[raymath] Vector2Angle() produces invalid results for some vectors

[AlxHnr]

Please, before submitting a new issue verify and check:

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• My code has no errors or misuse of raylib

Issue description

Vector2Angle() works fine for most values, but fails to compute the angle for certain others. I'm testing with valid normalized vectors which work fine with libraries like glm, online math sites and computing it by hand.

Environment

Fedora 36, Mesa 22.1.7, llvmpipe (LLVM 14.0.0).

Issue Screenshot

Output of the code sample below:

raylib angle: 5.73751
glm angle: 0.545679
angle computed with raylib primitives: 0.545679
----
raylib angle: -1.86645
glm angle: 1.86645
angle computed with raylib primitives: 1.86645

Code Example

#include <cmath>
#include <iostream>

#include <raymath.h>
#include <glm/vec2.hpp>
#include <glm/geometric.hpp>
#include <glm/gtx/vector_angle.hpp>

void wrongNumbers() {
  /* raylib example. */
  {
    Vector2 a = Vector2Normalize({-0.96f, -0.28f});
    Vector2 b = Vector2Normalize({-0.97f, 0.26f});
    float angle = Vector2Angle(a, b);
    std::cout << "raylib angle: " << angle << std::endl;
  }

  /* glm example. */
  {
    glm::vec2 a = glm::normalize(glm::vec2{-0.96f, -0.28f});
    glm::vec2 b = glm::normalize(glm::vec2{-0.97f, 0.26f});
    float angle = glm::angle(a, b);
    std::cout << "glm angle: " << angle << std::endl;
  }

  /* Angle computed with raylib primitives. */
  {
    Vector2 a = Vector2Normalize({-0.96f, -0.28f});
    Vector2 b = Vector2Normalize({-0.97f, 0.26f});
    float angle = acos(Vector2DotProduct(a, b));
    std::cout << "angle computed with raylib primitives: " << angle << std::endl;
  }
}

void goodButNegative() {
  /* raylib example. */
  {
    Vector2 a = Vector2Normalize({0.85f, 0.53f});
    Vector2 b = Vector2Normalize({0.26f, -0.97f});
    float angle = Vector2Angle(a, b);
    std::cout << "raylib angle: " << angle << std::endl;
  }

  /* glm example. */
  {
    glm::vec2 a = glm::normalize(glm::vec2{0.85f, 0.53f});
    glm::vec2 b = glm::normalize(glm::vec2{0.26f, -0.97f});
    float angle = glm::angle(a, b);
    std::cout << "glm angle: " << angle << std::endl;
  }

  /* Angle computed with raylib primitives. */
  {
    Vector2 a = Vector2Normalize({0.85f, 0.53f});
    Vector2 b = Vector2Normalize({0.26f, -0.97f});
    float angle = acos(Vector2DotProduct(a, b));
    std::cout << "angle computed with raylib primitives: " << angle << std::endl;
  }
}

int main() {
  wrongNumbers();
  std::cout << "----" << std::endl;
  goodButNegative();
}