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Hot zones ​

A hot zone is a module that hurts a lot when you change it: many things depend on it, it depends on many things itself, and ideally it's also large or sits in a cycle. The hotness score ranks modules so the worst offenders surface first.

Score ​

For each module:

couplingScore = log2(fanIn + 1) * log2(fanOut + 1)        // 0..1 normalized per project
hotnessScore  = couplingScore * (1 + 0.5 * inCycle) * sizeFactor

Where:

  • fanIn — number of modules importing this one.
  • fanOut — number of modules this one imports.
  • inCycle — boolean: is this module part of any SCC?
  • sizeFactor — log-scaled lines-of-code, normalized to ≈ 0.5..1.5.

log2(x+1) softens the long tail — a module with 200 importers isn't 100× worse than one with 20. The product (fan-in × fan-out, log-scaled) captures the "stuck in the middle" feeling: a module that's both heavily depended on and heavily depends on others is harder to refactor than one that's just heavy in one direction.

The +50% bump for being in a cycle reflects that cycles compound the cost of any change. Size matters less than coupling, hence sizeFactor is a multiplier, not a primary term.

Ranking ​

Top-N modules by hotnessScore are the project's hot zones. The desktop app highlights them with a warm color in the graph and lists them in the Insights panel. The CLI exposes them via the hotZones field of the JSON envelope.

What hot zones are not ​

  • They aren't bugs. A heavily-used utility is supposed to have high fan-in. A coordinator file is supposed to have high fan-out. Hot zones are attention pointers, not "fix this."
  • They aren't directly tied to grade. A project can have legitimate hot zones (a router, a context provider) and still grade A. The grade is dominated by cycles and layer violations.

When hot zones become recommendations ​

A hot zone is promoted to a recommendation only when extra signals pile up. The clearest case is split-god-module: fan-in in the top 5%, fan-in ≥ 8, and ≥ 300 lines of code. All three thresholds must hit. We deliberately don't recommend "do something about hot zone X" without a concrete pattern — that would just be noise.

See also ​