package main

import (
	"encoding/json"
	"github.com/gorilla/mux"
	"log"
	"math"
	"net/http"
	"strconv"
)

// Defining the constants used in the NFW profile
const (
	sigma float64 = 200
	f0    float64 = 0.1
	G     float64 = 4.302e-3
	rS    float64 = 1e4
)

// The actual NFW profile
// Uses the given distance to the middle of the galaxy to calculate the probability that a star is generated
func rho(r float64) float64 {
	a := (1) / (math.Sqrt(2*math.Pi) * sigma)
	b := math.Exp(-phi(r) / math.Pow(sigma, 2))
	return a * b
}

// A subset of the NFW function in order to handle the 0 case
func phi(x float64) float64 {
	if x == 0 {
		return -4 * math.Pi * f0 * G * math.Pow(rS, 2)
	}
	a := -(4 * math.Pi * f0 * G * math.Pow(rS, 3)) / x
	b := math.Log(1.0 + (x / rS))
	return a * b
}

// NFW is the corresponding HTTP-handler to the NFW Profile
// It uses the given distance to the Midpoint of the galaxy to calculate and return the probability that a star
// is generated.
func NFW(w http.ResponseWriter, r *http.Request) {
	params := mux.Vars(r)
	distanceToMidpoint, err := strconv.ParseFloat(params["r"], 64)
	if err != nil {
		panic(err)
	}
	var returnValue float64 = rho(distanceToMidpoint)
	err = json.NewEncoder(w).Encode(returnValue)
	if err != nil {
		panic(err)
	}
}

func main() {
	router := mux.NewRouter()
	router.HandleFunc("/NFW/{r}", NFW).Methods("GET")
	log.Fatal(http.ListenAndServe(":8000", router))
}