Include screenshot as example

.gitignore

@@ -0,0 +1,3 @@
+.cache
+compile_commands.json
+main

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "intern/aliases"]
+	path = intern/aliases
+	url = https://git.thewizardapprentice.com/abdelrahman/c-cpp-aliases.git

README.md

@@ -0,0 +1,7 @@
+# Arrow drawing test
+
+Test drawing arrows in SDL.
+
+To draw an arrow, click the mouse to start drawing, move your mouse to the desired location, then click again to finalise the drawing.
+
+![Example result](./result.png)

build

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+bear -- ./compile $@

compile

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+CC=clang
+CFLAGS="-g -Wall -Iintern $(pkg-config --cflags sdl2)"
+LIBS="$(pkg-config --libs sdl2) -lm"
+SRC=*.c
+OUT=main
+
+(set -x ; $CC $CFLAGS $LIBS $SRC -o $OUT)

drawing.c

@@ -0,0 +1,156 @@
+#include "drawing.h"
+#include "aliases/aliases.h"
+#include <SDL_render.h>
+#include <math.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define square(x) (x * x)
+
+bool init_arrows_arr(arrows_t *lines, u64 capacity) {
+  u64 size = capacity * sizeof(arrow_t);
+
+  lines->lines = (arrow_t *)malloc(size);
+  if (!(lines->lines)) {
+    return false;
+  }
+
+  memset(lines->lines, 0, size);
+
+  lines->count = 0;
+  lines->capacity = capacity;
+
+  return true;
+}
+
+void add_arrow(arrows_t *lines, arrow_t ln) {
+  if (lines->count == lines->capacity) {
+    arrow_t *ptr = lines->lines;
+
+    u64 new_capacity =
+        (lines->capacity + DEFAULT_ARROWS_CAPACITY) * sizeof(arrow_t);
+
+    lines->lines = (arrow_t *)realloc(lines->lines, new_capacity);
+    if (!(lines->lines)) {
+      lines->lines = ptr;
+      return;
+    }
+  }
+
+  lines->lines[(lines->count)++] = (arrow_t){
+      (point_t){ln.origin.x, ln.origin.y},
+      (vec2_t){ln.direction.x, ln.direction.y},
+  };
+}
+
+void draw_arrows(SDL_Renderer *renderer, const arrows_t *arrows,
+                 colour_t colour) {
+  for (u64 i = 0; i < arrows->count; ++i) {
+    const arrow_t *ln = &(arrows->lines[i]);
+
+    draw_arrow(renderer, ln, colour);
+  }
+}
+
+INTERNAL f32 vec2_magnitude(const vec2_t *vec) {
+  return sqrtf((f32)vec->x * vec->x + (f32)vec->y * vec->y);
+}
+
+INTERNAL void draw_arrow_head(SDL_Renderer *renderer, const arrow_t *arrow,
+                              const point_t *arrow_end) {
+  // m = line_slope
+  // dx = change_in_x
+  // dy = change_in_y
+  //
+  // m = dy / dx
+  //
+  // for two perpendicular lines, their slopes would be m and -1/m
+  //
+  // To draw a perpendicular line with fixed length:
+  // dy / dx = -1 / m
+  // (rearranged) => dx = -m * dy
+  //
+  // From Pythagorean theorem:
+  // let c = desired_length
+  //
+  // c^2 = dx^2 + dy^2
+  // (dx substituted) => c^2 = (-m * dy)^2 + dy^2
+  // c^2 = m^2 * dy^2 + dy^2
+  // c^2 = dy^2 * (m^2 + 1)
+  // (rearranged) dy^2 = c^2 / (m^2 + 1)
+  // dy = sqrt(c^2 / (m^2 + 1))
+  //
+  // Based on this answer from StackOverflow:
+  // https://stackoverflow.com/a/57065334
+
+  vec2_t unit_vector = {1, 1};
+  if (arrow->direction.y != 0) {
+    unit_vector.y = arrow->direction.y / abs(arrow->direction.y);
+  }
+
+  point_t arrow_tip = (point_t){-1, -1};
+  point_t arrow_base_p0 = (point_t){-1, -1};
+  point_t arrow_base_p1 = (point_t){-1, -1};
+  f32 half_length = 20.0f;
+
+  if (arrow->direction.x == 0) {
+    arrow_tip.x = arrow_end->x;
+    arrow_tip.y = arrow_end->y + half_length * unit_vector.y;
+    arrow_base_p0.x = arrow_end->x - half_length;
+    arrow_base_p0.y = arrow_end->y;
+    arrow_base_p1.x = arrow_end->x + half_length;
+    arrow_base_p1.y = arrow_end->y;
+  } else {
+    unit_vector.x = arrow->direction.x / abs(arrow->direction.x);
+
+    // Calcualte arrow tip
+    f32 dx0_squared = square((f32)(arrow->direction.x));
+    f32 dy0_squared = square((f32)(arrow->direction.y));
+
+    f32 dy = sqrtf(square(half_length) / (dx0_squared / dy0_squared + 1.0f)) *
+             unit_vector.y;
+    f32 dx = (arrow->direction.y != 0)
+                 ? dy * arrow->direction.x / arrow->direction.y
+                 : half_length * unit_vector.x;
+
+    // Draw perpendicular line
+    f32 slope = (f32)(arrow->direction.y) / arrow->direction.x;
+
+    f32 perpendicular_dy =
+        sqrtf(square(half_length) / (square((f32)slope) + 1.0f));
+    f32 perpendicular_dx = -1.0f * slope * perpendicular_dy;
+
+    arrow_tip.x = arrow_end->x + dx;
+    arrow_tip.y = arrow_end->y + dy;
+    arrow_base_p0.x = arrow_end->x - perpendicular_dx;
+    arrow_base_p0.y = arrow_end->y - perpendicular_dy;
+    arrow_base_p1.x = arrow_end->x + perpendicular_dx;
+    arrow_base_p1.y = arrow_end->y + perpendicular_dy;
+  }
+
+  SDL_RenderDrawLine(renderer, arrow_end->x, arrow_end->y, arrow_base_p0.x,
+                     arrow_base_p0.y);
+  SDL_RenderDrawLine(renderer, arrow_end->x, arrow_end->y, arrow_base_p1.x,
+                     arrow_base_p1.y);
+  SDL_RenderDrawLine(renderer, arrow_tip.x, arrow_tip.y, arrow_base_p0.x,
+                     arrow_base_p0.y);
+  SDL_RenderDrawLine(renderer, arrow_tip.x, arrow_tip.y, arrow_base_p1.x,
+                     arrow_base_p1.y);
+}
+
+void draw_arrow(SDL_Renderer *renderer, const arrow_t *arrow, colour_t colour) {
+  SDL_SetRenderDrawColor(renderer, colour.colour.r, colour.colour.g,
+                         colour.colour.b, colour.colour.a);
+
+  point_t end = (point_t){
+      arrow->origin.x + arrow->direction.x,
+      arrow->origin.y + arrow->direction.y,
+  };
+
+  if (vec2_magnitude(&(arrow->direction)) != 0) {
+    draw_arrow_head(renderer, arrow, &end);
+  }
+
+  SDL_RenderDrawLine(renderer, arrow->origin.x, arrow->origin.y, end.x, end.y);
+}

drawing.h

@@ -0,0 +1,55 @@
+#ifndef DRAWING_H
+#define DRAWING_H
+
+#include "aliases/aliases.h"
+#include <SDL_pixels.h>
+#include <SDL_render.h>
+#include <stdbool.h>
+
+#define DEFAULT_ARROWS_CAPACITY 1024
+
+#define init_arrows_default(arrows)                                            \
+  init_arrows_arr(arrows, DEFAULT_ARROWS_CAPACITY)
+#define init_arrows_with_capacity(arrows, capacity)                            \
+  init_arrows_arr(arrows, capacity)
+
+typedef struct colour colour_t;
+struct colour {
+  union {
+    u32 abgr;
+    SDL_Color colour;
+  };
+};
+
+typedef struct point point_t;
+struct point {
+  i32 x;
+  i32 y;
+};
+
+typedef struct vec2 vec2_t;
+struct vec2 {
+  i32 x;
+  i32 y;
+};
+
+typedef struct arrow arrow_t;
+struct arrow {
+  point_t origin;
+  vec2_t direction;
+};
+
+typedef struct arrows arrows_t;
+struct arrows {
+  arrow_t *lines;
+  u64 count;
+  u64 capacity;
+};
+
+bool init_arrows_arr(arrows_t *lines, u64 capacity);
+void add_arrow(arrows_t *lines, arrow_t ln);
+void draw_arrows(SDL_Renderer *renderer, const arrows_t *arrows,
+                 colour_t colour);
+void draw_arrow(SDL_Renderer *renderer, const arrow_t *arrow, colour_t colour);
+
+#endif // !DRAWING_H

intern/aliases

@@ -0,0 +1 @@
+Subproject commit f95f3aa499910286876c1f2cdceea8146ebcf7b1

main.c

@@ -1,11 +1,27 @@
+#include "SDL_mouse.h"
+#include "aliases/aliases.h"
+#include "drawing.h"
 #include <SDL2/SDL.h>
 #include <SDL2/SDL_events.h>
 #include <SDL2/SDL_render.h>
 #include <SDL2/SDL_video.h>
 #include <stdbool.h>
+#include <stdlib.h>
 
-#define WINDOW_WIDTH 800
-#define WINDOW_HEIGHT 600
+#define WINDOW_WIDTH 1280
+#define WINDOW_HEIGHT 720
+
+typedef struct state state_t;
+struct state {
+  i32 mouse_x;
+  i32 mouse_y;
+  i32 current_origin_x;
+  i32 current_origin_y;
+  bool running;
+  bool drawing;
+  arrows_t arrows;
+  arrow_t tmp_arrow;
+};
 
 int main(void) {
   SDL_Init(SDL_INIT_EVERYTHING);
@@ -17,15 +33,55 @@ int main(void) {
   SDL_Renderer *renderer = SDL_CreateRenderer(
       window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
 
-  bool running = true;
+  state_t st = {
+      .mouse_x = -1,
+      .mouse_y = -1,
+      .current_origin_x = -1,
+      .current_origin_y = -1,
+      .running = true,
+      .drawing = false,
+      .arrows = {0},
+      .tmp_arrow = {0},
+  };
 
-  SDL_Event event = {};
+  SDL_Event event = {0};
 
-  while (running) {
+  if (!init_arrows_default(&(st.arrows))) {
+    return EXIT_FAILURE;
+  }
+
+  while (st.running) {
     while (SDL_PollEvent(&event)) {
       switch (event.type) {
       case SDL_QUIT:
-        running = false;
+        st.running = false;
+        break;
+      case SDL_MOUSEBUTTONUP:
+        if (event.button.button == SDL_BUTTON_LEFT) {
+          if (st.drawing) {
+            add_arrow(&(st.arrows), st.tmp_arrow);
+
+            st.tmp_arrow = (arrow_t){0};
+
+            st.drawing = false;
+          } else {
+            st.current_origin_x = event.button.x;
+            st.current_origin_y = event.button.y;
+
+            st.drawing = true;
+          }
+        } else if (event.button.button == SDL_BUTTON_RIGHT) {
+          st.current_origin_x = -1;
+          st.current_origin_y = -1;
+
+          st.drawing = false;
+        }
+
+        break;
+      case SDL_MOUSEMOTION:
+        st.mouse_x = event.motion.x;
+        st.mouse_y = event.motion.y;
+
         break;
       }
     }
@@ -34,6 +90,16 @@ int main(void) {
 
     SDL_RenderClear(renderer);
 
+    if (st.drawing) {
+      st.tmp_arrow.origin = (point_t){st.current_origin_x, st.current_origin_y};
+      st.tmp_arrow.direction = (vec2_t){st.mouse_x - st.current_origin_x,
+                                        st.mouse_y - st.current_origin_y};
+
+      draw_arrow(renderer, &(st.tmp_arrow), (colour_t){.abgr = 0xff0000ff});
+    }
+
+    draw_arrows(renderer, &(st.arrows), (colour_t){.abgr = 0xff000000});
+
     SDL_RenderPresent(renderer);
   }
 
@@ -43,5 +109,5 @@ int main(void) {
 
   SDL_Quit();
 
-  return 0;
+  return EXIT_SUCCESS;
 }