Doom (id Software's Linux source release, linuxdoom-1.10)

id Software's released Doom source is version 1.10 of the Linux port: doomdef.h sets VERSION to 110, and John Carmack's README, signed 1997-12-23, says the code "only compiles and runs on linux" because the DOS code's sound library could not be released. doom/source-version

Value: 110

Every other Doom claim here reads this Linux code. The DOS 1.9 executable is not in the release, so a claim holds for DOS only as far as the two share game code.

Cited by: Doom

Doom's game logic runs in tics of 1/35 second (TICRATE 35). doom/tic-rate

About: Doom Running

Value: 35 tics/s

Cited by: Doom

Doom's random numbers come from rndtable, a fixed table of 256 bytes, walked by two indices: each call to P_Random or M_Random adds 1 to its own index, wraps it at 256, and returns that table entry. doom/rng

About: Doom Random number generator

Value: 256 entries

Because the index steps before the read, the first call after a reset returns entry 1 (8), and entry 0 comes up only on the 256th call. The comment "Which one is deterministic?" is id's own.

Cited by: Doom

Doom's play code (weapon damage, aim spread, pain) rolls P_Random, and M_Random serves the rest: the screen wipe, sound pitch, intermission animations, and the status bar. doom/rng-streams

About: Doom Random number generator

The split keeps cosmetic draws off the play sequence, so a wipe or a sound doesn't shift a later damage roll. The code shows the split; why id made it is not stated beyond the comment quoted under doom/rng.

Cited by: Doom

G_InitNew resets both of Doom's random indices to 0, and it runs when a new game starts, when a saved game loads, when a demo starts playing, and when the command line starts a game directly (a warp or autostart, or a net game). doom/rng-reset

About: Doom Random number generator

Value: 0

Moving to the next map does not call G_InitNew, so the play index carries over between maps within a game.

Cited by: Doom

Doom's rndtable is not a permutation: its 256 entries hold 166 distinct values, 101 of them once, 45 twice, 16 three times, 3 four times, and 1 five times; the value 145 appears 5 times and the value 0 twice. doom/rng-table

About: Doom Random number generator

{
 "distinct": 166,
 "once": 101,
 "twice": 45,
 "thrice": 16,
 "four-times": 3,
 "five-times": 1
}

Final Fantasy's battle table is likewise not a permutation (see ff1/rng-table), but Doom's repeats far more.

Each bullet from Doom's pistol, chaingun, or shotgun deals 5*(P_Random()%3+1), so 5, 10, or 15; the shotgun fires 7 such pellets. Over one pass of the table the multipliers 1, 2, and 3 come up 75, 88, and 93 times. doom/bullet-damage

About: Doom Damage formula

[
 5,
 10,
 15
]

The counts make the mean over a full pass 5 × 530/256 ≈ 10.35, a little above the 10 a uniform byte would give. No defense stat enters; only a player's armor reduces damage (doom/armor).

Cited by: Doom

Doom's fist deals (P_Random()%10+1)<<1, 2 to 20, multiplied by 10 while the player has the berserk strength power; the chainsaw deals 2 to 20. doom/melee-damage

About: Doom Damage formula

[
 2,
 20
]

A Doom projectile that hits a shootable thing, other than a monster of its shooter's species, deals ((P_Random()%8)+1) times its type's damage value: a rocket's 20 gives 20 to 160, a plasma ball's 5 gives 5 to 40, a BFG ball's 100 gives 100 to 800, and an imp's fireball's 3 gives 3 to 24. doom/missile-damage

About: Doom Damage formula

[
 1,
 8
]

The roll happens only when the projectile touches a shootable thing; a rocket's blast (doom/rocket-blast) and the BFG's tracers (doom/bfg-spray) come on top.

Cited by: Doom

A Doom rocket's explosion deals 128 minus the distance to each shootable thing in sight of the blast, measured as the larger of the x and y gaps less the thing's radius, and the Cyberdemon and Spider Mastermind take none. doom/rocket-blast

About: Doom Damage formula

Value: 128 HP

The distance is in whole map units and clamps at 0, so a thing touching the blast takes 128. The same blast hurts the player who fired it, since only the two bosses are skipped.

Cited by: Doom

When a Doom BFG ball's explosion reaches its third frame, 40 traces fan across 90° around the ball's heading, each starting from the player who fired it and reaching 1,024 units, and each trace that finds a target deals the sum of 15 rolls of 1 to 8, so 15 to 120. doom/bfg-spray

About: Doom Damage formula

[
 15,
 120
]

The comment on L793 says mo->target "is the originator (player)". The traces start where the player stands when the spray runs, so the player's position, not the ball's, decides what they reach.

Cited by: Doom

Doom's monster attacks use the same multiple-of-a-base form: a zombieman's bullet deals ((P_Random()%5)+1)*3, 3 to 15; an imp's claw (P_Random()%8+1)*3, 3 to 24; a demon's bite ((P_Random()%10)+1)*4, 4 to 40; and a Baron of Hell's claw (P_Random()%8+1)*10, 10 to 80, an attack function (A_BruisAttack) the code shares with Doom II's Hell Knight (MT_KNIGHT). doom/monster-damage

About: Doom Damage formula

The names come from the cast call's list in f_finale.c and d_englsh.h. The Baron of Hell's claw is its melee branch; at range it throws MT_BRUISERSHOT (damage value 8) instead.

Damage to a Doom player is halved on the easiest skill (sk_baby), and then armor takes a share: a third with the armor pickup (type 1) and half with the MegaArmor (type 2), until its points run out; monsters have no armor or defense value. doom/armor

About: Doom Damage formula

P_GiveArmor sets the points to 100 times the type (L258), so 100 for armor and 200 for MegaArmor. God mode and invulnerability ignore damage under 1000 (L847-L852).

Cited by: Difficulty level

Doom's bullets and melee are hitscan: P_LineAttack traces a line in the same tic, out to 2,048 units for guns and 64 for melee, and damages the first shootable thing whose height spans the shot's slope; rockets, plasma, and BFG balls are objects spawned by P_SpawnPlayerMissile that move each tic at their type's speed (20, 25, and 25 units per tic). doom/hitscan

About: Doom Hitscan

{
 "guns": 2048,
 "melee": 64
}

A trace stops at the first solid wall, or at a two-sided line whose opening the slope misses, before it reaches anything behind. Melee range is 64 units.

Cited by: Doom, Hitscan

Doom aims vertically for the player: P_BulletSlope traces 1,024 units straight ahead, then 5.625° (1<<26) to the left, then to the right, and takes the slope to the middle of the first shootable thing's visible part, within ±100/160; a bullet keeps the player's facing and takes only that slope, but a player's rocket, plasma, or BFG ball also turns to the side trace that found the target and flies level when none did. doom/autoaim

About: Doom Autoaim

Value: 5.625 °

The ±100/160 limit is the comment's "view angles"; two-sided lines narrow it as the trace passes ledges (PTR_AimTraverse, L842-L857). A positive angle step turns counterclockwise, so the first side trace looks left. The shotgun calls P_BulletSlope once for all its pellets.

Cited by: Autoaim, Doom, Perfect Dark

Doom's shotgun turns each of its 7 pellets by (P_Random()-P_Random())<<18, up to about 5.6° either side, at one shared vertical slope; the pistol and chaingun turn their shots the same way except while player->refire is 0, which covers the pistol's first shot and the chaingun's first two in a burst of held fire, and those fly straight. doom/shotgun-spread

About: Doom Hitscan

Value: 5.6 °

The difference of two rolls makes small turns likelier than large ones. Releasing fire resets refire to 0, so tapping the pistol keeps every shot exact. The code's Doom II super shotgun (A_FireShotgun2) fires 20 pellets, turns them by twice as much (<<19), and spreads their slopes too.

Cited by: Hitscan

Each tic a Doom player stands on the floor, P_MovePlayer adds a thrust of the command's forward and side moves times 2,048 (1/32 unit per tic for each step of move), the side thrust at 90° to the facing; a keyboard sets the forward move to 25 walking or 50 running and the side move to 24 or 40, and the player gets no thrust in the air. doom/thrust

About: Doom Running

{
 "forward": [
  25,
  50
 ],
 "side": [
  24,
  40
 ]
}

The ticcmd tables are read from the Linux source's G_BuildTiccmd. Mouse and joystick input add to the same forward and side moves.

After a Doom thing moves on the floor, its momentum is multiplied by FRICTION (0xe800, 0.90625) each tic, and set to 0 once both axes are under STOPSPEED (1/16 unit per tic) with no move commanded; friction skips missiles, a charging lost soul, anything in the air, and a corpse halfway off a step still moving faster than 1/4 unit per tic on an axis, and the no-momentum cheat stops a player outright. Each axis of momentum is capped at 30 units per tic. doom/friction

About: Doom Running

Value: 0.90625 ×/tic

The corpse test compares the floor the corpse rests on with its own sector's floor. The cap clamps x and y separately, so a diagonal move can exceed 30 units per tic in total.

Computed from the thrust and friction, not measured: with a thrust of a each tic and friction 0.90625, a Doom player on the floor settles at the speed v where a = 0.09375·v, about 16.7 units per tic (583 units/s) running forward and 8.3 walking; neither comes near the 30-unit cap. doom/run-speed

About: Doom Running

Value: 16.7 units/tic

Fixed-point truncation in FixedMul shaves the result slightly. A probe in a source port that keeps vanilla physics could measure it (T4).

Doom applies forward and side thrust at 90° to each other and caps each move separately, so running forward (50) while running sideways (40) gives √(50² + 40²) ≈ 64, 1.28 times the forward run alone. doom/strafe-running

About: Doom Strafe-running

Value: 1.28 ×

Friction scales both axes alike, so the ratio holds for the settled speed too (about 21.3 units per tic against 16.7). Read from the Linux source's input code.

Cited by: Doom, Strafe-running

Holding Doom's strafe modifier makes the turn keys add a side move, and the strafe keys add theirs on top, so a turn key and a strafe key pressed together reach 80, clamped to MAXPLMOVE (50); a run then moves at √(50² + 50²) ≈ 70.7, 1.41 times the forward run alone. doom/sr50

About: Doom Strafe-running

Value: 50

The name SR50 (strafe-running at side move 50) is players' usage, not the code's. Mouse movement with strafe held adds mousex*2 to the same total (L406-L407) and meets the same clamp. Read from the Linux source's input code.

Cited by: Doom, Strafe-running

When damage with a source hits a Doom monster and the monster survives, it makes that source its target, player or monster alike, unless the source is the monster itself or Doom II's Arch-vile (MT_VILE), or the monster's threshold from an earlier switch is still above 0; a damaged Arch-vile ignores its own threshold. Damage with no source never retargets. Each switch sets the threshold to 100; A_Chase lowers it by 1 per call and clears it at once when the target is gone or dead. doom/infighting

About: Doom Monster infighting

Value: 100

The same code makes a monster turn on a player, so infighting is the general rule rather than a special case. A_Chase runs once per chase state, which lasts several tics, so 100 chase steps is longer than the comment's 3 seconds. The Arch-vile tests are Doom II code in the shared source and never fire in Doom (1993).

Cited by: Doom, Monster infighting

A Doom projectile that touches a monster of its shooter's own type explodes without dealing damage, so those projectiles can't start infighting within a species; a hitscan attack has no such check. doom/same-species

About: Doom Monster infighting

A zombieman's bullet can therefore hit another zombieman and turn it (doom/infighting). The MT_PLAYER exception lets one player's rockets hit another. A rocket's blast is separate (P_RadiusAttack) and has no species check. The code also counts MT_KNIGHT (Doom II's Hell Knight, f_finale.c L348) as the Baron's species.

Cited by: Monster infighting

A Doom monster that survives a hit enters its pain state when P_Random() is below its pain chance, out of 256, unless it is a lost soul in mid-charge; the table gives the zombieman and imp 200, the demon 180, the shotgun guy 170, the cacodemon 128, the Baron of Hell 50, the Spider Mastermind 40, and the Cyberdemon 20, and the lost soul's 256 always succeeds. The pain sets MF_JUSTHIT, so the monster's next missile-range check passes at once if it can see its target. doom/pain-chance

About: Doom Pain chance

{
 "zombieman": 200,
 "imp": 200,
 "demon": 180,
 "shotgun-guy": 170,
 "cacodemon": 128,
 "baron": 50,
 "spider-mastermind": 40,
 "cyberdemon": 20,
 "lost-soul": 256
}

The pain state interrupts the monster's attack or chase. The missile-range check skips the reaction-time and distance tests when MF_JUSTHIT is set. The player's own entry is 255 (info.c L1117).

Cited by: Doom, Pain chance, Quake II

A Doom monster spawns with its type's reactiontime (8 for the zombieman) except on Nightmare, where it stays 0; each call of A_Chase lowers it by 1, and while it is above 0 the missile-range check refuses a ranged attack, unless the monster has flinched from a hit (its pain-chance roll, which sets MF_JUSTHIT) since the check last ran. Melee attacks don't check it. doom/reaction-time

About: Doom Reaction delay

Value: 8

A_Chase runs as the action of a monster's chase frames, so the count falls once per chase frame, not once per tic.

Cited by: Reaction delay