implment DPCM

src/dac.rs

@@ -1,4 +1,38 @@
 use crate::interface::DacInterface;
+
+pub enum Direction {
+    Up,
+    Down,
+}
+
+
+pub struct DpcmSample {
+    direction: Direction,
+    // only 3 bits
+    step_count: u8,
+}
+
+impl DpcmSample {
+    pub fn new(byte: u8, sub_index: usize) -> Self {
+        let data = if sub_index == 0 {
+            byte & 0x0F
+        } else {
+            byte >> 4
+        };
+        let direction = match data & 0x1 {
+            0x1 => Direction::Up,
+            0x0 => Direction::Down,
+            _ => Direction::Up,
+        };
+        let step_count = data >> 1;
+
+        Self {
+            direction,
+            step_count,
+        }
+    }
+}
+
 pub struct Dac<'a, T: DacInterface> {
     output: T,
     data: Option<&'a [u8]>,
@@ -31,3 +65,70 @@ impl<'a, T: DacInterface> Dac<'a, T> {
         }
     }
 }
+
+pub struct DpcmDac<'a, T: DacInterface> {
+    output: T,
+    data: Option<&'a [u8]>,
+    index: usize,
+    prev_amplitude: usize,
+    step_size: usize,
+    min: usize,
+    max: usize,
+}
+
+impl<'a, T: DacInterface> DpcmDac<'a, T> {
+    pub fn new(output: T) -> Self {
+        Self {
+            output,
+            data: None,
+            index: 0,
+            prev_amplitude: 0x80,
+            step_size: 0x3,
+            min: 0x19, 
+            max: 0xe6,
+        }
+    }
+    pub fn load_data(&mut self, data: &'a [u8]) {
+        self.data = Some(data);
+    }
+    pub fn seek_to_sample(&mut self, index: usize) {
+        self.index = index;
+    }
+
+    // output the next sampe. returns true if there are samples remaining, false if there are no samples remaining.
+    pub fn output_next(&mut self) -> bool {
+        if let Some(data) = self.data {
+            let sub_index = 1 - self.index % 2;
+
+            let sample = DpcmSample::new(data[self.index / 2], sub_index);
+            let new_amplitude = match sample.direction {
+                Direction::Down => self.prev_amplitude.saturating_sub(sample.step_count as usize * self.step_size).max(self.min),
+                Direction ::Up => (self.prev_amplitude + (sample.step_count as usize * self.step_size)).min(self.max),
+            };
+
+            // calculate normalized amplitude and write out
+            let normalized_amplitude = (new_amplitude - self.min) * 100 / (self.max - self.min);
+            self.output.write_amplitude(normalized_amplitude as u8);
+            self.prev_amplitude = new_amplitude;             
+            
+
+            // increment the sample index
+            let mut samples_remaining = true;
+            self.index = self.index + 1;
+
+            // reset the index to 0 if we roll over
+            if (self.index >= data.len()  * 2) {
+                self.index = 0;
+                self.prev_amplitude = 0x74;
+                samples_remaining = false;
+            }            
+            
+            // return whether or not we have samples remaining
+            samples_remaining
+        }
+        else {
+            // if the sample failed to load, we (duh) have no more samples remaining
+            false
+        }
+    }
+}