From 1136acfc365f357d2df13a263714e8ae0614c4f9 Mon Sep 17 00:00:00 2001
From: yum <yum.food.vr@gmail.com>
Date: Sun, 26 Feb 2023 19:42:33 -0800
Subject: Add retainDuration option to CaptureParams

This allows users to retain a suffix of the PCM buffer after a VAD
segmentation event, reducing some instances of words being lost at
the start of the next VAD window.
---
 Whisper/API/MfStructs.h                 |  3 +++
 Whisper/MF/AudioBuffer.h                | 15 +++++++++++++++
 Whisper/Whisper/ContextImpl.capture.cpp |  5 ++++-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/Whisper/API/MfStructs.h b/Whisper/API/MfStructs.h
index 39255de..c23d633 100644
--- a/Whisper/API/MfStructs.h
+++ b/Whisper/API/MfStructs.h
@@ -28,6 +28,9 @@ namespace Whisper
 		float maxDuration = 3.0f;
 		float dropStartSilence = 0.25f;
 		float pauseDuration = 0.333f;
+		// After audio is segmented using VAD, as many as this many seconds of
+		// audio will be retained as the input to the next transcription window.
+		float retainDuration = 0.25f;
 		// Flags for the audio capture
 		uint32_t flags = 0;
 	};
diff --git a/Whisper/MF/AudioBuffer.h b/Whisper/MF/AudioBuffer.h
index 77be1e0..63c4a8c 100644
--- a/Whisper/MF/AudioBuffer.h
+++ b/Whisper/MF/AudioBuffer.h
@@ -48,12 +48,27 @@ namespace Whisper
 		void dropFirst(size_t len)
 		{
 			assert(len <= mono.size());
+			if (len >= mono.size()) {
+				mono.clear();
+				return;
+			}
 			size_t remainder = mono.size() - len;
 			auto tmp = std::vector<float>(remainder);
 			memcpy(tmp.data(), mono.data() + len, remainder);
 			mono = std::move(tmp);
 		}
 
+		void retainLast(size_t len)
+		{
+			if (len >= mono.size()) {
+				return;
+			}
+			size_t prefix_len = mono.size() - len;
+			auto tmp = std::vector<float>(len);
+			memcpy(tmp.data(), mono.data() + prefix_len, len);
+			mono = std::move(tmp);
+		}
+
 		void normalize()
 		{
 			const auto &min = *std::min_element(mono.begin(), mono.end());
diff --git a/Whisper/Whisper/ContextImpl.capture.cpp b/Whisper/Whisper/ContextImpl.capture.cpp
index bc88249..0100fcd 100644
--- a/Whisper/Whisper/ContextImpl.capture.cpp
+++ b/Whisper/Whisper/ContextImpl.capture.cpp
@@ -53,6 +53,7 @@ namespace
 	struct CaptureParams
 	{
 		uint32_t minDuration, maxDuration, dropStartSilence, pauseDuration;
+		uint32_t retainDuration;
 		uint32_t flags;
 
 		CaptureParams( const sCaptureParams& cp )
@@ -64,6 +65,8 @@ namespace
 			__m128i ints = _mm_cvtps_epi32( floats );
 			store16( &minDuration, ints );
 
+			retainDuration = std::round(retainDuration * SAMPLE_RATE);
+
 			flags = cp.flags;
 		}
 	};
@@ -142,7 +145,7 @@ namespace
 			buffer.pcm.normalize();
 			SubmitThreadpoolWork( work );
 			pcmStartTime = nextSampleTime;
-			pcm.clear();
+			pcm.retainLast(captureParams.retainDuration);
 			vad.clear();
 			return S_OK;
 		}
-- 
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