Task 19: Update TTS generation endpoint to accept edited scripts

- Modified TTSRequest to accept optional segments array
- Updated generate_audiobook endpoint to use segments if provided
- Added generate_audiobook_from_segments method to TTSService
- Ensured backward compatibility with text-based generation
- Added comprehensive tests for segment-based generation
- All tests passing (7/7)

Requirements: 15.7

backend/test_segment_based_generation.py

@@ -0,0 +1,164 @@
+"""
+Test for segment-based audiobook generation (Task 19).
+
+Tests that the TTS endpoint can accept edited script segments
+and generate audio from them, while maintaining backward compatibility
+with text-based generation.
+"""
+import pytest
+from fastapi.testclient import TestClient
+from main import app
+
+client = TestClient(app)
+
+
+def test_generate_audiobook_with_segments():
+    """Test that audiobook can be generated from edited script segments."""
+    # Prepare request with segments
+    request_data = {
+        "segments": [
+            {"id": "seg_0", "speaker": "Alice", "text": "Hello, how are you?"},
+            {"id": "seg_1", "speaker": "Bob", "text": "I'm doing great, thanks!"},
+            {"id": "seg_2", "speaker": "Narrator", "text": "They continued their conversation."}
+        ],
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"},
+            {"id": "Bob", "voice": "am_adam"},
+            {"id": "Narrator", "voice": "af_sarah"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should succeed
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "audio/wav"
+    assert "audiobook.wav" in response.headers.get("content-disposition", "")
+    assert len(response.content) > 0
+
+
+def test_generate_audiobook_with_text_backward_compatibility():
+    """Test that audiobook generation still works with text (backward compatibility)."""
+    # Prepare request with text (old format)
+    request_data = {
+        "text": "Alice: Hello there!\nBob: Hi Alice!",
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"},
+            {"id": "Bob", "voice": "am_adam"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should succeed
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "audio/wav"
+    assert len(response.content) > 0
+
+
+def test_generate_audiobook_segments_take_precedence():
+    """Test that segments are used when both segments and text are provided."""
+    # Prepare request with both segments and text
+    request_data = {
+        "segments": [
+            {"id": "seg_0", "speaker": "Alice", "text": "From segments"}
+        ],
+        "text": "Alice: From text",
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should succeed and use segments
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "audio/wav"
+
+
+def test_generate_audiobook_missing_both_segments_and_text():
+    """Test that request fails when neither segments nor text is provided."""
+    request_data = {
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should fail with validation error
+    assert response.status_code == 400
+    assert "ValidationError" in response.json()["error"]
+
+
+def test_generate_audiobook_empty_segments():
+    """Test that request fails when segments array is empty."""
+    request_data = {
+        "segments": [],
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should fail - empty segments should be treated as not provided
+    assert response.status_code == 400
+
+
+def test_edited_script_preserves_speaker_assignments():
+    """Test that edited speaker assignments in segments are preserved."""
+    # Segments where user has changed speaker assignments
+    request_data = {
+        "segments": [
+            {"id": "seg_0", "speaker": "Bob", "text": "This was originally Alice's line"},
+            {"id": "seg_1", "speaker": "Alice", "text": "This was originally Bob's line"}
+        ],
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"},
+            {"id": "Bob", "voice": "am_adam"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should succeed with edited assignments
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "audio/wav"
+    assert len(response.content) > 0
+
+
+def test_segment_with_unmapped_speaker_uses_default():
+    """Test that segments with unmapped speakers fall back to default voice."""
+    request_data = {
+        "segments": [
+            {"id": "seg_0", "speaker": "UnknownSpeaker", "text": "Hello from unknown"}
+        ],
+        "lang": "a",
+        "roles": [
+            {"id": "Alice", "voice": "af_bella"}
+        ],
+        "speed": 1.0
+    }
+    
+    response = client.post("/api/tts", json=request_data)
+    
+    # Should succeed using default voice
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "audio/wav"
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])