crc32_combine64
Compute the CRC-32 of A followed by B without reading either sequence.
Full chapter and shared rules ↗57ad356e10ec68ee · Source at the matching commit ↗Read the function’s preconditions and results first. Expand the shared rules below for buffer and ownership requirements used throughout this API group.
uLong ZEXPORT crc32_combine64(uLong crc1, uLong crc2, z_off64_t len2);
Compute the CRC-32 of A followed by B without reading either sequence.
Domain Restricted domain. Effects: pure computation, reading only fixed library tables.
Pre For the concatenation result, the low 32 bits of crc1 and crc2
are the independently computed CRC-32 checksums of A and B, and len2 is the
length of B. The fixed CRC tables are present. Negative lengths are included
for rejection, with no checksum restriction.
Post A negative length returns 0. Otherwise return the CRC-32 of A followed by B. Empty B has CRC 0 and leaves A's checksum unchanged.
Effects Preserve all memory and ownership. Read no buffers or stream state.
The generated operator satisfies crc32_combine_op's input requirement.
For arbitrary checksums at length zero, the implementation returns the XOR of the low 32 bits. A wrong length has no concatenation guarantee. Zero is also a possible valid CRC, so negative-length rejection has no distinct error code.
Shared conditions and scope
API manual · Contract conventions · Buffer utilities
Compute a checksum a buffer at a time, or combine the checksums of independently processed byte sequences without rereading them. The byte sequences A and B below are ordinary data; the contracts use the Adler-32 or CRC-32 of those sequences.
| Task | Entries |
|---|---|
| Update Adler-32 | Size-type length, 32-bit length |
| Combine Adler-32 | Ordinary offset, explicit 64-bit offset |
| Update CRC-32 | Size-type length, 32-bit length |
| Combine CRC-32 | Ordinary offset, explicit 64-bit offset |
| Reuse a CRC combination operator | Generate, generate with 64-bit length, apply |
| Inspect the byte table | get_crc_table |
These focused contracts follow adler32.c, crc32.c and zlib.h in the default LP64 Linux
static build, with the pinned source and listed patches. Source readings are to confirm
by proof. No checksum entry writes state, allocates, frees or performs I/O. Exceptional
numeric results are ordinary values, rather than error codes. Domain and effects remain
independent as explained in contract conventions.
The practical checksum contracts use actual byte sequences A and B. Each checksum is computed independently, starting at 1 for Adler-32 or 0 for CRC-32. These contracts deliberately omit functional promises for forged checksums. The Lean targets also describe arbitrary seeds and exact scalar results. Both the Adler entries and their Lean targets require zero length for NULL initialization queries. See scope deviations and public goals.
In this build, uLong and z_size_t are unsigned 64-bit types; uInt is
unsigned 32-bit. z_off_t and z_off64_t are signed 64-bit. Input checksum bits
above bit 31 are ignored; returned checksums fit in 32 bits. The explicit
*64 declarations depend on the header's large-file feature macros.
Formal contract
ZlibF.Spec.Public.Goal.crc32_combine64
Proof status: Registered proof · Evidence and remaining work
/-- `uLong crc32_combine64(uLong crc1, uLong crc2, z_off64_t len2)`; full combine contract. -/
def Goal.crc32_combine64 : Prop := CrcCombine Zlib.Crc32.crc32_combine64