package middleware

import "net/http"

// DefaultHeaders returns a middleware that adds the given headers to every
// outgoing request. Existing headers on the request are not overwritten.
func DefaultHeaders(headers http.Header) Middleware {
	return func(next http.RoundTripper) http.RoundTripper {
		return RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
			// Clone lazily on the first header we actually add, so that
			// RoundTrippers never mutate the caller's request.
			cloned := false
			for key, values := range headers {
				if req.Header.Get(key) != "" {
					continue
				}
				if !cloned {
					req = req.Clone(req.Context())
					cloned = true
				}
				for _, v := range values {
					req.Header.Add(key, v)
				}
			}
			return next.RoundTrip(req)
		})
	}
}

// UserAgent returns a middleware that sets the User-Agent header on every
// outgoing request, unless one is already present.
func UserAgent(ua string) Middleware {
	return DefaultHeaders(http.Header{
		"User-Agent": {ua},
	})
}