add cossin LUT

.gitignore

@@ -1,3 +1,4 @@
 /target
 /dsp/target
 .gdb_history
+/dsp/src/cossin_table.txt

dsp/build.rs

@@ -0,0 +1,43 @@
+use std::f64::consts::PI;
+use std::fs::File;
+use std::io::prelude::*;
+use std::path::Path;
+
+const TABLE_DEPTH: usize = 8;
+const TABLE_SIZE: usize = 1 << TABLE_DEPTH;
+// Treat sin and cos as unsigned values since the sign will always be
+// positive in the range [0, pi/4).
+const SINCOS_MAX: f64 = u16::MAX as f64;
+
+fn main() {
+    let path = Path::new("src").join("cossin_table.txt");
+    let display = path.display();
+
+    let mut file = match File::create(&path) {
+        Err(why) => panic!("failed to write to {}: {}", display, why),
+        Ok(file) => file,
+    };
+
+    match file.write_all("[\n".as_bytes()) {
+        Err(why) => panic!("failed to write to {}: {}", display, why),
+        Ok(_) => (),
+    }
+
+    let phase_delta = PI / 4. / TABLE_SIZE as f64;
+    let phase_offset = phase_delta / 2.;
+    for i in 0..TABLE_SIZE {
+        let phase = phase_offset + phase_delta * (i as f64);
+        let cos = ((phase.cos() - 0.5) * 2. * SINCOS_MAX).round() as u16;
+        let sin = (phase.sin() * SINCOS_MAX).round() as u16;
+        let s = format!("    ({}, {}),\n", cos, sin);
+        match file.write_all(s.as_bytes()) {
+            Err(why) => panic!("failed to write to {}: {}", display, why),
+            Ok(_) => (),
+        }
+    }
+
+    match file.write_all("]\n".as_bytes()) {
+        Err(why) => panic!("failed to write to {}: {}", display, why),
+        Ok(_) => (),
+    }
+}

dsp/src/lib.rs

@@ -2,9 +2,25 @@
 #![cfg_attr(feature = "nightly", feature(asm, core_intrinsics))]
 
 pub type Complex<T> = (T, T);
+
+/// Round up half.
+///
+/// # Arguments
+///
+/// `x` - Value to shift and round.
+/// `shift` - Number of bits to right shift `x`.
+///
+/// # Returns
+///
+/// Shifted and rounded value.
+pub fn shift_round(x: i32, shift: i32) -> i32 {
+    (x + (1 << (shift - 1))) >> shift
+}
+
 pub mod iir;
 pub mod lockin;
 pub mod pll;
+pub mod trig;
 pub mod unwrap;
 
 #[cfg(test)]

dsp/src/trig.rs

@@ -0,0 +1,128 @@
+use super::{shift_round, Complex};
+use core::mem::swap;
+
+const PHASE_BITS: i32 = 20;
+const LUT_DEPTH: i32 = 8;
+const LUT_SIZE: usize = 1 << LUT_DEPTH as usize;
+const OCTANT_BITS: i32 = 3;
+const INTERPOLATION_BITS: i32 = PHASE_BITS - LUT_DEPTH - OCTANT_BITS;
+static COSSIN_TABLE: [(u16, u16); LUT_SIZE] = include!("cossin_table.txt");
+
+// Approximate pi/4 with an integer multiplier and right bit
+// shift. The numerator is designed to saturate the i32 range.
+const PI_4_NUMERATOR: i32 = 50;
+const PI_4_RIGHT_SHIFT: i32 = 6;
+
+/// Compute the cosine and sine of an angle.
+///
+/// # Arguments
+///
+/// `phase` - 20-bit fixed-point phase value.
+///
+/// # Returns
+///
+/// The cos and sin values of the provided phase as a `Complex<i32>`
+/// value.
+pub fn cossin(phase: i32) -> Complex<i32> {
+    let mut phase = phase;
+    let octant = (
+        (phase & (1 << (PHASE_BITS - 1))) >> (PHASE_BITS - 1),
+        (phase & (1 << (PHASE_BITS - 2))) >> (PHASE_BITS - 2),
+        (phase & (1 << (PHASE_BITS - 3))) >> (PHASE_BITS - 3),
+    );
+
+    // Mask off octant bits. This leaves the angle in the range [0,
+    // pi/4).
+    phase &= (1 << (PHASE_BITS - OCTANT_BITS)) - 1;
+
+    if octant.2 == 1 {
+        // phase = pi/4 - phase
+        phase = (1 << (INTERPOLATION_BITS + LUT_DEPTH)) - 1 - phase;
+    }
+
+    let interpolation: i32 = phase & ((1 << INTERPOLATION_BITS) - 1);
+
+    phase >>= INTERPOLATION_BITS;
+
+    let (mut cos, mut sin) = {
+        let lookup = COSSIN_TABLE[phase as usize];
+        (
+            // 1/2 < cos(0<=x<=pi/4) <= 1. So, to spread out the cos
+            // values and use the space more efficiently, we can
+            // subtract 1/2 and multiply by 2. Therefore, we add 1
+            // back in here. The sin values must be multiplied by 2 to
+            // have the same scale as the cos values.
+            lookup.0 as i32 + u16::MAX as i32,
+            (lookup.1 as i32) << 1,
+        )
+    };
+
+    // The phase values used for the LUT are adjusted up by half the
+    // phase step. The interpolation must accurately reflect this. So,
+    // an interpolation phase offset less than half the maximum
+    // involves a negative phase offset. The rest us a non-negative
+    // phase offset.
+    let interpolation_factor =
+        (interpolation - (1 << (INTERPOLATION_BITS - 1))) * PI_4_NUMERATOR;
+    let dsin = shift_round(
+        cos * interpolation_factor,
+        LUT_DEPTH + INTERPOLATION_BITS + PI_4_RIGHT_SHIFT,
+    );
+    let dcos = shift_round(
+        -sin * interpolation_factor,
+        LUT_DEPTH + INTERPOLATION_BITS + PI_4_RIGHT_SHIFT,
+    );
+
+    cos += dcos;
+    sin += dsin;
+
+    if octant.1 ^ octant.2 == 1 {
+        swap(&mut sin, &mut cos);
+    }
+
+    if octant.0 ^ octant.1 == 1 {
+        cos *= -1;
+    }
+
+    if octant.0 == 1 {
+        sin *= -1;
+    }
+
+    (cos, sin)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use core::f64::consts::PI;
+
+    #[test]
+    fn error_max_rms_all_phase() {
+        let max_amplitude: f64 = ((1 << 15) - 1) as f64;
+        let mut rms_err: Complex<f64> = (0., 0.);
+
+        for i in 0..(1 << PHASE_BITS) {
+            let phase = i as i32;
+            let radian_phase: f64 =
+                2. * PI * (phase as f64 + 0.5) / ((1 << PHASE_BITS) as f64);
+
+            let actual: Complex<f64> = (
+                max_amplitude * radian_phase.cos(),
+                max_amplitude * radian_phase.sin(),
+            );
+            let computed = cossin(phase);
+
+            let err = (
+                computed.0 as f64 / 4. - actual.0,
+                computed.1 as f64 / 4. - actual.1,
+            );
+            rms_err.0 += err.0 * err.0 / (1 << PHASE_BITS) as f64;
+            rms_err.1 += err.1 * err.1 / (1 << PHASE_BITS) as f64;
+
+            assert!(err.0.abs() < 0.89);
+            assert!(err.1.abs() < 0.89);
+        }
+        assert!(rms_err.0.sqrt() < 0.41);
+        assert!(rms_err.1.sqrt() < 0.41);
+    }
+}