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/// Describes a type that can linearly interpolate (and extrapolate) based on
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/// two values and a `f32` factor.
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pub trait Lerp: Copy {
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/// Interpolate/extrapolate between `a` and `b` and `t` as the factor.
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fn lerp(a: Self, b: Self, t: f32) -> Self;
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/// Interpolate between `a` and `b` by bounding the factor `t` in the range
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/// `0..=1.0`.
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fn lerp_bounded(a: Self, b: Self, t: f32) -> Self
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where
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Self: Sized,
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{
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match t {
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t if t < 0.0 => a,
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t if t > 1.0 => b,
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t => Self::lerp(a, b, t),
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}
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}
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}
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/// Type that can compute an inverse of linear interpolation.
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pub trait InvLerp: Copy {
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/// Find a factor between `0.0` and `1.0` that defines the position of
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/// `value` in the `min` and `max` closed interval.
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fn inv_lerp(min: Self, max: Self, value: Self) -> f32;
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}
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macro_rules! impl_lerp_for_int {
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($int: ident) => {
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impl Lerp for $int {
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fn lerp(a: Self, b: Self, t: f32) -> Self {
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(a as f32 + t * (b - a) as f32) as Self
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}
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}
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impl InvLerp for $int {
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fn inv_lerp(min: Self, max: Self, value: Self) -> f32 {
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(value - min) as f32 / (max - min) as f32
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}
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}
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};
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}
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macro_rules! impl_lerp_for_uint {
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($uint: ident) => {
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impl Lerp for $uint {
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fn lerp(a: Self, b: Self, t: f32) -> Self {
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if a <= b {
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(a as f32 + t * (b - a) as f32) as Self
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} else {
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(a as f32 - t * (a - b) as f32) as Self
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}
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}
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}
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impl InvLerp for $uint {
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fn inv_lerp(min: Self, max: Self, value: Self) -> f32 {
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if min <= max {
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(value - min) as f32 / (max - min) as f32
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} else {
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(value - max) as f32 / (min - max) as f32
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}
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}
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}
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};
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}
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impl_lerp_for_int!(i32);
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impl_lerp_for_uint!(u8);
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impl_lerp_for_uint!(u16);
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impl_lerp_for_uint!(u32);
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn lerp_for_int_and_uint() {
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assert_eq!(i32::lerp(0, 8, 0.5), 4);
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assert_eq!(i32::lerp(0, 8, -1.0), -8);
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assert_eq!(i32::lerp(8, 0, 0.5), 4);
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assert_eq!(u32::lerp(0, 8, 0.5), 4);
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assert_eq!(u32::lerp(8, 0, -1.0), 16);
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}
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#[test]
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fn inv_lerp_for_int_and_uint() {
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assert!((i32::inv_lerp(0, 8, 4) - 0.5).abs() < f32::EPSILON);
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assert!((i32::inv_lerp(0, 8, -8) - -1.0).abs() < f32::EPSILON);
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assert!((i32::inv_lerp(8, 0, 4) - 0.5).abs() < f32::EPSILON);
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assert!((u32::inv_lerp(0, 8, 4) - 0.5).abs() < f32::EPSILON);
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}
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}
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