Wrap some ffmpeg code to do raw audio rate conversion

setup/build_environment.py

@@ -143,6 +143,9 @@ hunspell_lib_dirs = []
 hyphen_inc_dirs = []
 hyphen_lib_dirs = []
 
+ffmpeg_inc_dirs = []
+ffmpeg_lib_dirs = []
+
 uchardet_inc_dirs, uchardet_lib_dirs, uchardet_libs = [], [], ['uchardet']
 
 openssl_inc_dirs, openssl_lib_dirs = [], []
@@ -206,6 +209,10 @@ else:
     uchardet_lib_dirs = pkgconfig_lib_dirs('uchardet', '', '/usr/lib')
     uchardet_libs = pkgconfig_libs('uchardet', '', '')
 
+if os.path.exists(os.path.join(sw, 'ffmpeg')):
+    ffmpeg_inc_dirs = [os.path.join(sw, 'ffmpeg', 'include')] + ffmpeg_inc_dirs
+    ffmpeg_lib_dirs = [os.path.join(sw, 'ffmpeg', 'lib')] + ffmpeg_lib_dirs
+
 
 if 'PODOFO_PREFIX' in os.environ:
     os.environ['PODOFO_LIB_DIR'] = os.path.join(os.environ['PODOFO_PREFIX'], 'lib')

setup/extensions.json

@@ -16,6 +16,14 @@
         "lib_dirs": "!hyphen_lib_dirs",
 		"needs_c": "99"
     },
+    {
+        "name": "ffmpeg",
+        "sources": "calibre/utils/ffmpeg.c",
+		"libraries": "avcodec avutil swresample",
+        "inc_dirs": "!ffmpeg_inc_dirs",
+        "lib_dirs": "!ffmpeg_lib_dirs",
+		"needs_c": "99"
+    },
     {
         "name": "uchardet",
         "sources": "calibre/ebooks/uchardet.c",

src/calibre/utils/ffmpeg.c

@@ -0,0 +1,88 @@
+/*
+ * ffmpeg.c
+ * Copyright (C) 2024 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+
+#define UNICODE
+#define PY_SSIZE_T_CLEAN
+
+#include <Python.h>
+
+#include <libavutil/audio_fifo.h>
+#include <libavcodec/avcodec.h>
+#include <libavutil/samplefmt.h>
+#include <libavutil/common.h>
+#include <libavutil/channel_layout.h>
+#include <libavutil/opt.h>
+#include <libavutil/mem.h>
+#include <libavutil/fifo.h>
+#include <libswresample/swresample.h>
+
+static PyObject*
+averror_as_python(int errnum) {
+    char buf[4096];
+    av_strerror(errnum, buf, sizeof(buf));
+    PyErr_SetString(PyExc_Exception, buf);
+    return NULL;
+}
+
+static PyObject*
+resample_raw_audio_16bit(PyObject *self, PyObject *args) {
+    int input_sample_rate, output_sample_rate, input_num_channels = 1, output_num_channels = 1;
+    Py_buffer inb = {0};
+    if (!PyArg_ParseTuple(args, "y*ii|ii", &inb, &input_sample_rate, &output_sample_rate, &input_num_channels, &output_num_channels)) return NULL;
+    int64_t output_size = av_rescale_rnd(inb.len, output_sample_rate, input_sample_rate, AV_ROUND_UP);
+    uint8_t *output = av_malloc(output_size);  // we have to use av_malloc as it aligns to vector boundaries
+    if (!output) { PyBuffer_Release(&inb); return PyErr_NoMemory(); }
+    AVChannelLayout input_layout = {0}, output_layout = {0};
+    av_channel_layout_default(&input_layout, input_num_channels);
+    av_channel_layout_default(&output_layout, output_num_channels);
+    SwrContext *swr_ctx = NULL;
+    static const enum AVSampleFormat fmt = AV_SAMPLE_FMT_S16;
+    const int bytes_per_sample = av_get_bytes_per_sample(fmt);
+    int ret = swr_alloc_set_opts2(&swr_ctx,
+            &output_layout, fmt, output_sample_rate,
+            &input_layout, fmt, input_sample_rate,
+            0, NULL);
+    if (ret != 0) { av_free(output); PyBuffer_Release(&inb); return averror_as_python(ret); }
+#define free_resources av_free(output); PyBuffer_Release(&inb); swr_free(&swr_ctx);
+    if ((ret = swr_init(swr_ctx)) < 0) { free_resources; return averror_as_python(ret); }
+    const uint8_t *input = inb.buf;
+    ret = swr_convert(swr_ctx,
+            &output, output_size / (output_num_channels * bytes_per_sample),
+            &input,  inb.len / (input_num_channels * bytes_per_sample)
+            );
+    if (ret < 0) { free_resources; return averror_as_python(ret); }
+    output_size = ret * output_num_channels * bytes_per_sample;
+    PyObject *ans = PyBytes_FromStringAndSize((char*)output, output_size);
+    free_resources;
+#undef free_resources
+    return ans;
+}
+
+
+// Boilerplate {{{
+static int
+exec_module(PyObject *module) { return 0; }
+
+CALIBRE_MODINIT_FUNC PyInit_ffmpeg(void) {
+    static PyModuleDef_Slot slots[] = { {Py_mod_exec, exec_module}, {0, NULL} };
+    static PyMethodDef methods[] = {
+        {"resample_raw_audio_16bit", (PyCFunction)resample_raw_audio_16bit, METH_VARARGS,
+        "resample_raw_audio(input_data, input_sample_rate, output_sample_rate, input_num_channels=1, output_num_channels=1) -> Return resampled raw audio data."
+        },
+        {0}  /* Sentinel */
+    };
+    static struct PyModuleDef module_def = {
+        .m_base     = PyModuleDef_HEAD_INIT,
+        .m_name     = "ffmpeg",
+        .m_doc      = "Wrapper for the ffmpeg C libraries",
+        .m_methods  = methods,
+        .m_slots    = slots,
+    };
+    return PyModuleDef_Init(&module_def);
+}
+// }}}