3 files changed, 84 insertions, 7 deletions

diff --git a/README.markdown b/README.markdown
index 887b833..6ac6da7 100644
--- a/README.markdown
+++ b/README.markdown
@@ -15,6 +15,23 @@ To get zipcode info using `curl` and [`jq`](https://stedolan.github.io/jq/):
 curl --silent http://localhost:8080/zip/18101 | jq '.'
 ```
 
+# Response
+
+Your response will look something like this:
+
+```json
+{
+	"latitude": 40.602847,
+	"longitude": -75.47022,
+	"city": "Allentown",
+	"state": "PA"
+}
+```
+
+If the zipcode you request isn't in the database, you'll get a `404` status with an empty body.
+
+Add a `?distance=` parameter to your request to see how far away two zipcodes are. The response format will be the same as above with an additional `distance` key that contains info about the second zipcode as well as `miles` and `kilometers` keys. If the second zipcode can't be found, the `distance` key won't be included.
+
 # Data
 
 This project uses public-domain zipcode data from the [Zip Code Database Project](http://zips.sourceforge.net).
diff --git a/database.go b/database.go
index 964915c..f0016bc 100644
--- a/database.go
+++ b/database.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/csv"
+	"math"
 	"os"
 	"strconv"
 	"strings"
@@ -14,6 +15,24 @@ type ZipcodeDetails struct {
 	State     string  `json:"state"`
 }
 
+const EarthRadiusMiles float64 = 3960
+const EarthRadiusKm float64 = 6373
+const DegToRad float64 = math.Pi / 180
+
+func (zip1 *ZipcodeDetails) DistanceTo(zip2 *ZipcodeDetails) (float64, float64) {
+	// the arc distance ψ = arccos(sin φ_1 sin φ_2 cos(θ_1-θ_2) + cos φ_1 cos φ_2)
+	// where
+	//		φ_k = 90° - latitude_k
+	//		θ_k = longitude_k
+	// (both in radians)
+	theta1 := zip1.Longitude * DegToRad
+	theta2 := zip2.Longitude * DegToRad
+	phi1 := (90 - zip1.Latitude) * DegToRad
+	phi2 := (90 - zip2.Latitude) * DegToRad
+	arc := math.Acos(math.Sin(phi1)*math.Sin(phi2)*math.Cos(theta1-theta2) + math.Cos(phi1)*math.Cos(phi2))
+	return arc * EarthRadiusMiles, arc * EarthRadiusKm
+}
+
 type ZipcodeDatabase struct {
 	Zipcodes map[string]*ZipcodeDetails
 }
diff --git a/main.go b/main.go
index 18342ed..0df0cce 100644
--- a/main.go
+++ b/main.go
@@ -8,6 +8,17 @@ import (
 	"strings"
 )
 
+type DistanceZipcode struct {
+	ZipcodeDetails
+	Miles      float64 `json:"miles"`
+	Kilometers float64 `json:"kilometers"`
+}
+
+type DistanceResponse struct {
+	ZipcodeDetails
+	Distance DistanceZipcode `json:"distance"`
+}
+
 func main() {
 	db := getInitializedDatabase()
 	http.HandleFunc("/zip/", func(w http.ResponseWriter, r *http.Request) {
@@ -37,25 +48,55 @@ func getListeningAddress() string {
 }
 
 func handleZipcodeRequest(response http.ResponseWriter, request *http.Request, db *ZipcodeDatabase) {
-	zip := zipcodeForRequest(request)
-	if details := db.Find(zip); details == nil {
+	zip := zipcodeForPath(request, db)
+	if zip == nil {
 		http.Error(response, "", 404)
 	} else {
 		setResponseHeaders(response)
-		sendZipcodeDetails(details, response)
+		if zip2 := zipcodeForQuery(request, db); zip2 != nil {
+			sendDistanceResponse(zip, zip2, response)
+		} else {
+			sendJson(response, zip)
+		}
 	}
 }
 
+func sendDistanceResponse(zip1, zip2 *ZipcodeDetails, writer http.ResponseWriter) {
+	mi, km := zip1.DistanceTo(zip2)
+	response := &DistanceResponse{}
+	response.ZipcodeDetails = *zip1
+	response.Distance.ZipcodeDetails = *zip2
+	response.Distance.Miles = mi
+	response.Distance.Kilometers = km
+	sendJson(writer, response)
+}
+
+func sendJson(writer http.ResponseWriter, data interface{}) {
+	bytes, _ := json.MarshalIndent(data, "", "  ")
+	fmt.Fprintf(writer, string(bytes))
+}
+
 func setResponseHeaders(response http.ResponseWriter) {
 	response.Header().Set("Access-Control-Allow-Origin", "*")
 	response.Header().Set("Content-Type", "application/json")
 }
 
-func zipcodeForRequest(request *http.Request) string {
+func extractZipFromPath(request *http.Request) string {
 	return request.URL.Path[len("/zip/"):]
 }
 
-func sendZipcodeDetails(details *ZipcodeDetails, response http.ResponseWriter) {
-	data, _ := json.MarshalIndent(details, "", "  ")
-	fmt.Fprintf(response, string(data))
+func zipcodeForPath(request *http.Request, db *ZipcodeDatabase) *ZipcodeDetails {
+	return db.Find(extractZipFromPath(request))
+}
+
+func extractZipFromQuery(request *http.Request) string {
+	if param := request.URL.Query()["distance"]; param != nil {
+		return param[0]
+	} else {
+		return ""
+	}
+}
+
+func zipcodeForQuery(request *http.Request, db *ZipcodeDatabase) *ZipcodeDetails {
+	return db.Find(extractZipFromQuery(request))
 }