Struct Meta

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pub struct Meta(/* private fields */);

Expand description

The meta byte in front of every stored string.

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impl Meta

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pub const fn new(kind: Kind, enc: Encoding, has_expiry: bool) -> Meta

Build the byte for a type, an encoding and the presence of a deadline.

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pub const fn string(enc: Encoding, has_expiry: bool) -> Meta

The byte for a string, which is what everything in strings.rs writes.

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pub const fn slot(kind: Kind, has_expiry: bool) -> Meta

The byte for a value that lives in a slab, with the record holding a number that says where.

The encoding bits are written as zero and mean nothing here. A set’s encoding is which of the three representations it is in, and that is a property of the body and not of the record, so OBJECT ENCODING follows the number and asks. Keeping a copy of it in these two bits would want the record rewritten every time a set was promoted, for a command nobody calls in a loop, and two places to disagree about the same fact.

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pub const fn from_byte(b: u8) -> Meta

Read the byte back.

An unknown encoding is impossible from our own writer, so the two spare bit patterns fall to raw, which is the reading that returns the bytes unchanged rather than reinterpreting them as something else.

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pub const fn byte(self) -> u8

The raw byte, as stored.

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pub const fn encoding(self) -> Encoding

Which encoding this value carries.

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pub const fn kind(self) -> Kind

Which type this key holds.

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pub const fn has_expiry(self) -> bool

Whether a deadline follows.

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pub const fn has_access(self) -> bool

Whether an access field follows the deadline.

Everything this crate writes sets it, so in a running server it is always true and the reader that checks it is checking something that cannot happen. It is here anyway because a bit in the byte costs nothing and the alternative was a flag day: without it, the day the field arrived, every reader had to agree with every writer at exactly the same moment.

It is not a file format concern. These records live in the arena and never reach a file, and the on disk record in yo_format has its own layout and its own versioning.

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pub const fn is_cold(self) -> bool

Whether the payload is a place on the file rather than the value.

The one question a point read asks before it decides whether it is going to touch a device, and the answer comes out of the byte the lookup has already fetched.

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