diff options
Diffstat (limited to 'gl/lib')
| -rw-r--r-- | gl/lib/randperm.c | 149 |
1 files changed, 142 insertions, 7 deletions
diff --git a/gl/lib/randperm.c b/gl/lib/randperm.c index 97c8d9a8b..26181b8a6 100644 --- a/gl/lib/randperm.c +++ b/gl/lib/randperm.c @@ -19,9 +19,11 @@ #include <config.h> +#include "hash.h" #include "randperm.h" #include <limits.h> +#include <stdlib.h> #include "xalloc.h" @@ -57,6 +59,94 @@ randperm_bound (size_t h, size_t n) return bound; } +/* Swap elements I and J in array V. */ + +static void +swap (size_t *v, size_t i, size_t j) +{ + size_t t = v[i]; + v[i] = v[j]; + v[j] = t; +} + +/* Structures and functions for a sparse_map abstract data type that's + used to effectively swap elements I and J in array V like swap(), + but in a more memory efficient manner (when the number of permutations + performed is significantly less than the size of the input). */ + +struct sparse_ent_ +{ + size_t index; + size_t val; +}; + +static size_t +sparse_hash_ (void const *x, size_t table_size) +{ + struct sparse_ent_ const *ent = x; + return ent->index % table_size; +} + +static bool +sparse_cmp_ (void const *x, void const *y) +{ + struct sparse_ent_ const *ent1 = x; + struct sparse_ent_ const *ent2 = y; + return ent1->index == ent2->index; +} + +typedef Hash_table sparse_map; + +/* Initialize the structure for the sparse map, + when a best guess as to the number of entries + specified with SIZE_HINT. */ + +static sparse_map * +sparse_new (size_t size_hint) +{ + return hash_initialize (size_hint, NULL, sparse_hash_, sparse_cmp_, free); +} + +/* Swap the values for I and J. If a value is not already present + then assume it's equal to the index. Update the value for + index I in array V. */ + +static void +sparse_swap (sparse_map *sv, size_t* v, size_t i, size_t j) +{ + struct sparse_ent_ *v1 = hash_delete (sv, &(struct sparse_ent_) {i,0}); + struct sparse_ent_ *v2 = hash_delete (sv, &(struct sparse_ent_) {j,0}); + + /* FIXME: reduce the frequency of these mallocs. */ + if (!v1) + { + v1 = xmalloc (sizeof *v1); + v1->index = v1->val = i; + } + if (!v2) + { + v2 = xmalloc (sizeof *v2); + v2->index = v2->val = j; + } + + size_t t = v1->val; + v1->val = v2->val; + v2->val = t; + if (!hash_insert (sv, v1)) + xalloc_die (); + if (!hash_insert (sv, v2)) + xalloc_die (); + + v[i] = v1->val; +} + +static void +sparse_free (sparse_map *sv) +{ + hash_free (sv); +} + + /* From R, allocate and return a malloc'd array of the first H elements of a random permutation of N elements. H must not exceed N. Return NULL if H is zero. */ @@ -79,21 +169,66 @@ randperm_new (struct randint_source *r, size_t h, size_t n) default: { + /* The algorithm is essentially the same in both + the sparse and non sparse case. In the sparse case we use + a hash to implement sparse storage for the set of n numbers + we're shuffling. When to use the sparse method was + determined with the help of this script: + + #!/bin/sh + for n in $(seq 2 32); do + for h in $(seq 2 32); do + test $h -gt $n && continue + for s in o n; do + test $s = o && shuf=shuf || shuf=./shuf + num=$(env time -f "$s:${h},${n} = %e,%M" \ + $shuf -i0-$((2**$n-2)) -n$((2**$h-2)) | wc -l) + test $num = $((2**$h-2)) || echo "$s:${h},${n} = failed" >&2 + done + done + done + + This showed that if sparseness = n/h, then: + + sparseness = 128 => .125 mem used, and about same speed + sparseness = 64 => .25 mem used, but 1.5 times slower + sparseness = 32 => .5 mem used, but 2 times slower + + Also the memory usage was only significant when n > 128Ki + */ + bool sparse = (n >= (128 * 1024)) && (n / h >= 32); + size_t i; + sparse_map *sv; - v = xnmalloc (n, sizeof *v); - for (i = 0; i < n; i++) - v[i] = i; + if (sparse) + { + sv = sparse_new (h * 2); + if (sv == NULL) + xalloc_die (); + v = xnmalloc (h, sizeof *v); + } + else + { + sv = NULL; /* To placate GCC's -Wuninitialized. */ + v = xnmalloc (n, sizeof *v); + for (i = 0; i < n; i++) + v[i] = i; + } for (i = 0; i < h; i++) { size_t j = i + randint_choose (r, n - i); - size_t t = v[i]; - v[i] = v[j]; - v[j] = t; + if (sparse) + sparse_swap (sv, v, i, j); + else + swap (v, i, j); } - v = xnrealloc (v, h, sizeof *v); + if (sparse) + sparse_free (sv); + else + v = xnrealloc (v, h, sizeof *v); } break; } |