Fix GAM splines_non_negative missing from actual_params by correcting boolean[] parameter serialization

[Copilot]

GH Issue: #16602

GAM models were returning splines_non_negative as None in actual_params even when explicitly set. The issue was in core parameter JSON serialization: boolean[] values were wrapped but emitted as null.

• Root cause

  • water.IcedWrapper did not correctly support boolean[] in JSON output (writeUnwrappedJSON returned null for boolean arrays).
  • Model parameter metadata relies on this wrapper for polymorphic actual_value serialization, so splines_non_negative was dropped to None in clients.

• Changes made

  • h2o-core/src/main/java/water/IcedWrapper.java
    • Added full boolean[] support in IcedWrapper:
      • store wrapped boolean arrays (b_ar)
      • return boolean arrays from get()
      • include boolean array handling in toString()
      • serialize boolean arrays as JSON booleans ([true,false,...]) instead of null
  • h2o-core/src/test/java/water/IcedWrapperTest.java
    • Added targeted unit coverage for:
      • round-trip retrieval of wrapped boolean[]
      • JSON output format for wrapped boolean[]
  • h2o-py/tests/testdir_algos/gam/pyunit_gam_splines_non_negative_actual_params.py
    • Added a GAM regression test that trains with splines_non_negative=[True, False] and verifies the value is preserved in:
      • model.actual_params["splines_non_negative"]
      • model.parms["splines_non_negative"]["actual_value"]

• Illustrative behavior

  IcedWrapper w = new IcedWrapper(new boolean[]{true, false});
  String json = new String(w.writeUnwrappedJSON(new AutoBuffer()).buf());
  // before: null
  // after:  [true,false]

[!WARNING]

[maurever]

@copilot , changes look good. Could you add a test where the actual parameters in the GAM algorithm are correct? Especially, the splines_non_negative parameter is correct in actual_params.

[Copilot]

@copilot , changes look good. Could you add a test where the actual parameters in the GAM algorithm are correct? Especially, the splines_non_negative parameter is correct in actual_params.

Added in commit 8776e30.

I introduced a focused GAM regression test at h2o-py/tests/testdir_algos/gam/pyunit_gam_splines_non_negative_actual_params.py that trains a GAM with splines_non_negative=[True, False] and asserts this value is preserved in both actual_params["splines_non_negative"] and parms["splines_non_negative"]["actual_value"].

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR fixes incorrect JSON serialization of boolean[] values in water.IcedWrapper, which caused certain model parameters (notably GAM’s splines_non_negative) to appear as None/missing in actual_params despite being explicitly set. The change improves parameter metadata fidelity for clients that rely on IcedWrapper.writeUnwrappedJSON for polymorphic actual_value serialization.

Changes:

• Add first-class boolean[] support to water.IcedWrapper (store, retrieve via get(), stringify, and serialize as a JSON boolean array).
• Add JUnit coverage validating boolean[] round-trip via get() and exact JSON output.
• Add a Python regression test ensuring GAM preserves splines_non_negative in both actual_params and parms[..]["actual_value"].

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
h2o-core/src/main/java/water/IcedWrapper.java	Implements boolean[] wrapping and correct unwrapped JSON serialization ([true,false]), preventing loss of boolean-array parameter values in model metadata.
h2o-core/src/test/java/water/IcedWrapperTest.java	Adds targeted unit tests for boolean[] retrieval and JSON output to prevent regressions in core serialization behavior.
h2o-py/tests/testdir_algos/gam/pyunit_gam_splines_non_negative_actual_params.py	Adds an end-to-end GAM regression test validating client-visible parameter preservation in actual_params and parameter schema metadata.

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[tomasfryda]

LGTM, thank you @maurever !

[valenad1]

Thank you for the fix!

Suggestion: reuse the existing AutoBuffer.putJSONAZ(boolean[]) instead of a new helper

The fix is correct, but writeBooleanArrayAsJSON duplicates logic that already exists in AutoBuffer.putJSONAZ(boolean[]) (AutoBuffer.java:1791) — same [, comma-join, ] with null → putJNULL(), identical output. That's actually the method (added in 2015) that makes serializing boolean[] possible in the first place; it's just currently private.

Every other array case/if-else in writeUnwrappedJSON already delegates to an AutoBuffer.putJSONA* method (putJSONA4, putJSONA8, putJSONA8d, …), so the boolean[] case being an inline loop is the lone exception. It'd be more consistent to delegate too.

IcedWrapper is in the same water package as AutoBuffer, so this doesn't need public — just drop private to make it package-private, matching putJSONA4/putJSONA8/putJSONA4f:

// AutoBuffer.java:1791
AutoBuffer putJSONAZ(boolean[] b) {   // was: private

Then in IcedWrapper the helper can go away entirely:

else if (t.equals("Bo"))
  return ab.putJSONAZ(b_ar);

Not a blocker — the current code works — just a maintainability/consistency cleanup.

(Drafted with Claude Code.)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[maurever]

@tomasfryda @valenad1, could you please review again? Thank you.