fix: tiers show incorrect volumes if denominator differs (#18041)

* use correct denom for previous tier and show correct number

* test the func

frontend/src/scenes/billing/billing-utils.spec.ts

@@ -1,4 +1,10 @@
-import { convertAmountToUsage, convertUsageToAmount, projectUsage, summarizeUsage } from './billing-utils'
+import {
+    convertAmountToUsage,
+    convertLargeNumberToWords,
+    convertUsageToAmount,
+    projectUsage,
+    summarizeUsage,
+} from './billing-utils'
 import tk from 'timekeeper'
 import { dayjs } from 'lib/dayjs'
 import billingJson from '~/mocks/fixtures/_billing_v2.json'
@@ -159,3 +165,13 @@ describe('convertAmountToUsageWithPercentDiscount', () => {
         }
     )
 })
+
+describe('convertLargeNumberToWords', () => {
+    it('should convert large numbers to words', () => {
+        expect(convertLargeNumberToWords(250, null, true, 'survey')).toEqual('First 250 surveys/mo')
+        expect(convertLargeNumberToWords(500, 250, true, 'survey')).toEqual('251-500')
+        expect(convertLargeNumberToWords(1000, 500, true, 'survey')).toEqual('501-1k')
+        expect(convertLargeNumberToWords(10000, 1000, true, 'survey')).toEqual('1-10k')
+        expect(convertLargeNumberToWords(10000000, 1000000, true, 'survey')).toEqual('1-10 million')
+    })
+})

frontend/src/scenes/billing/billing-utils.ts

@@ -185,14 +185,20 @@ export const convertLargeNumberToWords = (
     }
 
     let denominator = 1
-
     if (num >= 1000000) {
         denominator = 1000000
     } else if (num >= 1000) {
         denominator = 1000
     }
 
-    return `${previousNum ? `${(previousNum / denominator).toFixed(0)}-` : multipleTiers ? 'First ' : ''}${(
+    let prevDenominator = 1
+    if (previousNum && previousNum >= 1000000) {
+        prevDenominator = 1000000
+    } else if (previousNum && previousNum >= 1000) {
+        prevDenominator = 1000
+    }
+
+    return `${previousNum ? `${((previousNum + 1) / prevDenominator).toFixed(0)}-` : multipleTiers ? 'First ' : ''}${(
         num / denominator
     ).toFixed(0)}${denominator === 1000000 ? ' million' : denominator === 1000 ? 'k' : ''}${
         !previousNum && multipleTiers ? ` ${productType}s/mo` : ''