// Copyright 2015 Matthew Holt and The Caddy Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package push

import (
	"strings"
)

// linkResource contains the results of a parsed Link header.
type linkResource struct {
	uri    string
	params map[string]string
}

// parseLinkHeader is responsible for parsing Link header
// and returning list of found resources.
//
// Accepted formats are:
//
//     Link: <resource>; as=script
//     Link: <resource>; as=script,<resource>; as=style
//     Link: <resource>;<resource2>
//
// where <resource> begins with a forward slash (/).
func parseLinkHeader(header string) []linkResource {
	resources := []linkResource{}

	if header == "" {
		return resources
	}

	for _, link := range strings.Split(header, comma) {
		l := linkResource{params: make(map[string]string)}

		li, ri := strings.Index(link, "<"), strings.Index(link, ">")
		if li == -1 || ri == -1 {
			continue
		}

		l.uri = strings.TrimSpace(link[li+1 : ri])

		for _, param := range strings.Split(strings.TrimSpace(link[ri+1:]), semicolon) {
			parts := strings.SplitN(strings.TrimSpace(param), equal, 2)
			key := strings.TrimSpace(parts[0])
			if key == "" {
				continue
			}
			if len(parts) == 1 {
				l.params[key] = key
			}
			if len(parts) == 2 {
				l.params[key] = strings.TrimSpace(parts[1])
			}
		}

		resources = append(resources, l)
	}

	return resources
}

const (
	comma     = ","
	semicolon = ";"
	equal     = "="
)