[vlc-devel] [RFC 3/3] vlc_arrays: append items in amortized O(1)
Romain Vimont
rom1v at videolabs.io
Thu Jul 19 17:57:18 CEST 2018
Use exponential growth to append items in _amortized constant time_,
instead of realloc() on every append/remove.
The strategy is similar to the one used by ARRAY_* macros (and C++
std::vector, Java ArrayList, Rust std::vec, etc.).
Also expose vlc_array_reserve() to avoid unnecessary reallocations, and
vlc_array_shrink_to_fit() to give a better control on the array size.
---
include/vlc_arrays.h | 107 +++++++++++++++++++++------
include/vlc_common.h | 9 ++-
src/test/arrays.c | 168 ++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 259 insertions(+), 25 deletions(-)
diff --git a/include/vlc_arrays.h b/include/vlc_arrays.h
index ce2936f1452..ce22c2e1a92 100644
--- a/include/vlc_arrays.h
+++ b/include/vlc_arrays.h
@@ -25,6 +25,9 @@
#ifndef VLC_ARRAYS_H_
#define VLC_ARRAYS_H_
+#include <vlc_common.h>
+#include <assert.h>
+
/**
* \file
* This file defines functions, structures and macros for handling arrays in vlc
@@ -257,12 +260,16 @@ static inline void *realloc_or_free( void *p, size_t sz )
************************************************************************/
typedef struct vlc_array_t
{
+ size_t i_capacity;
size_t i_count;
void ** pp_elems;
} vlc_array_t;
+#define VLC_ARRAY_MAX_LENGTH (SIZE_MAX / sizeof(void *))
+
static inline void vlc_array_init( vlc_array_t * p_array )
{
+ p_array->i_capacity = 0;
p_array->i_count = 0;
p_array->pp_elems = NULL;
}
@@ -308,21 +315,75 @@ static inline ssize_t vlc_array_find( const vlc_array_t *ar,
return -1;
}
+static inline int vlc_array_resize_internal(vlc_array_t *array, size_t target)
+{
+ void **pp = (void **) vlc_realloc(array->pp_elems, target, sizeof(void *));
+ if (unlikely(!pp))
+ return -1;
+
+ array->pp_elems = pp;
+ array->i_capacity = target;
+ return 0;
+}
+
+static inline int vlc_array_reserve(vlc_array_t *array, size_t min_capacity)
+{
+ if (min_capacity <= array->i_capacity)
+ return 0; /* nothing to do */
+
+ if (min_capacity > VLC_ARRAY_MAX_LENGTH)
+ return -1;
+
+ if (min_capacity < 10)
+ min_capacity = 10; /* do not allocate tiny arrays */
+
+ /* multiply by 1.5 first (this may not overflow due to the way
+ * VLC_ARRAY_MAX_LENGTH is defined, unless sizeof(void *) == 1, which will
+ * never happen) */
+ size_t new_capacity = array->i_capacity + (array->i_capacity >> 1);
+ /* if it's still not sufficient */
+ if (new_capacity < min_capacity)
+ new_capacity = min_capacity;
+ else if (new_capacity > VLC_ARRAY_MAX_LENGTH)
+ /* the capacity must never exceed VLC_ARRAY_MAX_LENGTH */
+ new_capacity = VLC_ARRAY_MAX_LENGTH;
+
+ return vlc_array_resize_internal(array, new_capacity);
+}
+
+static inline int vlc_array_shrink_to_internal(vlc_array_t *array,
+ size_t target)
+{
+ if (array->i_capacity == target)
+ return 0;
+
+ if (target == 0)
+ {
+ vlc_array_clear(array);
+ return 0;
+ }
+
+ return vlc_array_resize_internal(array, target);
+}
+
+static inline int vlc_array_shrink_to_fit(vlc_array_t *array)
+{
+ return vlc_array_shrink_to_internal(array, array->i_count);
+}
+
/* Write */
static inline int vlc_array_insert( vlc_array_t *ar, void *elem, int idx )
{
- void **pp = (void **)realloc( ar->pp_elems,
- sizeof( void * ) * (ar->i_count + 1) );
- if( unlikely(pp == NULL) )
+ if (unlikely(vlc_array_reserve(ar, ar->i_count + 1)))
return -1;
+ void **pp = ar->pp_elems;
size_t tail = ar->i_count - idx;
if( tail > 0 )
memmove( pp + idx + 1, pp + idx, sizeof( void * ) * tail );
- pp[idx] = elem;
+ ar->pp_elems[idx] = elem;
ar->i_count++;
- ar->pp_elems = pp;
return 0;
}
@@ -334,13 +395,10 @@ static inline void vlc_array_insert_or_abort( vlc_array_t *ar, void *elem, int i
static inline int vlc_array_append( vlc_array_t *ar, void *elem )
{
- void **pp = (void **)realloc( ar->pp_elems,
- sizeof( void * ) * (ar->i_count + 1) );
- if( unlikely(pp == NULL) )
+ if (unlikely(vlc_array_reserve(ar, ar->i_count + 1)))
return -1;
- pp[ar->i_count++] = elem;
- ar->pp_elems = pp;
+ ar->pp_elems[ar->i_count++] = elem;
return 0;
}
@@ -350,6 +408,22 @@ static inline void vlc_array_append_or_abort( vlc_array_t *ar, void *elem )
abort();
}
+/* shrink when necessary after remove() */
+static inline int vlc_array_autoshrink_internal(vlc_array_t *array)
+{
+ if (array->i_capacity <= 10)
+ return 0; /* do not shrink to tiny length */
+
+ if (array->i_count + (array->i_count >> 1) + 5 > array->i_capacity)
+ return 0; /* no need to shrink */
+
+ size_t target = array->i_count <= 6 ? 10 : array->i_count + 5;
+ if (target >= array->i_capacity)
+ return 0; /* do not grow */
+
+ return vlc_array_shrink_to_internal(array, target);
+}
+
static inline void vlc_array_remove( vlc_array_t *ar, size_t idx )
{
void **pp = ar->pp_elems;
@@ -359,18 +433,7 @@ static inline void vlc_array_remove( vlc_array_t *ar, size_t idx )
memmove( pp + idx, pp + idx + 1, sizeof( void * ) * tail );
ar->i_count--;
-
- if( ar->i_count > 0 )
- {
- pp = (void **)realloc( pp, sizeof( void * ) * ar->i_count );
- if( likely(pp != NULL) )
- ar->pp_elems = pp;
- }
- else
- {
- free( pp );
- ar->pp_elems = NULL;
- }
+ vlc_array_autoshrink_internal(ar);
}
diff --git a/include/vlc_common.h b/include/vlc_common.h
index c88503c1a55..5d379865819 100644
--- a/include/vlc_common.h
+++ b/include/vlc_common.h
@@ -937,8 +937,6 @@ static inline bool mul_overflow(unsigned long long a, unsigned long long b,
VLC_API char const * vlc_error( int ) VLC_USED;
-#include <vlc_arrays.h>
-
/* MSB (big endian)/LSB (little endian) conversions - network order is always
* MSB, and should be used for both network communications and files. */
@@ -1118,6 +1116,12 @@ static inline void *vlc_alloc(size_t count, size_t size)
return mul_overflow(count, size, &size) ? NULL : malloc(size);
}
+VLC_USED VLC_MALLOC
+static inline void *vlc_realloc(void *ptr, size_t count, size_t size)
+{
+ return mul_overflow(count, size, &size) ? NULL : realloc(ptr, size);
+}
+
/*****************************************************************************
* I18n stuff
*****************************************************************************/
@@ -1180,6 +1184,7 @@ VLC_API const char * VLC_Compiler( void ) VLC_USED;
/*****************************************************************************
* Additional vlc stuff
*****************************************************************************/
+#include "vlc_arrays.h"
#include "vlc_messages.h"
#include "vlc_objects.h"
#include "vlc_variables.h"
diff --git a/src/test/arrays.c b/src/test/arrays.c
index 82277bbbc12..b7f5b829bd4 100644
--- a/src/test/arrays.c
+++ b/src/test/arrays.c
@@ -1,5 +1,5 @@
/*****************************************************************************
- * arrays.h : Test for ARRAY_* macros
+ * arrays.h : Test for VLC arrays
*****************************************************************************
* Copyright (C) 2018 VLC authors and VideoLAN
*
@@ -158,10 +158,176 @@ static void test_array_bsearch(void)
ARRAY_RESET(array);
}
+static void test_vlc_array_insert_remove(void)
+{
+ vlc_array_t array;
+ vlc_array_init(&array);
+
+ char data[32];
+
+ vlc_array_append(&array, &data[0]);
+ assert(vlc_array_count(&array) == 1);
+ assert(vlc_array_get(&array, 0) == &data[0]);
+
+ vlc_array_remove(&array, 0);
+ assert(vlc_array_count(&array) == 0);
+
+ vlc_array_append(&array, &data[1]);
+ vlc_array_append(&array, &data[2]);
+ vlc_array_append(&array, &data[3]);
+ vlc_array_remove(&array, 1);
+ assert(vlc_array_count(&array) == 2);
+ assert(vlc_array_get(&array, 0) == &data[1]);
+ assert(vlc_array_get(&array, 1) == &data[3]);
+
+ vlc_array_insert(&array, &data[4], 1);
+ assert(vlc_array_count(&array) == 3);
+ assert(vlc_array_get(&array, 0) == &data[1]);
+ assert(vlc_array_get(&array, 1) == &data[4]);
+ assert(vlc_array_get(&array, 2) == &data[3]);
+
+ vlc_array_clear(&array);
+}
+
+static void test_vlc_array_find(void)
+{
+ vlc_array_t array;
+ vlc_array_init(&array);
+
+ char data[10];
+
+ for (int i = 0; i < 10; ++i)
+ vlc_array_append(&array, &data[i]);
+
+ ssize_t idx;
+
+ idx = vlc_array_find(&array, &data[0]);
+ assert(idx == 0);
+
+ idx = vlc_array_find(&array, &data[1]);
+ assert(idx == 1);
+
+ idx = vlc_array_find(&array, &data[4]);
+ assert(idx == 4);
+
+ idx = vlc_array_find(&array, &data[9]);
+ assert(idx == 9);
+
+ idx = vlc_array_find(&array, "some other pointer");
+ assert(idx == -1);
+
+ vlc_array_clear(&array);
+}
+
+static void test_vlc_array_grow()
+{
+ vlc_array_t array;
+ vlc_array_init(&array);
+
+ char data;
+
+ for (int i = 0; i < 50; ++i)
+ vlc_array_append(&array, &data); /* append */
+
+ assert(vlc_array_count(&array) == 50);
+
+ for (int i = 0; i < 25; ++i)
+ vlc_array_insert(&array, &data, 20); /* insert from the middle */
+
+ assert(vlc_array_count(&array) == 75);
+
+ for (int i = 0; i < 25; ++i)
+ vlc_array_insert(&array, &data, 0); /* prepend */
+
+ assert(vlc_array_count(&array) == 100);
+
+ for (int i = 0; i < 50; ++i)
+ vlc_array_remove(&array, 20); /* remove from the middle */
+
+ assert(vlc_array_count(&array) == 50);
+
+ for (int i = 0; i < 25; ++i)
+ vlc_array_remove(&array, 0); /* remove from the head */
+
+ assert(vlc_array_count(&array) == 25);
+
+ for (int i = 24; i >= 0; --i)
+ vlc_array_remove(&array, i); /* remove from the tail */
+
+ assert(vlc_array_count(&array) == 0);
+
+ vlc_array_clear(&array);
+}
+
+static void test_vlc_array_exp_growth()
+{
+ vlc_array_t array;
+ vlc_array_init(&array);
+
+ char data;
+ size_t old_capacity = array.i_capacity;
+ int realloc_count = 0;
+ for (int i = 0; i < 10000; ++i)
+ {
+ vlc_array_append(&array, &data);
+ if (array.i_capacity != old_capacity)
+ {
+ realloc_count++;
+ old_capacity = array.i_capacity;
+ }
+ }
+
+ /* Test specifically for an expected growth factor of 1.5. In practice, the
+ * result is even lower (19) due to the first alloc of size 10 */
+ assert(realloc_count <= 23); /* ln(10000) / ln(1.5) ~= 23 */
+
+ realloc_count = 0;
+ for (int i = 9999; i >= 0; --i)
+ {
+ vlc_array_remove(&array, i);
+ if (array.i_capacity != old_capacity)
+ {
+ realloc_count++;
+ old_capacity = array.i_capacity;
+ }
+ }
+
+ /* Same expectation for removals */
+ assert(realloc_count <= 23);
+
+ vlc_array_clear(&array);
+}
+
+static void test_vlc_array_reserve()
+{
+ vlc_array_t array;
+ vlc_array_init(&array);
+
+ vlc_array_reserve(&array, 800);
+ assert(array.i_capacity >= 800);
+
+ size_t initial_capacity = array.i_capacity;
+
+ char data;
+ for (int i = 0; i < 800; ++i)
+ {
+ vlc_array_append(&array, &data);
+ assert(array.i_capacity == initial_capacity); /* no realloc */
+ }
+
+ vlc_array_clear(&array);
+}
+
int main(void)
{
test_array_insert_remove();
test_array_foreach();
test_array_find();
test_array_bsearch();
+
+ test_vlc_array_insert_remove();
+ test_vlc_array_find();
+ test_vlc_array_grow();
+ test_vlc_array_reserve();
+ test_vlc_array_exp_growth();
}
--
2.18.0
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