/*
Copyright 2013 Google Inc.

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 groupcache

import (
	"fmt"
	"hash/crc32"
	"io/ioutil"
	"net/http"
	"net/url"
	"strings"
	"sync"

	"code.google.com/p/goprotobuf/proto"

	pb "github.com/golang/groupcache/groupcachepb"
)

// TODO: make this configurable?
const defaultBasePath = "/_groupcache/"

// HTTPPool implements PeerPicker for a pool of HTTP peers.
type HTTPPool struct {
	// Context optionally specifies a context for the server to use when it
	// receives a request.
	// If nil, the server uses a nil Context.
	Context func(*http.Request) Context

	// Transport optionally specifies an http.RoundTripper for the client
	// to use when it makes a request.
	// If nil, the client uses http.DefaultTransport.
	Transport func(Context) http.RoundTripper

	// base path including leading and trailing slash, e.g. "/_groupcache/"
	basePath string

	// this peer's base URL, e.g. "https://example.net:8000"
	self string

	mu    sync.Mutex
	peers []string
}

var httpPoolMade bool

// NewHTTPPool initializes an HTTP pool of peers.
// It registers itself as a PeerPicker and as an HTTP handler with the
// http.DefaultServeMux.
// The self argument be a valid base URL that points to the current server,
// for example "http://example.net:8000".
func NewHTTPPool(self string) *HTTPPool {
	if httpPoolMade {
		panic("groupcache: NewHTTPPool must be called only once")
	}
	httpPoolMade = true
	p := &HTTPPool{basePath: defaultBasePath, self: self}
	RegisterPeerPicker(func() PeerPicker { return p })
	http.Handle(defaultBasePath, p)
	return p
}

// Set updates the pool's list of peers.
// Each peer value should be a valid base URL,
// for example "http://example.net:8000".
func (p *HTTPPool) Set(peers ...string) {
	p.mu.Lock()
	defer p.mu.Unlock()
	p.peers = append([]string{}, peers...)
}

func (p *HTTPPool) PickPeer(key string) (ProtoGetter, bool) {
	// TODO: make checksum implementation pluggable
	h := crc32.Checksum([]byte(key), crc32.IEEETable)
	p.mu.Lock()
	defer p.mu.Unlock()
	if len(p.peers) == 0 {
		return nil, false
	}
	n := int(h)
	if n < 0 {
		n *= -1
	}
	if peer := p.peers[n%len(p.peers)]; peer != p.self {
		// TODO: pre-build a slice of *httpGetter when Set()
		// is called to avoid these two allocations.
		return &httpGetter{p.Transport, peer + p.basePath}, true
	}
	return nil, false
}

func (p *HTTPPool) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// Parse request.
	if !strings.HasPrefix(r.URL.Path, p.basePath) {
		panic("HTTPPool serving unexpected path: " + r.URL.Path)
	}
	parts := strings.SplitN(r.URL.Path[len(p.basePath):], "/", 2)
	if len(parts) != 2 {
		http.Error(w, "bad request", http.StatusBadRequest)
		return
	}
	groupName, err := url.QueryUnescape(parts[0])
	if err != nil {
		http.Error(w, "decoding group: "+err.Error(), http.StatusBadRequest)
		return
	}
	key, err := url.QueryUnescape(parts[1])
	if err != nil {
		http.Error(w, "decoding key: "+err.Error(), http.StatusBadRequest)
		return
	}

	// Fetch the value for this group/key.
	group := GetGroup(groupName)
	if group == nil {
		http.Error(w, "no such group: "+groupName, http.StatusNotFound)
		return
	}
	var ctx Context
	if p.Context != nil {
		ctx = p.Context(r)
	}
	var value []byte
	err = group.Get(ctx, key, AllocatingByteSliceSink(&value))
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	// Write the value to the response body as a proto message.
	body, err := proto.Marshal(&pb.GetResponse{Value: value})
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	w.Header().Set("Content-Type", "application/x-protobuf")
	w.Write(body)
}

type httpGetter struct {
	transport func(Context) http.RoundTripper
	baseURL   string
}

func (h *httpGetter) Get(context Context, in *pb.GetRequest, out *pb.GetResponse) error {
	u := fmt.Sprintf(
		"%v%v/%v",
		h.baseURL,
		url.QueryEscape(in.GetGroup()),
		url.QueryEscape(in.GetKey()),
	)
	req, err := http.NewRequest("GET", u, nil)
	if err != nil {
		return err
	}
	tr := http.DefaultTransport
	if h.transport != nil {
		tr = h.transport(context)
	}
	res, err := tr.RoundTrip(req)
	if err != nil {
		return err
	}
	if res.StatusCode != http.StatusOK {
		return fmt.Errorf("server returned: %v", res.Status)
	}
	defer res.Body.Close()
	// TODO: avoid this garbage.
	b, err := ioutil.ReadAll(res.Body)
	if err != nil {
		return fmt.Errorf("reading response body: %v", err)
	}
	err = proto.Unmarshal(b, out)
	if err != nil {
		return fmt.Errorf("decoding response body: %v", err)
	}
	return nil
}