First Commit

Dockerfile

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+# syntax=docker/dockerfile:1
+
+FROM golang:1.17
+
+WORKDIR /factorial
+
+COPY go.mod ./
+COPY go.sum ./
+
+RUN go mod download
+
+COPY *.go ./
+
+RUN go build -o /factorial_serv

README.md

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+# Factorial RESTful API
+Takes a POST Request that takes json with the following structure: {"a":#,"b":#} (where # is an int) and 
+returns a json string containing the factorial of a and b in the same structure. When a negative number or 
+other incorrect input is given, the API returns an error.

go.mod

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+module github.com/ghfarrell/factorial_server
+
+go 1.17
+
+require github.com/julienschmidt/httprouter v1.3.0 // direct

go.sum

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+github.com/julienschmidt/httprouter v1.3.0 h1:U0609e9tgbseu3rBINet9P48AI/D3oJs4dN7jwJOQ1U=
+github.com/julienschmidt/httprouter v1.3.0/go.mod h1:JR6WtHb+2LUe8TCKY3cZOxFyyO8IZAc4RVcycCCAKdM=

main.go

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+package main
+
+import (
+	"context"
+	"encoding/json"
+	"log"
+	"net/http"
+
+	"github.com/julienschmidt/httprouter"
+)
+
+// type ContextKeyString is a type that is a string meant for use as the key in an https context value
+type ContextKeyString string
+type args struct {
+	A int `json:"a"`
+	B int `json:"b"`
+}
+type requestError struct {
+	Err string `json:"error"`
+}
+
+func factorial(a int, ch chan int) {
+	r := 1
+	for f := 1; f <= a; f++ {
+		r *= f
+	}
+	ch <- r
+}
+
+func postError(w http.ResponseWriter, r *http.Request, err string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusBadRequest)
+	json.NewEncoder(w).Encode(requestError{Err: err})
+}
+
+func middleware(f httprouter.Handle) httprouter.Handle {
+	return func(w http.ResponseWriter, r *http.Request, p httprouter.Params) {
+		var v args
+		dec := json.NewDecoder(r.Body)
+		dec.DisallowUnknownFields()
+		err := dec.Decode(&v)
+		if err != nil || v.A < 0 || v.B < 0 {
+			postError(w, r, "Incorrect input")
+		} else {
+			ctx := r.Context()
+			// using a new type definition to avoid compiler warnings
+			ctx = context.WithValue(ctx, ContextKeyString("a"), v.A)
+			ctx = context.WithValue(ctx, ContextKeyString("b"), v.B)
+			r = r.WithContext(ctx)
+			f(w, r, p)
+		}
+	}
+}
+
+func calculate(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
+	var v args
+	v.A = r.Context().Value(ContextKeyString("a")).(int)
+	v.B = r.Context().Value(ContextKeyString("b")).(int)
+	chanA := make(chan int)
+	chanB := make(chan int)
+	go factorial(v.A, chanA)
+	go factorial(v.B, chanB)
+	v.B = <-chanB
+	v.A = <-chanA
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(v)
+}
+
+func main() {
+	router := httprouter.New()
+	router.POST("/calculate", middleware(calculate))
+
+	log.Fatal(http.ListenAndServe(":8989", router))
+}