Optimize i7trivox.F: replace loops with intrinsics and fix boundary detection bug

[Copilot]

Description of the feature or the bug

The i7trivox.F routine contained hand-coded loops and repetitive calculations that could be replaced with Fortran intrinsic functions and array operations. Additionally, an operator precedence bug in boundary condition checking caused nodes at the maximum X boundary to be incorrectly excluded from voxel counting.

Description of the changes

Optimizations:

• Array initialization: Replace loop-based initialization with array slicing

  ! Before: loop with individual assignments
  ! After:
  IIX(1:NSN) = 0
  IIY(1:NSN) = 0
  IIZ(1:NSN) = 0

• Voxel index calculations: Combine MIN/MAX operations into single expressions, eliminating redundant assignments

  ! Before: two-step calculation with intermediate assignment
  IIX(I)=INT(NBX*(X(1,J)-XMINB)/(XMAXB-XMINB))
  IIX(I)=MAX(1,2+MIN(NBX,IIX(I)))
  
  ! After: single expression
  IIX(I)=MAX(1,MIN(NBX+2,2+INT(NBX*(X(1,J)-XMINB)/(XMAXB-XMINB))))

• Coordinate extraction: Group all coordinate extractions together for better cache locality before computing min/max bounds

Bug fixes:

• Boundary detection: Fix operator precedence in condition checking nodes outside reduced box. Due to .AND. having higher precedence than .OR., the original condition failed to detect nodes at maximum X boundary:

  ! Before (buggy): IIX==1 .OR. IIY==1 .OR. IIZ==1 .AND. IIX==NBX+2 .OR. ...
  ! After (fixed):  IIX==1 .OR. IIY==1 .OR. IIZ==1 .OR. IIX==NBX+2 .OR. ...

• Single-dimension grids: Replace confusing fallback values (-2, 1) with explicit full range (1, NBX+2) for clarity

Impact: Reduces ~14 lines of code, eliminates redundant memory writes in hot loops, and fixes incorrect boundary node counting.

Original prompt

Can you have a look a i7trivox.F?
I would like to look for optimization opportunities. In particular by using intrinsic fuctions instead of home made code snippet, and array operations instead of hand-made loops.

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