+//! IMA ADPCM decoding functionality.
+
+///! IMA ADPCM step change table.
+pub const IMA_STEPS: [i8; 16] = [
+ -1, -1, -1, -1, 2, 4, 6, 8,
+ -1, -1, -1, -1, 2, 4, 6, 8
+];
+
+///! IMA ADPCM step size table.
+pub const IMA_STEP_TABLE: [i32; 89] = [
+ 7, 8, 9, 10, 11, 12, 13, 14,
+ 16, 17, 19, 21, 23, 25, 28, 31,
+ 34, 37, 41, 45, 50, 55, 60, 66,
+ 73, 80, 88, 97, 107, 118, 130, 143,
+ 157, 173, 190, 209, 230, 253, 279, 307,
+ 337, 371, 408, 449, 494, 544, 598, 658,
+ 724, 796, 876, 963, 1060, 1166, 1282, 1411,
+ 1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
+ 3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
+ 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
+ 15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
+];
+
+///! Maximum step value for IMA ADPCM.
+pub const IMA_MAX_STEP: u8 = 88;
+
+///! Decoder for IMA ADPCM.
+pub struct IMAState {
+ ///! Current sample value.
+ pub predictor: i32,
+ ///! Current step index.
+ pub step: usize,
+}
+
+impl IMAState {
+ ///! Constructs a new instance of `IMAState`.
+ pub fn new() -> Self {
+ Self {
+ predictor: 0,
+ step: 0,
+ }
+ }
+ ///! Re-initialises decoder with new predictor and step values.
+ pub fn reset(&mut self, predictor: i16, step: u8) {
+ self.predictor = i32::from(predictor);
+ self.step = step.min(IMA_MAX_STEP) as usize;
+ }
+ ///! Computes a new sample value from an input nibble.
+ pub fn expand_sample(&mut self, nibble: u8) -> i16 {
+ let istep = (self.step as isize) + (IMA_STEPS[(nibble & 0xF) as usize] as isize);
+ let sign = (nibble & 8) != 0;
+ let diff = (i32::from(2 * (nibble & 7) + 1) * IMA_STEP_TABLE[self.step]) >> 3;
+ let sample = if !sign { self.predictor + diff } else { self.predictor - diff };
+ self.predictor = sample.max(i32::from(std::i16::MIN)).min(i32::from(std::i16::MAX));
+ self.step = istep.max(0).min(IMA_MAX_STEP as isize) as usize;
+ self.predictor as i16
+ }
+}
+
+