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Add fixed-size varint encoder function to simplify writing complex EBML element sizes

This commit is contained in:
Tangent 128 2017-08-06 01:20:51 -04:00
parent b5bbf952b0
commit 95fcc6c86b
1 changed files with 36 additions and 2 deletions
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38
src/ebml.rs
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@ -1,7 +1,7 @@
use bytes::{BigEndian, ByteOrder, BufMut};
use std::error::Error as ErrorTrait;
use std::fmt::{Display, Formatter, Result as FmtResult};
use std::io::{Cursor, Error as IoError, ErrorKind, Result as IoResult, Write};
use std::io::{Cursor, Error as IoError, ErrorKind, Result as IoResult, Write, Seek, SeekFrom};
pub const EBML_HEAD_ID: u64 = 0x0A45DFA3;
pub const VOID_ID: u64 = 0x6C;
@ -129,7 +129,7 @@ const SMALL_FLAG: u64 = 0x80;
const EIGHT_FLAG: u64 = 0x01 << (8*7);
const EIGHT_MAX: u64 = EIGHT_FLAG - 2;
/// Tries to write an EBML varint
/// Tries to write an EBML varint using minimal space
pub fn encode_varint<T: Write>(varint: Varint, output: &mut T) -> IoResult<usize> {
let (size, number) = match varint {
Varint::Unknown => (1, 0xFF),
@ -157,6 +157,40 @@ pub fn encode_varint<T: Write>(varint: Varint, output: &mut T) -> IoResult<usize
return output.write_all(&buffer.get_ref()[..size]).map(|()| size);
}
const FOUR_FLAG: u64 = 0x10 << (8*3);
const FOUR_MAX: u64 = FOUR_FLAG - 2;
// tries to write a varint with a fixed 4-byte representation
pub fn encode_varint_4<T: Write>(varint: Varint, output: &mut T) -> IoResult<usize> {
let number = match varint {
Varint::Unknown => FOUR_FLAG | (FOUR_FLAG - 1),
Varint::Value(too_big) if too_big > FOUR_MAX => {
return Err(IoError::new(ErrorKind::InvalidInput, WriteError::OutOfRange))
},
Varint::Value(value) => FOUR_FLAG | value
};
let mut buffer = Cursor::new([0; 4]);
buffer.put_u32::<BigEndian>(number as u32);
return output.write_all(&buffer.get_ref()[..]).map(|()| 4);
}
pub fn encode_element<T: Write + Seek, F: Fn(&mut T) -> IoResult<X>, X>(tag: u64, output: &mut T, content: F) -> IoResult<()> {
encode_varint(Varint::Value(tag), output)?;
encode_varint_4(Varint::Unknown, output)?;
let start = output.seek(SeekFrom::Current(0))?;
content(output)?;
let end = output.seek(SeekFrom::Current(0))?;
output.seek(SeekFrom::Start(start - 4))?;
encode_varint_4(Varint::Value(end - start), output)?;
output.seek(SeekFrom::Start(end))?;
Ok(())
}
pub fn encode_tag_header<T: Write>(tag: u64, size: Varint, output: &mut T) -> IoResult<usize> {
let id_size = encode_varint(Varint::Value(tag), output)?;
let size_size = encode_varint(size, output)?;