Fix cache token accounting and tiered pricing

frontend/src/features/models/utils/__tests__/tiered-pricing-multipliers.spec.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest'
+import {
+  cacheMultiplierFromPrice,
+  cachePriceFromInputMultiplier,
+} from '../tiered-pricing-multipliers'
+
+describe('cache input multipliers', () => {
+  it('converts an input multiplier to an actual cache price', () => {
+    expect(cachePriceFromInputMultiplier(4, 1.25)).toBe(5)
+    expect(cachePriceFromInputMultiplier(4, 0.1)).toBe(0.4)
+    expect(cachePriceFromInputMultiplier(4, 0)).toBe(0)
+  })
+
+  it('derives a multiplier from an existing cache price', () => {
+    expect(cacheMultiplierFromPrice(4, 5, 1.25)).toBe(1.25)
+    expect(cacheMultiplierFromPrice(4, 0.4, 0.1)).toBe(0.1)
+  })
+
+  it('uses the fallback when a multiplier cannot be derived', () => {
+    expect(cacheMultiplierFromPrice(0, 5, 1.25)).toBe(1.25)
+    expect(cacheMultiplierFromPrice(4, undefined, 0.1)).toBe(0.1)
+  })
+})

frontend/src/features/models/utils/tiered-pricing-multipliers.ts

@@ -0,0 +1,17 @@
+export function roundTierPrice(value: number): number {
+  return Number.parseFloat(Math.max(0, value).toFixed(6))
+}
+
+export function cachePriceFromInputMultiplier(inputPrice: number, multiplier: number): number {
+  const normalizedMultiplier = Number.isFinite(multiplier) && multiplier >= 0 ? multiplier : 0
+  return roundTierPrice(inputPrice * normalizedMultiplier)
+}
+
+export function cacheMultiplierFromPrice(
+  inputPrice: number,
+  cachePrice: number | undefined,
+  fallback: number,
+): number {
+  if (cachePrice == null || inputPrice <= 0) return fallback
+  return roundTierPrice(cachePrice / inputPrice)
+}