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// Copyright 2015 Ilkka Rauta // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. //! BitReader is a helper type to extract strings of bits from a slice of bytes. //! //! Here is how you read first a single bit, then three bits and finally four bits from a byte //! buffer: //! //! ``` //! use bitreader::BitReader; //! //! let slice_of_u8 = &[0b1000_1111]; //! let mut reader = BitReader::new(slice_of_u8); //! //! // You probably should use try! or some other error handling mechanism in real code if the //! // length of the input is not known in advance. //! let a_single_bit = reader.read_u8(1).unwrap(); //! assert_eq!(a_single_bit, 1); //! //! let more_bits = reader.read_u8(3).unwrap(); //! assert_eq!(more_bits, 0); //! //! let last_bits_of_byte = reader.read_u8(4).unwrap(); //! assert_eq!(last_bits_of_byte, 0b1111); //! ``` //! You can naturally read bits from longer buffer of data than just a single byte. //! //! As you read bits, the internal cursor of BitReader moves on along the stream of bits. Little //! endian format is assumed when reading the multi-byte values. BitReader supports reading maximum //! of 64 bits at a time (with read_u64). Reading signed values directly is not supported at the //! moment. //! //! The reads do not need to be aligned in any particular way. //! //! Reading zero bits is a no-op. //! //! You can also skip over a number of bits, in which case there is no arbitrary small limits like //! when reading the values to a variable. However, you can not seek past the end of the slice, //! either when reading or when skipping bits. //! //! Note that the code will likely not work correctly if the slice is longer than 2^61 bytes, but //! exceeding that should be pretty unlikely. Let's get back to this when people read exabytes of //! information one bit at a time. use std::fmt; use std::error::Error; use std::result; #[cfg(test)] mod tests; /// BitReader reads data from a byte slice at the granularity of a single bit. pub struct BitReader<'a> { bytes: &'a [u8], /// Position from the start of the slice, counted as bits instead of bytes position: u64, relative_offset: u64, } impl<'a> BitReader<'a> { /// Construct a new BitReader from a byte slice. The returned reader lives at most as long as /// the slice given to is valid. pub fn new(bytes: &'a [u8]) -> BitReader<'a> { BitReader { bytes: bytes, position: 0, relative_offset: 0, } } /// Returns a copy of current BitReader, with the difference that its position() returns /// positions relative to the position of the original BitReader at the construction time. /// After construction, both readers are otherwise completely independent, except of course /// for sharing the same source data. /// /// ``` /// use bitreader::BitReader; /// /// let bytes = &[0b11110000, 0b00001111]; /// let mut original = BitReader::new(bytes); /// assert_eq!(original.read_u8(4).unwrap(), 0b1111); /// assert_eq!(original.position(), 4); /// /// let mut relative = original.relative_reader(); /// assert_eq!(relative.position(), 0); /// /// assert_eq!(original.read_u8(8).unwrap(), 0); /// assert_eq!(relative.read_u8(8).unwrap(), 0); /// /// assert_eq!(original.position(), 12); /// assert_eq!(relative.position(), 8); /// ``` pub fn relative_reader(&self) -> BitReader<'a> { BitReader { bytes: self.bytes, position: self.position, relative_offset: self.position, } } /// Read at most 8 bits into a u8. pub fn read_u8(&mut self, bit_count: u8) -> Result<u8> { let value = try!(self.read_value(bit_count, 8)); Ok((value & 0xff) as u8) } /// Read at most 16 bits into a u16. pub fn read_u16(&mut self, bit_count: u8) -> Result<u16> { let value = try!(self.read_value(bit_count, 16)); Ok((value & 0xffff) as u16) } /// Read at most 32 bits into a u32. pub fn read_u32(&mut self, bit_count: u8) -> Result<u32> { let value = try!(self.read_value(bit_count, 32)); Ok((value & 0xffffffff) as u32) } /// Read at most 64 bits into a u64. pub fn read_u64(&mut self, bit_count: u8) -> Result<u64> { let value = try!(self.read_value(bit_count, 64)); Ok(value) } /// Read at most 8 bits into a i8. /// Assumes the bits are stored in two's complement format. pub fn read_i8(&mut self, bit_count: u8) -> Result<i8> { let value = try!(self.read_signed_value(bit_count, 8)); Ok((value & 0xff) as i8) } /// Read at most 16 bits into a i16. /// Assumes the bits are stored in two's complement format. pub fn read_i16(&mut self, bit_count: u8) -> Result<i16> { let value = try!(self.read_signed_value(bit_count, 16)); Ok((value & 0xffff) as i16) } /// Read at most 32 bits into a i32. /// Assumes the bits are stored in two's complement format. pub fn read_i32(&mut self, bit_count: u8) -> Result<i32> { let value = try!(self.read_signed_value(bit_count, 32)); Ok((value & 0xffffffff) as i32) } /// Read at most 64 bits into a i64. /// Assumes the bits are stored in two's complement format. pub fn read_i64(&mut self, bit_count: u8) -> Result<i64> { let value = try!(self.read_signed_value(bit_count, 64)); Ok(value) } /// Read a single bit as a boolean value. /// Interprets 1 as true and 0 as false. pub fn read_bool(&mut self) -> Result<bool> { match try!(self.read_value(1, 1)) { 0 => Ok(false), _ => Ok(true), } } /// Skip arbitrary number of bits. However, you can skip at most to the end of the byte slice. pub fn skip(&mut self, bit_count: u64) -> Result<()> { let end_position = self.position + bit_count; if end_position > self.bytes.len() as u64 * 8 { return Err(BitReaderError::NotEnoughData { position: self.position, length: (self.bytes.len() * 8) as u64, requested: bit_count, }); } self.position = end_position; Ok(()) } /// Returns the position of the cursor, or how many bits have been read so far. pub fn position(&self) -> u64 { self.position - self.relative_offset } /// Helper to make sure the "bit cursor" is exactly at the beginning of a byte, or at specific /// multi-byte alignment position. /// /// For example `reader.is_aligned(1)` returns true if exactly n bytes, or n * 8 bits, has been /// read. Similarly, `reader.is_aligned(4)` returns true if exactly n * 32 bits, or n 4-byte /// sequences has been read. /// /// This function can be used to validate the data is being read properly, for example by /// adding invocations wrapped into `debug_assert!()` to places where it is known the data /// should be n-byte aligned. pub fn is_aligned(&self, alignment_bytes: u32) -> bool { self.position % (alignment_bytes as u64 * 8) == 0 } fn read_signed_value(&mut self, bit_count: u8, maximum_count: u8) -> Result<i64> { let unsigned = try!(self.read_value(bit_count, maximum_count)); // Fill the bits above the requested bits with all ones or all zeros, // depending on the sign bit. let sign_bit = unsigned >> (bit_count - 1) & 1; let high_bits = if sign_bit == 1 { -1 } else { 0 }; Ok(high_bits << bit_count | unsigned as i64) } fn read_value(&mut self, bit_count: u8, maximum_count: u8) -> Result<u64> { if bit_count == 0 { return Ok(0); } if bit_count > maximum_count { return Err(BitReaderError::TooManyBitsForType { position: self.position, requested: bit_count, allowed: maximum_count, }); } let start_position = self.position; let end_position = self.position + bit_count as u64; if end_position > self.bytes.len() as u64 * 8 { return Err(BitReaderError::NotEnoughData { position: self.position, length: (self.bytes.len() * 8) as u64, requested: bit_count as u64, }); } let mut value: u64 = 0; for i in start_position..end_position { let byte_index = (i / 8) as usize; let byte = self.bytes[byte_index]; let shift = 7 - (i % 8); let bit = (byte >> shift) as u64 & 1; value = (value << 1) | bit; } self.position = end_position; Ok(value) } } /// Result type for those BitReader operations that can fail. pub type Result<T> = result::Result<T, BitReaderError>; /// Error enumeration of BitReader errors. #[derive(Debug,PartialEq,Copy,Clone)] pub enum BitReaderError { /// Requested more bits than there are left in the byte slice at the current position. NotEnoughData { position: u64, length: u64, requested: u64, }, /// Requested more bits than the returned variable can hold, for example more than 8 bits when /// reading into a u8. TooManyBitsForType { position: u64, requested: u8, allowed: u8, } } impl Error for BitReaderError { fn description(&self) -> &str { match *self { BitReaderError::NotEnoughData {..} => "Requested more bits than the byte slice has left", BitReaderError::TooManyBitsForType {..} => "Requested more bits than the requested integer type can hold", } } } impl fmt::Display for BitReaderError { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { //self.description().fmt(fmt) match *self { BitReaderError::NotEnoughData { position, length, requested } => write!(fmt, "BitReader: Requested {} bits with only {}/{} bits left (position {})", requested, length - position, length, position), BitReaderError::TooManyBitsForType { position, requested, allowed } => write!(fmt, "BitReader: Requested {} bits while the type can only hold {} (position {})", requested, allowed, position), } } } /// Helper trait to allow reading bits into a variable without explicitly mentioning its type. /// /// If you can't or want, for some reason, to use BitReader's read methods (`read_u8` etc.) but /// want to rely on type inference instead, you can use the ReadInto trait. The trait is /// implemented for all basic integer types (8/16/32/64 bits, signed/unsigned) /// and the boolean type. /// /// ``` /// use bitreader::{BitReader,ReadInto}; /// /// let slice_of_u8 = &[0b1110_0000]; /// let mut reader = BitReader::new(slice_of_u8); /// /// struct Foo { /// bar: u8, /// valid: bool, /// } /// /// // No type mentioned here, instead the type of bits is inferred from the type of Foo::bar, /// // and consequently the correct "overload" is used. /// let bits = ReadInto::read(&mut reader, 2).unwrap(); /// let valid = ReadInto::read(&mut reader, 1).unwrap(); /// /// let foo = Foo { bar: bits, valid: valid }; /// assert_eq!(foo.bar, 3); /// assert!(foo.valid); /// ``` pub trait ReadInto where Self: Sized { fn read(reader: &mut BitReader, bits: u8) -> Result<Self>; } // There's eight almost identical implementations, let's make this easier. macro_rules! impl_read_into { ($T:ty, $method:ident) => ( impl ReadInto for $T { fn read(reader: &mut BitReader, bits: u8) -> Result<Self> { reader.$method(bits) } } ) } impl_read_into!(u8, read_u8); impl_read_into!(u16, read_u16); impl_read_into!(u32, read_u32); impl_read_into!(u64, read_u64); impl_read_into!(i8, read_i8); impl_read_into!(i16, read_i16); impl_read_into!(i32, read_i32); impl_read_into!(i64, read_i64); // We can't cast to bool, so this requires a separate method. impl ReadInto for bool { fn read(reader: &mut BitReader, bits: u8) -> Result<Self> { match try!(reader.read_u8(bits)) { 0 => Ok(false), _ => Ok(true), } } }