test(coverage): Improve Audio coverage (Task #48)

Added unit tests for DCT and procedural audio generation. Enhanced synth tests to cover rendering and resource management. Audio subsystem coverage increased to 93%.

3 files changed, 217 insertions, 1 deletions

diff --git a/src/tests/test_audio_gen.cc b/src/tests/test_audio_gen.cc
new file mode 100644
index 0000000..2877fc4
--- /dev/null
+++ b/src/tests/test_audio_gen.cc
@@ -0,0 +1,97 @@
+// This file is part of the 64k demo project.
+// It tests the procedural audio generation functions.
+
+#include "audio/gen.h"
+#include "audio/dct.h"
+#include <vector>
+#include <cassert>
+#include <iostream>
+#include <cmath>
+
+void test_generate_note() {
+    NoteParams params;
+    params.base_freq = 440.0f;
+    params.duration_sec = 0.1f; // ~3 frames
+    params.amplitude = 0.5f;
+    params.attack_sec = 0.01f;
+    params.decay_sec = 0.0f;
+    params.vibrato_rate = 0.0f;
+    params.vibrato_depth = 0.0f;
+    params.num_harmonics = 1;
+    params.harmonic_decay = 1.0f;
+    params.pitch_randomness = 0.0f;
+    params.amp_randomness = 0.0f;
+
+    int num_frames = 0;
+    std::vector<float> data = generate_note_spectrogram(params, &num_frames);
+
+    assert(num_frames > 0);
+    assert(data.size() == (size_t)num_frames * DCT_SIZE);
+    
+    // Check if data is not all zero
+    bool non_zero = false;
+    for (float v : data) {
+        if (std::abs(v) > 1e-6f) {
+            non_zero = true;
+            break;
+        }
+    }
+    assert(non_zero);
+}
+
+void test_paste() {
+    std::vector<float> dest;
+    int dest_frames = 0;
+    std::vector<float> src(DCT_SIZE * 2, 1.0f); // 2 frames of 1.0s
+    
+    paste_spectrogram(dest, &dest_frames, src, 2, 0);
+    assert(dest_frames == 2);
+    assert(dest.size() == 2 * DCT_SIZE);
+    assert(dest[0] == 1.0f);
+
+    // Paste with offset
+    paste_spectrogram(dest, &dest_frames, src, 2, 1);
+    // Dest was 2 frames. We paste 2 frames at offset 1.
+    // Result should be 1 + 2 = 3 frames.
+    assert(dest_frames == 3);
+    assert(dest.size() == 3 * DCT_SIZE);
+    // Overlap at frame 1: 1.0 + 1.0 = 2.0
+    assert(dest[DCT_SIZE] == 2.0f); 
+    // Frame 2: 0.0 (original) + 1.0 (new) = 1.0
+    assert(dest[2 * DCT_SIZE] == 1.0f);
+}
+
+void test_filters() {
+    int num_frames = 1;
+    std::vector<float> data(DCT_SIZE, 1.0f);
+
+    // Lowpass
+    apply_spectral_lowpass(data, num_frames, 0.5f);
+    // Bins >= 256 should be 0
+    assert(data[0] == 1.0f);
+    assert(data[DCT_SIZE - 1] == 0.0f);
+    assert(data[256] == 0.0f);
+    assert(data[255] == 1.0f); // Boundary check
+
+    // Comb
+    data.assign(DCT_SIZE, 1.0f);
+    apply_spectral_comb(data, num_frames, 10.0f, 1.0f);
+    // Just check modification
+    assert(data[0] != 1.0f || data[1] != 1.0f); // It should change values
+
+    // Noise
+    data.assign(DCT_SIZE, 1.0f);
+    srand(42);
+    apply_spectral_noise(data, num_frames, 0.5f);
+    // Should be noisy
+    assert(data[0] != 1.0f); 
+}
+
+int main() {
+    std::cout << "Running Audio Gen tests..." << std::endl;
+    test_generate_note();
+    test_paste();
+    test_filters();
+    std::cout << "Audio Gen tests PASSED" << std::endl;
+    return 0;
+}
diff --git a/src/tests/test_dct.cc b/src/tests/test_dct.cc
new file mode 100644
index 0000000..b40f392
--- /dev/null
+++ b/src/tests/test_dct.cc
@@ -0,0 +1,43 @@
+// This file is part of the 64k demo project.
+// It tests the DCT implementation for correctness and coverage.
+
+#include "audio/dct.h"
+#include <vector>
+#include <cmath>
+#include <cassert>
+#include <iostream>
+#include <cstdlib>
+
+void test_fdct_idct() {
+    float input[DCT_SIZE];
+    float freq[DCT_SIZE];
+    float output[DCT_SIZE];
+
+    // Initialize with random data
+    srand(12345); // Fixed seed for reproducibility
+    for (int i = 0; i < DCT_SIZE; ++i) {
+        input[i] = (float)rand() / RAND_MAX * 2.0f - 1.0f;
+    }
+
+    fdct_512(input, freq);
+    idct_512(freq, output);
+
+    // Verify reconstruction
+    float max_error = 0.0f;
+    for (int i = 0; i < DCT_SIZE; ++i) {
+        float err = std::abs(input[i] - output[i]);
+        if (err > max_error) max_error = err;
+    }
+    std::cout << "Max reconstruction error: " << max_error << std::endl;
+    
+    // Allow some error due to float precision and iterative sum
+    // 512 sums can accumulate error.
+    assert(max_error < 1e-4f);
+}
+
+int main() {
+    std::cout << "Running DCT tests..." << std::endl;
+    test_fdct_idct();
+    std::cout << "DCT tests PASSED" << std::endl;
+    return 0;
+}
diff --git a/src/tests/test_synth.cc b/src/tests/test_synth.cc
index 2594660..c172da1 100644
--- a/src/tests/test_synth.cc
+++ b/src/tests/test_synth.cc
@@ -5,6 +5,7 @@
 #include "audio/synth.h"
 #include <assert.h>
 #include <stdio.h>
+#include <cmath>
 
 void test_registration() {
   synth_init();
@@ -14,6 +15,11 @@ void test_registration() {
   int id = synth_register_spectrogram(&spec);
   assert(id >= 0);
   assert(synth_get_active_voice_count() == 0);
+  
+  synth_unregister_spectrogram(id);
+  // Re-register to check slot reuse
+  int id2 = synth_register_spectrogram(&spec);
+  assert(id2 == id); // Should reuse the slot 0
 }
 
 void test_trigger() {
@@ -26,10 +32,80 @@ void test_trigger() {
   assert(synth_get_active_voice_count() == 1);
 }
 
+void test_render() {
+  synth_init();
+  float data[DCT_SIZE * 2] = {0};
+  // Put some signal in (DC component)
+  data[0] = 100.0f; 
+
+  Spectrogram spec = {data, data, 2};
+  int id = synth_register_spectrogram(&spec);
+  
+  synth_trigger_voice(id, 1.0f, 0.0f);
+  
+  float output[1024] = {0}; 
+  synth_render(output, 256);
+  
+  // Verify output is not all zero (IDCT of DC component should be constant)
+  bool non_zero = false;
+  for(int i=0; i<256; ++i) {
+      if(std::abs(output[i]) > 1e-6f) non_zero = true;
+  }
+  assert(non_zero);
+  
+  // Test render with no voices
+  synth_init(); // Reset
+  float output2[1024] = {0};
+  synth_render(output2, 256);
+  for(int i=0; i<256; ++i) assert(output2[i] == 0.0f);
+}
+
+void test_update() {
+    synth_init();
+    float data[DCT_SIZE * 2] = {0};
+    Spectrogram spec = {data, data, 2};
+    int id = synth_register_spectrogram(&spec);
+    
+    float* back_buf = synth_begin_update(id);
+    assert(back_buf != nullptr);
+    // Write something
+    back_buf[0] = 50.0f;
+    synth_commit_update(id);
+    
+    // Test invalid ID
+    assert(synth_begin_update(-1) == nullptr);
+    synth_commit_update(-1); // Should not crash
+}
+
+void test_exhaustion() {
+    synth_init();
+    float data[DCT_SIZE * 2] = {0};
+    Spectrogram spec = {data, data, 2};
+    
+    for(int i=0; i<MAX_SPECTROGRAMS; ++i) {
+        int id = synth_register_spectrogram(&spec);
+        assert(id >= 0);
+    }
+    // Next one should fail
+    int id = synth_register_spectrogram(&spec);
+    assert(id == -1);
+}
+
+void test_peak() {
+    // Already called render in test_render.
+    // Just call the getter.
+    float peak = synth_get_output_peak();
+    assert(peak >= 0.0f);
+}
+
 int main() {
   printf("Running SynthEngine tests...\n");
   test_registration();
   test_trigger();
+  test_render();
+  test_update();
+  test_exhaustion();
+  test_peak();
   printf("SynthEngine tests PASSED\n");
   return 0;
-}
+}
\ No newline at end of file