//! Bit streams.

use crate::durandal::err::*;

/// Reads `width` bits from `b` starting at bit `cr_ptr` into an integer.
///
/// # Errors
///
/// The function will return an error if `width` is 0 or more than 64, or if
/// `(cr_ptr + width - 1) / 8` would overflow an index into `b`.
pub fn read_bits(b: &[u8], cr_ptr: usize, width: u8) -> ResultS<(usize, u64)>
{
   if width < 1 || width > 64 {
      bail!("invalid number of bits");
   }

   let nx_ptr = cr_ptr + usize::from(width);
   let ed_ptr = nx_ptr - 1;

   let first_byte = cr_ptr / 8;
   let last_byte  = ed_ptr / 8;

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

   let first_bits = cr_ptr as u32 % 8;
   let last_bits  = nx_ptr as u32 % 8;

   let last_mask = ((1u128 << last_bits) - 1) as u64;

   let num_bytes = last_byte - first_byte;

   let bytes = get_bytes(b, num_bytes, first_byte);

   let bits = u64::from_be_bytes(bytes);
   let bits = bits.wrapping_shl(first_bits);
   let bits = bits >> (64 - width);

   // special case for over byte boundary
   let bits = if num_bytes == 8 {
      let lbyt = u64::from(b[last_byte]);
      let lbyt = lbyt >> (8 - last_bits);
      let lbyt = lbyt & last_mask;

      (bits & !last_mask) | lbyt
   } else {
      bits
   };

   Ok((nx_ptr, bits))
}

fn get_bytes(b: &[u8], num_bytes: usize, f: usize) -> [u8; 8]
{
   match num_bytes {
      8 |
      7 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5], b[f+6], b[f+7]],
      6 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5], b[f+6],      0],
      5 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4], b[f+5],           0, 0],
      4 => [b[f], b[f+1], b[f+2], b[f+3], b[f+4],                0, 0, 0],
      3 => [b[f], b[f+1], b[f+2], b[f+3],                     0, 0, 0, 0],
      2 => [b[f], b[f+1], b[f+2],                          0, 0, 0, 0, 0],
      1 => [b[f], b[f+1],                               0, 0, 0, 0, 0, 0],
      0 => [b[f],                                    0, 0, 0, 0, 0, 0, 0],
      _ => panic!("invalid number of bytes to read ({})", num_bytes),
   }
}

#[test]
fn bit_tests()
{
   const INPUT: &[u8] = &[0b011_00101, 0b10101010, 0b00010000, 0b00000000,
                          0b11111111, 0b11100001, 0b10101100, 0b00110011,
                          0b10_1001_01, 0b11100_000, 0b00000111, 0b000000_01,
                          0b11001010, 0b10101111, 0b00101011, 0b0_1101010,
                          0b11010101, 0b10100011, 0b01010101, 0b11_000001];

   let (p, n) = read_bits(INPUT, 0, 3).unwrap();
   assert_eq!(n, 0b011);

   let (p, n) = read_bits(INPUT, p, 63).unwrap();
   assert_eq!(n, 0x16A8_4003_FF86_B0CE);

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

   let (p, n) = read_bits(INPUT, p, 7).unwrap();
   assert_eq!(n, 0b0111100);

   let (p, n) = read_bits(INPUT, p, 17).unwrap();
   assert_eq!(n, 0b00000000111000000);

   let (p, n) = read_bits(INPUT, p, 27).unwrap();
   assert_eq!(n, 0b011100101010101111001010110);

   let (p, n) = read_bits(INPUT, p, 33).unwrap();
   assert_eq!(n, 0b110101011010101101000110101010111);

   let (p, n) = read_bits(INPUT, p, 6).unwrap();
   assert_eq!(n, 1);

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

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

// EOF