Maraiah/maraiah/bit.rs

//! Bit streams.

use crate::err::*;

/// Reads `width` bits from `b` starting at bit `cr_bit` into an integer, most
/// significant bit first.
///
/// # Errors
///
/// The function will return an error if `width` is more than 64, or if
/// `(cr_bit + width - 1) / 8` would overflow an index into `b`.
pub fn read_bits_b(b: &[u8], cr_bit: usize, width: u8) -> ResultS<u64>
{
	if width == 0 {
		return Ok(0);
	}

	if width > 64 {
		bail!("invalid number of bits");
	}

	let last = (cr_bit + usize::from(width) - 1) / 8;

	if last >= b.len() {
		bail!("not enough data");
	}

	let mut byte_ptr = cr_bit / 8;
	let mut bits_ptr = cr_bit % 8;

	let mut res = 0;

	for _ in 0..width {
		res <<= 1;

		if b[byte_ptr] & (1 << bits_ptr) != 0 {
			res |= 1;
		}

		bits_ptr += 1;

		if bits_ptr > 7 {
			bits_ptr  = 0;
			byte_ptr += 1;
		}
	}

	Ok(res)
}

/// The same as `read_bits_b`, but least-significant bit first.
pub fn read_bits_l(b: &[u8], cr_bit: usize, width: u8) -> ResultS<u64>
{
	if width == 0 {
		Ok(0)
	} else {
		let res = read_bits_b(b, cr_bit, width)?;

		Ok(reverse_bits(res) >> (64 - width))
	}
}

// FIXME: change this to u64::reverse_bits when stabilized
const fn reverse_bits(v: u64) -> u64
{
	let v = v >> 1 & 0x5555_5555_5555_5555 | ((v & 0x5555_5555_5555_5555) << 1);
	let v = v >> 2 & 0x3333_3333_3333_3333 | ((v & 0x3333_3333_3333_3333) << 2);
	let v = v >> 4 & 0x0F0F_0F0F_0F0F_0F0F | ((v & 0x0F0F_0F0F_0F0F_0F0F) << 4);
	v.swap_bytes()
}

#[test]
fn bit_tests()
{
	const INPUT: &[u8] = &[0b01100101, 0b10101010, 0b00010000, 0b00000000,
	                       0b11111111, 0b11100001, 0b10101100, 0b00110011,
	                       0b10100101, 0b11100000, 0b00000111, 0b00000001,
	                       0b11001010, 0b10101111, 0b00101011, 0b01101010,
	                       0b11010101, 0b10100011, 0b01010101, 0b11000001];

	let mut p = 0;

	let n = read_bits_b(INPUT, p, 3).unwrap();
	assert_eq!(n, 0b101);
	p += 3;

	let n = read_bits_b(INPUT, p, 63).unwrap();
	assert_eq!(n, 0b001100101010100001000000000001111111110000111001101011100110010);
	p += 63;

	let n = read_bits_b(INPUT, p, 4).unwrap();
	assert_eq!(n, 0b1001);
	p += 4;

	let n = read_bits_b(INPUT, p, 7).unwrap();
	assert_eq!(n, 0b0100000);
	p += 7;

	let n = read_bits_b(INPUT, p, 17).unwrap();
	assert_eq!(n, 0b11111100000100000);
	p += 17;

	let n = read_bits_b(INPUT, p, 27).unwrap();
	assert_eq!(n, 0b000101001111110101110101000);
	p += 27;

	let n = read_bits_b(INPUT, p, 33).unwrap();
	assert_eq!(n, 0b101011010101011110001011010101010);
	p += 33;

	let n = read_bits_b(INPUT, p, 6).unwrap();
	assert_eq!(n, 0b000011);
	p += 6;

	let e = read_bits_b(INPUT, p, 1);
	assert!(if let Err(_) = e {true} else {false});

	let e = read_bits_b(INPUT, p, 2);
	assert!(if let Err(_) = e {true} else {false});

	let mut p = 0;

	let n = read_bits_l(INPUT, 0, 3).unwrap();
	assert_eq!(n, 0b101);
	p += 3;

	let n = read_bits_l(INPUT, p, 63).unwrap();
	assert_eq!(n, 0b010011001110101100111000011111111100000000000100001010101001100);
	p += 63;

	let n = read_bits_l(INPUT, p, 4).unwrap();
	assert_eq!(n, 0b1001);
	p += 4;

	let n = read_bits_l(INPUT, p, 7).unwrap();
	assert_eq!(n, 0b0000010);
	p += 7;

	let n = read_bits_l(INPUT, p, 17).unwrap();
	assert_eq!(n, 0b00000100000111111);
	p += 17;

	let n = read_bits_l(INPUT, p, 27).unwrap();
	assert_eq!(n, 0b000101011101011111100101000);
	p += 27;

	let n = read_bits_l(INPUT, p, 33).unwrap();
	assert_eq!(n, 0b010101010110100011110101010110101);
	p += 33;

	let n = read_bits_l(INPUT, p, 6).unwrap();
	assert_eq!(n, 0b110000);
	p += 6;

	let e = read_bits_l(INPUT, p, 1);
	assert!(if let Err(_) = e {true} else {false});

	let e = read_bits_l(INPUT, p, 2);
	assert!(if let Err(_) = e {true} else {false});
}

// EOF