feat: remove selected audio DC offset with double precision analysis

docs/AUDIO_EDITOR.md

@@ -29,3 +29,7 @@ Reverse selection changes only the chosen source frames in a new local snapshot.
 ## Phase 73
 
 Invert polarity negates selected float samples; it is not time reversal. Over-range finite samples remain over-range; no implicit limiting is performed. Outside-selection frames are bit-preserved in the copied snapshot.
+
+## Phase 74
+
+Remove DC offset subtracts the mean of the selected range from only that range. This is a constant offset correction, not a high-pass filter. Selection boundaries may acquire discontinuities; use fades or zero-crossing tools where appropriate. Finite over-range results are retained; float overflow rejects the edit.

docs/ROADMAP.md

@@ -1762,12 +1762,12 @@ Implemented: selected-range polarity inversion preserves source length and every
 Measure and subtract the selected range mean using double precision accumulation, preserve other frames and reject invalid ranges atomically.
 
 ### Tasks and acceptance criteria
-- [ ] Implement bounded C99 analysis/processing and accessible FLTK controls.
+- [x] Implement bounded C99 analysis/processing and accessible FLTK controls.
-- [ ] Preserve source files, atomic error handling, history and callback safety.
+- [x] Preserve source files, atomic error handling, history and callback safety.
-- [ ] Pass meaningful core and GUI checks; document exact behavior and limits.
+- [x] Pass meaningful core and GUI checks; document exact behavior and limits.
 
 ### Implementation status
-Planned.
+Implemented: double-precision selected-range mean subtraction in a new sample snapshot; nonfinite results reject without mutation. Core residual mean/exact surrounding samples and GUI processing/undo pass.
 
 ---
 

include/xtc/xtc_sample.h

@@ -15,7 +15,7 @@ void xtc_sample_destroy(xtc_sample *sample);
 xtc_result xtc_sample_info(const xtc_sample *sample,unsigned int *rate,size_t *frames);
 xtc_result xtc_sample_read(const xtc_sample *sample,size_t frame,float *out);
 xtc_result xtc_sample_analyze(const xtc_sample *sample,size_t begin,size_t end,xtc_sample_stats *out);
-typedef enum { XTC_SAMPLE_REVERSE, XTC_SAMPLE_INVERT } xtc_sample_operation;
+typedef enum { XTC_SAMPLE_REVERSE, XTC_SAMPLE_INVERT, XTC_SAMPLE_REMOVE_DC } xtc_sample_operation;
 /* Independent full-source result; only [begin,end) changes. Never mutates input.
  * amount is reserved/ignored for reversal. Failure preserves *out. */
 xtc_result xtc_sample_process(const xtc_sample *sample,size_t begin,size_t end,

src/audio/sample.c

@@ -53,14 +53,15 @@ xtc_result xtc_sample_write(const xtc_sample *s,size_t begin,size_t end,const ch
 xtc_result xtc_sample_process(const xtc_sample *s,size_t begin,size_t end,
                              xtc_sample_operation operation,double amount,xtc_sample **out)
 {
-    xtc_sample *copy;size_t i;xtc_result r=XTC_OK;
+    xtc_sample *copy;size_t i;xtc_result r=XTC_OK;xtc_sample_stats stats;
-    if(!range(s,begin,end)||!out||(operation<XTC_SAMPLE_REVERSE||operation>XTC_SAMPLE_INVERT)||!isfinite(amount))return XTC_ERR_INVALID_ARGUMENT;
+    if(!range(s,begin,end)||!out||(operation<XTC_SAMPLE_REVERSE||operation>XTC_SAMPLE_REMOVE_DC)||!isfinite(amount))return XTC_ERR_INVALID_ARGUMENT;
+    r=xtc_sample_analyze(s,begin,end,&stats);if(r!=XTC_OK)return r;
     copy=calloc(1,sizeof(*copy));if(!copy)return XTC_ERR_OUT_OF_MEMORY;
     copy->wave=s->wave;copy->wave.samples=malloc(s->wave.frames*sizeof(float));
     if(!copy->wave.samples){free(copy);return XTC_ERR_OUT_OF_MEMORY;}
     for(i=0;i<s->wave.frames;++i)copy->wave.samples[i]=s->wave.samples[i];
     for(i=begin;i<end;++i){double value;
-        switch(operation){case XTC_SAMPLE_INVERT:value=-(double)s->wave.samples[i];break;case XTC_SAMPLE_REVERSE:value=s->wave.samples[end-1-(i-begin)];break;default:value=0;r=XTC_ERR_INVALID_ARGUMENT;break;}
+        switch(operation){case XTC_SAMPLE_REMOVE_DC:value=(double)s->wave.samples[i]-stats.mean;break;case XTC_SAMPLE_INVERT:value=-(double)s->wave.samples[i];break;case XTC_SAMPLE_REVERSE:value=s->wave.samples[end-1-(i-begin)];break;default:value=0;r=XTC_ERR_INVALID_ARGUMENT;break;}
         if(!isfinite(value)||fabs(value)>3.4028234663852886e38){r=XTC_ERR_CAPACITY;break;}
         copy->wave.samples[i]=(float)value;
     }

src/gui/sample_editor.cpp

@@ -43,6 +43,7 @@ SampleEditor::SampleEditor(xtc_sample *sample):Fl_Double_Window(1100,650,"xtc
     auto *export_button=new Fl_Button(434,520,200,30,"Export selected frames...");export_button->callback([](Fl_Widget *,void *c){auto *e=static_cast<SampleEditor *>(c);const char *path=fl_input("NEW absolute WAV path (selection only)","");if(path){auto r=e->export_selection(path);if(r!=XTC_OK)fl_alert("Export failed: %s",xtc_result_string(r));}},this);
     auto *reverse=new Fl_Button(16,562,160,30,"Reverse selection");reverse->callback([](Fl_Widget *,void *c){auto r=static_cast<SampleEditor *>(c)->process(XTC_SAMPLE_REVERSE);if(r!=XTC_OK)fl_alert("Reversal failed: %s",xtc_result_string(r));},this);
     auto *invert=new Fl_Button(188,562,160,30,"Invert polarity");invert->callback([](Fl_Widget *,void *c){auto r=static_cast<SampleEditor *>(c)->process(XTC_SAMPLE_INVERT);if(r!=XTC_OK)fl_alert("Polarity edit failed: %s",xtc_result_string(r));},this);
+    auto *dc=new Fl_Button(360,562,160,30,"Remove DC offset");dc->callback([](Fl_Widget *,void *c){auto r=static_cast<SampleEditor *>(c)->process(XTC_SAMPLE_REMOVE_DC);if(r!=XTC_OK)fl_alert("DC removal failed: %s",xtc_result_string(r));},this);
     auto *undo=new Fl_Button(850,604,110,30,"Undo / redo");undo->callback([](Fl_Widget *,void *c){auto r=static_cast<SampleEditor *>(c)->undo_sample();if(r!=XTC_OK)fl_alert("No local sample edit to undo.");},this);
     auto *close=new Fl_Button(970,604,110,30,"Close");close->callback([](Fl_Widget *,void *c){static_cast<SampleEditor *>(c)->hide();},this);callback([](Fl_Widget *,void *c){static_cast<SampleEditor *>(c)->hide();},this);
     Fl_Group::end();xtc_ui::style(this);refresh();

tests/integration/test_gui.cpp

@@ -195,6 +195,7 @@ static int sample_editor_tests()
     MainWindow window(false,"");window.show();auto *s=timeline(&window);char path[4096];CHECK(realpath("examples/projects/original-pluck.wav",path));
     CHECK(window.import_audio(path)==XTC_OK);
     Fl::add_timeout(.1,[](void *){auto *e=dynamic_cast<SampleEditor *>(Fl::modal());CHECK(e!=nullptr);if(e){CHECK(e->select(10,100)==XTC_OK);float before=0,after=0;CHECK(xtc_sample_read(e->sample(),10,&before)==XTC_OK);
+        CHECK(e->process(XTC_SAMPLE_REMOVE_DC)==XTC_OK);CHECK(e->undo_sample()==XTC_OK);
         CHECK(e->process(XTC_SAMPLE_INVERT)==XTC_OK);CHECK(e->undo_sample()==XTC_OK);
         CHECK(e->process(XTC_SAMPLE_REVERSE)==XTC_OK);CHECK(e->undo_sample()==XTC_OK);CHECK(xtc_sample_read(e->sample(),10,&after)==XTC_OK && before==after);CHECK(e->undo_sample()==XTC_OK);CHECK(e->select(100,10)==XTC_ERR_INVALID_ARGUMENT);CHECK(e->view(10,8)==XTC_OK);CHECK(e->view(e->frames(),1)==XTC_ERR_INVALID_ARGUMENT);e->hide();}},nullptr);
     CHECK(window.open_sample_editor()==XTC_OK);CHECK(s->selected_clip()!=UINT64_MAX);

tests/unit/test_sample.c

@@ -40,5 +40,10 @@ int main(void)
      assert(xtc_sample_process(q,1,7,XTC_SAMPLE_INVERT,0,&twice)==XTC_OK);
      for(size_t j=0;j<8;++j){assert(xtc_sample_read(twice,j,&v)==XTC_OK && v==values[j]);}
      xtc_sample_destroy(q);xtc_sample_destroy(twice);}
+    {xtc_sample *q=NULL;xtc_sample_stats after;assert(xtc_sample_analyze(s,1,7,&stats)==XTC_OK);
+     assert(xtc_sample_process(s,1,7,XTC_SAMPLE_REMOVE_DC,0,&q)==XTC_OK);
+     assert(xtc_sample_analyze(q,1,7,&after)==XTC_OK && fabs(after.mean)<.000001);
+     for(size_t j=0;j<8;++j){assert(xtc_sample_read(q,j,&v)==XTC_OK && fabs(v-(j>=1&&j<7 ? values[j]-stats.mean : values[j]))<.000001);}
+     xtc_sample_destroy(q);}
     xtc_sample_destroy(s);assert(unlink(path)==0 && rmdir(root)==0);puts("Sample editor core tests passed.");return 0;
 }