Test Results: Doom's PRNG

Implementation

Doom by id Software may be a cult classic, but the way it handles random numbers is unexpectedly primitive.

Within the game's binary is a table listing 256 "random" bytes, and the game keeps track of the current position in the table. When a random number is needed, it simply returns the value at the current position and moves to the next entry. No other calculations are done.

This might seem like a terrible way to handle randomness, especially in a video game, but it does allow for some interesting features. For example, this ensures that online games are syncronized, and it's also the secret behind Doom's ability to record gameplay.

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Test Results

Period Length Test
SEED	Number indicates how many results could be obtained before the generator fell into a loop.
1138	256	FAIL
65535	256	FAIL
8675309	256	FAIL
16777216	256	FAIL
123456789	256	FAIL

Dice Roll Test
	This is a surprisingly good distribution, suggesting that the table was created using values from an actual PRNG.

Dartboard Test
SEED	Minimum of 6,000 darts placed needed to pass.
1138	126	FAIL
65535	126	FAIL
8675309	126	FAIL
16777216	126	FAIL
123456789	126	FAIL

Crush Test
SEED	70% or higher required to pass.
1138	4%	FAIL
65535	4%	FAIL
8675309	4%	FAIL
16777216	4%	FAIL
123456789	4%	FAIL

Plot Test
	With its small range of values, Doom's PRNG didn't have a chance at passing this test.

Example Output
0	8	109	220	222	241	149	107	75	248	254	140	16	66	74	21	211	47	80	242
154	27	205	128	161	89	77	36	95	110	85	48	212	140	211	249	22	79	200	50
28	188	52	140	202	120	68	145	62	70	184	190	91	197	152	224	149	104	25	178
252	182	202	182	141	197	4	81	181	242	145	42	39	227	156	198	225	193	219	93
122	175	249	0	175	143	70	239	46	246	163	53	163	109	168	135	2	235	25	92
20	145	138	77	69	166	78	176	173	212	166	113	94	161	41	50	239	49	111	164
70	60	2	37	171	75	136	156	11	56	42	146	138	229	73	146	77	61	98	196
135	106	63	197	195	86	96	203	113	101	170	247	181	113	80	250	108	7	255	237
129	226	79	107	112	166	103	241	24	223	239	120	198	58	60	82	128	3	184	66
143	224	145	224	81	206	163	45	63	90	168	114	59	33	159	95	28	139	123	98
125	196	15	70	194	253	54	14	109	226	71	17	161	93	186	87	244	138	20	52
123	251	26	36	17	46	52	231	232	76	31	221	84	37	216	165	212	106	197	242
98	43	39	175	254	145	190	84	118	222	187	136	120	163	236	249	0	8	109	220
222	241	149	107	75	248	254	140	16	66	74	21	211	47	80	242	154	27	205	128
161	89	77	36	95	110	85	48	212	140	211	249	22	79	200	50	28	188	52	140
202	120	68	145	62	70	184	190	91	197	152	224	149	104	25	178	252	182	202	182
141	197	4	81	181	242	145	42	39	227	156	198	225	193	219	93	122	175	249	0
175	143	70	239	46	246	163	53	163	109	168	135	2	235	25	92	20	145	138	77
69	166	78	176	173	212	166	113	94	161	41	50	239	49	111	164	70	60	2	37
171	75	136	156	11	56	42	146	138	229	73	146	77	61	98	196	135	106	63	197