hackerrank-coding-challenge

Challenge Description

We​ ​would​ ​like​ ​to​ ​have​ ​a​ ​restful​ ​API​ ​for​ ​our​ ​statistics.​ ​The​ ​main​ ​use​ ​case​ ​for​ ​our​ ​API​ ​is​ ​to calculate​ ​realtime​ ​statistic​ ​from​ ​the​ ​last​ ​60​ ​seconds.​ ​There​ ​will​ ​be​ ​two​ ​APIs,​ ​one​ ​of​ ​them​ ​is called​ ​every​ ​time​ ​a​ ​transaction​ ​is​ ​made.​ ​It​ ​is​ ​also​ ​the​ ​sole​ ​input​ ​of​ ​this​ ​rest​ ​API.​ ​The​ ​other​ ​one returns​ ​the​ ​statistic​ ​based​ ​of​ ​the​ ​transactions​ ​of​ ​the​ ​last​ ​60​ ​seconds.

Specs

1. POST​ ​/transactions Every​ ​Time​ ​a​ ​new​ ​transaction​ ​happened,​ ​this​ ​endpoint​ ​will​ ​be​ ​called.

{
    ​​​​​​​​​​​​​"amount":​ ​12.3,
​ ​​ ​​  ​"timestamp":​ ​1478192204000 
}

• amount​​​​- ​​​is​​ a​ ​double ​​specifying ​​the ​​amount
• timestamp​​​-​​​transaction​​ time​ ​in ​​epoch ​​in ​​millis​ ​in ​​UTC ​​time​​zone​

Returns:​ ​Empty​ ​body​ ​with​ ​either​ ​201​ ​or​ ​204.

• 201​ ​-​ ​in​ ​case​ ​of​ ​success
• 204​ ​-​ ​if​ ​transaction​ ​is​ ​older​ ​than​ ​60​ ​seconds

2. GET /statistics This​ ​is​ ​the​ ​main​ ​endpoint​ ​of​ ​this​ ​task,​ ​this​ ​endpoint​ ​have​ ​to​ ​execute​ ​in​ ​constant​ ​time​ ​and memory​ ​(O(1)).​ ​It​ ​returns​ ​the​ ​statistic​ ​based​ ​on​ ​the​ ​transactions​ ​which​ ​happened​ ​in​ ​the​ ​last​ ​60 seconds.

{
    ​"sum":​ ​1000,
    ​"avg":​ ​100, 
    "max":​ ​200, 
    "min":​ ​50, 
    "count":​ ​10
}
 ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​ ​​

Returns:

• sum: ​​​​is ​​a ​​double ​​specifying​ ​the ​​total ​​sum​ ​of ​​transaction​​ value​ ​in ​​the ​​last ​​60 ​​seconds
• avg: ​​​​is ​​a ​​double ​​specifying​​ the​​ average​​ amount​​ of​​ transaction ​​value ​​in ​​the ​​last ​​60 seconds
• max​​​: is​​ a​​ double​​ specifying​​ single​​ highest​​ transaction​​ value ​​in​​ the ​​last ​​60​​ seconds
• min​​​​is​​: ​​double​​ specifying ​​single​​ lowest ​​transaction​​ value​​ in ​​the ​​last ​​60​​ seconds
• count​​​​: ​​a​​ long​​ specifying​​ the​​ total ​​number​​ of​​ transactions​​ happened​​ in​​ the​​ last ​​60 seconds

Requirements

For​​ the​​ rest​​ api,​​the ​​biggest​ ​and​​ maybe ​​hardest ​​requirement ​​is​​ to ​​make ​​the ​​​​GET​ ​/statistics execute​ ​in​ ​constant​ ​time​ ​and​ ​space.​ ​The​ ​best​ ​solution​ ​would​ ​be​ ​O(1).​ ​It​ ​is​ ​very​ ​recommended​ ​to tackle​ ​the​ ​O(1)​ ​requirement​ ​as​ ​the​ ​last​ ​thing​ ​to​ ​do​ ​as​ ​it​ ​is​ ​not​ ​the​ ​only​ ​thing​ ​which​ ​will​ ​be​ ​rated​ ​in the​ ​code​ ​challenge. Other​ ​requirements,​ ​which​ ​are​ ​obvious,​ ​but​ ​also​ ​listed​ ​here​ ​explicitly:

• The​ ​API​ ​have​ ​to​ ​be​ ​threadsafe​ ​with​ ​concurrent​ ​requests
• The​ ​API​ ​have​ ​to​ ​function​ ​properly,​ ​with​ ​proper​ ​result
• The​ ​project​ ​should​ ​be​ ​buildable,​ ​and​ ​tests​ ​should​ ​also​ ​complete​ ​successfully.​ ​e.g.​ ​If maven​ ​is​ ​used,​ ​then​ ​mvn​ ​clean​ ​install​ ​should​ ​complete​ ​successfully.
• The API should be able to deal with time discrepancy, which means, at any point of time, we​ ​could​ ​receive​ ​a​ ​transaction​ ​which​ ​have​ ​a​ ​timestamp​ ​of​ ​the​ ​past
• Make​ ​sure​ ​to​ ​send​ ​the​ ​case​ ​in​ ​memory​ ​solution​ ​without​ ​database​ ​(including​ ​in-memory database)
• Endpoints​ ​have​ ​to​ ​execute​ ​in​ ​constant​ ​time​ ​and​ ​memory​ ​(O(1))
• Please​ ​complete​ ​the​ ​challenge​ ​using​ ​Java

Build and Run

1. Build mvn clean install
2. Run mvn spring-boot:run