From 44ed65a545970829322098e22d10947e6d545d9a Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 23 Aug 2015 13:02:13 -0400
Subject: [PATCH] Avoid use of float arithmetic in bipartite_match.c.
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.
---
src/backend/lib/bipartite_match.c | 75 +++++++++++++++++++------------
src/include/lib/bipartite_match.h | 16 ++++---
2 files changed, 55 insertions(+), 36 deletions(-)
diff --git a/src/backend/lib/bipartite_match.c b/src/backend/lib/bipartite_match.c
index 037dd1de30b..317222f57ac 100644
--- a/src/backend/lib/bipartite_match.c
+++ b/src/backend/lib/bipartite_match.c
@@ -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;
}
diff --git a/src/include/lib/bipartite_match.h b/src/include/lib/bipartite_match.h
index 373bbede1e1..c106a7e41d1 100644
--- a/src/include/lib/bipartite_match.h
+++ b/src/include/lib/bipartite_match.h
@@ -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 */
--
GitLab