Fix moncon_cutoff declared as integer, truncating 1e-8 to 0

[sbryngelson]

Summary

Severity: CRITICAL — completely disables the THINC monotonicity constraint.

File: src/common/m_constants.fpp, line 47

moncon_cutoff is declared as integer but assigned the real literal 1e-8_wp. Fortran silently truncates this to 0. Every comparison like moncon > moncon_cutoff is then always true (since moncon_cutoff = 0), so the THINC monotonicity constraint is never enforced.

Before

integer, parameter :: moncon_cutoff = 1e-8_wp  ! becomes 0

After

real(wp), parameter :: moncon_cutoff = 1e-8_wp  ! stays 1e-8

Why this went undetected

Fortran truncates real-to-integer conversions at compile time with at most a warning (often suppressed). THINC still produces plausible-looking results without the monotonicity constraint — the interface compression just allows more extrema than intended, which is hard to notice visually.

Test plan

• Run THINC interface compression test case and verify monotonicity constraint activates
• Check that interface overshoots are now suppressed

🤖 Generated with Claude Code

Fixes #1195

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Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review Type consistency The cutoff constant moncon_cutoff was changed from integer to real to fix truncation. Verify all uses throughout the codebase treat moncon_cutoff as a real (no local integer declarations or implicit conversions). Search for any places that compare or assign this constant to integer variables or use integer literals in the same expressions, as those will reintroduce logic bugs. Magnitude / precision check Ensure the chosen value (1e-8_wp) is appropriate for the working precision (wp) used across targets. If wp can be single precision on some builds, this cutoff may underflow relative to epsilon; validate the value against machine epsilon or test cases to confirm the monotonicity limiter becomes active as intended.

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Pull request overview

Fixes a constant type bug in the shared m_constants module where moncon_cutoff was declared as an integer while being assigned a working-precision real literal, which effectively truncated the cutoff to 0 and disabled THINC’s monotonicity constraint.

Changes:

• Change moncon_cutoff from integer, parameter to real(wp), parameter so 1e-8_wp is preserved.
• Restores intended thresholding behavior for THINC monotonicity checks (used in src/simulation/m_muscl.fpp).

Comments suppressed due to low confidence (1)

src/common/m_constants.fpp:47

• The inline comment has a spelling/grammar issue: “extremas” should be “extrema” (plural of extremum).

    real(wp), parameter :: moncon_cutoff = 1e-8_wp !< Monotonicity constraint's limiter to prevent extremas in THINC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (e12577c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1174   +/-   ##
=======================================
  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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