fix(parser): bind Kentaro alternative cost to mana value

[matthewevans]

Fixes #5899.

Summary by CodeRabbit

• Bug Fixes

  • Fixed alternative spell costs that use {X} to correctly match the affected spell’s mana value.
  • Casting affected spells now presents the correct alternative cost, avoids duplicate X prompts, and deducts the proper amount of mana.

• Tests

  • Added parser and integration coverage for Kentaro’s alternative cost behavior.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: b53d787c-4751-4786-af98-35fc105c9f88

📥 Commits

Reviewing files that changed from the base of the PR and between ad753b7 and f5815dd.

📒 Files selected for processing (1)

• crates/engine/tests/integration/issue_5899_kentaro_alternative_cost.rs

📝 Walkthrough

Walkthrough

Kentaro’s {X} alternative cost now binds to the affected spell’s mana value. The parser converts this form to SelfManaValue, and casting resolves it before payment. Parser and integration tests cover the corrected flow.

Changes

Kentaro alternative cost

Layer / File(s)	Summary
Parse spell-mana-value alternative costs crates/engine/src/parser/oracle_static/cost_mod.rs, crates/engine/src/parser/oracle_static/tests.rs	The parser recognizes a standalone {X} clause bound to a spell’s mana value and converts it to SelfManaValue. Parser tests cover Kentaro’s Samurai filter and dynamic cost.
Resolve self-referential costs during casting crates/engine/src/game/casting.rs, crates/engine/src/game/keywords.rs	Granted alternative costs resolve self-referential mana placeholders against the affected spell before affordability and payment. Documentation describes nested AbilityCost resolution.
Validate Kentaro casting behavior crates/engine/tests/integration/issue_5899_kentaro_alternative_cost.rs, crates/engine/tests/integration/main.rs	The integration test verifies the optional alternative cost, casting completion without a second X prompt, and mana consumption. The test module is registered in the engine suite.

Estimated code review effort: 2 (Simple) | ~15 minutes

Mergeability Score: 🟡 Moderate · up to ad753

The regression test does not distinguish Kentaro’s alternative cost from the spell’s normal cost, so an incorrect cast path could still pass. Merge readiness therefore depends on updating the test to require the colored normal cost to fail while the generic alternative cost succeeds.

Possibly related PRs

• phase-rs/phase#7298: Both changes cover mana-value/X cost binding in the engine’s casting-cost flow.
• phase-rs/phase#7030: Both changes modify casting cost handling and payability in casting.rs.
• phase-rs/phase#6516: Both changes modify cost resolution logic in casting.rs.

Suggested labels: bug

Suggested reviewers: lgray, ntindle, dripsmvcp

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the parser fix that binds Kentaro’s alternative cost to the spell’s mana value.
Linked Issues check	✅ Passed	The parser, engine changes, and regression tests address issue #5899 by creating a mana-value-based alternative cost without an extra X prompt.
Out of Scope Changes check	✅ Passed	All changes support issue #5899 through parser logic, cost resolution, documentation, and targeted regression tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ship/5899-kentaro-alt-cost

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/engine/tests/integration/issue_5899_kentaro_alternative_cost.rs`:
- Around line 20-29: Update the test setup around the Test Samurai created by
add_creature_to_hand so its normal mana cost has mana value 3 but requires
colored mana, while preserving the pool of only three colorless mana; ensure the
cast therefore succeeds only through Kentaro’s resolved generic alternative
cost.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 8f628df9-0cf5-4688-8ad6-c495363ccbd8

📥 Commits

Reviewing files that changed from the base of the PR and between 1e7a9a6 and ad753b7.

📒 Files selected for processing (6)

• crates/engine/src/game/casting.rs
• crates/engine/src/game/keywords.rs
• crates/engine/src/parser/oracle_static/cost_mod.rs
• crates/engine/src/parser/oracle_static/tests.rs
• crates/engine/tests/integration/issue_5899_kentaro_alternative_cost.rs
• crates/engine/tests/integration/main.rs

[github-actions]

Generated for head f5815ddfc40db128f2ceec94e487c9a4dfbf5d49.

Parse changes introduced by this PR · 1 card(s), 2 signature(s) (baseline: main 1e7a9a648cd1)

🟢 Added (1 signature)

• 1 card · ➕ static/CastWithAlternativeCost(Mana { cost: SelfManaValue }) · added: CastWithAlternativeCost(Mana { cost: SelfManaValue }) (affects=you control Samurai)
  • Affected (first 3): Kentaro, the Smiling Cat

🔴 Removed (1 signature)

• 1 card · ➖ ability/PayCost · removed: PayCost
  • Affected (first 3): Kentaro, the Smiling Cat