Contracts

ROOKIE HACKS II · BEST HEDERA

split expenses with friends on-chain — every edit, settle, and delete kept in an immutable audit log

fig. 1 — every edit appended to an immutable, on-chain audit logHedera · tamper-proof

created0xA1…Dinner · ₹2,400 · split 3 ways#1041

edited0xB2…amount → ₹2,700#1042

settled0xC3…paid ₹900 share#1043

settled0xA1…paid ₹900 share#1044

↳ each entry hash-chained to the last — no silent edits

01THE PROBLEMsomeone always edits the record

Splitting expenses is easy until someone disputes a transaction — or quietly edits a record nobody else can see.

Contracts moves the whole expense-splitting workflow onto a blockchain, where the ledger's immutability means no transaction can be altered without a visible, permanent trace. Built at Rookie Hacks II 2022, where it won Best Blockchain Project Using Hedera.

02THE ARCHITECTUREHedera contracts as the database

Instead of a traditional database, the backbone is Hedera-deployed Solidity contracts. Users authenticate with blockchain accounts; every mutation — an edit, a deletion, a settlement — is captured in an immutable audit log, so all participants share one tamper-proof history.

AUTH

Hedera

↓

RECORD

add / edit / settle a shared expense

Solidity

↓

APPEND

every mutation logged immutably

audit log

↓

VIEW

one transparent history for all

Next.js

STACK — Solidity · Hedera · Next.js · Express · Node · TailwindCSS

03THE HARD PARTHedera, newly launched

I built the full frontend — the blockchain auth flow and the transaction UI. The challenge was Hedera's nascent tooling: sparse docs for a freshly-launched platform meant first-principles debugging to get auth and contract calls working together:

blockchain auth

wallet-account sign-in, no passwords

contract-as-database

expenses persisted as on-chain state

immutable audit log

edits and settlements traceable forever

sparse-docs debugging

first-principles work on a new platform

04IN THE WILDplates 01–04 · Rookie Hacks II

[next study — Hospitatva→][view the source→]