fix trigonometry functions so angles correspond with GL coordinates, add function to calculate angle between two vectors

src/math.cpp

@@ -2,10 +2,15 @@
 
 glm::vec2 sb::velocity_to_delta(float angle, float magnitude)
 {
-    return {glm::sin(angle) * magnitude, -glm::cos(angle) * magnitude};
+    return {glm::sin(angle) * magnitude, glm::cos(angle) * magnitude};
 }
 
 glm::vec2 sb::velocity_to_delta(glm::vec2 velocity)
 {
     return velocity_to_delta(velocity.x, velocity.y);
 }
+
+float sb::angle_between(glm::vec2 start, glm::vec2 end)
+{
+    return glm::atan(end.x - start.x, end.y - start.y);
+}

src/math.hpp

@@ -10,8 +10,26 @@
 
   [math.hpp]
 
-  For math helper functions that require only GLM primitives.
-    
+  For math helper functions that require only GLM primitives, especially trigonometric functions.
+  Angle values are in radians. 0 is directly up on the screen in GL coordinates, and the angle increases
+  clockwise.
+
+  This test of points on a square and one point below the square should print the expected output:
+
+    glm::vec2 a {5.0f, 5.0f}, b {1.0f, 1.0f}, c {1.0f, 5.0f}, d {5.0f, 1.0f}, e {5.0f, -1.0f};
+    std::cout << glm::degrees(sb::angle_between(a, b)) << " " << glm::degrees(sb::angle_between(b, a)) << " " <<
+      glm::degrees(sb::angle_between(a, c)) << " " << glm::degrees(sb::angle_between(c, a)) << " " <<
+      glm::degrees(sb::angle_between(b, e)) << " " << glm::degrees(sb::angle_between(e, b)) << std::endl <<
+      sb::velocity_to_delta(sb::angle_between(a, b), 4.0f * glm::sqrt(2.0f)) << " " <<
+      sb::velocity_to_delta(sb::angle_between(b, a), 4.0f * glm::sqrt(2.0f)) << " " <<
+      sb::velocity_to_delta(sb::angle_between(d, a), 4.0f) << " " <<
+      sb::velocity_to_delta(sb::angle_between(b, e), 1.0f) << std::endl;
+
+  That should print:
+
+    -135 45 -90 90 116.565 -63.435
+    {-4, -4} {4, 4} {0, 4} {0.894427, -0.447214}
+  
 */
 
 #pragma once
@@ -28,4 +46,7 @@ namespace sb
 
     /* Convert a vector containing angle and magnitude to an X and Y offset vector. */
     glm::vec2 velocity_to_delta(glm::vec2);
+
+    /* Get the angle between two vectors, or the angle the first would rotate to to point toward the second. */
+    float angle_between(glm::vec2, glm::vec2);
 }