除位置视为0值计算伴随式
int index_val = recd[j];
if (index_val == -1) index_val = index_of[0]; // 删除位置视为0
if (index_val != -1) {
s[i] ^= alpha_to[(index_val + i * j) % NN];
}
}
if (s[i] != 0)
syn_error = 1;
s[i] = index_of[s[i]];
}
if (syn_error) {
err_flag = 1;
d[0] = 0;
d[1] = s[1];
elp[0][0] = 0;
elp[1][0] = 1;
for (i = 1; i < NN - KK; i++) {
elp[0][i] = -1;
elp[1][i] = 0;
}
l[0] = 0;
l[1] = 0;
u_lu[0] = -1;
u_lu[1] = 0;
u = 0;
// Initialize with erasures if any
if (erasure_count > 0) {
l[1] = erasure_count;
elp[1][0] = 1; // σ_0 = 1
// σ(x) = ∏_{i=0}^{v-1} (1 - X_i x)
for (i = 0; i < erasure_count; i++) {
int Xi = erasures[i]; // X_i = α^(erasures[i]) is not used directly
int alpha_Xi = alpha_to[Xi]; // actual element value
for (j = l[1]; j > 0; j--) {
// 使用域元素乘法而不是指数的加法
if (elp[1][j-1] != -1) {
int temp = index_of[elp[1][j-1]];
if (temp != -1) {
elp[1][j] ^= alpha_to[(temp + Xi) % NN];
}
}
}
// Also multiply the constant term
if (elp[1][0] != -1) {
int temp = index_of[elp[1][0]];
if (temp != -1) {
elp[1][0] = alpha_to[(temp + Xi) % NN];
}
}
}
// Compute initial discrepancy
if (s[1] != -1) {
d[1] = alpha_to[s[1]];
for (i = 1; i <= erasure_count; i++) {
if (elp[1][i] != 0) {
int temp = index_of[elp[1][i]];
if (temp != -1) {
d[1] ^= alpha_to[(temp + i) % NN];
}
}
}
d[1] = index_of[d[1]];
} else {
d[1] = -1;
}
}
do {
u++;
if (d[u] == -1) {
l[u + 1] = l[u];
for (i = 0; i <= l[u]; i++) {
elp[u + 1][i] = elp[u][i];
elp[u][i] = index_of[elp[u][i]];
}
}
else {
q = u - 1;
while ((d[q] == -1) && (q > 0))
q--;
if (q > 0) {
j = q;
do {
j--;
if ((d[j] != -1) && (u_lu[q] < u_lu[j]))
q = j;
} while (j > 0);
}
if (l[u] > l[q] + u - q)
l[u + 1] = l[u];
else
l[u + 1] = l[q] + u - q;
for (i = 0; i < NN - KK; i++)
elp[u + 1][i] = 0;
for (i = 0; i <= l[q]; i++) {
if (elp[q][i] != -1) {
elp[u + 1][i + u - q] = alpha_to[(d[u] + NN - d[q] + elp[q][i]) % NN];
}
}
for (i = 0; i <= l[u]; i++) {
elp[u + 1][i] ^= elp[u][i];
elp[u][i] = index_of[elp[u][i]];
}
}
u_lu[u + 1] = u - l[u + 1];
if (u < NN - KK) {
if (s[u + 1] != -1)
d[u + 1] = alpha_to[s[u + 1]];
else
d[u + 1] = 0;
for (i = 1; i <= l[u + 1]; i++) {
if (elp[u + 1][i] != 0) {
int temp = index_of[elp[u + 1][i]];
if (temp != -1 && s[u + 1 - i] != -1) {
d[u + 1] ^= alpha_to[(s[u + 1 - i] + temp) % NN];
}
}
}
d[u + 1] = index_of[d[u + 1]];
}
} while ((u < NN - KK) && (l[u + 1] <= TT + erasure_count));
u++;
if (l[u] <= TT + erasure_count) {
for (i = 0; i <= l[u]; i++)
elp[u][i] = index_of[elp[u][i]];
for (i = 1; i <= l[u]; i++)
reg[i] = elp[u][i];
count = 0;
for (i = 1; i <= NN; i++) {
q = 1;
for (j = 1; j <= l[u]; j++) {
if (reg[j] != -1) {
reg[j] = (reg[j] + j) % NN;
q ^= alpha_to[reg[j]];
}
}
if (!q) {
root[count] = i;
loc[count] = NN - i;
count++;
}
}
if (count == l[u]) {
for (i = 1; i <= l[u]; i++) {
if (s[i] != -1)
z[i] = alpha_to[s[i]];
else
z[i] = 0;
for (j = 1; j < i; j++) {
if (elp[u][i - j] != -1 && s[j] != -1) {
z[i] ^= alpha_to[(elp[u][i - j] + s[j]) % NN];
}
}
z[i] = index_of[z[i]];
}
for (i = 0; i < NN; i++) {
err[i] = 0;
if (recd[i] != -1)
recd[i] = alpha_to[recd[i]];
else
recd[i] = 0;
}
// 正确的Forney算法实现
for (i = 0; i < count; i++) {
// 分子计算
err[loc[i]] = alpha_to[z[1]];
for (j = 2; j <= count; j++) {
if (z[j] != -1) {
err[loc[i]] ^= alpha_to[(z[j] + j * root[i]) % NN];
}
}
if (err[loc[i]] != 0) {
// 转换为指数形式
err[loc[i]] = index_of[err[loc[i]]];
// 分母计算 - 使用经典实现
q = 0; /* form denominator of error term */
for (j = 0; j < count; j++) {
if (j != i) {
// 计算 1 - X_j * X_i^{-1} = 1 - α^{loc[j] - loc[i]}
int diff = (loc[j] - loc[i] + NN) % NN;
int term = 1 ^ alpha_to[diff];
q = (q + index_of[term]) % NN;
}
}
// 最终错误值计算
err[loc[i]] = alpha_to[(err[loc[i]] - (q + loc[i]) + 2 * NN) % NN];
// 纠正错误
recd[loc[i]] ^= err[loc[i]];
}
}
}
else {
err_flag = 2;
for (i = 0; i < NN; i++) {
if (recd[i] != -1)
recd[i] = alpha_to[recd[i]];
else
recd[i] = 0;
}
}
}
else {
err_flag = 2;
for (i = 0; i < NN; i++) {
if (recd[i] != -1)
recd[i] = alpha_to[recd[i]];
else
recd[i] = 0;
}
}
}
else {
for (i = 0; i < NN; i++) {
if (recd[i] != -1)
recd[i] = alpha_to[recd[i]];
else
recd[i] = 0;
}
}
// 重新计算伴随式以验证校正是否成功
int syn_error_after = 0;
for (i = 1; i <= NN - KK; i++) {
int s_after = 0;
for (j = 0; j < NN; j++) {
int index_val = index_of[recd[j]];
if (index_val == -1) continue;
s_after ^= alpha_to[(index_val + i * j) % NN];
}
if (s_after != 0) {
syn_error_after = 1;
break;
}
}
if (syn_error_after) {
err_flag = 2; // 校正失败
}
}
int main() {
register int i;
int acterr_num, err_pos[NN];
srand((unsigned int)time(NULL)); // Initialize random seed
/* generate the Galois Field GF(2**MM) */
generate_gf();
/* compute the generator polynomial */
gen_poly();
do {
// ====== 重置全局状态 ======
memset(data, 0, sizeof(data));
memset(bb, 0, sizeof(bb));
memset(recd, 0, sizeof(recd));
err_flag = 0;
// =========================
printf("\nReed-Solomon code is (%d,%d) over GF(2^%d).\n", NN, KK, MM);
printf("This can correct up to %d errors.\n", TT);
printf("\nEnter number of errors to inject (0-%d): ", TT);
scanf_s("%d", &acterr_num);
if (acterr_num > TT) {
printf("Error: Maximum %d errors can be corrected\n", TT);
continue;
}
if (acterr_num > 0) {
printf("Enter positions of errors (0-%d):\n", NN - 1);
for (i = 0; i < acterr_num; i++) {
printf("Error %d position: ", i + 1);
scanf_s("%d", &err_pos[i]);
if (err_pos[i] < 0 || err_pos[i] >= NN) {
printf("Invalid position! Must be 0-%d\n", NN - 1);
i--;
}
}
}
// Generate test data
for (i = 0; i < KK; i++)
data[i] = rand() % (NN + 1);
// Backup original data before encoding
dtype original_data[KK];
memcpy(original_data, data, sizeof(data));
printf("\nOriginal data (first 10 symbols):\n");
for (i = 0; i < 10 && i < KK; i++)
printf("%d ", data[i]);
printf("\n");
// Encode data
encode_rs();
// Form transmitted codeword
for (i = 0; i < NN - KK; i++)
recd[i] = bb[i];
for (i = NN - KK; i < NN; i++)
recd[i] = data[i - (NN - KK)];
// Make a copy for decoding
int recd_copy[NN];
for (i = 0; i < NN; i++)
recd_copy[i] = recd[i];
// Inject errors
printf("\nInjecting %d errors...\n", acterr_num);
for (i = 0; i < acterr_num; i++) {
int pos = err_pos[i];
int orig_val = recd[pos];
// 确保错误值在有效范围内
int error_val = (rand() % (NN - 1)) + 1; // 1 to NN-1
// 使用异或操作注入错误
recd[pos] ^= error_val;
printf(" Position %d: %d → %d (error = %d)\n",
pos, orig_val, recd[pos], error_val);
}
// Convert to index form for decoding
int recd_index[NN];
for (i = 0; i < NN; i++) {
recd_index[i] = recd[i];
recd[i] = index_of[recd[i]];
}
// Decode
decode_rs();
// Print decoding result
switch (err_flag) {
case 0:
printf("\nStatus: No errors detected\n");
break;
case 1:
printf("\nStatus: Errors corrected successfully\n");
break;
case 2:
printf("\nStatus: Too many errors to correct\n");
break;
}
// Print decoded data (first 10 symbols) - now from recd array
printf("\nDecoded data (first 10 symbols):\n");
for (i = NN - KK; i < NN - KK + 10 && i < NN; i++)
printf("%d ", recd[i]);
printf("\n");
// Print original data (first 10 symbols) from backup
printf("Original data (first 10 symbols):\n");
for (i = 0; i < 10 && i < KK; i++)
printf("%d ", original_data[i]);
printf("\n");
// Verify decoding - Compare decoded data with original data
int errors_data = 0;
for (i = 0; i < KK; i++) {
int decoded_value = recd[i + (NN - KK)];
if (decoded_value != original_data[i]) {
errors_data++;
printf("Data position %d: Original=%d, Decoded=%d\n",
i + (NN - KK), original_data[i], decoded_value);
}
}
// Verify parity symbols
int errors_parity = 0;
for (i = 0; i < NN - KK; i++) {
if (recd[i] != recd_copy[i]) {
errors_parity++;
printf("Parity position %d: Original=%d, Decoded=%d\n",
i, recd_copy[i], recd[i]);
}
}
if (errors_data == 0 && errors_parity == 0) {
printf("\nVerification: ALL symbols match original!\n");
} else {
printf("\nVerification: %d data symbols and %d parity symbols differ from original\n",
errors_data, errors_parity);
}
printf("\nPress ESC to exit, any other key to continue...");
} while (_getch() != 27);
return 0;
}
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