Enable zooming by drawing a rectangle with the mouse.

Makefile

@@ -1,2 +1,2 @@
 main: main.c
-	gcc main.c -o main -I./raylib/include -L./raylib/lib -lraylib -lm
+	gcc -Wall -Wpedantic main.c -o main -I./raylib/include -L./raylib/lib -lraylib -lm

main.c

@@ -1,20 +1,33 @@
 #include <assert.h>
 #include <math.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include "raylib.h"
 
 #define WIDTH 800
 #define HEIGHT 600
-#define MAX_ITER 50 
+#define MAX_ITER 50
+#define SELECTED CLITERAL(Color){ 112, 31, 126, 100 }    // Dark Purple
 
-int mandelbrot(double x, double y) {
+typedef struct RectangleD {
+    double x;                // Rectangle top-left corner position x
+    double y;                // Rectangle top-left corner position y
+    double width;            // Rectangle width
+    double height;           // Rectangle height
+} RectangleD;
+
+float min(float x, float y) {
+    return (x < y) ? x : y;
+}
+
+int mandelbrot(double x, double y, int max_iter) {
     int k;
     double u = 0.0;
     double v = 0.0;
     double u2 = 0.0;
     double v2 = 0.0;
 
-    for (k = 1; k < MAX_ITER && (u2 + v2 < 4.0); k++) {
+    for (k = 1; k < max_iter && (u2 + v2 < 4.0); k++) {
         v = 2 * u * v + y;
         u = u2 - v2 + x;
         u2 = u * u;
@@ -25,22 +38,29 @@ int mandelbrot(double x, double y) {
 }
 
 
-int main() {
-    InitWindow(WIDTH, HEIGHT, "Hello, raylib!");
-    
-    double re_start = -2.0;
-    double re_end = 1.0;
-    double im_start = -1.0;
-    double im_end = 1.0;
+void measure_time() {
+    // struct timespec start, end;
+    // double time_spent;
+    // clock_gettime(CLOCK_MONOTONIC, &start);
+    // clock_gettime(CLOCK_MONOTONIC, &end);
+    // time_spent = (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) / 1e9;
+    // printf("Execution time: %.9f seconds\n", time_spent);
+}
 
-    while (!WindowShouldClose()) {
-        BeginDrawing();
-        ClearBackground(RAYWHITE);
-        for (int x = 0; x < WIDTH; ++x) {
-            for (int y = 0; y < HEIGHT; ++y) {
-                double zreal = re_start + (x / (float) WIDTH) * (re_end - re_start);
-                double zimag = im_start + (y / (float) HEIGHT) * (im_end - im_start);
-                int m = mandelbrot(zreal, zimag);
+void render_mandelbrot(RenderTexture2D *target, RectangleD *area) {
+    double re_start = area->x;
+    double re_end = area->x + area->width;
+    double im_start = area->y;
+    double im_end = area->y + area->height;
+    int width = GetScreenWidth();
+    int height = GetScreenHeight();
+
+    BeginTextureMode(*target);
+        for (int x = 0; x < width; ++x) {
+            for (int y = 0; y < height; ++y) {
+                double zreal = re_start + (x / (double) width) * (re_end - re_start);
+                double zimag = im_start + (y / (double) height) * (im_end - im_start);
+                int m = mandelbrot(zreal, zimag, MAX_ITER);
                 assert(m >= 0 && m <= MAX_ITER);
 
                 unsigned char c = 255 - ((m * 255) / MAX_ITER);
@@ -48,6 +68,55 @@ int main() {
                 DrawPixel(x, y, color);
             }
         }
+    EndTextureMode();
+}
+
+
+int main() {
+
+    bool mouse_pressed = false;
+    Vector2 selection_start = { 0 };
+
+    RectangleD rect_mandelbrot = { -2.0, -1.0, 3.0, 2.0 };
+    Rectangle rect_selected = { 0 };
+    
+
+    InitWindow(WIDTH, HEIGHT, "Mandelbrot Viewer");
+    RenderTexture2D target = LoadRenderTexture(800, 600);
+    render_mandelbrot(&target, &rect_mandelbrot);
+
+    while (!WindowShouldClose()) {
+
+        if (IsMouseButtonPressed(MOUSE_BUTTON_LEFT)) {
+            selection_start = GetMousePosition();
+        }
+
+        if (IsMouseButtonDown(MOUSE_BUTTON_LEFT)) {
+            mouse_pressed = true;
+            rect_selected.x = min(selection_start.x, GetMouseX());
+            rect_selected.y = min(selection_start.y, GetMouseY());
+            rect_selected.width = abs(selection_start.x - GetMouseX());
+            rect_selected.height = rect_selected.width * (HEIGHT / (float) WIDTH);
+        } 
+
+        if (IsMouseButtonReleased(MOUSE_BUTTON_LEFT)) {
+            rect_mandelbrot.x += (rect_mandelbrot.width * (double) (rect_selected.x / GetScreenWidth()));
+            rect_mandelbrot.y += (rect_mandelbrot.height * (double) (rect_selected.y / GetScreenHeight()));
+            rect_mandelbrot.width *= ((double) rect_selected.width / GetScreenWidth());
+            rect_mandelbrot.height *= ((double) rect_selected.height / GetScreenHeight());
+
+            render_mandelbrot(&target, &rect_mandelbrot);
+        }
+
+        BeginDrawing();
+        ClearBackground(RAYWHITE);
+        DrawTextureRec(target.texture, (Rectangle) { 0, 0, (float)target.texture.width, (float)-target.texture.height }, (Vector2) { 0, 0 }, WHITE);
+
+        if (mouse_pressed) {
+            DrawRectangleRec(rect_selected, SELECTED);
+            mouse_pressed = false;
+        }
+
         EndDrawing();
     }