No one will notice until they've got two stations traveling down the
highway together blinking on and off of their Nuvi screen!
Lynn (D) - KJ4ERJ - Things always go wrong at the worst possible moment
Scott Miller wrote:
> The T2 does indeed use a hash when creating D607 waypoints. I talked to
> Garmin about changing the spec to use a 32-bit ID before it became
> public, but apparently they didn't do it. I calculated the hash
> collision probability and collisions will certainly happen, but I don't
> think they'll be frequent enough to be noticeable.
>
> Scott
>
> Bob Poortinga wrote:
>
>>
>>
>> Does anyone have a copy of the Garmin FMI specification verion 1.0 which is
>> applicable to the Nuvi 3xx (including 350) series?
>>
>> I was looking that the FMI 2.5 spec (supported by Nuvi 12xx, 13xx, and 14xx)
>> and here is snippet of the specification for the Create Waypoint Protocol:
>>
>> ----------------------------------------------------------
>> typedef struct /* D607 */
>> {
>> uint16 unique_id;
>> map_symbol symbol;
>> sc_position_type posn;
>> uint16 cats;
>> uchar_t8 name[/* 31 bytes, null-terminated */];
>> uchar_t8 comment[/* variable length, null-terminated string, 51 bytes
>> max */];
>> } waypoint_data_type;
>>
>> The unique_id is a unique identifier for the waypoint. If the specified
>> unique_id is already in use, then the existing waypoint will be modified
>> instead of creating a new waypoint.
>> ----------------------------------------------------------
>>
>> Please re-read that last sentence. It seems to me that if the OT2 is to
>> be compatible with FMI 2.5 devices, it needs to maintain a list of waypoint
>> unique_id's which can be used to update a waypoint instead of creating a
>> new one (breadcrumb). I doubt the Tracker2 has enough RAM to keep a long
>> list of waypoint ids. I wonder if anyone has a good algorithm for generating
>> unique 16 bit numbers given a station or object name so that maintaining
>> a list is not required. Since there is no perfect hash function, I wonder
>> if there is one that is good enough for this application? Thoughts?
>>
>> --
>> Bob Poortinga K9SQL <http://www.linkedin.com/in/bobpoortinga>
>> Bloomington, Indiana US
>>
>>
>>
>
>
>
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