Random number generator with 3 inputs

Question

I am looking for a random number generator with 3 inputs for a terrain generator. The inputs are an x, y (position) and a seed value. And the function returns back a random number from 0-1.

So far I found this question, but this has 2 inputs. Although I can combine x and y to a single number, to get two inputs, this will restrict the choices I will have for x, y as they need to be sufficiently large (the terrain is infinite).

Is there any random function that takes in 3 inputs, or would I need to end up using the 2 input version?

What type are the three inputs? uint32/uint64/double/something else? — Leo
– Leo, Commented Mar 10, 2022 at 10:57
Your intention to pass a seed value for every invocation of your generator indicates a conceptual misunderstanding of how PRNGs work. Frequent manual seeding will not increase the quality of the pseudo-random numbers, and in fact usually does the opposite. — pjs
– pjs, Commented Mar 10, 2022 at 14:21
@pjs I don't think the intention is to increase the quality of the numbers. It is to have a repeatable way to generate a 2D map from different seeds while being able to generate the points in an arbitrary order (like when the player moves around in directions they pick). Kind of like a CTR mode of a cipher but non-cryptographic. — Leo
– Leo, Commented Mar 10, 2022 at 14:29
This is similar to an earlier SO question #61969876. You can combine x and y with the Cantor pairing function. Then use the result of the pairing function as an offset into some pseudo-random serie. Pseudo-random generators that efficiently support this arbitrary access into their serie can be CTRs, or MRGs such as MRG32k3a. That way, only a single seed value is required for the whole game. — jpmarinier
– jpmarinier, Commented Mar 10, 2022 at 16:43

Leo · Accepted Answer · 2022-03-10 11:21:05Z

2

Something like this should work. It takes three 32-bit integers, and outputs a single 32-bit integer. If you want to turn the output into a double between 0 and 1, just divide by UINT32_MAX.

The input and output sizes can be adjusted.

You can also tweak the balance between output quality and speed. You'll notice that the middle section is just repeated 3 lines, remove or add more of them to make the output more or less biased.

Here's the C code.

uint32_t rotl32(uint32_t n, uint8_t k) {
  uint32_t a = n << k;
  uint32_t b = n >> (32 - k);
  return a | b;
}

uint32_t three_input_random(uint32_t x, uint32_t y, uint32_t z) {
  uint32_t a = x;
  uint32_t b = y;
  uint32_t c = z;

  b ^= rotl32(a + c, 7);
  c ^= rotl32(b + a, 9);
  a ^= rotl32(c + b, 18);
  b ^= rotl32(a + c, 7);
  c ^= rotl32(b + a, 9);
  a ^= rotl32(c + b, 18);
  b ^= rotl32(a + c, 7);
  c ^= rotl32(b + a, 9);
  a ^= rotl32(c + b, 18);

  return a + b + c + x + y + z;
}

answered Mar 10, 2022 at 11:21

Leo

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1 Comment

Anderson Green Over a year ago

I found many implementations of noise functions in GLSL that can be used for this purpose as well.

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