Hoist protocol dispatch into module-level tables - #118
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_ethertype_class() and _ip_protocol_class() re-ran their deferred imports and rebuilt a dict literal on every call. They run once per layer per frame, so this was the hottest path in the library — a corpus profile put _ip_protocol_class at the top by tottime. Build each table once, on first use, and reduce the call to a lookup. The imports stay inside the build function, so the layer modules remain acyclic exactly as before; only the rebuilding goes away. The IPv6-only gating is preserved by construction rather than by a per-call membership test: the IPv4 table is the IPv6 table minus the extension-header numbers, so an IPv4 packet with protocol=0 still cannot decode a Hop-by-Hop layer. That invariant now has a table-level test alongside the existing public-API one. Measured on this machine (200k calls, best of run): _ip_protocol_class 1439 -> 104 ns/call (13.9x) _ethertype_class 712 -> 86 ns/call (8.3x) corpus walk 58,300 -> 76,500 frames/sec (1.31x) The issue quotes 91x/42x for the two functions; those figures compare against a bare dict lookup, while the numbers above are through the function call, whose overhead now dominates at ~90-100 ns. The end-to-end corpus figure is the one that matters for #86. Closes#82. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01QJnVMNGwTRDktC4rkABtgt
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Summary
_ethertype_class()and_ip_protocol_class()re-ran their deferred imports and rebuilt adictliteral on every call — once per layer per frame, the hottest path in the library. Both now build their table once, on first use, and reduce the call to a lookup. Internal only; no API change.What's included
layer2/ethernet.py:_ETHERTYPE_CLASSES, populated by_build_ethertype_classes()on first use. The VLAN tag types fold into the same table (dict.fromkeys) instead of a separate membership test.layer3/ip.py:_IPV6_PROTOCOL_CLASSESand_IPV4_PROTOCOL_CLASSES. The gating is baked into the tables — the IPv4 table is the IPv6 one minus_IPV6_ONLY_NUMBERS— so it costs nothing per call and cannot drift.test_ipv4_never_dispatches_extension_headers).Measured
200k calls, best of run, CPython 3.12 on this machine:
_ip_protocol_class_ethertype_classA note on the issue's 91×/42×: those compare against a bare dict lookup. The figures above are measured through the function call, whose overhead (~90–100 ns) now dominates — the table lookup itself is a few ns. The end-to-end corpus number is the one #86 will gate on.
Verification
uv run ruff checkanduv run ruff format --checkare cleanuv run mypyis clean (strict)uv run pytest— 748 passedCHANGELOG.mdentry under## [Unreleased]Notes
#87 (Tier 2) generalises this into a public registry. This lands the table as a private implementation detail so the registry design isn't forced now; the shapes are deliberately compatible.
Closes#82.
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https://claude.ai/code/session_01QJnVMNGwTRDktC4rkABtgt
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