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Read more about the Cauchy matrix: https://en.wikipedia.org/wiki/Cauchy_matrix The Art of Computer Programming Volume 1: Fundamental Algorithms Donald E. Knuth - 1997 Paragraph 1.2.3: Sums and Products Page 37, Cauchy's matrix: $a_{ij} = \frac{1}{x_i + y_j}$ Page 38, Exercise 41: $b_{ij} = \frac{\prod_{k=1}^{n}{(x_j + y_k)(x_k + y_i)}}{(x_j + y_i)\prod_{k \ne j}^{n}{(x_j - x_k)}\prod_{k \ne i}^{n}{(y_i - y_k)}}$
104 lines
3.4 KiB
C++
104 lines
3.4 KiB
C++
/*
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Cauchy Reed Solomon Erasure Coding
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Copyright 2023 Ahmet Inan <inan@aicodix.de>
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*/
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#pragma once
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namespace CODE {
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template <typename GF>
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struct CauchyReedSolomonEncoder
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{
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typedef typename GF::value_type value_type;
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typedef typename GF::ValueType ValueType;
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typedef typename GF::IndexType IndexType;
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// $a_{ij} = \frac{1}{x_i + y_j}$
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IndexType cauchy_matrix(int i, int j)
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{
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ValueType row(i), col(ValueType::N - j);
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return rcp(index(row + col));
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}
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void operator()(const ValueType *data, ValueType *block, int block_num, int block_len, int block_cnt)
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{
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assert(block_num <= ValueType::N - block_cnt);
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for (int k = 0; k < block_cnt; k++) {
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IndexType a_ik = cauchy_matrix(block_num, k);
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for (int j = 0; j < block_len; j++) {
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if (k)
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block[j] = fma(a_ik, data[block_len*k+j], block[j]);
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else
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block[j] = a_ik * data[block_len*k+j];
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}
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}
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}
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void operator()(const value_type *data, value_type *block, int block_num, int block_len, int block_cnt)
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{
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(*this)(reinterpret_cast<const ValueType *>(data), reinterpret_cast<ValueType *>(block), block_num, block_len, block_cnt);
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}
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void operator()(const void *data, void *block, int block_number, int block_bytes, int block_count)
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{
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assert(block_bytes % sizeof(value_type) == 0);
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(*this)(reinterpret_cast<const value_type *>(data), reinterpret_cast<value_type *>(block), block_number, block_bytes / sizeof(value_type), block_count);
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}
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};
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template <typename GF>
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struct CauchyReedSolomonDecoder
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{
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typedef typename GF::value_type value_type;
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typedef typename GF::ValueType ValueType;
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typedef typename GF::IndexType IndexType;
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// $b_{ij} = \frac{\prod_{k=1}^{n}{(x_j + y_k)(x_k + y_i)}}{(x_j + y_i)\prod_{k \ne j}^{n}{(x_j - x_k)}\prod_{k \ne i}^{n}{(y_i - y_k)}}$
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IndexType inverse_cauchy_matrix(const ValueType *rows, int i, int j, int n)
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{
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ValueType col_i(ValueType::N - i);
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IndexType prod_xy(0);
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for (int k = 0; k < n; k++) {
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ValueType col_k(ValueType::N - k);
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prod_xy *= index(rows[j] + col_k) * index(rows[k] + col_i);
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}
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IndexType prod_x(0);
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for (int k = 0; k < n; k++) {
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if (k != j) {
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prod_x *= index(rows[j] + rows[k]);
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}
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}
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IndexType prod_y(0);
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for (int k = 0; k < n; k++) {
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if (k != i) {
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ValueType col_k(ValueType::N - k);
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prod_y *= index(col_i + col_k);
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}
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}
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return prod_xy / (index(rows[j] + col_i) * prod_x * prod_y);
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}
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void operator()(ValueType *data, const ValueType *blocks, const ValueType *block_nums, int block_len, int block_cnt)
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{
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for (int i = 0; i < block_cnt; i++) {
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for (int k = 0; k < block_cnt; k++) {
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IndexType b_ik = inverse_cauchy_matrix(block_nums, i, k, block_cnt);
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for (int j = 0; j < block_len; j++) {
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if (k)
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data[j] = fma(b_ik, blocks[block_len*k+j], data[j]);
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else
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data[j] = b_ik * blocks[block_len*k+j];
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}
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}
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data += block_len;
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}
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}
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void operator()(value_type *data, const value_type *blocks, const value_type *block_nums, int block_len, int block_cnt)
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{
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(*this)(reinterpret_cast<ValueType *>(data), reinterpret_cast<const ValueType *>(blocks), reinterpret_cast<const ValueType *>(block_nums), block_len, block_cnt);
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}
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void operator()(void *data, const void *blocks, const value_type *block_numbers, int block_bytes, int block_count)
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{
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assert(block_bytes % sizeof(value_type) == 0);
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(*this)(reinterpret_cast<value_type *>(data), reinterpret_cast<const value_type *>(blocks), block_numbers, block_bytes / sizeof(value_type), block_count);
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}
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};
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}
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