# snappy.crc32c

> A module of snappy's API.

CRC-32C (Castagnoli, polynomial 0x1EDC6F41 / reflected 0x82F63B78), init/xorout 0xFFFFFFFF. Table-driven with a slice-by-8 fast path.

This is the checksum the Snappy **framing format** masks and stores per
chunk — not the same polynomial as plain CRC-32 (gzip/zlib/PNG use
0xEDB88320; do not reuse hashes.mojo's `crc32` here).

`crc32c(data, seed=0)` matches the well-known check value
`crc32c(b"123456789") == 0xE3069283`. Passing a previous result as `seed`
chains across buffers: `crc32c(b, seed=crc32c(a)) == crc32c(a + b)`.

## Structs

### `Crc32c`

```mojo
struct Crc32c
```

Incremental CRC-32C. `seed` is the previous crc32c result (0 to start).

Implements: `Copyable`, `Movable`

#### `__init__`

```mojo
def __init__(out self, seed: UInt32 = UInt32(0))
```

#### `update`

```mojo
def update(mut self, data: Span[UInt8])
```

#### `finish`

```mojo
def finish(self) -> UInt32
```

## Functions

### `crc32c`

```mojo
def crc32c(data: Span[UInt8], seed: UInt32 = UInt32(0)) -> UInt32
```

### `mask_crc32c`

```mojo
def mask_crc32c(crc: UInt32) -> UInt32
```

Snappy framing-format masking: rotate right 15, then add a magic constant (mod 2**32). Never store or compare a raw crc32c in a frame — always mask on write, unmask on read.

### `unmask_crc32c`

```mojo
def unmask_crc32c(masked: UInt32) -> UInt32
```

Inverse of `mask_crc32c`.