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Avoid use of float arithmetic in bipartite_match.c.

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Since the distances used in this algorithm are small integers (not more
than the size of the U set, in fact), there is no good reason to use float
arithmetic for them.  Use short ints instead: they're smaller, faster, and
require no special portability assumptions.

Per testing by Greg Stark, which disclosed that the code got into an
infinite loop on VAX for lack of IEEE-style float infinities.  We don't
really care all that much whether Postgres can run on a VAX anymore,
but there seems sufficient reason to change this code anyway.

In passing, make a few other small adjustments to make the code match
usual Postgres coding style a bit better.
This commit is contained in:
Tom Lane 2015-08-23 13:02:13 -04:00
parent 5956b7f9e8
commit 44ed65a545
2 changed files with 55 additions and 36 deletions
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75
src/backend/lib/bipartite_match.c
View File

@ -16,17 +16,20 @@
*/
#include "postgres.h"
#include <float.h>
#include <math.h>
#include <limits.h>
#include "lib/bipartite_match.h"
#include "miscadmin.h"
#include "utils/builtins.h"
/*
* The distances computed in hk_breadth_search can easily be seen to never
* exceed u_size. Since we restrict u_size to be less than SHRT_MAX, we
* can therefore use SHRT_MAX as the "infinity" distance needed as a marker.
*/
#define HK_INFINITY SHRT_MAX
static bool hk_breadth_search(BipartiteMatchState *state);
static bool hk_depth_search(BipartiteMatchState *state, int u, int depth);
static bool hk_depth_search(BipartiteMatchState *state, int u);
/*
* Given the size of U and V, where each is indexed 1..size, and an adjacency
@ -37,26 +40,29 @@ BipartiteMatch(int u_size, int v_size, short **adjacency)
{
BipartiteMatchState *state = palloc(sizeof(BipartiteMatchState));
Assert(u_size < SHRT_MAX);
Assert(v_size < SHRT_MAX);
if (u_size < 0 || u_size >= SHRT_MAX ||
v_size < 0 || v_size >= SHRT_MAX)
elog(ERROR, "invalid set size for BipartiteMatch");
state->u_size = u_size;
state->v_size = v_size;
state->matching = 0;
state->adjacency = adjacency;
state->pair_uv = palloc0((u_size + 1) * sizeof(short));
state->pair_vu = palloc0((v_size + 1) * sizeof(short));
state->distance = palloc((u_size + 1) * sizeof(float));
state->queue = palloc((u_size + 2) * sizeof(short));
state->matching = 0;
state->pair_uv = (short *) palloc0((u_size + 1) * sizeof(short));
state->pair_vu = (short *) palloc0((v_size + 1) * sizeof(short));
state->distance = (short *) palloc((u_size + 1) * sizeof(short));
state->queue = (short *) palloc((u_size + 2) * sizeof(short));
while (hk_breadth_search(state))
{
int u;
for (u = 1; u <= u_size; ++u)
for (u = 1; u <= u_size; u++)
{
if (state->pair_uv[u] == 0)
if (hk_depth_search(state, u, 1))
if (hk_depth_search(state, u))
state->matching++;
}
CHECK_FOR_INTERRUPTS(); /* just in case */
}
@ -79,19 +85,23 @@ BipartiteMatchFree(BipartiteMatchState *state)
pfree(state);
}
/*
* Perform the breadth-first search step of H-K matching.
* Returns true if successful.
*/
static bool
hk_breadth_search(BipartiteMatchState *state)
{
int usize = state->u_size;
short *queue = state->queue;
float *distance = state->distance;
short *distance = state->distance;
int qhead = 0; /* we never enqueue any node more than once */
int qtail = 0; /* so don't have to worry about wrapping */
int u;
distance[0] = get_float4_infinity();
distance[0] = HK_INFINITY;
for (u = 1; u <= usize; ++u)
for (u = 1; u <= usize; u++)
{
if (state->pair_uv[u] == 0)
{
@ -99,7 +109,7 @@ hk_breadth_search(BipartiteMatchState *state)
queue[qhead++] = u;
}
else
distance[u] = get_float4_infinity();
distance[u] = HK_INFINITY;
}
while (qtail < qhead)
@ -111,45 +121,52 @@ hk_breadth_search(BipartiteMatchState *state)
short *u_adj = state->adjacency[u];
int i = u_adj ? u_adj[0] : 0;
for (; i > 0; --i)
for (; i > 0; i--)
{
int u_next = state->pair_vu[u_adj[i]];
if (isinf(distance[u_next]))
if (distance[u_next] == HK_INFINITY)
{
distance[u_next] = 1 + distance[u];
Assert(qhead < usize + 2);
queue[qhead++] = u_next;
Assert(qhead <= usize + 2);
}
}
}
}
return !isinf(distance[0]);
return (distance[0] != HK_INFINITY);
}
/*
* Perform the depth-first search step of H-K matching.
* Returns true if successful.
*/
static bool
hk_depth_search(BipartiteMatchState *state, int u, int depth)
hk_depth_search(BipartiteMatchState *state, int u)
{
float *distance = state->distance;
short *distance = state->distance;
short *pair_uv = state->pair_uv;
short *pair_vu = state->pair_vu;
short *u_adj = state->adjacency[u];
int i = u_adj ? u_adj[0] : 0;
short nextdist;
if (u == 0)
return true;
if (distance[u] == HK_INFINITY)
return false;
nextdist = distance[u] + 1;
if ((depth % 8) == 0)
check_stack_depth();
check_stack_depth();
for (; i > 0; --i)
for (; i > 0; i--)
{
int v = u_adj[i];
if (distance[pair_vu[v]] == distance[u] + 1)
if (distance[pair_vu[v]] == nextdist)
{
if (hk_depth_search(state, pair_vu[v], depth + 1))
if (hk_depth_search(state, pair_vu[v]))
{
pair_vu[v] = u;
pair_uv[u] = v;
@ -158,6 +175,6 @@ hk_depth_search(BipartiteMatchState *state, int u, int depth)
}
}
distance[u] = get_float4_infinity();
distance[u] = HK_INFINITY;
return false;
}

16
src/include/lib/bipartite_match.h
View File

@ -11,7 +11,7 @@
/*
* Given a bipartite graph consisting of nodes U numbered 1..nU, nodes V
* numbered 1..nV, and an adjacency map of undirected edges in the form
* adjacency[u] = [n, v1, v2, v3, ... vn], we wish to find a "maximum
* adjacency[u] = [k, v1, v2, v3, ... vk], we wish to find a "maximum
* cardinality matching", which is defined as follows: a matching is a subset
* of the original edges such that no node has more than one edge, and a
* matching has maximum cardinality if there exists no other matching with a
@ -24,21 +24,23 @@
* the problem of planning a collection of grouping sets with the provably
* minimal number of sort operations.
*/
typedef struct bipartite_match_state
typedef struct BipartiteMatchState
{
/* inputs: */
int u_size; /* size of U */
int v_size; /* size of V */
short **adjacency; /* adjacency[u] = [k, v1,v2,v3,...,vk] */
/* outputs: */
int matching; /* number of edges in matching */
short **adjacency; /* adjacency[u] = [n, v1,v2,v3,...,vn] */
short *pair_uv; /* pair_uv[u] -> v */
short *pair_vu; /* pair_vu[v] -> u */
float *distance; /* distance[u], float so we can have +inf */
/* private state for matching algorithm: */
short *distance; /* distance[u] */
short *queue; /* queue storage for breadth search */
} BipartiteMatchState;
BipartiteMatchState *BipartiteMatch(int u_size, int v_size, short **adjacency);
extern BipartiteMatchState *BipartiteMatch(int u_size, int v_size, short **adjacency);
void BipartiteMatchFree(BipartiteMatchState *state);
extern void BipartiteMatchFree(BipartiteMatchState *state);
#endif /* BIPARTITE_MATCH_H */