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git-subtree-dir: src/simplicity git-subtree-split: 86ac0f92c4b2b6d694dca85dce3b6909aed25894
54 lines
1.9 KiB
C
54 lines
1.9 KiB
C
/* This modules defines a structure representing bit strings. */
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#ifndef SIMPLICITY_BITSTRING_H
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#define SIMPLICITY_BITSTRING_H
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#include <stddef.h>
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#include <stdint.h>
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#include <limits.h>
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#include <stdbool.h>
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#include "simplicity_assert.h"
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/* Represents a bitstring of length 'len' bits using an array of unsigned char.
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* The bit at index 'n', where 0 <= 'n' < 'len', is located at bit '1 << (CHAR_BIT - 1 - (offset + n) % CHAR_BIT)' of
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* array element 'arr[(offset + n) / CHAR_BIT]'.
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* Other bits in the array may be any value.
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*
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* Invariant: len <= 2^31
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* offset + length <= SIZE_MAX
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* 0 < len implies unsigned char arr[(offset + len - 1) / CHAR_BIT + 1];
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*/
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typedef struct bitstring {
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const unsigned char* arr;
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size_t len;
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size_t offset;
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} bitstring;
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/* Return the nth bit from a bitstring.
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*
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* Precondition: NULL != s
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* n < s->len;
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*/
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static inline bool getBit(const bitstring *s, size_t n) {
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size_t total_offset = s->offset + n;
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simplicity_assert(n < s->len);
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return 1 & (s->arr[total_offset / CHAR_BIT] >> (CHAR_BIT - 1 - (total_offset % CHAR_BIT)));
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}
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/* Return 8 bits from a bitstring staring from the nth bit.
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*
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* Precondition: NULL != s
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* n + 8 <= s->len;
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*/
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static inline uint_fast8_t getByte(const bitstring *s, size_t n) {
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simplicity_assert(8 <= s->len);
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simplicity_assert(n <= s->len - 8);
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size_t total_offset = s->offset + n;
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if (total_offset % CHAR_BIT <= CHAR_BIT - 8) {
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return (uint_fast8_t)(0xff & (s->arr[total_offset / CHAR_BIT] >> (CHAR_BIT - 8 - (total_offset % CHAR_BIT))));
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} else {
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/* CHAR_BIT < total_offset % CHAR_BIT + 8 */
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return (uint_fast8_t)(0xff & (((1U * s->arr[total_offset / CHAR_BIT]) << (total_offset % CHAR_BIT - (CHAR_BIT - 8)))
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}
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}
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#endif
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