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Simplify btree_base_field_type<>

pull/5/head
Josh MacDonald 2011-12-14 13:47:35 -08:00
parent 9fe3bb3b33
commit 89a0af9086
1 changed files with 16 additions and 35 deletions
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51
btree.h
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@ -316,38 +316,6 @@ static bool btree_compare_keys(
return key_comparer::bool_compare(comp, x, y);
}
// btree_is_node_big<> is a recursive template to determine whether a
// node of TargetNodeSize bytes needs a larger base_fields type
// (uint16, instead of uint8) to accomodate >= 256 values per node.
template <int TargetNodeSize, int ValueSize>
struct btree_is_node_big :
btree_is_node_big<(TargetNodeSize / 2), (ValueSize / 2)> {
};
template <int TargetNodeSize>
struct btree_is_node_big<TargetNodeSize, 1> {
enum {
// In the base case, TargetNodeSize corresponds to single-byte
// entries and is the maximum number of values.
is_big = base::integral_constant<bool, (TargetNodeSize >= 256)>::value,
};
};
// A helper for sizing the btree node's base_fields. The "type"
// typedef in this struct is an integral type large enough to hold as
// many ValueSize-values as will fit a node of TargetNodeSize bytes.
template <int TargetNodeSize, int ValueSize>
struct btree_base_field_type {
enum {
// "value_space" is the maximum leaf node count. leaf nodes have
// a greatest maximum of the node types.
value_space = TargetNodeSize - 2 * sizeof(void*),
};
typedef typename base::if_<
btree_is_node_big<value_space, ValueSize>::is_big,
uint16,
uint8>::type type;
};
template <typename Key, typename Compare,
typename Alloc, int TargetNodeSize, int ValueSize>
struct btree_common_params {
@ -365,12 +333,21 @@ struct btree_common_params {
typedef Key key_type;
typedef ssize_t size_type;
typedef ptrdiff_t difference_type;
typedef typename btree_base_field_type<TargetNodeSize, ValueSize>::type
base_field_type;
enum {
kTargetNodeSize = TargetNodeSize,
// Available space for values. This is largest for leaf nodes,
// which has overhead no fewer than two pointers.
kNodeValueSpace = TargetNodeSize - 2 * sizeof(void*),
};
// This is an integral type large enough to hold as many
// ValueSize-values as will fit a node of TargetNodeSize bytes.
typedef typename base::if_<
(kNodeValueSpace / ValueSize) >= 256,
uint16,
uint8>::type node_count_type;
};
// A parameters structure for holding the type parameters for a btree_map.
@ -541,7 +518,7 @@ class btree_node {
linear_search_type, binary_search_type>::type search_type;
struct base_fields {
typedef typename Params::base_field_type field_type;
typedef typename Params::node_count_type field_type;
// A boolean indicating whether the node is a leaf or not.
bool leaf;
@ -1479,6 +1456,10 @@ class btree : public Params::key_compare {
COMPILE_ASSERT(kNodeValues <
(1 << (8 * sizeof(typename base_fields::field_type))),
target_node_size_too_large);
// Test the assumption made in setting kNodeValueSpace.
COMPILE_ASSERT(sizeof(base_fields) >= 2 * sizeof(void*),
node_space_assumption_incorrect);
};
////