From 9b81de3789d203c88adbde9e461290859d211b0a Mon Sep 17 00:00:00 2001
From: Nick Black <dankamongmen@gmail.com>
Date: Sun, 22 Dec 2019 08:08:53 -0500
Subject: [PATCH] O(1) passive damage detection #189 (#197)

* notcurses: set up lastframe #189
* render: o(1) take no prisoners damage detection #189
---
 doc/man/man1/notcurses-demo.1           |   6 +-
 src/demo/demo.c                         |  30 ++++-
 src/demo/demo.h                         |   2 +-
 src/demo/{bleachworm.c => witherworm.c} | 155 ++++++++++++---------
 src/lib/egcpool.h                       |   6 +
 src/lib/fade.c                          |   2 -
 src/lib/internal.h                      |  52 +++----
 src/lib/libav.c                         |   1 -
 src/lib/notcurses.c                     | 108 ++-------------
 src/lib/render.c                        | 172 +++++++++++++++++-------
 10 files changed, 289 insertions(+), 245 deletions(-)
 rename src/demo/{bleachworm.c => witherworm.c} (92%)

diff --git a/doc/man/man1/notcurses-demo.1 b/doc/man/man1/notcurses-demo.1
index 04821d856..304b0eced 100644
--- a/doc/man/man1/notcurses-demo.1
+++ b/doc/man/man1/notcurses-demo.1
@@ -6,6 +6,7 @@ notcurses-demo \- Show off some notcurses features
 [ \fB\-p \fIpath \fR]
 [ \fB\-d \fIdelaymult \fR]
 [ \fB\-k \fR]
+[ \fB\-c \fR]
 [ \fB\-h / \fB\-\-help \fR]
 .IR demospec
 .SH OPTIONS
@@ -20,6 +21,9 @@ Apply a (floating-point) multiplier to the standard delay of 1s.
 Inhibit use of the alternate screen. Necessary if you want the output left
 on your terminal after the program exits.
 .TP
+.BR \-c
+Do not attempt to seed the PRNG. This is useful when benchmarking.
+.TP
 .BR \-h ", " \-\-help
 Print a usage message, and exit with success.
 .TP
@@ -45,7 +49,7 @@ contains a set of text-based demonstrations of capabilities from the notcurses l
 .P
 (s)liders—a missing-piece puzzle made up of colorful blocks
 .P
-(b)leachworm—a great Nothing slowly robs the world of color
+(w)hiteworm—a great Nothing slowly robs the world of color
 .P
 (v)iew—images and a video are rendered as text
 .P
diff --git a/src/demo/demo.c b/src/demo/demo.c
index a3b297802..ef59df680 100644
--- a/src/demo/demo.c
+++ b/src/demo/demo.c
@@ -10,6 +10,10 @@
 #include <notcurses.h>
 #include "demo.h"
 
+// ansi terminal definition-4-life
+static const int MIN_SUPPORTED_ROWS = 25;
+static const int MIN_SUPPORTED_COLS = 80;
+
 static const char DEFAULT_DEMO[] = "iemlubgswvpo";
 static char datadir[PATH_MAX] = "/usr/share/notcurses"; // FIXME
 
@@ -46,11 +50,12 @@ struct timespec demodelay = {
 static void
 usage(const char* exe, int status){
   FILE* out = status == EXIT_SUCCESS ? stdout : stderr;
-  fprintf(out, "usage: %s [ -h ] [ -k ] [ -d mult ] [ -f renderfile ] demospec\n", exe);
+  fprintf(out, "usage: %s [ -h ] [ -k ] [ -d mult ] [ -c ] [ -f renderfile ] demospec\n", exe);
   fprintf(out, " -h: this message\n");
   fprintf(out, " -k: keep screen; do not switch to alternate\n");
   fprintf(out, " -d: delay multiplier (float)\n");
   fprintf(out, " -f: render to file in addition to stdout\n");
+  fprintf(out, " -c: constant PRNG seed, useful for benchmarking\n");
   fprintf(out, "all demos are run if no specification is provided\n");
   fprintf(out, " b: run box\n");
   fprintf(out, " e: run eagles\n");
@@ -63,7 +68,7 @@ usage(const char* exe, int status){
   fprintf(out, " s: run shuffle\n");
   fprintf(out, " u: run uniblock\n");
   fprintf(out, " v: run view\n");
-  fprintf(out, " w: run bleachworm\n");
+  fprintf(out, " w: run witherworm\n");
   exit(status);
 }
 
@@ -183,7 +188,7 @@ ext_demos(struct notcurses* nc, const char* demos){
       case 'l': ret = luigi_demo(nc); break;
       case 'v': ret = view_demo(nc); break;
       case 'e': ret = eagle_demo(nc); break;
-      case 'w': ret = bleachworm_demo(nc); break;
+      case 'w': ret = witherworm_demo(nc); break;
       case 'p': ret = panelreel_demo(nc); break;
       default:
         fprintf(stderr, "Unknown demo specification: %c\n", *demos);
@@ -208,13 +213,17 @@ ext_demos(struct notcurses* nc, const char* demos){
 // if it's NULL, there were valid options, but no spec.
 static const char*
 handle_opts(int argc, char** argv, notcurses_options* opts){
+  bool constant_seed = false;
   int c;
   memset(opts, 0, sizeof(*opts));
-  while((c = getopt(argc, argv, "hkd:f:p:")) != EOF){
+  while((c = getopt(argc, argv, "hckd:f:p:")) != EOF){
     switch(c){
       case 'h':
         usage(*argv, EXIT_SUCCESS);
         break;
+      case 'c':
+        constant_seed = true;
+        break;
       case 'k':
         opts->inhibit_alternate_screen = true;
         break;
@@ -244,6 +253,9 @@ handle_opts(int argc, char** argv, notcurses_options* opts){
         usage(*argv, EXIT_FAILURE);
     }
   }
+  if(!constant_seed){
+    srand(time(NULL)); // a classic blunder lol
+  }
   const char* demos = argv[optind];
   return demos;
 }
@@ -252,7 +264,6 @@ handle_opts(int argc, char** argv, notcurses_options* opts){
 int main(int argc, char** argv){
   struct notcurses* nc;
   notcurses_options nopts;
-  struct ncplane* ncp;
   if(!setlocale(LC_ALL, "")){
     fprintf(stderr, "Couldn't set locale based on user preferences\n");
     return EXIT_FAILURE;
@@ -267,8 +278,9 @@ int main(int argc, char** argv){
   if((nc = notcurses_init(&nopts, stdout)) == NULL){
     return EXIT_FAILURE;
   }
-  if((ncp = notcurses_stdplane(nc)) == NULL){
-    fprintf(stderr, "Couldn't get standard plane\n");
+  int dimx, dimy;
+  notcurses_term_dim_yx(nc, &dimy, &dimx);
+  if(dimy < MIN_SUPPORTED_ROWS || dimx < MIN_SUPPORTED_COLS){
     goto err;
   }
   // no one cares about the leaderscreen. 1s max.
@@ -306,6 +318,10 @@ int main(int argc, char** argv){
   return EXIT_SUCCESS;
 
 err:
+  notcurses_term_dim_yx(nc, &dimy, &dimx);
   notcurses_stop(nc);
+  if(dimy < MIN_SUPPORTED_ROWS || dimx < MIN_SUPPORTED_COLS){
+    fprintf(stderr, "At least an 80x25 terminal is required (current: %dx%d)\n", dimx, dimy);
+  }
   return EXIT_FAILURE;
 }
diff --git a/src/demo/demo.h b/src/demo/demo.h
index e68f04d23..be778b762 100644
--- a/src/demo/demo.h
+++ b/src/demo/demo.h
@@ -16,7 +16,7 @@ extern struct timespec demodelay;
 char* find_data(const char* datum);
 
 int unicodeblocks_demo(struct notcurses* nc);
-int bleachworm_demo(struct notcurses* nc);
+int witherworm_demo(struct notcurses* nc);
 int box_demo(struct notcurses* nc);
 int maxcolor_demo(struct notcurses* nc);
 int grid_demo(struct notcurses* nc);
diff --git a/src/demo/bleachworm.c b/src/demo/witherworm.c
similarity index 92%
rename from src/demo/bleachworm.c
rename to src/demo/witherworm.c
index d1c3ca24f..aa680e172 100644
--- a/src/demo/bleachworm.c
+++ b/src/demo/witherworm.c
@@ -160,81 +160,112 @@ lightup_surrounding_cells(struct ncplane* n, const cell* cells, int y, int x){
   return 0;
 }
 
+typedef struct snake {
+  cell lightup[13];
+  int x, y;
+  uint64_t channels;
+  int prevx, prevy;
+} snake;
+
+static void
+init_snake(snake* s, int dimy, int dimx){
+  for(size_t i = 0 ; i < sizeof(s->lightup) / sizeof(*s->lightup) ; ++i){
+    cell_init(&s->lightup[i]);
+  }
+  // start it in the lower center of the screen
+  s->x = (random() % (dimx / 2)) + (dimx / 4);
+  s->y = (random() % (dimy * 2 / 3)) + (dimy / 3);
+  s->channels = 0;
+  channels_set_fg_rgb(&s->channels, 255, 255, 255);
+  channels_set_bg_rgb(&s->channels, 20, 20, 20);
+  s->prevx = 0;
+  s->prevy = 0;
+}
+
+static int
+snakey_top(struct notcurses* nc, snake* s){
+  struct ncplane* n = notcurses_stdplane(nc);
+  get_surrounding_cells(n, s->lightup, s->y, s->x);
+  ncplane_cursor_move_yx(n, s->y, s->x);
+  if(lightup_surrounding_cells(n, s->lightup, s->y, s->x)){
+    return -1;
+  }
+  return 0;
+}
+
+static int
+snakey(struct notcurses* nc, snake* s, int dimy, int dimx, const struct timespec* iterdelay){
+  struct ncplane* n = notcurses_stdplane(nc);
+  int oldy, oldx;
+  clock_nanosleep(CLOCK_MONOTONIC, 0, iterdelay, NULL);
+  cell c = CELL_TRIVIAL_INITIALIZER;
+  do{ // force a move
+    oldy = s->y;
+    oldx = s->x;
+    // FIXME he ought be weighted to avoid light; he's a snake after all
+    int direction = random() % 4;
+    switch(direction){
+      case 0: --s->y; break;
+      case 1: ++s->x; break;
+      case 2: ++s->y; break;
+      case 3: --s->x; break;
+    }
+    // keep him away from the sides due to width irregularities
+    if(s->x < (dimx / 4)){
+      s->x = dimx / 4;
+    }else if(s->x >= dimx * 3 / 4){
+      s->x = dimx * 3 / 4;
+    }
+    if(s->y < 0){
+      s->y = 0;
+    }else if(s->y >= dimy){
+      s->y = dimy - 1;
+    }
+    ncplane_cursor_move_yx(n, s->y, s->x);
+    ncplane_at_cursor(n, &c);
+    // don't allow the snake into the summary zone (test for walls)
+    if(wall_p(n, &c)){
+      s->x = oldx;
+      s->y = oldy;
+    }
+  }while((oldx == s->x && oldy == s->y) || (s->x == s->prevx && s->y == s->prevy));
+  s->prevy = oldy;
+  s->prevx = oldx;
+  cell_release(n, &c);
+  return 0;
+}
+
 // each snake wanders around aimlessly, prohibited from entering the summary
 // section. it ought light up the cells around it; to do this, we keep an array
 // of 13 cells with the original colors, which we tune up for the duration of
 // our colocality (unless they're summary area walls).
 static void *
 snake_thread(void* vnc){
+  const int snakecount = 3; // FIXME base count off area
   struct notcurses* nc = vnc;
   struct ncplane* n = notcurses_stdplane(nc);
-  cell lightup[13];
-  size_t i;
-  for(i = 0 ; i < sizeof(lightup) / sizeof(*lightup) ; ++i){
-    cell_init(&lightup[i]);
-  }
   int dimy, dimx;
   ncplane_dim_yx(n, &dimy, &dimx);
-  int x, y;
-  // start it in the lower center of the screen
-  x = (random() % (dimx / 2)) + (dimx / 4);
-  y = (random() % (dimy / 2)) + (dimy / 2);
-  cell head = CELL_TRIVIAL_INITIALIZER;
-  uint64_t channels = 0;
-  channels_set_fg_rgb(&channels, 255, 255, 255);
-  channels_set_bg_rgb(&channels, 20, 20, 20);
-  cell_prime(n, &head, "א", 0, channels);
-  cell c = CELL_TRIVIAL_INITIALIZER;
+  snake snakes[snakecount];
+  for(int s = 0 ; s < snakecount ; ++s){
+    init_snake(&snakes[s], dimy, dimx);
+  }
   struct timespec iterdelay = { .tv_sec = 0, .tv_nsec = 1000000000ul / 20, };
-  int prevx = 0, prevy = 0;
   while(true){
     pthread_testcancel();
-    get_surrounding_cells(n, lightup, y, x);
-    ncplane_cursor_move_yx(n, y, x);
-    ncplane_at_cursor(n, &c);
-    if(lightup_surrounding_cells(n, lightup, y, x)){
+    for(int s = 0 ; s < snakecount ; ++s){
+      if(snakey_top(nc, &snakes[s])){
+        return NULL;
+      }
+    }
+    if(notcurses_render(nc)){
       return NULL;
     }
-    notcurses_render(nc);
-    int oldy, oldx;
-    clock_nanosleep(CLOCK_MONOTONIC, 0, &iterdelay, NULL);
-    do{ // force a move
-      oldy = y;
-      oldx = x;
-      // FIXME he ought be weighted to avoid light; he's a snake after all
-      int direction = random() % 4;
-      switch(direction){
-        case 0: --y; break;
-        case 1: ++x; break;
-        case 2: ++y; break;
-        case 3: --x; break;
+    for(int s = 0 ; s < snakecount ; ++s){
+      if(snakey(nc, &snakes[s], dimy, dimx, &iterdelay)){
+        return NULL;
       }
-      // keep him away from the sides due to width irregularities
-      if(x < (dimx / 4)){
-        x = dimx / 4;
-      }else if(x >= dimx * 3 / 4){
-        x = dimx * 3 / 4;
-      }
-      if(y < 0){
-        y = 0;
-      }else if(y >= dimy){
-        y = dimy - 1;
-      }
-      ncplane_cursor_move_yx(n, y, x);
-      ncplane_at_cursor(n, &c);
-      // don't allow the snake into the summary zone (test for walls)
-      if(wall_p(n, &c)){
-        x = oldx;
-        y = oldy;
-      }
-    }while((oldx == x && oldy == y) || (x == prevx && y == prevy));
-    prevy = oldy;
-    prevx = oldx;
-  }
-  cell_release(n, &head); // FIXME won't be released when cancelled
-  cell_release(n, &c); // FIXME won't be released when cancelled
-  for(i = 0 ; i < sizeof(lightup) / sizeof(*lightup) ; ++i){
-    cell_release(n, &lightup[i]);
+    }
   }
   return NULL;
 }
@@ -306,7 +337,7 @@ message(struct ncplane* n, int maxy, int maxx, int num, int total,
 }
 
 // Much of this text comes from http://kermitproject.org/utf8.html
-int bleachworm_demo(struct notcurses* nc){
+int witherworm_demo(struct notcurses* nc){
   static const char* strs[] = {
     "Война и мир",
     "Бра́тья Карама́зовы",
@@ -537,8 +568,8 @@ int bleachworm_demo(struct notcurses* nc){
     NULL
   };
   const char** s;
-  const int steps[] = { 0x100, 0x100, 0x40000, 0x10001, };
-  const int starts[] = { 0x004000, 0x000040, 0x010101, 0x400040, };
+  const int steps[] = { 0x10040, 0x100, 0x100, 0x10001, };
+  const int starts[] = { 0x10101, 0x004000, 0x000040, 0x400040, };
 
   struct ncplane* n = notcurses_stdplane(nc);
   size_t i;
diff --git a/src/lib/egcpool.h b/src/lib/egcpool.h
index 22281a3fd..5e97235c6 100644
--- a/src/lib/egcpool.h
+++ b/src/lib/egcpool.h
@@ -230,6 +230,12 @@ egcpool_dump(egcpool* pool){
   pool->poolused = 0;
 }
 
+static inline const char*
+egcpool_extended_gcluster(const egcpool* pool, const cell* c){
+  uint32_t idx = cell_egc_idx(c);
+  return pool->pool + idx;
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/lib/fade.c b/src/lib/fade.c
index 97a6011f0..2ecc4a65d 100644
--- a/src/lib/fade.c
+++ b/src/lib/fade.c
@@ -134,7 +134,6 @@ int ncplane_fadein(ncplane* n, const struct timespec* ts){
         }
       }
     }
-    ncplane_updamage(n);
     notcurses_render(n->nc);
     uint64_t nextwake = (iter + 1) * nanosecs_step + startns;
     struct timespec sleepspec;
@@ -204,7 +203,6 @@ int ncplane_fadeout(ncplane* n, const struct timespec* ts){
         }
       }
     }
-    ncplane_updamage(n);
     cell* c = &n->defcell;
     if(!cell_fg_default_p(c)){
       channels_get_fg_rgb(pp.channels[pp.cols * y], &r, &g, &b);
diff --git a/src/lib/internal.h b/src/lib/internal.h
index c67611377..74e9755f7 100644
--- a/src/lib/internal.h
+++ b/src/lib/internal.h
@@ -51,7 +51,6 @@ typedef struct ncplane {
   uint32_t attrword;    // same deal as in a cell
   void* userptr;        // slot for the user to stick some opaque pointer
   cell defcell;         // cell written anywhere that fb[i].gcluster == 0
-  unsigned char* damage;// damage map, one per row
   struct notcurses* nc; // notcurses object of which we are a part
 } ncplane;
 
@@ -80,20 +79,32 @@ typedef struct ncvisual {
 
 typedef struct notcurses {
   pthread_mutex_t lock;
-  int ttyfd;      // file descriptor for controlling tty, from opts->ttyfp
-  FILE* ttyfp;    // FILE* for controlling tty, from opts->ttyfp
-  FILE* ttyinfp;  // FILE* for processing input
-  unsigned char* damage;   // damage map (row granularity)
+  ncplane* top;   // the contents of our topmost plane (initially entire screen)
+  ncplane* stdscr;// aliases some plane from the z-buffer, covers screen
+
+  // we keep a copy of the last rendered frame. this facilitates O(1)
+  // notcurses_at_yx() and O(1) damage detection (at the cost of some memory).
+  cell* lastframe;// last rendered framebuffer, NULL until first render
+  int lfdimx;     // dimensions of lastframe, unchanged by screen resize
+  int lfdimy;     // lfdimx/lfdimy are 0 until first render
+  egcpool pool;   // duplicate EGCs into this pool
+
+  // we assemble the encoded output in a POSIX memstream, and keep it around
+  // between uses. this could be a problem if it ever tremendously spiked, but
+  // that's a highly unlikely situation.
   char* mstream;  // buffer for rendering memstream, see open_memstream(3)
   FILE* mstreamfp;// FILE* for rendering memstream
   size_t mstrsize;// size of rendering memstream
-  int colors;     // number of colors usable for this screen
+
   ncstats stats;  // some statistics across the lifetime of the notcurses ctx
   ncstats stashstats; // cumulative stats, unaffected by notcurses_reset_stats()
+
   // We verify that some terminfo capabilities exist. These needn't be checked
   // before further use; just use tiparm() directly.
+  int colors;     // number of colors terminfo reported usable for this screen
   char* cup;      // move cursor
-  bool RGBflag;   // terminfo-reported "RGB" flag for 24bpc directcolor
+  char* cuf;      // move n cells right
+  char* cuf1;     // move 1 cell right
   char* civis;    // hide cursor
   // These might be NULL, and we can more or less work without them. Check!
   char* clearscr; // erase screen and home cursor
@@ -117,12 +128,15 @@ typedef struct notcurses {
   char* italoff;  // CELL_STYLE_ITALIC (disable)
   char* smkx;     // enter keypad transmit mode (keypad_xmit)
   char* rmkx;     // leave keypad transmit mode (keypad_local)
-  struct termios tpreserved; // terminal state upon entry
-  bool suppress_banner; // from notcurses_options
+  bool RGBflag;   // terminfo-reported "RGB" flag for 24bpc directcolor
   bool CCCflag;   // terminfo-reported "CCC" flag for palette set capability
-  ncplane* top;   // the contents of our topmost plane (initially entire screen)
-  ncplane* stdscr;// aliases some plane from the z-buffer, covers screen
+
+  int ttyfd;      // file descriptor for controlling tty, from opts->ttyfp
+  FILE* ttyfp;    // FILE* for controlling tty, from opts->ttyfp
+  FILE* ttyinfp;  // FILE* for processing input
   FILE* renderfp; // debugging FILE* to which renderings are written
+  struct termios tpreserved; // terminal state upon entry
+  bool suppress_banner; // from notcurses_options
   unsigned char inputbuf[BUFSIZ];
   // we keep a wee ringbuffer of input queued up for delivery. if
   // inputbuf_occupied == sizeof(inputbuf), there is no room. otherwise, data
@@ -161,19 +175,6 @@ fbcellidx(const ncplane* n, int row, int col){
   return row * n->lenx + col;
 }
 
-// set all elements of a damage map true or false
-static inline void
-flash_damage_map(unsigned char* damage, int count, bool val){
-  if(val){
-    memset(damage, 0xff, sizeof(*damage) * count);
-  }else{
-    memset(damage, 0, sizeof(*damage) * count);
-  }
-}
-
-// mark all lines of the notcurses object touched by this plane as damaged
-void ncplane_updamage(ncplane* n);
-
 // For our first attempt, O(1) uniform conversion from 8-bit r/g/b down to
 // ~2.4-bit 6x6x6 cube + greyscale (assumed on entry; I know no way to
 // even semi-portably recover the palette) proceeds via: map each 8-bit to
@@ -223,8 +224,7 @@ term_emit(const char* name __attribute__ ((unused)), const char* seq,
 
 static inline const char*
 extended_gcluster(const ncplane* n, const cell* c){
-  uint32_t idx = cell_egc_idx(c);
-  return n->pool.pool + idx;
+  return egcpool_extended_gcluster(&n->pool, c);
 }
 
 #define NANOSECS_IN_SEC 1000000000
diff --git a/src/lib/libav.c b/src/lib/libav.c
index 3b3ad8aa3..edd2ecafb 100644
--- a/src/lib/libav.c
+++ b/src/lib/libav.c
@@ -354,7 +354,6 @@ int ncvisual_render(const ncvisual* ncv, int begy, int begx, int leny, int lenx)
       cell_release(ncv->ncp, &c);
     }
   }
-  flash_damage_map(ncv->ncp->damage + ncv->placey, y - ncv->placey, true);
   return 0;
 }
 
diff --git a/src/lib/notcurses.c b/src/lib/notcurses.c
index f4f90f8bc..3b570529e 100644
--- a/src/lib/notcurses.c
+++ b/src/lib/notcurses.c
@@ -262,9 +262,7 @@ term_verify_seq(char** gseq, const char* name){
 static void
 free_plane(ncplane* p){
   if(p){
-    ncplane_updamage(p);
     egcpool_dump(&p->pool);
-    free(p->damage);
     free(p->fb);
     free(p);
   }
@@ -287,13 +285,6 @@ ncplane_create(notcurses* nc, int rows, int cols, int yoff, int xoff){
     return NULL;
   }
   memset(p->fb, 0, fbsize);
-  p->damage = malloc(sizeof(*p->damage) * rows);
-  if(p->damage == NULL){
-    free(p->fb);
-    free(p);
-    return NULL;
-  }
-  flash_damage_map(p->damage, rows, false);
   p->userptr = NULL;
   p->leny = rows;
   p->lenx = cols;
@@ -341,8 +332,7 @@ ncplane* notcurses_newplane(notcurses* nc, int rows, int cols,
   return n;
 }
 
-// can be used on stdscr, unlike ncplane_resize() which prohibits it. sets all
-// members of the plane's damage map to damaged.
+// can be used on stdscr, unlike ncplane_resize() which prohibits it.
 static int
 ncplane_resize_internal(ncplane* n, int keepy, int keepx, int keepleny,
                        int keeplenx, int yoff, int xoff, int ylen, int xlen){
@@ -377,19 +367,6 @@ ncplane_resize_internal(ncplane* n, int keepy, int keepx, int keepleny,
   if(fb == NULL){
     return -1;
   }
-  // if we're the standard plane, the updamage can be charged at the end. it's
-  // unsafe to call now anyway, because if we shrank, the notcurses damage map
-  // has already been shrunk down
-  if(n != n->nc->stdscr){
-    ncplane_updamage(n); // damage any lines we were on
-  }
-  unsigned char* tmpdamage;
-  if((tmpdamage = realloc(n->damage, sizeof(*n->damage) * ylen)) == NULL){
-    free(fb);
-    return -1;
-  }
-  n->damage = tmpdamage;
-  flash_damage_map(n->damage, ylen, true);
   // update the cursor, if it would otherwise be off-plane
   if(n->y >= ylen){
     n->y = ylen - 1;
@@ -411,7 +388,6 @@ ncplane_resize_internal(ncplane* n, int keepy, int keepx, int keepleny,
     n->lenx = xlen;
     n->leny = ylen;
     free(preserved);
-    ncplane_updamage(n); // damage any lines we're now on
     return 0;
   }
   // we currently have maxy rows of maxx cells each. we will be keeping rows
@@ -448,7 +424,6 @@ ncplane_resize_internal(ncplane* n, int keepy, int keepx, int keepleny,
   n->lenx = xlen;
   n->leny = ylen;
   free(preserved);
-  ncplane_updamage(n); // damage any lines we're now on
   return 0;
 }
 
@@ -489,19 +464,6 @@ int notcurses_resize(notcurses* n, int* rows, int* cols){
   if(keepx > oldcols){
     keepx = oldcols;
   }
-  unsigned char* tmpdamage;
-  if((tmpdamage = malloc(sizeof(*n->damage) * *rows)) == NULL){
-    return -1;
-  }
-  if(oldcols < *cols){ // all are busted if rows got bigger
-    free(n->damage);
-    flash_damage_map(tmpdamage, *rows, true);
-  }else if(oldrows <= *rows){ // new rows are pre-busted, old are straight
-    memcpy(tmpdamage, n->damage, oldrows * sizeof(*tmpdamage));
-    flash_damage_map(tmpdamage + oldrows, *rows - oldrows, true);
-    free(n->damage);
-  }
-  n->damage = tmpdamage;
   if(ncplane_resize_internal(n->stdscr, 0, 0, keepy, keepx, 0, 0, *rows, *cols)){
     return -1;
   }
@@ -593,6 +555,8 @@ interrogate_terminfo(notcurses* nc, const notcurses_options* opts){
   term_verify_seq(&nc->clearscr, "clear");
   term_verify_seq(&nc->cleareol, "el");
   term_verify_seq(&nc->clearbol, "el1");
+  term_verify_seq(&nc->cuf, "cuf"); // n non-destructive spaces
+  term_verify_seq(&nc->cuf1, "cuf1"); // non-destructive space
   if(prep_special_keys(nc)){
     return -1;
   }
@@ -725,6 +689,10 @@ notcurses* notcurses_init(const notcurses_options* opts, FILE* outfp){
   ret->ttyinfp = stdin; // FIXME
   ret->mstream = NULL;
   ret->mstrsize = 0;
+  ret->lastframe = NULL;
+  ret->lfdimy = 0;
+  ret->lfdimx = 0;
+  egcpool_init(&ret->pool);
   if(make_nonblocking(ret->ttyinfp)){
     free(ret);
     return NULL;
@@ -783,16 +751,10 @@ notcurses* notcurses_init(const notcurses_options* opts, FILE* outfp){
     free_plane(ret->top);
     goto err;
   }
-  if((ret->damage = malloc(sizeof(*ret->damage) * ret->stdscr->leny)) == NULL){
-    free_plane(ret->top);
-    goto err;
-  }
   if((ret->mstreamfp = open_memstream(&ret->mstream, &ret->mstrsize)) == NULL){
-    free(ret->damage);
     free_plane(ret->top);
     goto err;
   }
-  flash_damage_map(ret->damage, ret->stdscr->leny, false);
   // term_emit("clear", ret->clear, ret->ttyfp, false);
   ret->suppress_banner = opts->suppress_bannner;
   if(!opts->suppress_bannner){
@@ -856,10 +818,11 @@ int notcurses_stop(notcurses* nc){
       nc->top = p->z;
       free_plane(p);
     }
-    free(nc->damage);
     if(nc->mstreamfp){
       fclose(nc->mstreamfp);
     }
+    egcpool_dump(&nc->pool);
+    free(nc->lastframe);
     free(nc->mstream);
     input_free_esctrie(&nc->inputescapes);
     ret |= pthread_mutex_destroy(&nc->lock);
@@ -955,7 +918,6 @@ int ncplane_set_default(ncplane* ncp, const cell* c){
   if(ret < 0){
     return -1;
   }
-  ncplane_updamage(ncp);
   return ret;
 }
 
@@ -991,7 +953,6 @@ int ncplane_move_above_unsafe(ncplane* restrict n, ncplane* restrict above){
   *an = n->z; // splice n out
   n->z = above; // attach above below n
   *aa = n; // spline n in above
-  ncplane_updamage(n); // conservative (we might not actually be visible)
   return 0;
 }
 
@@ -1004,7 +965,6 @@ int ncplane_move_below_unsafe(ncplane* restrict n, ncplane* restrict below){
   *an = n->z; // splice n out
   n->z = below->z; // reattach subbelow list to n
   below->z = n; // splice n in below
-  ncplane_updamage(n); // conservative (we might not actually be visible)
   return 0;
 }
 
@@ -1016,7 +976,6 @@ int ncplane_move_top(ncplane* n){
   *an = n->z; // splice n out
   n->z = n->nc->top;
   n->nc->top = n;
-  ncplane_updamage(n);
   return 0;
 }
 
@@ -1032,7 +991,6 @@ int ncplane_move_bottom(ncplane* n){
   }
   *an = n;
   n->z = NULL;
-  ncplane_updamage(n);
   return 0;
 }
 
@@ -1062,24 +1020,6 @@ ncplane_cursor_stuck(const ncplane* n){
   return (n->x == n->lenx && n->y == n->leny);
 }
 
-int cell_duplicate(ncplane* n, cell* targ, const cell* c){
-  cell_release(n, targ);
-  targ->attrword = c->attrword;
-  targ->channels = c->channels;
-  if(cell_simple_p(c)){
-    targ->gcluster = c->gcluster;
-    return !!c->gcluster;
-  }
-  size_t ulen = strlen(extended_gcluster(n, c));
-// fprintf(stderr, "[%s] (%zu)\n", extended_gcluster(n, c), strlen(extended_gcluster(n, c)));
-  int eoffset = egcpool_stash(&n->pool, extended_gcluster(n, c), ulen);
-  if(eoffset < 0){
-    return -1;
-  }
-  targ->gcluster = eoffset + 0x80;
-  return ulen;
-}
-
 static inline void
 cell_set_wide(cell* c){
   c->channels |= CELL_WIDEASIAN_MASK;
@@ -1129,7 +1069,6 @@ int ncplane_putc(ncplane* n, const cell* c){
       cell_release(n, candidate);
     }
   }
-  n->damage[n->y] = true;
   advance_cursor(n, cols);
   ncplane_unlock(n);
   return cols;
@@ -1174,13 +1113,6 @@ int ncplane_cursor_at(const ncplane* n, cell* c, char** gclust){
   return 0;
 }
 
-void cell_release(ncplane* n, cell* c){
-  if(!cell_simple_p(c)){
-    egcpool_release(&n->pool, cell_egc_idx(c));
-    c->gcluster = 0; // don't subject ourselves to double-release problems
-  }
-}
-
 int cell_load(ncplane* n, cell* c, const char* gcluster){
   cell_release(n, c);
   int bytes;
@@ -1536,33 +1468,12 @@ int ncplane_box(ncplane* n, const cell* ul, const cell* ur,
   return 0;
 }
 
-// mark all lines of the notcurses object touched by this plane as damaged
-void ncplane_updamage(ncplane* n){
-  int drangelow = n->absy;
-  int drangehigh = n->absy + n->leny;
-  if(drangehigh > n->nc->stdscr->leny){
-    drangehigh = n->nc->stdscr->leny;
-  }
-  if(drangelow < n->nc->stdscr->absy){
-    drangelow = n->nc->stdscr->absy;
-  }
-  if(drangelow > n->nc->stdscr->absy + n->nc->stdscr->leny - 1){
-    drangelow = n->nc->stdscr->absy + n->nc->stdscr->leny - 1;
-  }
-  flash_damage_map(n->nc->damage + drangelow, drangehigh - drangelow, true);
-}
-
 int ncplane_move_yx(ncplane* n, int y, int x){
   if(n == n->nc->stdscr){
     return -1;
   }
-  ncplane_updamage(n); // damage any lines we are currently on
-  bool movedy = n->absy != y;
   n->absy = y;
   n->absx = x;
-  if(movedy){
-    ncplane_updamage(n);
-  }
   return 0;
 }
 
@@ -1602,7 +1513,6 @@ void ncplane_erase(ncplane* n){
   egcpool_init(&n->pool);
   cell_load(n, &n->defcell, egc);
   free(egc);
-  ncplane_updamage(n);
   ncplane_unlock(n);
 }
 
diff --git a/src/lib/render.c b/src/lib/render.c
index dff9fc0bc..e8eef21f9 100644
--- a/src/lib/render.c
+++ b/src/lib/render.c
@@ -1,4 +1,5 @@
 #include <ctype.h>
+#include <limits.h>
 #include <unistd.h>
 #include <sys/poll.h>
 #include "internal.h"
@@ -98,24 +99,33 @@ prep_optimized_palette(notcurses* nc, FILE* out __attribute__ ((unused))){
   return 0;
 }
 
-/*
 // reshape the shadow framebuffer to match the stdplane's dimensions, throwing
 // away the old one.
 static int
 reshape_shadow_fb(notcurses* nc){
-  const size_t size = sizeof(nc->shadowbuf) * nc->stdscr->leny * nc->stdscr->lenx;
-  cell* fb = malloc(size);
+  if(nc->lfdimx == nc->stdscr->lenx && nc->lfdimy == nc->stdscr->leny){
+    return 0; // no change
+  }
+  const size_t size = sizeof(*nc->lastframe) * nc->stdscr->leny * nc->stdscr->lenx;
+  cell* fb = realloc(nc->lastframe, size);
   if(fb == NULL){
+    free(nc->lastframe);
+    nc->lastframe = NULL;
+    nc->lfdimx = 0;
+    nc->lfdimy = 0;
     return -1;
   }
-  free(nc->shadowbuf);
-  nc->shadowbuf = fb;
-  nc->shadowy = nc->stdscr->leny;
-  nc->shadowx = nc->stdscr->lenx;
+  nc->lastframe = fb;
+  // FIXME more memset()tery than we need, both wasting work and wrecking
+  // damage detection for the upcoming render
+  memset(nc->lastframe, 0, size);
+  nc->lastframe = fb;
+  nc->lfdimy = nc->stdscr->leny;
+  nc->lfdimx = nc->stdscr->lenx;
   memset(fb, 0, size);
+  egcpool_dump(&nc->pool);
   return 0;
 }
-*/
 
 // Find the topmost cell for this coordinate by walking down the z-buffer,
 // looking for an intersecting ncplane. Once we've found one, check it for
@@ -135,9 +145,8 @@ reshape_shadow_fb(notcurses* nc){
 // whichever one occurs at the top with a non-transparent α (α < 3). To effect
 // tail recursion, though, we instead write first, and then recurse, blending
 // as we descend. α <= 0 is opaque. α >= 3 is fully transparent.
-static ncplane*
-dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha,
-                 bool* damage){
+static inline ncplane*
+dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha){
   // once we decide on our glyph, it cannot be changed by anything below, so
   // lock in this plane for the actual cell return.
   ncplane* glyphplane = NULL;
@@ -164,20 +173,12 @@ dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha,
               c->attrword = vis->attrword;
               cell_set_fchannel(c, cell_get_fchannel(vis)); // FIXME blend it in
               falpha -= (CELL_ALPHA_TRANSPARENT - nalpha); // FIXME blend it in
-              if(p->damage[poffy]){
-                *damage = true;
-                p->damage[poffy] = false;
-              }
             }
           }
         }
         if(balpha > 0 && (nalpha = cell_get_bg_alpha(vis)) < CELL_ALPHA_TRANSPARENT){
           cell_set_bchannel(c, cell_get_bchannel(vis)); // FIXME blend it in
           balpha -= (CELL_ALPHA_TRANSPARENT - nalpha);
-          if(p->damage[poffy]){
-            *damage = true;
-            p->damage[poffy] = false;
-          }
         }
         if((falpha <= 0 && balpha <= 0) || !p->z){ // done!
           return glyphplane ? glyphplane : p;
@@ -191,10 +192,9 @@ dig_visible_cell(cell* c, int y, int x, ncplane* p, int falpha, int balpha,
 }
 
 static inline ncplane*
-visible_cell(cell* c, int y, int x, ncplane* n, bool* damage){
+visible_cell(cell* c, int y, int x, ncplane* n){
   cell_init(c);
-  return dig_visible_cell(c, y, x, n, CELL_ALPHA_TRANSPARENT,
-                          CELL_ALPHA_TRANSPARENT, damage);
+  return dig_visible_cell(c, y, x, n, CELL_ALPHA_TRANSPARENT, CELL_ALPHA_TRANSPARENT);
 }
 
 // write the cell's UTF-8 grapheme cluster to the provided FILE*. returns the
@@ -385,6 +385,74 @@ term_fg_rgb8(notcurses* nc, FILE* out, unsigned r, unsigned g, unsigned b){
   return 0;
 }
 
+static inline void
+pool_release(egcpool* pool, cell* c){
+  if(!cell_simple_p(c)){
+    egcpool_release(pool, cell_egc_idx(c));
+    c->gcluster = 0; // don't subject ourselves to double-release problems
+  }
+}
+
+void cell_release(ncplane* n, cell* c){
+  pool_release(&n->pool, c);
+}
+
+// Duplicate one cell onto another, possibly crossing ncplanes.
+static inline int
+cell_duplicate_far(egcpool* tpool, cell* targ, const ncplane* splane, const cell* c){
+  pool_release(tpool, targ);
+  targ->attrword = c->attrword;
+  targ->channels = c->channels;
+  if(cell_simple_p(c)){
+    targ->gcluster = c->gcluster;
+    return !!c->gcluster;
+  }
+  size_t ulen = strlen(extended_gcluster(splane, c));
+// fprintf(stderr, "[%s] (%zu)\n", extended_gcluster(n, c), strlen(extended_gcluster(n, c)));
+  int eoffset = egcpool_stash(tpool, extended_gcluster(splane, c), ulen);
+  if(eoffset < 0){
+    return -1;
+  }
+  targ->gcluster = eoffset + 0x80;
+  return ulen;
+}
+
+// Duplicate one cell onto another when they share a plane. Convenience wrapper.
+int cell_duplicate(ncplane* n, cell* targ, const cell* c){
+  return cell_duplicate_far(&n->pool, targ, n, c);
+}
+
+// the heart of damage detection. compare two cells (from two different planes)
+// for equality. if they are equal, return 0. otherwise, dup the second onto
+// the first and return non-zero.
+static int
+cellcmp_and_dupfar(egcpool* dampool, cell* damcell, const ncplane* srcplane,
+                   const cell* srccell){
+  if(damcell->attrword == srccell->attrword){
+    if(damcell->channels == srccell->channels){
+      bool damsimple = cell_simple_p(damcell);
+      bool srcsimple = cell_simple_p(srccell);
+      if(damsimple == srcsimple){
+        if(damsimple){
+          if(damcell->gcluster == srccell->gcluster){
+            return 0; // simple match
+          }
+        }else{
+          const char* damegc = egcpool_extended_gcluster(dampool, damcell);
+          const char* srcegc = extended_gcluster(srcplane, srccell);
+          assert(strcmp(damegc, "三体"));
+          assert(strcmp(srcegc, "三体"));
+          if(strcmp(damegc, srcegc) == 0){
+            return 0; // EGC match
+          }
+        }
+      }
+    }
+  }
+  cell_duplicate_far(dampool, damcell, srcplane, srccell);
+  return 1;
+}
+
 static inline int
 notcurses_render_internal(notcurses* nc){
   int ret = 0;
@@ -405,40 +473,55 @@ notcurses_render_internal(notcurses* nc){
   // cells and the current cell uses no defaults, or if both the current and
   // the last used both defaults.
   bool fgelidable = false, bgelidable = false, defaultelidable = false;
-  /*if(nc->stdscr->leny != nc->shadowy || nc->stdscr->lenx != nc->shadowx){
-    reshape_shadow_fb(nc);
-  }*/
+  // if this fails, struggle bravely on. we can live without a lastframe.
+  reshape_shadow_fb(nc);
   for(y = 0 ; y < nc->stdscr->leny ; ++y){
-    bool linedamaged = false; // have we repositioned the cursor to start line?
-    bool newdamage = nc->damage[y];
-// fprintf(stderr, "nc->damage[%d] (%p) = %u\n", y, nc->damage + y, nc->damage[y]);
-    if(newdamage){
-      nc->damage[y] = 0;
-    }
-    // move to the beginning of the line, in case our accounting was befouled
-    // by wider- (or narrower-) than-reported characters
+    // how many characters have we elided? it's not worthwhile to invoke a
+    // cursor movement with cup if we only elided one or two. set to INT_MAX
+    // whenever we're on a new line.
+    int needmove = INT_MAX;
     for(x = 0 ; x < nc->stdscr->lenx ; ++x){
       unsigned r, g, b, br, bg, bb;
       ncplane* p;
       cell c; // no need to initialize
-      p = visible_cell(&c, y, x, nc->top, &newdamage);
-      assert(p);
+      p = visible_cell(&c, y, x, nc->top);
       // don't try to print a wide character on the last column; it'll instead
       // be printed on the next line. they probably shouldn't be admitted, but
       // we can end up with one due to a resize.
+      // FIXME but...print what, exactly, instead?
       if((x + 1 >= nc->stdscr->lenx && cell_double_wide_p(&c))){
-        continue;
+        continue; // needmove will be reset as we restart the line
       }
-      if(!linedamaged){
-        if(newdamage){
-          term_emit("cup", tiparm(nc->cup, y, x), out, false);
-          nc->stats.cellelisions += x;
-          nc->stats.cellemissions += (nc->stdscr->lenx - x);
-          linedamaged = true;
-        }else{
+      // lastframe has already been sized to match the current size, so no need
+      // to check whether we're within its bounds. just check the cell.
+      if(nc->lastframe){
+        cell* oldcell = &nc->lastframe[fbcellidx(nc->stdscr, y, x)];
+        if(cellcmp_and_dupfar(&nc->pool, oldcell, p, &c) == 0){
+          // no need to emit a cell; what we rendered appears to already be
+          // here. no updates are performed to elision state nor lastframe.
+          ++nc->stats.cellelisions;
+          if(needmove < INT_MAX){
+            ++needmove;
+          }
+          if(cell_double_wide_p(&c)){
+            if(needmove < INT_MAX){
+              ++needmove;
+            }
+            ++nc->stats.cellelisions;
+            ++x;
+          }
           continue;
         }
       }
+      ++nc->stats.cellemissions;
+      if(needmove > 8){ // FIXME cuf and cuf1 aren't guaranteed!
+        term_emit("cup", tiparm(nc->cup, y, x), out, false);
+      }else if(needmove > 1){
+        term_emit("cuf", tiparm(nc->cuf, needmove), out, false);
+      }else if(needmove){
+        term_emit("cuf1", tiparm(nc->cuf1), out, false);
+      }
+      needmove = 0;
       // set the style. this can change the color back to the default; if it
       // does, we need update our elision possibilities.
       bool normalized;
@@ -498,9 +581,6 @@ notcurses_render_internal(notcurses* nc){
         ++x;
       }
     }
-    if(linedamaged == false){
-      nc->stats.cellelisions += x;
-    }
   }
   ret |= fflush(out);
   fflush(nc->ttyfp);