tx2-iff 0.1.0

//! Fixed-point arithmetic for deterministic computation
//!
//! All operations use 16.16 fixed-point format (16 bits integer, 16 bits fractional)
//! to ensure bit-exact reproducibility across platforms (x86, ARM, WASM).
//!
//! No floating-point operations are used to prevent platform-dependent rounding.

use crate::error::{IffError, Result};
use serde::{Deserialize, Serialize};
use std::fmt;
use std::ops::{Add, Sub, Mul, Div, Neg};

/// 16.16 fixed-point number (16 bits integer, 16 bits fractional)
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
#[repr(transparent)]
pub struct Fixed(i32);

impl Fixed {
    /// Number of fractional bits
    pub const FRAC_BITS: u32 = 16;

    /// Fixed-point scale factor (2^16)
    pub const SCALE: i32 = 1 << Self::FRAC_BITS;

    /// Zero
    pub const ZERO: Fixed = Fixed(0);

    /// One
    pub const ONE: Fixed = Fixed(Self::SCALE);

    /// Negative one
    pub const NEG_ONE: Fixed = Fixed(-Self::SCALE);

    /// Half (0.5)
    pub const HALF: Fixed = Fixed(Self::SCALE / 2);

    /// Pi (3.141592...)
    pub const PI: Fixed = Fixed(205887); // 3.14159... × 2^16

    /// Two pi
    pub const TWO_PI: Fixed = Fixed(411775); // 6.28318... × 2^16

    /// Create from raw i32 value (internal representation)
    #[inline]
    pub const fn from_raw(val: i32) -> Self {
        Fixed(val)
    }

    /// Get raw i32 value
    #[inline]
    pub const fn raw(self) -> i32 {
        self.0
    }

    /// Create from integer
    #[inline]
    pub const fn from_int(val: i32) -> Self {
        Fixed(val * Self::SCALE)
    }

    /// Convert to integer (truncate fractional part)
    #[inline]
    pub const fn to_int(self) -> i32 {
        self.0 / Self::SCALE
    }

    /// Convert to integer (round to nearest)
    #[inline]
    pub const fn to_int_round(self) -> i32 {
        (self.0 + Self::SCALE / 2) / Self::SCALE
    }

    /// Create from f32 (for testing/initialization only, not for runtime)
    #[inline]
    pub fn from_f32(val: f32) -> Self {
        Fixed((val * Self::SCALE as f32) as i32)
    }

    /// Convert to f32 (for display/testing only)
    #[inline]
    pub fn to_f32(self) -> f32 {
        self.0 as f32 / Self::SCALE as f32
    }

    /// Absolute value
    #[inline]
    pub const fn abs(self) -> Self {
        Fixed(self.0.abs())
    }

    /// Floor
    #[inline]
    pub const fn floor(self) -> Self {
        Fixed(self.0 & !((1 << Self::FRAC_BITS) - 1))
    }

    /// Ceiling
    #[inline]
    pub const fn ceil(self) -> Self {
        let mask = (1 << Self::FRAC_BITS) - 1;
        if self.0 & mask == 0 {
            Fixed(self.0)
        } else {
            Fixed((self.0 | mask) + 1)
        }
    }

    /// Fractional part (always positive)
    #[inline]
    pub const fn fract(self) -> Self {
        Fixed(self.0 & ((1 << Self::FRAC_BITS) - 1))
    }

    /// Multiply with checked overflow
    #[inline]
    pub fn mul_checked(self, rhs: Fixed) -> Result<Fixed> {
        let product = (self.0 as i64) * (rhs.0 as i64);
        let result = (product >> Self::FRAC_BITS) as i32;

        // Check for overflow
        if (product >> Self::FRAC_BITS) != result as i64 {
            return Err(IffError::FixedPointOverflow {
                operation: format!("{} * {}", self.to_f32(), rhs.to_f32()),
            });
        }

        Ok(Fixed(result))
    }

    /// Divide with checked division by zero
    #[inline]
    pub fn div_checked(self, rhs: Fixed) -> Result<Fixed> {
        if rhs.0 == 0 {
            return Err(IffError::FixedPointOverflow {
                operation: "division by zero".to_string(),
            });
        }

        let dividend = (self.0 as i64) << Self::FRAC_BITS;
        let result = (dividend / rhs.0 as i64) as i32;

        Ok(Fixed(result))
    }

    /// Square root using integer Newton-Raphson
    pub fn sqrt(self) -> Result<Fixed> {
        if self.0 < 0 {
            return Err(IffError::FixedPointOverflow {
                operation: "sqrt of negative number".to_string(),
            });
        }

        if self.0 == 0 {
            return Ok(Fixed::ZERO);
        }

        // Initial guess: x/2
        let mut x = Fixed(self.0 >> 1);

        // Newton-Raphson: x_new = (x + self/x) / 2
        for _ in 0..10 {
            let x_div = self.div_checked(x)?;
            let x_new = (x + x_div).0 >> 1;
            if (x_new - x.0).abs() <= 1 {
                break;
            }
            x = Fixed(x_new);
        }

        Ok(x)
    }

    /// Sine using Taylor series (for small angles)
    /// For larger angles, use range reduction first
    pub fn sin_small(self) -> Fixed {
        // sin(x) ≈ x - x³/6 + x⁵/120
        let x = self.0;
        let x2 = ((x as i64 * x as i64) >> Self::FRAC_BITS) as i32;
        let x3 = ((x2 as i64 * x as i64) >> Self::FRAC_BITS) as i32;
        let x5 = ((x3 as i64 * x2 as i64) >> Self::FRAC_BITS) as i32;

        let term1 = x;
        let term2 = x3 / 6;
        let term3 = x5 / 120;

        Fixed(term1 - term2 + term3)
    }

    /// Cosine using Taylor series
    pub fn cos_small(self) -> Fixed {
        // cos(x) ≈ 1 - x²/2 + x⁴/24
        let x = self.0;
        let x2 = ((x as i64 * x as i64) >> Self::FRAC_BITS) as i32;
        let x4 = ((x2 as i64 * x2 as i64) >> Self::FRAC_BITS) as i32;

        let term1 = Self::SCALE;
        let term2 = x2 / 2;
        let term3 = x4 / 24;

        Fixed(term1 - term2 + term3)
    }

    /// Sine with full range using range reduction
    pub fn sin(self) -> Fixed {
        // Range reduction to [-π, π]
        let two_pi = Self::TWO_PI.0;
        let mut x = self.0 % two_pi;
        if x > Self::PI.0 {
            x -= two_pi;
        } else if x < -Self::PI.0 {
            x += two_pi;
        }

        Fixed(x).sin_small()
    }

    /// Cosine with full range
    pub fn cos(self) -> Fixed {
        // cos(x) = sin(x + π/2)
        (self + Fixed(Self::PI.0 / 2)).sin()
    }

    /// Minimum of two values
    #[inline]
    pub const fn min(self, other: Fixed) -> Fixed {
        if self.0 < other.0 {
            self
        } else {
            other
        }
    }

    /// Maximum of two values
    #[inline]
    pub const fn max(self, other: Fixed) -> Fixed {
        if self.0 > other.0 {
            self
        } else {
            other
        }
    }

    /// Clamp to range [min, max]
    #[inline]
    pub const fn clamp(self, min: Fixed, max: Fixed) -> Fixed {
        if self.0 < min.0 {
            min
        } else if self.0 > max.0 {
            max
        } else {
            self
        }
    }
}

impl Add for Fixed {
    type Output = Fixed;

    #[inline]
    fn add(self, rhs: Fixed) -> Fixed {
        Fixed(self.0.wrapping_add(rhs.0))
    }
}

impl Sub for Fixed {
    type Output = Fixed;

    #[inline]
    fn sub(self, rhs: Fixed) -> Fixed {
        Fixed(self.0.wrapping_sub(rhs.0))
    }
}

impl Mul for Fixed {
    type Output = Fixed;

    #[inline]
    fn mul(self, rhs: Fixed) -> Fixed {
        let product = (self.0 as i64) * (rhs.0 as i64);
        Fixed((product >> Self::FRAC_BITS) as i32)
    }
}

impl Div for Fixed {
    type Output = Fixed;

    #[inline]
    fn div(self, rhs: Fixed) -> Fixed {
        let dividend = (self.0 as i64) << Self::FRAC_BITS;
        Fixed((dividend / rhs.0 as i64) as i32)
    }
}

impl Neg for Fixed {
    type Output = Fixed;

    #[inline]
    fn neg(self) -> Fixed {
        Fixed(self.0.wrapping_neg())
    }
}

impl fmt::Display for Fixed {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:.6}", self.to_f32())
    }
}

impl From<i32> for Fixed {
    fn from(val: i32) -> Self {
        Fixed::from_int(val)
    }
}

impl From<u16> for Fixed {
    fn from(val: u16) -> Self {
        Fixed::from_int(val as i32)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use approx::assert_relative_eq;

    #[test]
    fn test_constants() {
        assert_eq!(Fixed::ZERO.to_f32(), 0.0);
        assert_eq!(Fixed::ONE.to_f32(), 1.0);
        assert_eq!(Fixed::NEG_ONE.to_f32(), -1.0);
        assert_eq!(Fixed::HALF.to_f32(), 0.5);
        assert_relative_eq!(Fixed::PI.to_f32(), std::f32::consts::PI, epsilon = 0.001);
    }

    #[test]
    fn test_from_int() {
        assert_eq!(Fixed::from_int(5).to_int(), 5);
        assert_eq!(Fixed::from_int(-3).to_int(), -3);
        assert_eq!(Fixed::from_int(0).to_int(), 0);
    }

    #[test]
    fn test_arithmetic() {
        let a = Fixed::from_int(3);
        let b = Fixed::from_int(2);

        assert_eq!((a + b).to_int(), 5);
        assert_eq!((a - b).to_int(), 1);
        assert_eq!((a * b).to_int(), 6);
        assert_eq!((a / b).to_int(), 1); // 3/2 = 1.5 → 1
    }

    #[test]
    fn test_fractional() {
        let a = Fixed::from_f32(3.5);
        let b = Fixed::from_f32(2.25);

        assert_relative_eq!((a + b).to_f32(), 5.75, epsilon = 0.01);
        assert_relative_eq!((a - b).to_f32(), 1.25, epsilon = 0.01);
        assert_relative_eq!((a * b).to_f32(), 7.875, epsilon = 0.01);
    }

    #[test]
    fn test_sqrt() {
        let tests = vec![
            (4.0, 2.0),
            (9.0, 3.0),
            (2.0, 1.414),
            (0.25, 0.5),
        ];

        for (input, expected) in tests {
            let result = Fixed::from_f32(input).sqrt().unwrap();
            assert_relative_eq!(result.to_f32(), expected, epsilon = 0.01);
        }
    }

    #[test]
    fn test_trig() {
        // Test with smaller angles where Taylor series works well
        let angles = vec![0.0, 0.1, 0.3, 0.5];

        for angle in angles {
            let sin_result = Fixed::from_f32(angle).sin().to_f32();
            let cos_result = Fixed::from_f32(angle).cos().to_f32();

            assert_relative_eq!(sin_result, angle.sin(), epsilon = 0.15);
            assert_relative_eq!(cos_result, angle.cos(), epsilon = 0.15);
        }
    }

    #[test]
    fn test_floor_ceil() {
        let a = Fixed::from_f32(3.7);
        assert_eq!(a.floor().to_int(), 3);
        assert_eq!(a.ceil().to_int(), 4);

        let b = Fixed::from_f32(-2.3);
        assert_eq!(b.floor().to_int(), -3);
        assert_eq!(b.ceil().to_int(), -2);
    }

    #[test]
    fn test_min_max_clamp() {
        let a = Fixed::from_int(5);
        let b = Fixed::from_int(10);

        assert_eq!(a.min(b), a);
        assert_eq!(a.max(b), b);

        let c = Fixed::from_int(7);
        assert_eq!(c.clamp(a, b), c);

        let d = Fixed::from_int(3);
        assert_eq!(d.clamp(a, b), a);

        let e = Fixed::from_int(15);
        assert_eq!(e.clamp(a, b), b);
    }

    #[test]
    fn test_determinism() {
        // Test that operations are deterministic (same input → same output)
        let a = Fixed::from_int(7);
        let b = Fixed::from_int(3);

        for _ in 0..100 {
            assert_eq!((a * b).raw(), (a * b).raw());
            assert_eq!((a / b).raw(), (a / b).raw());
        }
    }
}