Articles & guides

Notes from behind the judge.

Deep dives on the architecture behind ls-judge — how we run untrusted code safely, how the fleet scales, and the strange corners of running 100+ language runtimes on one host.

·7 min read

Sliding window, from brute force to O(n)

The sliding-window pattern is the 'grow, then shrink' trick that solves most substring and subarray problems in a single pass. Walk it from the naive O(n²) loop to O(n) with a concrete example — every snippet is a click away from running live.
patternssliding-windowalgorithmstutorial
·9 min read

Why we run every code submission in a Firecracker microVM

Every submission on ls-judge runs in its own Firecracker microVM — the same primitive AWS Lambda uses. Here is why we picked the hardware-isolation route over containers, and what it cost us to get there.
firecrackersandboxingsecurityarchitecture