107 lines
3.2 KiB
Rust
107 lines
3.2 KiB
Rust
//! Bit streams.
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use crate::durandal::err::*;
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/// Reads `width` bits from `b` starting at bit `cr_bit` into an integer.
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///
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/// # Errors
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///
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/// The function will return an error if `width` is 0 or more than 64, or if
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/// `(cr_bit + width - 1) / 8` would overflow an index into `b`.
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pub fn read_bits(b: &[u8], cr_bit: usize, width: u8) -> ResultS<(usize, u64)>
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{
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if width < 1 || width > 64 {
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bail!("invalid number of bits");
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}
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let nx_bit = cr_bit + usize::from(width);
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let ed_bit = nx_bit - 1;
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let cr_byte = cr_bit / 8;
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let ed_byte = ed_bit / 8;
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check_data!(ed_byte + 1, b);
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let cr_bits = cr_bit as u32 % 8;
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let nx_bits = nx_bit as u32 % 8;
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let nx_mask = ((1_u128 << nx_bits) - 1) as u64;
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let num_bytes = ed_byte - cr_byte;
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let bytes = get_bytes(b, num_bytes, cr_byte);
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let bits = u64::from_be_bytes(bytes);
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let bits = bits.wrapping_shl(cr_bits);
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let bits = bits >> (64 - width);
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// special case for over byte boundary
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if num_bytes == 8 {
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let lbyt = u64::from(b[ed_byte]);
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let lbyt = lbyt >> (8 - nx_bits);
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let lbyt = lbyt & nx_mask;
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Ok((nx_bit, bits & !nx_mask | lbyt))
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} else {
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Ok((nx_bit, bits))
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}
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}
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fn get_bytes(b: &[u8], num_bytes: usize, f: usize) -> [u8; 8]
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{
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match num_bytes {
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8 |
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7 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5], b[f+6], b[f+7]],
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6 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5], b[f+6], 0],
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5 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5], 0, 0],
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4 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], 0, 0, 0],
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3 => [b[f], b[f+1], b[f+2], b[f+3], 0, 0, 0, 0],
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2 => [b[f], b[f+1], b[f+2], 0, 0, 0, 0, 0],
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1 => [b[f], b[f+1], 0, 0, 0, 0, 0, 0],
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0 => [b[f], 0, 0, 0, 0, 0, 0, 0],
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_ => panic!("invalid number of bytes to read ({})", num_bytes),
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}
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}
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#[test]
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fn bit_tests()
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{
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const INPUT: &[u8] = &[0b011_00101, 0b10101010, 0b00010000, 0b00000000,
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0b11111111, 0b11100001, 0b10101100, 0b00110011,
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0b10_1001_01, 0b11100_000, 0b00000111, 0b000000_01,
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0b11001010, 0b10101111, 0b00101011, 0b0_1101010,
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0b11010101, 0b10100011, 0b01010101, 0b11_000001];
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let (p, n) = read_bits(INPUT, 0, 3).unwrap();
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assert_eq!(n, 0b011);
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let (p, n) = read_bits(INPUT, p, 63).unwrap();
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assert_eq!(n, 0x16A8_4003_FF86_B0CE);
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let (p, n) = read_bits(INPUT, p, 4).unwrap();
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assert_eq!(n, 0b1001);
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let (p, n) = read_bits(INPUT, p, 7).unwrap();
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assert_eq!(n, 0b0111100);
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let (p, n) = read_bits(INPUT, p, 17).unwrap();
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assert_eq!(n, 0b00000000111000000);
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let (p, n) = read_bits(INPUT, p, 27).unwrap();
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assert_eq!(n, 0b011100101010101111001010110);
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let (p, n) = read_bits(INPUT, p, 33).unwrap();
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assert_eq!(n, 0b110101011010101101000110101010111);
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let (p, n) = read_bits(INPUT, p, 6).unwrap();
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assert_eq!(n, 1);
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let e = read_bits(INPUT, p, 1);
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assert!(if let Err(_) = e {true} else {false});
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let e = read_bits(INPUT, p, 2);
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assert!(if let Err(_) = e {true} else {false});
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}
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// EOF
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