Class: Doom::Game::Random

Inherits:
Object
  • Object
show all
Defined in:
lib/doom/game/random.rb

Overview

DOOM's P_Random -- a fixed 256-entry table walked by an index, not a PRNG.

The point is determinism: two peers running the same tics with the same starting index observe identical results, which is what lockstep networking requires. Kernel#rand cannot give us that.

Matches m_random.c. Only the game-state RNG lives here; presentation-only randomness (screen melt) stays on Kernel#rand since it never feeds the simulation.

Constant Summary collapse

RNDTABLE = 
[
  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
].freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(seed = 0) ⇒ Random

Returns a new instance of Random.



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# File 'lib/doom/game/random.rb', line 41

def initialize(seed = 0)
  reset(seed)
end

Instance Attribute Details

#index ⇒ Object

Index into RNDTABLE. Part of the game state -- include it in desync hashes, and save/restore it alongside player positions.



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# File 'lib/doom/game/random.rb', line 39

def index
  @index
end

Instance Method Details

#byte ⇒ Object

P_Random(): next table entry, 0-255.



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# File 'lib/doom/game/random.rb', line 50

def byte
  @index = (@index + 1) & 0xff
  RNDTABLE[@index]
end

#diff ⇒ Object

Signed difference of two draws, -255..255. DOOM uses this shape a lot (P_Random() - P_Random()) for spread and jitter.



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# File 'lib/doom/game/random.rb', line 57

def diff
  byte - byte
end

#rand(arg = nil) ⇒ Object

Kernel#rand-compatible surface so call sites read the same as before.

rand       -> Float in [0, 1)
rand(n)    -> Integer in 0...n     (DOOM's P_Random()%n)
rand(a..b) -> Integer in a..b



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# File 'lib/doom/game/random.rb', line 66

def rand(arg = nil)
  case arg
  when nil
    byte / 256.0
  when ::Range
    lo = arg.begin
    hi = arg.exclude_end? ? arg.end - 1 : arg.end
    span = hi - lo + 1
    span <= 0 ? lo : lo + (byte % span)
  else
    n = arg.to_i
    n <= 0 ? 0 : byte % n
  end
end

#reset(seed = 0) ⇒ Object 



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# File 'lib/doom/game/random.rb', line 45

def reset(seed = 0)
  @index = seed & 0xff
end