// Copyright 2016 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

//! Bit lengths.

use crate::{error::InputTooLongError, polyfill};

/// The length of something, in bits.
///
/// This can represent a bit length that isn't a whole number of bytes.
#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
#[repr(transparent)]
pub struct BitLength<T = usize>(T);

pub(crate) trait FromByteLen<T>: Sized {
    /// Constructs a `BitLength` from the given length in bytes.
    ///
    /// Fails if `bytes * 8` is too large for a `T`.
    fn from_byte_len(bytes: T) -> Result<Self, InputTooLongError<T>>;
}

impl FromByteLen<usize> for BitLength<usize> {
    #[inline]
    fn from_byte_len(bytes: usize) -> Result<Self, InputTooLongError> {
        match bytes.checked_mul(8) {
            Some(bits) => Ok(Self(bits)),
            None => Err(InputTooLongError::new(bytes)),
        }
    }
}

impl FromByteLen<u64> for BitLength<u64> {
    #[inline]
    fn from_byte_len(bytes: u64) -> Result<Self, InputTooLongError<u64>> {
        match bytes.checked_mul(8) {
            Some(bits) => Ok(Self(bits)),
            None => Err(InputTooLongError::new(bytes)),
        }
    }
}

impl FromByteLen<usize> for BitLength<u64> {
    #[inline]
    fn from_byte_len(bytes: usize) -> Result<Self, InputTooLongError<usize>> {
        match polyfill::u64_from_usize(bytes).checked_mul(8) {
            Some(bits) => Ok(Self(bits)),
            None => Err(InputTooLongError::new(bytes)),
        }
    }
}

impl<T> BitLength<T> {
    /// Constructs a `BitLength` from the given length in bits.
    #[inline]
    pub const fn from_bits(bits: T) -> Self {
        Self(bits)
    }
}

impl<T: Copy> BitLength<T> {
    /// The number of bits this bit length represents, as the underlying type.
    #[inline]
    pub fn as_bits(self) -> T {
        self.0
    }
}

// Lengths measured in bits, where all arithmetic is guaranteed not to
// overflow.
impl BitLength<usize> {
    #[cfg(feature = "alloc")]
    #[inline]
    pub(crate) fn half_rounded_up(&self) -> Self {
        let round_up = self.0 & 1;
        Self((self.0 / 2) + round_up)
    }

    /// The bit length, rounded up to a whole number of bytes.
    #[inline]
    pub const fn as_usize_bytes_rounded_up(&self) -> usize {
        // Equivalent to (self.0 + 7) / 8, except with no potential for
        // overflow and without branches.

        // Branchless round_up = if self.0 & 0b111 != 0 { 1 } else { 0 };
        let round_up = ((self.0 >> 2) | (self.0 >> 1) | self.0) & 1;

        (self.0 / 8) + round_up
    }

    #[cfg(feature = "alloc")]
    #[inline]
    pub(crate) fn try_sub_1(self) -> Result<Self, crate::error::Unspecified> {
        let sum = self.0.checked_sub(1).ok_or(crate::error::Unspecified)?;
        Ok(Self(sum))
    }
}

impl BitLength<u64> {
    pub fn to_be_bytes(self) -> [u8; 8] {
        self.0.to_be_bytes()
    }
}

#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
impl From<BitLength<usize>> for BitLength<u64> {
    fn from(BitLength(value): BitLength<usize>) -> Self {
        BitLength(polyfill::u64_from_usize(value))
    }
}

impl TryFrom<BitLength<u64>> for BitLength<core::num::NonZeroU64> {
    type Error = <core::num::NonZeroU64 as TryFrom<u64>>::Error;

    fn try_from(BitLength(value): BitLength<u64>) -> Result<Self, Self::Error> {
        value.try_into().map(BitLength)
    }
}

const _TEST_AS_USIZE_BYTES_ROUNDED_UP_EVEN: () =
    assert!(BitLength::from_bits(8192).as_usize_bytes_rounded_up() == 8192 / 8);
const _TEST_AS_USIZE_BYTES_ROUNDED_UP_ONE_BIT_HIGH: () =
    assert!(BitLength::from_bits(8192 + 1).as_usize_bytes_rounded_up() == (8192 / 8) + 1);
const _TEST_AS_USIZE_BYTES_ROUNDED_UP_SEVEN_BITS_HIGH: () =
    assert!(BitLength::from_bits(8192 + 7).as_usize_bytes_rounded_up() == (8192 / 8) + 1);