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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