Fix s_write_intf_data_file using wrong loop index for distance check

[sbryngelson]

Summary

Severity: HIGH — interface contour extraction uses stale loop index, produces wrong point distances.

File: src/post_process/m_data_output.fpp, lines 1549-1563

In s_write_intf_data_file, the Euclidean distance euc_d between a candidate interface point and existing stored points is computed using x_d1(i) and y_d1(i), where i is stale from a previous outer loop (value = m+1 after the maxalpha search loop). This references out-of-bounds or wrong data. Additionally, euc_d is computed once outside the inner loop, so the distance check is the same for all stored points.

Before

! euc_d uses stale 'i' from the previous loop, not the stored-point index
euc_d = sqrt((x_cc(j) - x_d1(i))**2 + (y_cc(k) - y_d1(i))**2)
tgp = sqrt(dx(j)**2 + dy(k)**2)
do i = 1, counter        ! i is now a different loop variable
    if (euc_d < tgp) then
        cycle             ! wrong: should exit, not cycle
    elseif (euc_d > tgp .and. i == counter) then
        ! add point

After

tgp = sqrt(dx(j)**2 + dy(k)**2)
do i = 1, counter
    euc_d = sqrt((x_cc(j) - x_d1(i))**2 + (y_cc(k) - y_d1(i))**2)
    if (euc_d < tgp) then
        exit              ! too close to existing point, reject candidate
    elseif (i == counter) then
        ! add point — passed all distance checks

Three fixes:

1. Moved euc_d computation inside the loop so it checks distance to each stored point
2. Changed cycle to exit for correct early termination (reject if too close to any point)
3. Removed dead code in the counter == 0 branch (distance to self)

Test plan

• Run post-processing with interface data output
• Verify extracted contour points are properly spaced

🤖 Generated with Claude Code

Fixes #1211

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Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review Logic change The inner-loop termination was changed from cycle to exit. Verify this breaks only the inner do i loop (so the candidate point is rejected) and does not skip needed iterations of the outer loops. Confirm intended behaviour when multiple stored points are checked — ensure candidates are only added when they pass all checks. Distance calculation The Euclidean distance euc_d is now computed inside the loop using the stored-point index i, which fixes the stale-index bug. Ensure the indices used (x_cc(j) - x_d1(i) and y_cc(k) - y_d1(i)) match the grid indexing convention in the rest of the module. Performance Issue The code computes sqrt for every stored-point comparison. For many stored points this may be expensive; consider comparing squared distances instead to avoid repeated square-root operations.

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Pull request overview

This PR fixes a critical bug in the interface data output logic where distance calculations were using stale loop indices, potentially causing incorrect spacing validation of interface contour points.

Changes:

• Moved Euclidean distance computation inside the loop over stored points to use the correct index
• Replaced cycle with exit to properly terminate when a candidate point is too close to existing points
• Removed redundant distance calculations and simplified the conditional logic

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (2c3fcc9).

Files with missing lines	Patch %	Lines
src/post_process/m_data_output.fpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1191   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20496    -2     
  Branches     1990     1990           
=======================================
  Hits         9030     9030           
+ Misses      10329    10327    -2     
  Partials     1139     1139           

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