//! Fixed point numbers.

use std::{fmt::{self, Write},
          ops};

macro_rules! define_fixed_type {
   (
      $(#[$outer:meta])*
      struct $Type:ident ($IT:ident) : $UT:ident, $LT:ident, $FracBits:expr;
   ) => {
      $(#[$outer])*
      #[derive(Clone, PartialEq, serde::Serialize)]
      pub struct $Type($IT);

      impl $Type
      {
         /// The number of fractional bits in this type.
         pub const FRACBITS: $UT = $FracBits;
         /// The integer mask for the fractional bits.
         pub const FRACMASK: $UT = (1 << Self::FRACBITS) - 1;
         /// The representation of `1.0` in this type.
         pub const ONE:      $IT = 1 << Self::FRACBITS;

         /// Creates a value of this type with the bit pattern `bits`.
         #[inline]
         pub const fn from_bits(bits: $UT) -> Self {$Type(bits as $IT)}

         /// Returns the raw bit pattern.
         #[inline]
         pub fn to_bits(&self) -> $UT {self.0 as $UT}

         /// Sets the raw bit pattern to `bits`.
         #[inline]
         pub fn set_bits(&mut self, bits: $UT) {self.0 = bits as $IT}

         /// Returns the integral portion.
         #[inline]
         pub fn integ(&self) -> $LT {(self.0 >> Self::FRACBITS) as $LT}

         /// Returns the fractional portion.
         ///
         /// This is a value of `1` to `FRACMASK`.
         #[allow(trivial_numeric_casts)]
         pub fn fract(&self) -> u16 {(self.0 as $UT & Self::FRACMASK) as u16}

         /// Returns a multiplication by integer.
         ///
         /// Panics if the result overflows.
         #[inline]
         pub fn mul_i(&self, n: $LT) -> Self {$Type(self.0 * $IT::from(n))}

         /// Returns a division by integer.
         ///
         /// Panics if `n` is `0`.
         #[inline]
         pub fn div_i(&self, n: $LT) -> Self {$Type(self.0 / $IT::from(n))}

         #[inline]
         fn fx_div(x: $IT, y: $IT) -> $IT
         {
            (i64::from(x) * i64::from(Self::ONE) / i64::from(y)) as $IT
         }

         #[inline]
         fn fx_mul(x: $IT, y: $IT) -> $IT
         {
            (i64::from(x) * i64::from(y) / i64::from(Self::ONE)) as $IT
         }
      }

      impl From<$LT> for $Type
      {
         #[inline]
         fn from(n: $LT) -> Self {$Type($IT::from(n) << Self::FRACBITS)}
      }

      impl ops::Add for $Type
      {
         type Output = Self;

         #[inline]
         fn add(self, o: Self) -> Self {$Type(self.0 + o.0)}
      }

      impl ops::Sub for $Type
      {
         type Output = Self;

         #[inline]
         fn sub(self, o: Self) -> Self {$Type(self.0 - o.0)}
      }

      impl ops::Mul for $Type
      {
         type Output = Self;

         #[inline]
         fn mul(self, o: Self) -> Self {$Type(Self::fx_mul(self.0, o.0))}
      }

      impl ops::Div for $Type
      {
         type Output = Self;

         #[inline]
         fn div(self, o: Self) -> Self {$Type(Self::fx_div(self.0, o.0))}
      }

      impl ops::Neg for $Type
      {
         type Output = Self;

         #[inline]
         fn neg(self) -> Self {$Type(-self.0)}
      }

      impl ops::Not for $Type
      {
         type Output = Self;

         #[inline]
         fn not(self) -> Self {$Type(!self.0)}
      }

      impl fmt::Display for $Type
      {
         fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
         {
            let prec = f.precision().unwrap_or(1);
            let widt = f.width().unwrap_or(0);
            write!(f, "{:widt$}.", self.integ(), widt = widt)?;

            let mut k = self.to_bits();
            for _ in 0..prec {
               k &= Self::FRACMASK;
               k *= 10;
               let d = k >> Self::FRACBITS;
               let d = d % 10;
               f.write_char((d as u8 + b'0') as char)?;
            }

            Ok(())
         }
      }

      impl fmt::Debug for $Type
      {
         fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
         {
            fmt::Display::fmt(self, f)
         }
      }
   };
}

define_fixed_type! {
   /// A fixed point type representing an angle.
   ///
   /// The format of this type is `0.9s`, but because of the implementation,
   /// the real format is `7.9s`.
   struct Angle(i16) : u16, i8, 9;
}

define_fixed_type! {
   /// A fixed point type representing a world unit.
   ///
   /// The format of this type is `5.10s`. This has caused eternal suffering.
   struct Unit(i16) : u16, i8, 10;
}

define_fixed_type! {
   /// A generic fixed point type.
   ///
   /// The format of this type is `15.16s`.
   struct Fixed(i32) : u32, i16, 16;
}

#[test]
fn fixed_basic_ops()
{
   let seven_div_2 = 3 << Fixed::FRACBITS | Fixed::FRACMASK / 2 + 1;
   assert_eq!((Fixed::from(1) + 1.into()).to_bits(), 2 << Fixed::FRACBITS);
   assert_eq!((Fixed::from(2) - 1.into()).to_bits(), 1 << Fixed::FRACBITS);
   assert_eq!((Fixed::from(6) * 2.into()).to_bits(), 12 << Fixed::FRACBITS);
   assert_eq!((Fixed::from(6).mul_i(2)).to_bits(), 12 << Fixed::FRACBITS);
   assert_eq!((Fixed::from(7) / 2.into()).to_bits(), seven_div_2);
   assert_eq!((Fixed::from(7).div_i(2)).to_bits(), seven_div_2);
}

#[test]
#[should_panic]
#[allow(unused_must_use)]
fn fixed_overflow() {Fixed::from(i16::max_value()) + 1.into();}

#[test]
fn fixed_printing()
{
   assert_eq!(format!("{}", Fixed::from(6)), "6.0");
   assert_eq!(format!("{}", Fixed::from(7).div_i(2)), "3.5");
   assert_eq!(format!("{:7.7}", Fixed::from_bits(0xDEAD_BEEF)),
              "  -8531.7458343");
}

// EOF