hypot for radius

darts.h

#ifndef DARTS_H
#define DARTS_H

#include <stdint.h>

typedef struct
{
    float x;
    float y;
} coordinate_t;

uint8_t score(coordinate_t);
#endif

darts.c

#include "darts.h"
#include <math.h>
#include <stdbool.h>

const float INNER_RING = 1.0f;
const float MIDDLE_RING = 5.0f;
const float OUTER_RING = 10.0f;

static float radius(coordinate_t throw)
{
   return hypot(throw.x, throw.y);
}

static bool throw(float radius, float ring)
{
   return radius <= ring;
}

uint8_t score(coordinate_t dart_throw)
{
   float is_within = radius(dart_throw);

   if (throw(is_within, INNER_RING)) {
      return 10;
   }
   if (throw(is_within, MIDDLE_RING)) {
      return 5;
   }
   if (throw(is_within, OUTER_RING)) {
      return 1;
   }
   return 0;
}

This approach starts by defining const values for the rings. The const values are given meaningful names instead of using the float literals as magic numbers.

The radius function uses the hypot function to calculate the radius of the dart throw from the x and y coordinates.

The throw function returns if the radius passed in is within the ring passed in.

The score function uses the radius function to calculate the radius of the dart throw.

Due to the naming of the function and varables, the if statements read much like natural language.

The throw function is passed the dart throw radius and the ring. If it returns true, then the function returns with the score for throwing within that ring.

If the throw is not within a defined ring, then the function returns 0.