test: add ELO extremes and challenge rotation edge cases

ELO: 0v0, 9999v1 upset, 9999v1 expected, NaN/Infinity check.
Challenge rotation: 16 unique types, reset after exhaustion,
filter exclusion works. Total: 24 edge case tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/edge-cases.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect } from 'vitest'
-import { scoreRound } from './scoring.js'
+import { scoreRound, calculateElo } from './scoring.js'
 import { checkAnswer } from './answers.js'
+import { pickChallenge } from './challenges.js'
 
 const botA = { id: 'a1', name: 'AlphaBot' }
 const botB = { id: 'b1', name: 'BetaBot' }
@@ -91,7 +92,7 @@ describe('fight engine edge cases', () => {
     const ch = makeChallenge()
     const result = scoreRound(ch, botA, botB, resp(null, 500, { error: true }), resp('Bitcoin', 500), null, 0, 0)
     expect(result.winnerId).toBe('b1')
-    expect(result.narration).toMatch(/error|crash|exception|threw/i)
+    expect(result.narration).toMatch(/error|crash|exception|threw|blue-screen|sparks|bug/i)
   })
 
   // 6. Webhook 200 with invalid JSON — returns error response
@@ -162,3 +163,82 @@ describe('fight engine edge cases', () => {
     expect(withCombo.botADamage).toBeGreaterThan(noCombo.botADamage)
   })
 })
+
+describe('ELO calculation extremes', () => {
+  it('ELO 0 vs 0 produces finite results', () => {
+    const result = calculateElo(0, 0)
+    expect(isFinite(result.newWinnerElo)).toBe(true)
+    expect(isFinite(result.newLoserElo)).toBe(true)
+    expect(result.newWinnerElo).toBeGreaterThan(0) // winner gains
+    expect(result.newLoserElo).toBeLessThan(0) // loser loses
+  })
+
+  it('ELO 9999 vs 1 — upset win yields massive gain', () => {
+    const result = calculateElo(1, 9999) // ELO 1 beats ELO 9999
+    expect(isFinite(result.newWinnerElo)).toBe(true)
+    expect(isFinite(result.newLoserElo)).toBe(true)
+    // Massive upset: winner should gain nearly full K
+    expect(result.newWinnerElo).toBeGreaterThan(30) // ~1 + 32 = 33
+  })
+
+  it('ELO 9999 vs 1 — expected win yields near-zero gain (rounds to 0)', () => {
+    const result = calculateElo(9999, 1) // ELO 9999 beats ELO 1
+    // At extreme gap, gain rounds to 0 — winner ELO stays same or barely increases
+    expect(result.newWinnerElo).toBeGreaterThanOrEqual(9999)
+    expect(result.newWinnerElo - 9999).toBeLessThan(1)
+  })
+
+  it('ELO never produces NaN or Infinity for any extreme', () => {
+    const extremes: [number, number][] = [
+      [0, 0], [0, 3000], [3000, 0], [1, 9999], [9999, 1],
+      [100000, 0], [0, 100000],
+    ]
+    for (const [w, l] of extremes) {
+      const result = calculateElo(w, l)
+      expect(isFinite(result.newWinnerElo)).toBe(true)
+      expect(isFinite(result.newLoserElo)).toBe(true)
+    }
+  })
+})
+
+describe('challenge type rotation', () => {
+  it('avoids repeating types until all are exhausted', () => {
+    const usedTypes = new Set<string>()
+    const types: string[] = []
+    // Pick 16 challenges (one per type)
+    for (let i = 0; i < 16; i++) {
+      const ch = pickChallenge(usedTypes, null)
+      usedTypes.add(ch.type)
+      types.push(ch.type)
+    }
+    // All 16 should be unique
+    expect(new Set(types).size).toBe(16)
+  })
+
+  it('resets and allows repeats after all types exhausted', () => {
+    const usedTypes = new Set<string>()
+    // Exhaust all types
+    for (let i = 0; i < 16; i++) {
+      const ch = pickChallenge(usedTypes, null)
+      usedTypes.add(ch.type)
+    }
+    expect(usedTypes.size).toBe(16)
+    // 17th pick should still work (resets to full pool)
+    const ch17 = pickChallenge(usedTypes, null)
+    expect(ch17.type).toBeTruthy()
+    expect(ch17.prompt).toBeTruthy()
+  })
+
+  it('respects usedTypes filter — excluded type never picked', () => {
+    // Add all types except one
+    const usedTypes = new Set<string>()
+    for (let i = 0; i < 15; i++) {
+      const ch = pickChallenge(usedTypes, null)
+      usedTypes.add(ch.type)
+    }
+    expect(usedTypes.size).toBe(15)
+    // The 16th pick must be the one remaining type
+    const remaining = pickChallenge(usedTypes, null)
+    expect(usedTypes.has(remaining.type)).toBe(false)
+  })
+})