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Force extended value string linear search (not binary search) in one case: 

Fixed: { -2, -1, 0, 1, 3} (note gap) used a binary search (which would fail);

 Note:  { -2, -1, 0, 1, 2 ,3 } (no gap) allowed; will still do a direct access;

Also: Add a comment to README.developer extended value string section.

svn path=/trunk/; revision=44659
This commit is contained in:
Bill Meier 2012-08-24 20:44:33 +00:00
parent 7d891022e6
commit 0035a9076b
2 changed files with 26 additions and 3 deletions
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12
doc/README.developer
View File

@ -1895,9 +1895,9 @@ field would be set to NULL.
-- Extended value strings
You can also use an extended version of the value_string for faster lookups.
It requires a value_string as input.
It requires a value_string array as input.
If all of a contiguous range of values from min to max are present in the array
the value will be used as as a direct index into a value_string array.
in ascending order the value will be used as as a direct index into a value_string array.
If the values in the array are not contiguous (ie: there are "gaps"), but are
in ascending order a binary search will be used.
@ -1905,6 +1905,14 @@ in ascending order a binary search will be used.
Note: "gaps" in a value_string array can be filled with "empty" entries eg:
{value, "Unknown"} so that direct access to the array is is possible.
Note: the value_string array values are *unsigned*; IOW: -1 is greater than 0.
So:
{ -2, -1, 1, 2 }; wrong: linear search will be used (note gap)
{ 1, 2, -2, -1 }; correct: binary search will be used
As a special case:
{ -2, -1, 0, 1, 2 }; OK: direct(indexed) access will be used (note no gap)
The init macro (see below) will perform a check on the value string the first
time it is used to determine which search algorithm fits and fall back to a
linear search if the value_string does not meet the criteria above.

17
epan/value_string.c
View File

@ -285,6 +285,20 @@ _match_strval_ext_init(const guint32 val, const value_string_ext *a_vse)
* 1 Binary search, the values MUST be in numerical order.
* 2 The value used as an index(the value string MUST have all values between first and last defined in numerical order)
*/
/* Note: The value_string 'value' is *unsigned*.
*
* Special case:
* { -3, -2, -1, 0, 1, 2 } will be treated as "ascending ordered" (altho not really such)
* thus allowing as a 'direct' search.
*
* Note:
* { -3, -2, 0, 1, 2 } and { -3, -2, -1, 0, 2 } will both be considered as "out-of-order with gaps"
* thus requiring a 'linear' search.
* { 0, 1, 2, -3, -2 } and { 0, 2, -3, -2, -1 } will be considered "ascending ordered with gaps"
* thus allowing a 'binary' search.
*/
enum { VS_SEARCH = 0, VS_BIN_TREE, VS_INDEX } type = VS_INDEX;
guint32 prev_value;
@ -303,7 +317,8 @@ _match_strval_ext_init(const guint32 val, const value_string_ext *a_vse)
type = VS_BIN_TREE;
}
/* XXX: Should check for dups ?? */
if ((type == VS_BIN_TREE) && (prev_value > vs_p[i].value)) {
if ((type == VS_BIN_TREE) &&
((prev_value > vs_p[i].value) || (first_value > vs_p[i].value))) {
type = VS_SEARCH;
break;
}