diff --git a/Makefile b/Makefile
index 195aed7..cfc5da2 100644
--- a/Makefile
+++ b/Makefile
@@ -17,6 +17,7 @@ CFLAGS += -Wno-error=unused-but-set-variable
 CFLAGS += -Wno-error=unused-variable
 CFLAGS += -I$(MAKEFILE_PATH)/include
 CFLAGS += -I$(MAKEFILE_PATH)
+CFLAGS += -I$(dir $(GLFW))../include
 CXXFLAGS += -fno-exceptions
 LDFLAGS += -nostdlib++ -lm -static-libgcc
 ifeq ($(OS),Windows_NT)
diff --git a/include/opengl.h b/include/opengl.h
index 68348d5..f60157e 100644
--- a/include/opengl.h
+++ b/include/opengl.h
@@ -24,6 +24,10 @@ uint make_texture(const void * data,
 
 uint make_framebuffer(uint * texture, int length);
 
+void debug_framebuffer(int width, int height,
+                       int elements,
+                       const char * format);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/include/solution/2025/04/count.fs.glsl.h b/include/solution/2025/04/count.fs.glsl.h
new file mode 100644
index 0000000..e56a3d3
--- /dev/null
+++ b/include/solution/2025/04/count.fs.glsl.h
@@ -0,0 +1,19 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_src_solution_2025_04_count_fs_glsl_start __asm("_binary_src_solution_2025_04_count_fs_glsl_start");
+extern uint32_t _binary_src_solution_2025_04_count_fs_glsl_end __asm("_binary_src_solution_2025_04_count_fs_glsl_end");
+extern uint32_t _binary_src_solution_2025_04_count_fs_glsl_size __asm("_binary_src_solution_2025_04_count_fs_glsl_size");
+
+#define src_solution_2025_04_count_fs_glsl_start ((const char *)&_binary_src_solution_2025_04_count_fs_glsl_start)
+#define src_solution_2025_04_count_fs_glsl_end ((const char *)&_binary_src_solution_2025_04_count_fs_glsl_end)
+#define src_solution_2025_04_count_fs_glsl_size (src_solution_2025_04_count_fs_glsl_end - src_solution_2025_04_count_fs_glsl_start)
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/include/solution/2025/04/simulate.fs.glsl.h b/include/solution/2025/04/simulate.fs.glsl.h
new file mode 100644
index 0000000..0d90c8f
--- /dev/null
+++ b/include/solution/2025/04/simulate.fs.glsl.h
@@ -0,0 +1,19 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_src_solution_2025_04_simulate_fs_glsl_start __asm("_binary_src_solution_2025_04_simulate_fs_glsl_start");
+extern uint32_t _binary_src_solution_2025_04_simulate_fs_glsl_end __asm("_binary_src_solution_2025_04_simulate_fs_glsl_end");
+extern uint32_t _binary_src_solution_2025_04_simulate_fs_glsl_size __asm("_binary_src_solution_2025_04_simulate_fs_glsl_size");
+
+#define src_solution_2025_04_simulate_fs_glsl_start ((const char *)&_binary_src_solution_2025_04_simulate_fs_glsl_start)
+#define src_solution_2025_04_simulate_fs_glsl_end ((const char *)&_binary_src_solution_2025_04_simulate_fs_glsl_end)
+#define src_solution_2025_04_simulate_fs_glsl_size (src_solution_2025_04_simulate_fs_glsl_end - src_solution_2025_04_simulate_fs_glsl_start)
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/src/main.c b/src/main.c
index 1ff091d..5e6b520 100644
--- a/src/main.c
+++ b/src/main.c
@@ -29,6 +29,7 @@ const float triangle_array[] = {
 
 extern void solution_2025_01(unsigned int vertex_array);
 extern void solution_2025_03(unsigned int vertex_array);
+extern void solution_2025_04(unsigned int vertex_array);
 
 int main()
 {
@@ -56,6 +57,13 @@ int main()
 
   glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
 
+  int n;
+  glGetIntegerv(GL_NUM_EXTENSIONS, &n);
+  for (int i=0; i<n; i++)  {
+    const char* extension = (const char*)glGetStringi(GL_EXTENSIONS, i);
+    //printf("Ext %d: %s\n", i, extension);
+  }
+
   //////////////////////////////////////////////////////////////////////
   // buffers
   //////////////////////////////////////////////////////////////////////
@@ -88,7 +96,8 @@ int main()
       glfwSetWindowShouldClose(window, true);
 
     //solution_2025_01(vertex_array);
-    solution_2025_03(vertex_array);
+    //solution_2025_03(vertex_array);
+    solution_2025_04(vertex_array);
 
     break;
 
diff --git a/src/opengl.c b/src/opengl.c
index 15bce81..4ad1b6e 100644
--- a/src/opengl.c
+++ b/src/opengl.c
@@ -85,11 +85,14 @@ uint make_texture(const void * data,
   glGenTextures(1, &texture);
 
   glBindTexture(GL_TEXTURE_2D, texture);
-  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
-  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
+  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
 
+  float color[3] = {0.0, 0.0, 0.0};
+  glTexParameterfv(GL_TEXTURE_2D, GL_TEXTURE_BORDER_COLOR, color);
+
   glTexImage2D(GL_TEXTURE_2D, 0, internalformat, width, height, 0, format, type, data);
 
   glBindTexture(GL_TEXTURE_2D, 0);
@@ -107,8 +110,33 @@ uint make_framebuffer(uint * texture, int length)
   }
 
   GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
+  //printf("glCheckFramebufferStatus: %x\n", status);
   assert(status == GL_FRAMEBUFFER_COMPLETE);
 
   glBindFramebuffer(GL_FRAMEBUFFER, 0);
   return framebuffer;
 }
+
+void debug_framebuffer(int width, int height,
+                       int elements,
+                       const char * format)
+{
+  assert(elements == 1 || elements == 3);
+  uint gl_format = (elements == 1) ? GL_RED : GL_RGB;
+
+  float out[elements * width * height] = {};
+  glReadPixels(0, 0,
+               width, height,
+               gl_format,
+               GL_FLOAT,
+               out);
+
+  for (int y = 0; y < height; y++) {
+    for (int x = 0; x < width; x++) {
+      for (int i = 0; i < elements; i++) {
+        printf(format, out[(y * width + x) * elements + i]);
+      }
+    }
+    printf("\n");
+  }
+}
diff --git a/src/solution/2025/04/count.fs.glsl b/src/solution/2025/04/count.fs.glsl
new file mode 100644
index 0000000..4830b8f
--- /dev/null
+++ b/src/solution/2025/04/count.fs.glsl
@@ -0,0 +1,34 @@
+#version 330 core
+
+uniform sampler2D input_sampler_a;
+uniform sampler2D input_sampler_b;
+uniform vec4 dim; // w h halfpx_w halfpx_h
+
+uniform vec2 input_length;
+
+out vec4 fragment_result;
+
+vec2 halfpixel(float x, float y, vec4 dim)
+{
+  return vec2((x * 2.0 + 1.0) * dim.z,
+              (y * 2.0 + 1.0) * dim.w);
+}
+
+float sample(sampler2D sampler, float x, float y)
+{
+  return texture(sampler, halfpixel(x, y, dim)).x;
+}
+
+void main()
+{
+  float acc = 0.0;
+
+  for (float y = 0.0; y < input_length.y; y += 1.0) {
+    for (float x = 0.0; x < input_length.x; x += 1.0) {
+      acc += sample(input_sampler_a, x, y);
+      acc -= sample(input_sampler_b, x, y);
+    }
+  }
+
+  fragment_result = vec4(acc);
+}
diff --git a/src/solution/2025/04/simulate.fs.glsl b/src/solution/2025/04/simulate.fs.glsl
new file mode 100644
index 0000000..86cc126
--- /dev/null
+++ b/src/solution/2025/04/simulate.fs.glsl
@@ -0,0 +1,51 @@
+#version 330 core
+
+uniform sampler2D input_sampler;
+uniform vec4 dim; // w h halfpx_w halfpx_h
+
+out vec4 fragment_result;
+
+in vec2 f_position;
+
+vec2 halfpixel(float x, float y, vec4 dim)
+{
+  return vec2((x * 2.0 + 1.0) * dim.z,
+              (y * 2.0 + 1.0) * dim.w);
+}
+
+float sample(sampler2D sampler, float x, float y)
+{
+  return texture(sampler, halfpixel(x, y, dim)).x;
+}
+
+float neighbors(sampler2D sampler, float x, float y)
+{
+  float acc = 0.0;
+  acc += sample(sampler, x - 1, y - 1);
+  acc += sample(sampler, x + 0, y - 1);
+  acc += sample(sampler, x + 1, y - 1);
+  acc += sample(sampler, x + 1, y + 0);
+  acc += sample(sampler, x - 1, y + 0);
+  acc += sample(sampler, x - 1, y + 1);
+  acc += sample(sampler, x + 0, y + 1);
+  acc += sample(sampler, x + 1, y + 1);
+  return acc;
+}
+
+vec2 integer_coord(vec2 v, vec4 dim)
+{
+  return ((v * 0.5 + 0.5) - dim.zw) * dim.xy;
+}
+
+void main()
+{
+  vec2 c = integer_coord(f_position, dim);
+
+  float n = neighbors(input_sampler, c.x, c.y);
+
+  float v = sample(input_sampler, c.x, c.y);
+
+  float result = (n < 4) ? 0 : v;
+
+  fragment_result = vec4(result);
+}
diff --git a/src/solution/2025/04/solution.c b/src/solution/2025/04/solution.c
new file mode 100644
index 0000000..38eb17a
--- /dev/null
+++ b/src/solution/2025/04/solution.c
@@ -0,0 +1,207 @@
+#include <stdio.h>
+
+#include "glad.h"
+#include "input.h"
+#include "opengl.h"
+
+#include "puzzle/2025/04/input.h"
+#include "solution/generic.vs.glsl.h"
+#include "solution/2025/04/simulate.fs.glsl.h"
+#include "solution/2025/04/count.fs.glsl.h"
+
+static const int temp_width = 256;
+static const int temp_height = 256;
+
+static void draw(uint vertex_array,
+                 uint framebuffer,
+                 uint * texture,
+                 int texture_count)
+{
+  for (int i = 0; i < texture_count; i++) {
+    glActiveTexture(GL_TEXTURE0 + i);
+    glBindTexture(GL_TEXTURE_2D, texture[i]);
+  }
+
+  glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
+  glViewport(0, 0, temp_width, temp_height);
+
+  const uint draw_buffers0[1] = {GL_COLOR_ATTACHMENT0};
+  glDrawBuffers(1, draw_buffers0);
+
+  glBindVertexArray(vertex_array);
+  glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+}
+
+void solution_2025_04(unsigned int vertex_array)
+{
+  //////////////////////////////////////////////////////////////////////
+  // textures
+  //////////////////////////////////////////////////////////////////////
+
+  const char * input_start = puzzle_2025_04_input_start;
+  const int input_length = puzzle_2025_04_input_size;
+  //const char * input_start = "..@@.@@@@.\n@@@.@.@.@@\n@@@@@.@.@@\n@.@@@@..@.\n@@.@@@@.@@\n.@@@@@@@.@\n.@.@.@.@@@\n@.@@@.@@@@\n.@@@@@@@@.\n@.@.@@@.@.\n";
+  //const int input_length = 110;
+
+  int y = 0;
+  int x = 0;
+  int input_width = 0;
+  int input_height = 0;
+  float buf[temp_width * temp_height];
+  for (int i = 0; i < input_length; i++) {
+    char c = input_start[i];
+    if (c == '\n') {
+      y += 1;
+      input_width = x;
+      x = 0;
+    } else {
+      buf[y * temp_width + x] = (float)(int)(c == '@');
+      x += 1;
+    }
+  }
+  input_height = y;
+
+  uint texture_input = make_texture(buf,
+                                    GL_R32F,
+                                    temp_width,
+                                    temp_height,
+                                    GL_RED,
+                                    GL_FLOAT);
+  uint texture_a = make_texture(buf,
+                                GL_R32F,
+                                temp_width,
+                                temp_height,
+                                GL_RED,
+                                GL_FLOAT);
+  uint framebuffer_a = make_framebuffer(&texture_a, 1);
+  uint texture_b = make_texture(NULL,
+                                GL_R32F,
+                                temp_width,
+                                temp_height,
+                                GL_RED,
+                                GL_FLOAT);
+  uint framebuffer_b = make_framebuffer(&texture_b, 1);
+
+  uint texture_output = make_texture(NULL,
+                                     GL_R32F,
+                                     1,
+                                     1,
+                                     GL_RED,
+                                     GL_FLOAT);
+  uint framebuffer_output = make_framebuffer(&texture_output, 1);
+
+  uint framebuffers[] = { framebuffer_a, framebuffer_b };
+  uint textures[] = { texture_a, texture_b };
+
+  //////////////////////////////////////////////////////////////////////
+  // shaders
+  //////////////////////////////////////////////////////////////////////
+
+  uint p__simulate = compile_shader(src_solution_generic_vs_glsl_start,
+                                    src_solution_generic_vs_glsl_size,
+                                    src_solution_2025_04_simulate_fs_glsl_start,
+                                    src_solution_2025_04_simulate_fs_glsl_size);
+  uint p__simulate__input_sampler = glGetUniformLocation(p__simulate, "input_sampler");
+  uint p__simulate__dim = glGetUniformLocation(p__simulate, "dim");
+
+  uint p__count = compile_shader(src_solution_generic_vs_glsl_start,
+                                    src_solution_generic_vs_glsl_size,
+                                    src_solution_2025_04_count_fs_glsl_start,
+                                    src_solution_2025_04_count_fs_glsl_size);
+  uint p__count__input_sampler_a = glGetUniformLocation(p__count, "input_sampler_a");
+  uint p__count__input_sampler_b = glGetUniformLocation(p__count, "input_sampler_b");
+  uint p__count__dim = glGetUniformLocation(p__count, "dim");
+  uint p__count__input_length = glGetUniformLocation(p__count, "input_length");
+
+  //////////////////////////////////////////////////////////////////////
+  // part 1
+  //////////////////////////////////////////////////////////////////////
+  {
+    // simulate
+    glUseProgram(p__simulate);
+    glUniform1i(p__simulate__input_sampler, 0);
+    glUniform4f(p__simulate__dim,
+                temp_width,
+                temp_height,
+                0.5f / temp_width,
+                0.5f / temp_height);
+    draw(vertex_array, framebuffer_b, &texture_a, 1);
+    //debug_framebuffer(input_width, input_height, 1, "%1.0f");
+
+    // count
+    glUseProgram(p__count);
+    glUniform1i(p__count__input_sampler_a, 0);
+    glUniform1i(p__count__input_sampler_b, 1);
+    glUniform4f(p__count__dim,
+                temp_width,
+                temp_height,
+                0.5f / temp_width,
+                0.5f / temp_height);
+    glUniform2f(p__count__input_length,
+                input_width,
+                input_height);
+    uint count_textures[] = {texture_a, texture_b};
+    draw(vertex_array, framebuffer_output, count_textures, 2);
+    //debug_framebuffer(1, 1, 1, "%f");
+    float part1;
+    glReadPixels(0, 0, 1, 1, GL_RED, GL_FLOAT, &part1);
+    printf("part 1: %d\n", (int)part1);
+  }
+
+  //////////////////////////////////////////////////////////////////////
+  // part 2
+  //////////////////////////////////////////////////////////////////////
+
+  int ix = 0;
+
+  while (true) {
+    // simulate
+    glUseProgram(p__simulate);
+    glUniform1i(p__simulate__input_sampler, 0);
+    glUniform4f(p__simulate__dim,
+                temp_width,
+                temp_height,
+                0.5f / temp_width,
+                0.5f / temp_height);
+    draw(vertex_array, framebuffers[!ix], &textures[ix], 1);
+    //debug_framebuffer(input_width, input_height, 1, "%1.0f");
+
+    // count
+    glUseProgram(p__count);
+    glUniform1i(p__count__input_sampler_a, 0);
+    glUniform1i(p__count__input_sampler_b, 1);
+    glUniform4f(p__count__dim,
+                temp_width,
+                temp_height,
+                0.5f / temp_width,
+                0.5f / temp_height);
+    glUniform2f(p__count__input_length,
+                input_width,
+                input_height);
+    uint count_textures[] = {texture_a, texture_b};
+    draw(vertex_array, framebuffer_output, count_textures, 2);
+    float out;
+    glReadPixels(0, 0, 1, 1, GL_RED, GL_FLOAT, &out);
+    if (out == 0)
+      break;
+
+    ix = !ix;
+  }
+  // count
+  glUseProgram(p__count);
+  glUniform1i(p__count__input_sampler_a, 0);
+  glUniform1i(p__count__input_sampler_b, 1);
+  glUniform4f(p__count__dim,
+              temp_width,
+              temp_height,
+              0.5f / temp_width,
+              0.5f / temp_height);
+  glUniform2f(p__count__input_length,
+              input_width,
+              input_height);
+  uint count_textures[] = {texture_input, texture_b};
+  draw(vertex_array, framebuffer_output, count_textures, 2);
+  float part2;
+  glReadPixels(0, 0, 1, 1, GL_RED, GL_FLOAT, &part2);
+  printf("part 2: %d\n", (int)part2);
+}