Phase 16: advantage/disadvantage applies to any single die

applyRollMode (src/lib/dice/notation.ts) now rewrites the first single-die term
of any size (d4/d6/d20/...) to 2dN kh1/kl1, not just d20. Multi-die pools (2d6)
and terms with an existing keep/drop (4d6kh3) are left untouched. Dice page copy
updated; unit tests for d4/d6/d8 + skip cases; e2e toggles Advantage and rolls a
d4 → 2d4kh1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/features/dice/DicePage.tsx

@@ -92,7 +92,7 @@ export function DicePage() {
               variant={mode === 'advantage' ? 'primary' : 'secondary'}
               aria-pressed={mode === 'advantage'}
               onClick={() => toggleMode('advantage')}
-              title="Toggle advantage for d20 rolls"
+              title="Toggle advantage — rolls the next die twice, keep highest"
             >
               Advantage
             </Button>
@@ -101,14 +101,14 @@ export function DicePage() {
               variant={mode === 'disadvantage' ? 'primary' : 'secondary'}
               aria-pressed={mode === 'disadvantage'}
               onClick={() => toggleMode('disadvantage')}
-              title="Toggle disadvantage for d20 rolls"
+              title="Toggle disadvantage — rolls the next die twice, keep lowest"
             >
               Disadvantage
             </Button>
           </div>
           {mode !== 'normal' && (
             <p className="mt-2 text-xs text-accent">
-              {mode === 'advantage' ? 'Advantage' : 'Disadvantage'} is on — d20 rolls use 2d20{mode === 'advantage' ? 'kh1' : 'kl1'}.
+              {mode === 'advantage' ? 'Advantage' : 'Disadvantage'} is on — the next single die rolls twice, keeping the {mode === 'advantage' ? 'highest' : 'lowest'}.
             </p>
           )}
 

src/lib/dice/notation.test.ts

@@ -11,9 +11,18 @@ describe('applyRollMode', () => {
     expect(applyRollMode('2d6+3', 'advantage')).toBe('2d6+3');
     expect(applyRollMode('1d20+5', 'normal')).toBe('1d20+5');
   });
-  it('only transforms the first d20 term', () => {
+  it('only transforms the first die term', () => {
     expect(applyRollMode('1d20+1d20', 'advantage')).toBe('2d20kh1+1d20');
   });
+  it('applies to any single die, not just d20', () => {
+    expect(applyRollMode('1d4', 'advantage')).toBe('2d4kh1');
+    expect(applyRollMode('d6', 'disadvantage')).toBe('2d6kl1');
+    expect(applyRollMode('1d8+3', 'advantage')).toBe('2d8kh1+3');
+  });
+  it('does not touch multi-die pools or terms that already keep/drop', () => {
+    expect(applyRollMode('2d6+3', 'advantage')).toBe('2d6+3');
+    expect(applyRollMode('4d6kh3', 'advantage')).toBe('4d6kh3');
+  });
 });
 
 describe('parseDice', () => {

src/lib/dice/notation.ts

@@ -51,18 +51,21 @@ export class DiceParseError extends Error {}
 export type RollMode = 'normal' | 'advantage' | 'disadvantage';
 
 /**
- * Apply advantage/disadvantage to a d20 expression by turning the first d20 term
- * into 2d20kh1 / 2d20kl1. Non-d20 expressions are returned unchanged, so the
- * toggle only affects d20 checks (and keeps any modifiers, e.g. 1d20+5).
+ * Apply advantage/disadvantage by turning the first single-die term (any dN —
+ * d4, d6, d20, …) into 2dN kh1 / 2dN kl1, keeping any modifiers (e.g. 1d20+5).
+ * Only single-die terms (count 1 or omitted) are affected, since "advantage" on
+ * a pool like 2d6 isn't well-defined; terms that already carry a keep/drop
+ * modifier (e.g. 4d6kh3) are left untouched so we never double-apply.
  */
 export function applyRollMode(expression: string, mode: RollMode): string {
   if (mode === 'normal') return expression;
   const keep = mode === 'advantage' ? 'kh1' : 'kl1';
-  let replaced = false;
-  return expression.replace(/(^|[+\-\s])\d*d20\b/i, (m, pre: string) => {
-    if (replaced) return m;
-    replaced = true;
-    return `${pre}2d20${keep}`;
+  let done = false;
+  return expression.replace(/(^|[+\-\s])(\d*)d(\d+)((?:kh|kl|dh|dl)\d+)?/gi, (m, pre: string, count: string, sides: string, mod: string | undefined) => {
+    if (done || mod) return m;
+    if (count !== '' && count !== '1') return m; // only single-die terms
+    done = true;
+    return `${pre}2d${sides}${keep}`;
   });
 }