Three primitives:
simd_memmem(haystack, needle) — find the first occurrence
of needle in haystack. Specialised for the multipart
boundary scan, which is the dominant cost of parsing
multipart/form-data uploads (every body chunk must be scanned
for the per-request boundary delimiter --<boundary>).simd_percent_decode(input, out) — RFC 3986 §2.1
percent-decoder for URL-encoded query-string and form-body
fragments. Bulk-scans for % escape markers and copies
unescaped runs in one append_span call instead of
byte-by-byte.simd_cookie_scan(input) — split a Cookie /
Set-Cookie header value on ; delimiters in one pass,
returning the byte-offset list of separators so the caller
can build cookie name/value pairs without per-byte iteration.
Background
Mojo's stdlib Span[UInt8] doesn't ship a vectorised
memmem / percent-decode / cookie-split primitive yet. The
HTTP/1.1 parser hot path today loops byte-at-a-time for each
of these — fine for small payloads but linear in the input
size with a per-byte branch cost that dominates above ~4 KiB.
The "SIMD" here refers to an SSE4.2 / AVX2 vectorised inner
loop using PCMPESTRI / PSHUFB — that inner-loop swap
can land later. This module provides the clean public API +
correct scalar implementations + property tests. All future
SIMD acceleration plugs in behind the same function
signatures: a canonical scalar decoder first, with a SIMD swap
behind the same signatures.
What this module provides
simd_memmem(haystack, needle) -> Int — return the
byte-offset of the first match, or -1 on no match. Empty
needle returns 0 by convention (the empty string matches at
every position; we report the first). Linear-time
Rabin-Karp-flavoured scan: pre-computes a rolling hash of
the needle, walks the haystack with a sliding window,
byte-compares on hash hit. Slower than Boyer-Moore on
pathological adversarial input but no per-position table
setup cost.simd_percent_decode(input, out) raises HttpParseError —
appends the percent-decoded form of input to out.
Raises on malformed percent-escapes (lone % at end of
input, % followed by non-hex, etc.).HttpParseError — typed enum-style error. Variants:
TRAILING_PERCENT (lone % at end), INVALID_HEX
(% followed by a non-hex byte).simd_cookie_scan(input, mut offsets: List[Int]) — appends
the byte offsets of every ; to offsets. Caller
reconstructs the cookie name/value pairs by slicing
input[prev:offset].
These primitives don't touch the wire-protocol semantics — they
are byte-level helpers. Wiring into the multipart parser
(flare.http.multipart), the form decoder
(flare.http.form), and the cookie parser
(flare.http.cookie.parse_cookie_header) is a follow-up
commit that swaps the per-byte loops for these helpers without
changing public APIs.