86 lines
2.5 KiB
C
86 lines
2.5 KiB
C
#include "parameters.h"
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#include "repetition.h"
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#include <stddef.h>
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#include <stdint.h>
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#include <stdio.h>
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/**
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* @file repetition.c
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* @brief Implementation of repetition codes
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*/
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#define MASK_N2 ((((uint64_t) 1) << PARAM_N2) - 1)
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static inline int32_t popcount(uint64_t n);
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/**
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* @brief Encoding each bit in the message m using the repetition code
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*
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*
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* @param[out] em Pointer to an array that is the code word
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* @param[in] m Pointer to an array that is the message
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*/
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void PQCLEAN_HQC192_CLEAN_repetition_code_encode(uint64_t *em, const uint64_t *m) {
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uint64_t bit, idx_r;
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size_t pos_r;
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pos_r = 0;
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for (size_t i = 0; i < VEC_N1_SIZE_64; i++) {
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for (size_t j = 0; j < 64 && pos_r < PARAM_N1N2; j++) {
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bit = -((m[i] >> j) & 1);
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idx_r = (pos_r & 0x3f);
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em[(pos_r >> 6) + 0] ^= (bit & 0x7FFFFFFFFFFFFFFUL) << idx_r;
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em[(pos_r >> 6) + 1] ^= (bit & 0x3FFFFFFFFFFFFFFUL) >> ((63 - idx_r));
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pos_r += PARAM_N2;
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}
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}
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}
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/**
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* @brief Compute the Hamming weight of the 64-bit integer n
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*
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* The Hamming weight is computed using a trick described in
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* Henry S. Warren : "Hacker's Delight", chap 5., p. 66
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* @param[out] the Hamming weight of n
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* @param[in] a 64-bit integer n
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*/
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static inline int32_t popcount(uint64_t n) {
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n -= (n >> 1) & 0x5555555555555555UL;
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n = (n & 0x3333333333333333UL) + ((n >> 2) & 0x3333333333333333UL);
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n = (n + (n >> 4)) & 0x0f0f0f0f0f0f0f0fUL;
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return (n * 0x0101010101010101UL) >> 56;
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}
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/**
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* @brief Decoding the code words to a message using the repetition code
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*
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* We use a majority decoding. In fact we have that PARAM_N2 = 2 * PARAM_T + 1, thus,
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* if the Hamming weight of the vector is greater than PARAM_T, the code word is decoded
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* to 1 and 0 otherwise.
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*
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* @param[out] m Pointer to an array that is the message
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* @param[in] em Pointer to an array that is the code word
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*/
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void PQCLEAN_HQC192_CLEAN_repetition_code_decode(uint64_t *m, const uint64_t *em) {
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size_t t = 0;
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uint32_t b, bn, bi, c, cn, ci;
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uint64_t cx, ones;
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uint64_t mask;
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for (b = 0; b < PARAM_N1N2 - PARAM_N2 + 1; b += PARAM_N2) {
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bn = b >> 6;
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bi = b & 63;
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c = b + PARAM_N2 - 1;
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cn = c >> 6;
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ci = c & 63;
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cx = em[cn] << (63 - ci);
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mask = (uint64_t) (-((int64_t) (cn ^ (bn + 1))) >> 63); // cn != bn+1
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ones = popcount(((em[bn] >> bi) & MASK_N2) | (cx & ~mask));
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m[t >> 6] |= (uint64_t) ((((PARAM_T - ones) >> 31) & 1) << (t & 63));
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t++;
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}
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}
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