Struct BitWriter

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pub struct BitWriter { /* private fields */ }

Expand description

A growable, MSB-first bit buffer.

Uses a bit cache: bits accumulate in the low nbits positions of a 64-bit cache (the most-recently-written bit is least-significant within the valid region), and whole bytes are flushed off the top. A multi-bit write is a shift

OR + a short byte-flush loop — not one operation per bit. After every public write the invariant nbits < 8 and “bits above nbits are zero” hold.

Implementations§

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impl BitWriter

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pub fn new() -> Self

Creates an empty writer.

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pub fn with_capacity(bytes: usize) -> Self

Creates a writer with bytes pre-reserved — for the per-frame slice writer, so the CAVLC hot loop never pays a Vec realloc mid-frame.

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pub fn bit_len(&self) -> usize

Number of bits written so far.

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pub fn is_byte_aligned(&self) -> bool

true if the writer is on a byte boundary.

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pub fn write_bits(&mut self, value: u32, n: u32)

Writes the low n bits of value, most-significant first (u(n)). n <= 32.

Keeps up to 31 pending bits in cache and flushes a whole u32 word (4 bytes) at a time, like openh264’s BsWriteBits (WRITE_BE_32) — a quarter the Vec writes of byte-at-a-time flushing.

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pub fn write_bit(&mut self, bit: bool)

Writes a single bit (true => 1).

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pub fn append(&mut self, other: &BitWriter)

Appends every bit of other, in order, at this writer’s current position.

Neither writer need be byte-aligned. This is what lets a macroblock be encoded into a scratch writer before the syntax that must precede it is known (mb_skip_run is only decided once the macroblock’s own cost is), so the encoder can commit one encode rather than trialing and repeating it. Sound for CAVLC because a macroblock’s Exp-Golomb/VLC syntax does not depend on its bit position; an arithmetic coder (CABAC) could NOT be spliced this way, since its state is carried across the whole slice.

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pub fn write_ue(&mut self, value: u32)

Unsigned Exp-Golomb code ue(v).

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pub fn write_se(&mut self, value: i32)

Signed Exp-Golomb code se(v): 0->0, 1->1, -1->2, 2->3, -2->4, ....

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pub fn rbsp_trailing_bits(&mut self)

Writes the rbsp_trailing_bits(): a stop bit 1 then zero-pad to a byte.

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pub fn align_zero(&mut self)

Pads with zero bits to the next byte boundary (no stop bit), then flushes the pending whole bytes from the cache (the word-flush in write_bits can leave up to 31 pending bits).

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pub fn into_bytes(self) -> Vec<u8> ⓘ

Consumes the writer, returning the byte buffer. Flushes any whole bytes still pending in the cache; panics if a sub-byte remainder is left (call rbsp_trailing_bits or align_zero first).

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