diff --git a/2021/day11/aoc-c.c b/2021/day11/aoc-c.c
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+/* aoc-c.c: Advent of Code 2021, day 11 parts 1 & 2
+ *
+ * Copyright (C) 2021 Bruno Raoult ("br")
+ * Licensed under the GNU General Public License v3.0 or later.
+ * Some rights reserved. See COPYING.
+ *
+ * You should have received a copy of the GNU General Public License along with this
+ * program. If not, see <https://www.gnu.org/licenses/gpl-3.0-standalone.html>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later <https://spdx.org/licenses/GPL-3.0-or-later.html>
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <malloc.h>
+
+#include "debug.h"
+#include "bits.h"
+
+#define MAX_GRID 10
+
+static int grid[MAX_GRID][MAX_GRID];
+static int gsize;                                 /* grid size */
+
+static int stack[MAX_GRID * MAX_GRID];            /* flashing stack */
+static int nstack;                                /* current stack size */
+
+#define loop_for_grid(l, c, val)                \
+    for (l = 1; l <= gsize; ++l)                \
+        for (c = 1; c <= gsize; ++c)            \
+            val = grid[l][c];
+
+#define VALID(x, y)    ((x) >= 0 && (y) >= 0 && (x) < gsize && (y) < gsize)
+
+inline static int push(int l, int c)
+{
+    return ((stack[nstack++] = l * gsize + c));
+}
+
+inline static int pop()
+{
+    if (!nstack)
+        return -1;
+    return stack[--nstack];
+}
+
+static void print_grid(int (*arr)[MAX_GRID], int step)
+{
+    log_f(3, "step = %d\n", step);
+    for (int l = 0; l < gsize; ++l) {
+        for (int c = 0; c < gsize; ++c) {
+            log(3, "%2d ", arr[l][c]);
+        }
+        log(3, "\n");
+    }
+}
+
+inline static int inc_cell(int l, int c)
+{
+    if (VALID(l, c) && ++grid[l][c] == 10) {
+        push(l, c);
+        return 1;
+    }
+    return 0;
+}
+
+void flash_grid(int (*arr)[MAX_GRID], int step)
+{
+    log_f(3, "step = %d\n", step);
+    for (int l = 1; l <= gsize; ++l) {
+        for (int c = 1; c <= gsize; ++c) {
+            log(3, "%2d ", arr[l][c]);
+        }
+        log(3, "\n");
+    }
+}
+
+/* run 1 step */
+static void step(int nsteps)
+{
+    int vpop;
+    int flash = 0;
+    //int flashed[MAX_GRID][MAX_GRID];
+
+    //init_grid(flashed);
+    /* create an empty grid */
+
+    for (int step = 1; step <= nsteps; ++step) {
+        /* 1) increase all energy levels */
+        for (int l = 0; l < gsize; ++l) {
+            for (int c = 0; c < gsize; ++c) {
+                flash += inc_cell(l, c);
+            }
+        }
+
+        /* 2) perform BFS until stack is empty */
+        while ((vpop = pop()) !=  -1) {
+            int pl = vpop / gsize, pc = vpop % gsize;
+            for (int l = -1; l <= 1; ++l)
+                for (int c = -1; c <= 1; ++c)
+                    flash += inc_cell(pl + l, pc + c);
+        }
+        //print_grid(grid, 1);
+
+        /* 3) cleanup flashed cells */
+        for (int l = 0; l < gsize; ++l) {
+            for (int c = 0; c < gsize; ++c) {
+                if (grid[l][c] > 9)
+                    grid[l][c] = 0;
+            }
+        }
+        print_grid(grid, step);
+        printf("step = %d flashed = %d\n", step, flash);
+    }
+}
+
+static s64 doit(int part)
+{
+    int l = 0, c;
+    ssize_t len;
+    size_t alloc = 0;
+    char *buf = NULL, *p;
+
+    len = getline(&buf, &alloc, stdin);
+    gsize = len - 1;
+    for (l = 0; l < gsize; ++l) {
+        buf[gsize] = 0;
+        log_f(10, "len = %d str = [%s]\n", grid, buf);
+        for (c = 0, p = buf; c < gsize; c++, p++)
+            grid[l][c] = *p - '0';
+        getline(&buf, &alloc, stdin);
+    }
+    free(buf);
+    print_grid(grid, 0);
+    step(100);
+    print_grid(grid, 1);
+    return part;
+}
+
+static int usage(char *prg)
+{
+    fprintf(stderr, "Usage: %s [-d debug_level] [-p part]\n", prg);
+    return 1;
+}
+
+int main(int ac, char **av)
+{
+    int opt;
+    u32 exercise = 1, debug = 1;
+    s64 res;
+
+    while ((opt = getopt(ac, av, "d:p:")) != -1) {
+        switch (opt) {
+            case 'd':
+                debug = atoi(optarg);
+                break;
+            case 'p':                             /* 1 or 2 */
+                exercise = atoi(optarg);
+                if (exercise < 1 || exercise > 2)
+                    return usage(*av);
+                break;
+            default:
+                return usage(*av);
+        }
+    }
+    debug_level_set(debug);
+    if (optind < ac)
+        return usage(*av);
+
+    //init_borders();
+    res = doit(exercise);
+    printf("%s : res=%lu\n", *av, res);
+    exit (0);
+}