diff options
Diffstat (limited to 'src/common')
| -rw-r--r-- | src/common/util.c | 45 | ||||
| -rw-r--r-- | src/common/util.h | 4 |
2 files changed, 49 insertions, 0 deletions
diff --git a/src/common/util.c b/src/common/util.c index 50097dac93..c52b279f1c 100644 --- a/src/common/util.c +++ b/src/common/util.c @@ -513,6 +513,51 @@ round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor) return number; } +/** Return the lowest x in [INT64_MIN, INT64_MAX] such that x is at least + * <b>number</b>, and x modulo <b>divisor</b> == 0. */ +int64_t +round_int64_to_next_multiple_of(int64_t number, int64_t divisor) +{ + tor_assert(divisor > 0); + if (number >= 0 && INT64_MAX - divisor + 1 >= number) + number += divisor - 1; + number -= number % divisor; + return number; +} + +/** Transform a random value <b>p</b> from the uniform distribution in + * [0.0, 1.0[ into a Laplace distributed value with location parameter + * <b>mu</b> and scale parameter <b>b</b> in [-Inf, Inf[. */ +double +sample_laplace_distribution(double mu, double b, double p) +{ + tor_assert(p >= 0.0 && p < 1.0); + /* This is the "inverse cumulative distribution function" from: + * http://en.wikipedia.org/wiki/Laplace_distribution */ + return mu - b * (p > 0.5 ? 1.0 : -1.0) + * tor_mathlog(1.0 - 2.0 * fabs(p - 0.5)); +} + +/** Add random noise between INT64_MIN and INT64_MAX coming from a + * Laplace distribution with mu = 0 and b = <b>delta_f</b>/<b>epsilon</b> + * to <b>signal</b> based on the provided <b>random</b> value in + * [0.0, 1.0[. */ +int64_t +add_laplace_noise(int64_t signal, double random, double delta_f, + double epsilon) +{ + /* cast to int64_t intended */ + int64_t noise = sample_laplace_distribution( + 0.0, /* just add noise, no further signal */ + delta_f / epsilon, random); + if (noise > 0 && INT64_MAX - noise < signal) + return INT64_MAX; + else if (noise < 0 && INT64_MIN - noise > signal) + return INT64_MIN; + else + return signal + noise; +} + /** Return the number of bits set in <b>v</b>. */ int n_bits_set_u8(uint8_t v) diff --git a/src/common/util.h b/src/common/util.h index 921dd79da0..b8fd20fd7d 100644 --- a/src/common/util.h +++ b/src/common/util.h @@ -172,6 +172,10 @@ uint64_t round_to_power_of_2(uint64_t u64); unsigned round_to_next_multiple_of(unsigned number, unsigned divisor); uint32_t round_uint32_to_next_multiple_of(uint32_t number, uint32_t divisor); uint64_t round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor); +int64_t round_int64_to_next_multiple_of(int64_t number, int64_t divisor); +double sample_laplace_distribution(double mu, double b, double p); +int64_t add_laplace_noise(int64_t signal, double random, double delta_f, + double epsilon); int n_bits_set_u8(uint8_t v); /* Compute the CEIL of <b>a</b> divided by <b>b</b>, for nonnegative <b>a</b> |