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Fuse Energy

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Fuse Energy is a technology-driven energy company that supplies electricity to consumers, often incorporating smart pricing and app-based management of energy usage. It operates in the retail energy and clean energy sector, aiming to make power more affordable and accessible.

4 interview notes · updated Jul 2026

Fuse Energy·Software Engineer·Technical Phone Screen

Jun 2026
Got a system design coding question for a Software Engineer role at Fuse Energy. It was a meaty in-memory spreadsheet problem with dependency tracking, cycle detection, and undo support. More algorithmic depth than I expected for what felt like a mid-level role.
  • Design an in-memory spreadsheet that supports Set, Sum, Get, and Undo operations on cells and ranges, where formula cells are reactive, duplicate references count multiple times, and Sum must reject any operation that would create a cycle in the dependency graph.

“This one took me a while to even parse fully.”

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Fuse Energy·Software Engineer·Technical Phone Screen

Jun 2026
Fuse Energy had me build a mini spreadsheet engine from scratch, SET/GET/SUM operations, dependency tracking, cycle detection and all. Pretty meaty for a single coding round and the cycle handling piece is where I think most people would trip up.
  • Design and implement an in-memory spreadsheet supporting SET, GET, and SUM operations, where SUM can reference individual cells or rectangular ranges, and values update dynamically when dependencies change.
  • How do you detect and handle cyclic dependencies in the spreadsheet formula graph, for example if A1 depends on B1 and then B1 is set to depend on A1?

“This is a bigger problem than it looks at first glance.”

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Fuse Energy·Software Engineer·Technical Phone Screen

Apr 2026
Did a coding round for a Software Engineer role at Fuse Energy, building on a previous mini-spreadsheet problem. This one pushed further into graph territory and I had to think carefully about how dependency cycles actually propagate.
  • Given a spreadsheet that supports Set, Get, and SUM operations, extend it to detect circular dependencies. If assigning a SUM formula to a cell would create a cycle (direct or indirect) in the dependency graph, the operation should be rejected with an error.

“This was a follow-up to the basic spreadsheet problem so I thought I had a head start.”

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Fuse Energy·Software Engineer·Technical Phone Screen

Apr 2026
Fuse Energy had me extend a mini spreadsheet coding problem I'd apparently already done in a prior round, this time adding undo functionality. Felt like a reasonable follow-up but the history stack part tripped me up more than I expected.
  • You have a mini spreadsheet supporting Set, Get, and SUM operations. Now add an UNDO command that reverts the most recent Set or SUM operation, restoring both the cell's previous value and its formula state using a history stack.

“The core Set/Get/SUM part was fine since I'd seen it before.”

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