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common/random_select: central place for reservoir sampling.

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Turns out we can make quite a simple API out of it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2020-08-11 14:35:56 +09:30
parent 12d0d5c185
commit 496c0dd1e6
11 changed files with 76 additions and 59 deletions
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8
gossipd/seeker.c
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@@ -9,6 +9,7 @@
#include <ccan/tal/tal.h>
#include <common/decode_array.h>
#include <common/pseudorand.h>
#include <common/random_select.h>
#include <common/status.h>
#include <common/timeout.h>
#include <common/type_to_string.h>
@@ -454,7 +455,7 @@ static bool get_unannounced_nodes(const tal_t *ctx,
{
size_t num = 0;
u64 offset;
u64 threshold = pseudorand_u64();
double total_weight = 0.0;
/* Pick an example short_channel_id at random to query. As a
* side-effect this gets the node. */
@@ -475,11 +476,8 @@ static bool get_unannounced_nodes(const tal_t *ctx,
(*scids)[num++] = c->scid;
} else {
/* Maybe replace one: approx. reservoir sampling */
u64 p = pseudorand_u64();
if (p > threshold) {
if (random_select(1.0, &total_weight))
(*scids)[pseudorand(max)] = c->scid;
threshold = p;
}
}
}