etag.nx

etag.nx -- generate + compare HTTP ETag headers (RFC 7232). The ETag header identifies a specific version of a resource so conditional requests (If-None-Match / If-Match) can avoid re-sending bytes that haven't changed. Every static-file server needs this, every CDN lives or dies on it. Format (RFC 7232 §2.3): ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4" <- strong ETag: W/"3ad4a9c3" <- weak Strong ETags mean byte-exact match; weak (W/-prefixed) means semantically equivalent but possibly byte-different (e.g. same content reflowed whitespace). We generate ETags by: 1. sha256 (existing) of the file contents -> 32 bytes 2. hex-encode (existing) -> 64 ASCII chars 3. truncate to etag_size bytes (caller choice; 8-32 typical) 4. wrap in double quotes Comparison follows RFC 7232 §2.3.2: strong match requires both to be strong and byte-identical; weak match matches regardless of W/- prefix. Invariants: ET1 Output always ends with a trailing DQUOTE. ET2 Weak prefix is exactly "W/" (2 ASCII bytes, case-sensitive). ET3 Caller chooses truncation length; 8 hex chars (4 bytes of entropy) is the minimum practical, 32 (128 bits) is plenty.