1 files changed, 149 insertions, 0 deletions

diff --git a/timestretch.c b/timestretch.c
new file mode 100644
index 0000000..63ba059
--- /dev/null
+++ b/timestretch.c
@@ -0,0 +1,149 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+#include <float.h>
+#include <math.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <inttypes.h>
+
+// global values
+static short * g_overlap_buffer;
+static size_t  g_overlap;
+
+static size_t  g_input_length;  // Minimal length 
+static size_t  g_output_length;
+static size_t  g_corr_length;
+
+static size_t  g_skip;          // Per frame skip
+static size_t  g_offset;        // Offset into stream, lower bits
+static size_t  g_firstframe;    // If this is set, the caller should provide initial data in g_overlap_buffer
+
+/* some good default for mixing voice, values in micro seconds */
+#define OVERLAP            10   // overlapping length (music default = 12 ms)
+#define CORR_WINDOW        15   // overlapping correlation window length (music default = 28 ms)
+#define OUTPUT_LEN         30   // one processing sequence length in milliseconds (music default = 82 ms)
+
+// Returns the length of one output frame
+static size_t calc_convert_values( int sample_rate, float tempo ) {
+    g_overlap = ( sample_rate * OVERLAP ) / 1000;
+
+    free( g_overlap_buffer );
+    g_overlap_buffer = malloc( sizeof(short) * g_overlap );
+    g_firstframe = 1;
+
+    g_output_length = (sample_rate * OUTPUT_LEN ) / 1000;
+    if ( g_output_length < g_overlap)
+        g_output_length = g_overlap;
+    g_corr_length = ( sample_rate * CORR_WINDOW ) / 1000;
+
+    g_skip = (size_t)( tempo * (float)g_output_length * 65536.0 );
+    g_input_length = g_corr_length + g_output_length + g_overlap;
+    if( g_skip / 65536 > g_input_length )
+        g_input_length = g_skip / 65536;
+
+    return g_output_length;
+}
+
+/*
+  Example: tempo 1.5 with 30/15/10 => skip == 45, out = 30
+we found an offset of 5 msec
+
+  [#####OOOOOOOOOOVVVVVVVVVVVVVVVVVVVVmmmmmmmmmm?????????????????????????]
+                                               ^--skip
+  [###############OOOOOOOOOOVVVVVVVVVVVVVVVVVVVVmmmmmmmmmm???????????????]
+  [                                            #####OOOOOOOOOOVVVVVVVVVVVVVVVVVVVVmmmmmmmmmm?????????????????????????]
+*/
+
+static unsigned int find_corr_max(const short *input, const short *mixbuf) {
+    unsigned int i, j, offs = 0;
+    double corr_max = FLT_MIN;
+
+    // Scans for the best correlation value by testing each possible position
+    for (i = 0; i < g_corr_length; ++i) {
+        double acc, i_f = (double)i, sl_f = (double)g_corr_length;
+        double heur = 0.75 - ( i_f * i_f ) / ( sl_f * sl_f ) + i_f / sl_f;
+        int64_t acc_i;
+
+        for(j = 0, acc_i = 0; j < g_overlap; ++j )
+            acc_i += input[i+j] * mixbuf[j];
+
+        // boost middle of sequence by top of a flat parabola
+        // slope stolen from soundtouch
+        acc = (double)acc_i * heur;
+        if ( corr_max < acc ) {
+            offs = i;
+            corr_max = acc;
+        }
+    }
+// printf( "%03d %014.0lf\n", best_offs, best_corr );
+    return offs;
+}
+
+// Returns the amount of samples that can be discarded from begin of the input buffer
+size_t process_frame( short *input, short *output, short *overlap ) {
+    int i, i_ = (int)g_overlap;
+    unsigned int offset = 0;
+
+    // The first frame needs to be copied verbatim,
+    // we do not have anything to mix, yet.
+    if( g_firstframe ) {
+        memcpy( output, input, g_output_length * sizeof(short) );
+        g_firstframe = 0;
+    } else {
+        offset = find_corr_max( input, overlap );
+
+        // Mix end of last frame with begin of this frame
+        for (i = 0; i < (int)g_overlap ; ++i, --i_ )
+            output[i] = ( i_ * overlap[i] + i * input[i+offset] ) / (int)g_overlap;
+
+        // Copy rest of the input verbatim
+        memcpy( output + g_overlap, input + offset + g_overlap, ( g_output_length - g_overlap ) * sizeof(short) );
+    }
+
+    // Remember end of this frame for next frame
+    memcpy( overlap, input + offset + g_output_length, g_overlap * sizeof(short) );
+
+    // Remove the processed samples from the input buffer.
+    g_offset &= 0xffff;
+    g_offset += g_skip;
+    return g_offset / 65536;
+}
+
+int main( int args, char **argv ) {
+  size_t    out_chunk_size = calc_convert_values( 8000, 1.25f );
+  size_t    in_fill = 0;
+  short     outbuf[ g_output_length ];
+  short     inbuf [ g_input_length  ];
+
+  printf( "DEBUG: OL: %zd SWL: %zd SL: %zd SK: %zd ICM: %zd\n", g_overlap, g_output_length, g_corr_length, g_skip / 65536, g_input_length );
+
+  int fd_in  = open( "in.raw", O_RDONLY );
+  int fd_out = open( "out.raw", O_CREAT | O_WRONLY | O_TRUNC );
+
+  (void)args; (void)argv; (void)out_chunk_size;
+
+  while( 1 ) {
+    size_t processed;
+    size_t missing = g_input_length - in_fill;
+    size_t fromfd = read( fd_in, inbuf + in_fill, missing * sizeof(short) );
+
+    if( fromfd > 0 )
+        in_fill += fromfd / sizeof(short);
+    if( fromfd != missing * sizeof(short) ) {
+        write( fd_out, inbuf, in_fill * sizeof(short) );
+        close( fd_in ); close( fd_out );
+        exit(0);
+    }
+
+    // Do one cycle of processing and outputting
+    processed = process_frame( inbuf, outbuf, g_overlap_buffer );
+    write( fd_out, outbuf, g_output_length * sizeof(short) );
+
+    memmove( inbuf, inbuf + processed, ( in_fill - processed ) * sizeof(short) );
+    in_fill -= processed;
+  }
+
+  return 0;
+}