arrow-drawing-test/src/drawing.c

#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)

typedef struct line line_t;
struct line {
  point_t p0;
  point_t p1;
};

typedef struct triangle triangle_t;
struct triangle {
  point_t p0;
  point_t p1;
  point_t p2;
};

INTERNAL void draw_line(SDL_Renderer *renderer, line_t ln);
INTERNAL void draw_triangle(SDL_Renderer *renderer, triangle_t triangle);

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->capacity = new_capacity;
  }

  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 triangle_t calculate_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;
  }

  return (triangle_t){arrow_tip, arrow_base_p0, arrow_base_p1};
}

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,
  };
  line_t line = (line_t){arrow->origin, end};

  if (vec2_magnitude(&(arrow->direction)) != 0) {
    triangle_t triangle = calculate_arrow_head(renderer, arrow, &end);
    draw_triangle(renderer, triangle);
  }

  draw_line(renderer, line);
}

INTERNAL void draw_line(SDL_Renderer *renderer, line_t ln) {
  SDL_RenderDrawLine(renderer, ln.p0.x, ln.p0.y, ln.p1.x, ln.p1.y);
}

INTERNAL void draw_triangle(SDL_Renderer *renderer, triangle_t triangle) {
  line_t ln0 = (line_t){triangle.p0, triangle.p1};
  line_t ln1 = (line_t){triangle.p0, triangle.p2};
  line_t ln2 = (line_t){triangle.p1, triangle.p2};

  draw_line(renderer, ln0);
  draw_line(renderer, ln1);
  draw_line(renderer, ln2);
}