2.go

package main

/*
Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
*/

import (
	"net/http"
    "encoding/json"
)

func EulerTwoHandler(rw http.ResponseWriter, req *http.Request) {
    sum := fibSum(4000000)
	responseData := map[string]interface{} {
		"result" : "OK",
        "sum": sum,
	}

	jsonEncoder := json.NewEncoder(rw)
	jsonEncoder.Encode(responseData)
}

func fibSum(until int) int {
    a := 1
    b := 2
    c := a + b
    sum := 2 // to account for b

    for c < until {
        if c % 2 == 0 {
            sum += c
        }
        a = b
        b = c
        c = b + a
    }
    return sum
}