From d170afea8eedbdae81cda82d4700886b07f30fbc Mon Sep 17 00:00:00 2001
From: Trek H <trek.hopton@gmail.com>
Date: Fri, 27 Sep 2019 18:59:08 +0930
Subject: [PATCH] audio-player: added decBytes function

I added a function called decBytes to calculate the number of PCM bytes that will be generated from a given array of ADPCM bytes.
---
 cmd/audio-player/adpcm.js | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/cmd/audio-player/adpcm.js b/cmd/audio-player/adpcm.js
index 784145b7..eb11bd26 100644
--- a/cmd/audio-player/adpcm.js
+++ b/cmd/audio-player/adpcm.js
@@ -61,9 +61,10 @@ class Decoder {
         ];
     }
 
-    static get byteDepth() { return 2; } // We are working with 16-bit samples. TODO(Trek): make configurable.
-    static get headSize() { return 8; } // Number of bytes in the header of ADPCM.
-    static get chunkLenSize() { return 4; }
+    static get byteDepth() { return 2; }    // We are working with 16-bit samples. TODO(Trek): make configurable.
+    static get headSize() { return 8; }     // Number of bytes in the header of ADPCM.
+    static get chunkLenSize() { return 4; } // Length in bytes of the chunk length field in header.
+    static get compFact() { return 4; }     // In general ADPCM compresses by a factor of 4.
 
     // decodeSample takes 4 bits which represents a single ADPCM nibble, and returns a 16 bit decoded PCM sample.
     decodeSample(nibble) {
@@ -152,4 +153,27 @@ class Decoder {
             (b[2] << 16) |
             (b[3] << 24));
     }
+
+    // decBytes takes a parameter that is assumed to be a byte array containing one or more adpcm chunks.
+    // It reads the chunk lengths from the chunk headers to calculate and return the number of pcm bytes that are expected to be decoded from the adpcm.
+    static decBytes(b){
+        let chunkLen;
+        let n = 0;
+        for (let off = 0; off + Decoder.headSize <= b.length; off += chunkLen) {
+            chunkLen = Decoder.bytesToInt32(b.slice(off, off + Decoder.chunkLenSize))
+            if (off + chunkLen > b.length) {
+                break;
+            }
+
+            // Account for uncompressed sample in header.
+            n += Decoder.byteDepth;
+            // Account for PCM bytes that will result from decoding ADPCM.
+            n += (chunkLen - Decoder.headSize)*Decoder.compFact;
+            // Account for padding.
+            if(b[off+Decoder.chunkLenSize+3] == 0x01){
+                n -= Decoder.byteDepth;
+            }
+        }
+        return n;
+    }
 }
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