Fix simplex noise using mod(i,255) instead of iand(i,255)

[sbryngelson]

User description

Summary

• Fix 2D simplex noise in m_simplex_noise.fpp using mod(i, 255) which produces range 0-254, skipping index 255 of the permutation table.

Bug Details

The simplex noise implementation hashes grid coordinates into the permutation table using:

ii = mod(i, 255)    ! range 0..254
jj = mod(j, 255)    ! range 0..254

The permutation table p_vec has 256 entries (indices 0-255). Using mod(i, 255) gives range 0-254, so p_vec(255) is never accessed. This introduces a subtle bias in the noise distribution — one permutation entry is systematically excluded, slightly reducing the randomness of the noise pattern.

The standard simplex noise algorithm uses mod(i, 256) or equivalently iand(i, 255) (bitwise AND with 255) to wrap indices to the full 0-255 range.

Fix

Replace mod(i, 255) with iand(i, 255) which gives the correct 0-255 range and is also faster (bitwise operation vs division).

Test plan

• Simplex noise perturbation cases still produce valid noise fields
• All 256 permutation table entries are now reachable

🤖 Generated with Claude Code

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CodeAnt-AI Description

Fix incorrect index wrapping in 2D simplex noise so all permutation entries are used

What Changed

• Index wrapping for the simplex noise permutation now covers the full 0–255 range, so the previously skipped permutation entry is reachable
• Noise outputs no longer exclude one permutation value, removing a subtle bias in generated noise patterns
• Index wrapping uses a bitwise operation which is marginally faster than the previous modulus approach

Impact

✅ Correct simplex noise distribution
✅ Full permutation table coverage (0–255)
✅ Slightly faster noise generation

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Fixes #1223

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Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review Portability of bitwise wrap The new wrapping uses a bitwise AND (iand(i, 255)) which assumes two's-complement integer representation and that bitwise AND semantics match mathematical modulo 256 for all inputs (including negative i/j). On unusual platforms or non-standard integer representations this may differ from mod(i,256). Verify the intended behaviour for negative inputs and target compilers/platforms.

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Pull request overview

This PR fixes a bug in the 2D simplex noise implementation where mod(i, 255) incorrectly produces a range of 0-254, leaving one permutation table entry inaccessible and introducing bias in the noise distribution.

Changes:

• Replace mod(i, 255) with iand(i, 255) to correctly access all 256 permutation table entries (0-255)
• Replace mod(j, 255) with iand(j, 255) for the same reason
• Use faster bitwise AND operation instead of modulo division

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Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (590253b).

Files with missing lines	Patch %	Lines
src/pre_process/m_simplex_noise.fpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1218   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20498           
  Branches     1990     1990           
=======================================
  Hits         9030     9030           
  Misses      10329    10329           
  Partials     1139     1139           

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