feat: add fast random float gen based on pcg by sicsix · Pull Request #1158 · community-shaders/skyrim-community-shaders

sicsix · 2025-06-17T23:08:29Z

adds random float generators in range [0-1
uses pcg as a base to produce random bits then shifts those bits to produce multiple output floats cheaply

Summary by CodeRabbit

New Features
- Introduced new functions for generating random floating-point values, supporting single values as well as 2D, 3D, and 4D vectors.
Refactor
- Updated the random number generation logic to allow the random state to persist and be updated across calls.

coderabbitai · 2025-06-17T23:08:35Z

Walkthrough

The PCG random number generator function was refactored to update and persist its state externally via an inout parameter. New functions were introduced to generate 1D, 2D, 3D, and 4D floating-point random values from the PCG state, supporting both single-value and vector outputs, with optional access to the underlying random word.

Changes

File	Change Summary
package/Shaders/Common/Random.hlsli	Refactored `pcg` to use `inout uint state`; added `f1`, `f2`, `f3`, `f4` functions for float/vector random output

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant Random

    Caller->>Random: f1/f2/f3/f4(inout seed [, out word])
    Random->>Random: pcg(inout seed)
    Random-->>Caller: float/float2/float3/float4 (random values), [word]

Poem

A shuffle of bits, a dance of the seed,
Now PCG remembers each deed.
Floats in ones, twos, threes, and fours,
Randomness knocking at shader doors.
With every call, the numbers bloom—
A rabbit’s delight in a shader room!
🐇✨

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package/Shaders/Common/Random.hlsli (2 hunks)

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package/Shaders/Common/Random.hlsli

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coderabbitai

Actionable comments posted: 1

🧹 Nitpick comments (2)

package/Shaders/Common/Random.hlsli (2)
170-175: Mask mantissa for the second component for consistency & clarity

bits1 is built without the mantissa mask:
uint bits1 = word >> 9 | 0x3F800000u;
While word >> 9 is only 23 bits wide, masking expresses the intent and protects against accidental changes:
-    uint bits1 = word >> 9 | 0x3F800000u;
+    uint bits1 = (word >> 9) & 0x007FFFFFu | 0x3F800000u;
Apply the same pattern wherever a rotated/shifted word is converted to mantissa bits.

155-160: Minor: mark helper word as uint only when needed

word is an out parameter, but you often need it only when the caller asks for it.
An alternative is to overload with a defaulted parameter:
float f1(inout uint seed, out uint word = uint(0));
This would avoid duplicating wrapper overloads. Not critical, just a size/readability consideration.

Also applies to: 168-176, 184-194, 202-214

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Reviewing files that changed from the base of the PR and between 111b074 and f94bc88.

📒 Files selected for processing (1)

package/Shaders/Common/Random.hlsli (2 hunks)

⏰ Context from checks skipped due to timeout of 90000ms (3)

GitHub Check: Build plugin and addons
GitHub Check: Validate shader compilation (Flatrim, .github/configs/shader-validation.yaml)
GitHub Check: Validate shader compilation (VR, .github/configs/shader-validation-vr.yaml)

🔇 Additional comments (1)

package/Shaders/Common/Random.hlsli (1)

105-111: pcg stateful variant looks correct

The math matches the canonical PCG-XSH-RR 32-bit step and the state is correctly updated via the inout parameter.
No issues noticed here.

github-actions · 2025-06-17T23:43:38Z

✅ A pre-release build is available for this PR:
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feat: add fast random float gen based on pcg

f94bc88

coderabbitai Bot reviewed Jun 17, 2025

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Comment thread package/Shaders/Common/Random.hlsli Outdated

small tidy, clearer variable names

fcb0d0e

doodlum approved these changes Jun 17, 2025

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alandtse merged commit 33ab238 into community-shaders:dev Jun 18, 2025
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alandtse pushed a commit that referenced this pull request Jun 20, 2025

feat: add fast random float gen based on pcg (#1158)

f51d371

davo0411 pushed a commit to davo0411/skyrim-community-shaders that referenced this pull request Jun 21, 2025

feat: add fast random float gen based on pcg (community-shaders#1158)

b783657

alandtse pushed a commit to alandtse/open-shaders that referenced this pull request Jul 20, 2025

feat: add fast random float gen based on pcg (community-shaders#1158)

7f186a6

coderabbitai Bot mentioned this pull request Aug 13, 2025

feat: add lens effects #1363

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coderabbitai Bot mentioned this pull request Dec 25, 2025

test: add hlsl unit tests #1653

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feat: add fast random float gen based on pcg#1158

feat: add fast random float gen based on pcg#1158
alandtse merged 2 commits into
community-shaders:devfrom
sicsix:rand

sicsix commented Jun 17, 2025 •

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sicsix commented Jun 17, 2025 •

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