Pokemon D/P Great Marsh, Swarm, Trophy Garden finds...

[NekoSabrina]

Some time ago I had a few misadventures looking into how some stuff worked in the old Gen 4 games. Fast forward to now, where I have a little more experience looking at how games work thanks to studying some other games (specifically Mega Man Battle Network), I came back to take another look at this problem and actually found some interesting data

The highlighted bytes are the Great Marsh Pokemon, and it's rather interesting how these bytes behave based on the testing I did with them. The first byte controls the daily Pokemon for both Areas 1 and 2, the second Area 3, the third Areas 4 & 5, the fourth Area 6. In the configuration shown in this save file, Areas 1 and 6 had Croagunk, Area 2 had Yanma, Area 3 had Staravia, Area 4 had Drapion, Area 5 had Carnivine. Using an old Gen 4 save editor that had the capability, I changed each area's daily Pokemon to Kangaskhan and watched what happened. Editing Areas 1 and 2 changed the first byte (to 97 and E0 respectively), editing Area 3 changed the second byte from AE to DE, editing Areas 4 and 5 changed the third byte (to 3B and 76 respectively), editing Area 6 changed the fourth byte from B7 to AF.

I don't know exactly how the Pokemon are calculated from this (especially with the first and third bytes), but based on the old editor's data the Great Marsh Pokemon are arranged as follows:

Spoiler

00, 03, 07, 11/17, 1B/27 Croagunk

01, 04, 08, 12/18, 1C/28 Skorupi

02, 05, 09, 13/19, 1D/29 Carnivine

06, 0F/15, 1F/31 Golduck

0A/10, 1A/26, 1E/30 Roselia

0B/11, 10/16, 19/25 Staravia

0C/12 Toxicroak

0D/13 Drapion

0E/14 Exeggcute

14/20 Yanma

15/21 Shroomish

16/22 Paras

17/23 Kangaskhan

18/24 Gulpin

The next four bytes after the Great Marsh are the Swarms, which seems overly complicated for handling one daily Pokemon at a time (Makuhita, in the case of the save shown above), but I think PKHeX is actually able to manipulate this through Block Data (though you need to know what you're doing). Once boiled down, the Swarm values are as follows:

Spoiler

00 Doduo

01 Zigzagoon

02 Cubone

03 Nosepass

04 Phanpy

05 Dunsparce

06 Snubbull

07 Absol

08 Spoink

09 Drowzee

0A/10 Delibird

0B/11 Swinub

0C/12 Voltorb

0D/13 Farfetch'd

0E/14 Skitty

0F/15 Natu

10/16 Makuhita

11/17 Krabby

12/18 Spinda

13/19 Beldum

14/20 Pidgey

15/21 Corsola

16/22 Surskit

17/23 Lickitung

18/24 Smoochum

19/25 Electrike

1A/26 Slakoth

1B/27 Magnemite

72DC and 72DE are the Trophy Garden Pokemon, with C being the current day's Pokemon and E being the previous one. These are far more straighforward than the Marsh and Swarms, with basic hex values:

Spoiler

00 Eevee

01 Bonsly

02 Happiny

03 Meowth

04 Cleffa

05 Clefairy

06 Igglybuff

07 Plusle

08 Jigglypuff

09 Porygon

0A Castform

0B Minun

0C Mime Jr

0D Marill

0E Chansey

0F Azurill

FFFF No Previous Day Pokemon

Now one last interesting tidbit, I did find this data in the game's ram, but there's a bit of a catch

Unfortunately, this data floats around the 02274700-4800 range, so it is impossible to edit by Action Replay without a way to make the locations of these bytes static (even then, the complex nature of the Marsh and Swarm makes them difficult to even live edit in an emulator's memory viewer without knowing how they work). Still, it allows the ability to study this data with the game in motion (and for the record, that day's seed translates to Skorupi in Marsh Areas 1, 2, and 6; Paras in Area 3, Roselia in Area 4, Kangaskhan in Area 5, and a Dunsparce Swarm).

Unfortunately I don't know how all of this translates to Platinum, but it really seemed like this was an area of Gen 4 that hadn't gotten a lot of research, so hopefully it helps (and hopefully someone with a lot more time and patience than I do can finish the research on the Marsh).

[palimer6]

Funnily enough I also just so happened to look into the Great Marsh very recently and your old post helped a lot with the old save editor and its info.

Basically the only thing you're missing with your observations on what bytes get changed is that a pokemon appearing in a specific area does not change the whole byte, but only 5 bits of it. 

Read as little endian 80 ae 36 b7 becomes b7 36 ae 80 or in binary 1011 0111 0011 0110 1010 1110 1000 0000.

If we now split that into units of 5 bits we get 10 11011 10011 01101 01011 10100 00000 or 2 1B 13 0D 0B 14 00. The 2 shouldn't be used for Great Marsh calculation here.

These values line up with the pokemon you had in your areas if we assign the 5 bit blocks "in reverse". 1B is area 6, 13 is area 5, 0D is area 4, 0B is area 3, 14 is area 2, 00 is area 1.

The following table shows which Pokemon each 5 bit block corresponds to in both DP and Pt once you have the national dex.

Hex	Bin	Dec	DP	Pt
00	00000	0	Croagunk	Toxicroak
01	00001	1	Skorupi	Kecleon
02	00010	2	Carnivine	Kecleon
03	00011	3	Croagunk	Carnivine
04	00100	4	Skorupi	Skorupi
05	00101	5	Carnivine	Croagunk
06	00110	6	Golduck	Quagsire
07	00111	7	Croagunk	Drapion
08	01000	8	Skorupi	Skorupi
09	01001	9	Carnivine	Croagunk
0A	01010	10	Roselia	Quagsire
0B	01011	11	Staravia	Kangaskhan
0C	01100	12	Toxicroak	Paras
0D	01101	13	Drapion	Drapion
0E	01110	14	Exeggcute	Exeggcute
0F	01111	15	Golduck	Exeggcute
10	10000	16	Staravia	Skorupi
11	10001	17	Croagunk	Croagunk
12	10010	18	Skorupi	Skorupi
13	10011	19	Carnivine	Carnivine
14	10100	20	Yanma	Yanma
15	10101	21	Shroomish	Shroomish
16	10110	22	Paras	Paras
17	10111	23	Kangaskhan	Kangaskhan
18	11000	24	Gulpin	Gulpin
19	11001	25	Roselia	Tropius
1A	11010	26	Staravia	Gulpin
1B	11011	27	Croagunk	Shroomish
1C	11100	28	Skorupi	Skorupi
1D	11101	29	Carnivine	Carnivine
1E	11110	30	Roselia	Croagunk
1F	11111	31	Golduck	Tangela

I know that in Platinum, every Pokemon that isn't in the Sinnoh Dex gets replaced with Wooper before you get the National Dex. I don't know how it works with DP, as I was playing Platinum while looking into this.

I also went and tried this with manual hex editing of my save file (and fixing the checksum with PKHeX) and changing the area encounters worked like a charm.

[NekoSabrina]

Very interesting. So that's how the Great Marsh Pokemon are sorted out (and maybe my initial assumption about what each byte does was slightly off). I was figuring maybe there was maybe something to the little endian order of the bytes when grouped together, but studying them individually is easier to do the other way. Kinda curious what those random two extra bits do, if anything, though.

The post-National Dex slots in D/P are split between Wooper, Quagsire, Azurill, Marill, Bidoof, and Bibarel pre-National Dex but I don't know which ones correspond to which slot (only that Wooper and Azurill take two each and the others one apiece).