interpret logical plane line via scrolled_row() #443

src/lib/internal.h

@@ -57,8 +57,13 @@ struct esctrie;
 // A plane may be partially or wholly offscreen--this might occur if the
 // screen is resized, for example. Offscreen portions will not be rendered.
 // Accesses beyond the borders of a panel, however, are errors.
+//
+// The framebuffer 'fb' is a set of rows. For scrolling, we interpret it as a
+// circular buffer of rows. 'logrow' is the index of the row at the logical top
+// of the plane.
 typedef struct ncplane {
   cell* fb;             // "framebuffer" of character cells
+  int logrow;           // logical top row, starts at 0, add one for each scroll
   int x, y;             // current cursor location within this plane
   int absx, absy;       // origin of the plane relative to the screen
   int lenx, leny;       // size of the plane, [0..len{x,y}) is addressable

src/lib/notcurses.c

@@ -286,6 +286,7 @@ ncplane_create(notcurses* nc, ncplane* n, int rows, int cols,
   p->leny = rows;
   p->lenx = cols;
   p->x = p->y = 0;
+  p->logrow = 0;
   if( (p->bound = n) ){
     p->absx = xoff + n->absx;
     p->absy = yoff + n->absy;

src/lib/render.c

@@ -233,6 +233,13 @@ lock_in_highcontrast(cell* targc, struct crender* crender){
   }
 }
 
+// taking in a desired true row of the plane, find the line within the
+// plane's framebuffer, taking into account the current scrolling state.
+static inline int
+scrolled_row(const ncplane* p, int y){
+  return (y + p->logrow) % p->leny;
+}
+
 // Paints a single ncplane into the provided scratch framebuffer 'fb', and
 // ultimately 'lastframe' (we can't always write directly into 'lastframe',
 // because we need build state to solve certain cells, and need compare their
@@ -276,7 +283,7 @@ paint(ncplane* p, cell* lastframe, struct crender* rvec,
         continue;
       }
       struct crender* crender = &rvec[fbcellidx(absy, dstlenx, absx)];
-      const cell* vis = &p->fb[nfbcellidx(p, y, x)];
+      const cell* vis = &p->fb[nfbcellidx(p, scrolled_row(p, y), x)];
       // if we never loaded any content into the cell (or obliterated it by
       // writing in a zero), use the plane's base cell.
       if(vis->gcluster == 0 && !cell_wide_right_p(vis)){