wave: write FACT chunk for IEEE float

Per the RIFF/WAVE Rev. 3 documentation, non-PCM formats require a fact chunk, while PCM does not.
This is also what libsdnfile/audacity do

Reference: https://mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/WAVE.html (see the 'fact Chunk' section and linked Rev. 3 RIFF docs).

Author: mbeijen

Lib/test/test_wave.py

@@ -258,6 +258,58 @@ def test_open_in_write_raises(self):
             support.gc_collect()
             self.assertIsNone(cm.unraisable)
 
+    def test_ieee_float_has_fact_chunk(self):
+        nframes = 100
+        with tempfile.NamedTemporaryFile(delete_on_close=False) as fp:
+            filename = fp.name
+        self.addCleanup(unlink, filename)
+
+        with wave.open(filename, 'wb') as w:
+            w.setnchannels(1)
+            w.setsampwidth(2)
+            w.setframerate(22050)
+            w.setformat(wave.WAVE_FORMAT_IEEE_FLOAT)
+            w.writeframes(b'\x00\x00' * nframes)
+
+        with open(filename, 'rb') as f:
+            f.read(12)
+            fact_found = False
+            fact_samples = None
+            while True:
+                chunk_id = f.read(4)
+                if len(chunk_id) < 4:
+                    break
+                chunk_size = struct.unpack('<L', f.read(4))[0]
+                if chunk_id == b'fact':
+                    fact_found = True
+                    fact_samples = struct.unpack('<L', f.read(4))[0]
+                    break
+                f.seek(chunk_size + (chunk_size & 1), 1)
+
+        self.assertTrue(fact_found)
+        self.assertEqual(fact_samples, nframes)
+
+    def test_pcm_has_no_fact_chunk(self):
+        with tempfile.NamedTemporaryFile(delete_on_close=False) as fp:
+            filename = fp.name
+        self.addCleanup(unlink, filename)
+
+        with wave.open(filename, 'wb') as w:
+            w.setnchannels(1)
+            w.setsampwidth(2)
+            w.setframerate(22050)
+            w.writeframes(b'\x00\x00' * 100)
+
+        with open(filename, 'rb') as f:
+            f.read(12)
+            while True:
+                chunk_id = f.read(4)
+                if len(chunk_id) < 4:
+                    break
+                chunk_size = struct.unpack('<L', f.read(4))[0]
+                self.assertNotEqual(chunk_id, b'fact')
+                f.seek(chunk_size + (chunk_size & 1), 1)
+
 
 class WaveOpen(unittest.TestCase):
     def test_open_pathlike(self):

Lib/wave.py

@@ -468,6 +468,7 @@ def initfp(self, file):
         self._nframeswritten = 0
         self._datawritten = 0
         self._datalength = 0
+        self._fact_sample_count_pos = None
         self._headerwritten = False
 
     def __del__(self):
@@ -615,6 +616,9 @@ def _ensure_header_written(self, datasize):
                 raise Error('sampling rate not specified')
             self._write_header(datasize)
 
+    def _needs_fact_chunk(self):
+        return self._format == WAVE_FORMAT_IEEE_FLOAT
+
     def _write_header(self, initlength):
         assert not self._headerwritten
         self._file.write(b'RIFF')
@@ -625,12 +629,23 @@ def _write_header(self, initlength):
             self._form_length_pos = self._file.tell()
         except (AttributeError, OSError):
             self._form_length_pos = None
-        self._file.write(struct.pack('<L4s4sLHHLLHH4s',
-            36 + self._datalength, b'WAVE', b'fmt ', 16,
+        has_fact = self._needs_fact_chunk()
+        header_overhead = 36 + (12 if has_fact else 0)
+        self._file.write(struct.pack('<L4s4sLHHLLHH',
+            header_overhead + self._datalength, b'WAVE', b'fmt ', 16,
             self._format, self._nchannels, self._framerate,
             self._nchannels * self._framerate * self._sampwidth,
             self._nchannels * self._sampwidth,
-            self._sampwidth * 8, b'data'))
+            self._sampwidth * 8))
+        if has_fact:
+            self._file.write(b'fact')
+            self._file.write(struct.pack('<L', 4))
+            try:
+                self._fact_sample_count_pos = self._file.tell()
+            except (AttributeError, OSError):
+                self._fact_sample_count_pos = None
+            self._file.write(struct.pack('<L', self._nframes))
+        self._file.write(b'data')
         if self._form_length_pos is not None:
             self._data_length_pos = self._file.tell()
         self._file.write(struct.pack('<L', self._datalength))
@@ -641,8 +656,13 @@ def _patchheader(self):
         if self._datawritten == self._datalength:
             return
         curpos = self._file.tell()
+        header_overhead = 36 + (12 if self._needs_fact_chunk() else 0)
         self._file.seek(self._form_length_pos, 0)
-        self._file.write(struct.pack('<L', 36 + self._datawritten))
+        self._file.write(struct.pack('<L', header_overhead + self._datawritten))
+        if self._fact_sample_count_pos is not None:
+            self._file.seek(self._fact_sample_count_pos, 0)
+            nframes = self._datawritten // (self._nchannels * self._sampwidth)
+            self._file.write(struct.pack('<L', nframes))
         self._file.seek(self._data_length_pos, 0)
         self._file.write(struct.pack('<L', self._datawritten))
         self._file.seek(curpos, 0)