package httpr

import (
	"context"
	"errors"
	"net/http"
	"strings"
)

type path []string

// newPath ensures that a path provided is correct and splits it.
func newPath(path string) (path, error) {
	pathLen := len(path)

	if pathLen == 0 {
		return nil, errors.New("empty path is not allowed")
	}

	if path[0] != '/' {
		return nil, errors.New("path should start with a slash symbol \"/\"")
	}

	if strings.Count(path, "*") > 1 {
		return nil, errors.New("path can have only one catch-all parameter \"*\"")
	}

	if path[pathLen-1] == '/' {
		path = path[:pathLen-1]
	}

	parts := strings.Split(path, "/")

	parts[0] = "/"

	return parts, nil
}

// newServePath is a reduced version of newPath for ServeHTTP.
func newServePath(path string) (path, error) {
	if path[0] != '/' {
		return nil, errors.New("path should start with a slash symbol \"/\"")
	}

	path = strings.ReplaceAll(path, "//", "/")

	pathLen := len(path)
	if path[pathLen-1] == '/' {
		path = path[:pathLen-1]
	}

	parts := strings.Split(path, "/")

	parts[0] = "/"

	return parts, nil
}

// Params holds path parameters that are set as :key.
type Params map[string]string

type paramsKey struct{}

// ParamsKey is used as a key for Params in a request's Context.
var ParamsKey paramsKey = paramsKey{}

type node struct {
	endpoint string
	children []*node
	handler  http.HandlerFunc
}

func (n *node) get(path path) (http.HandlerFunc, Params) {
	pathLen := len(path)
	curNode := n

outer:
	for i := range path {
		// Check if this node is a catch-all endpoint.
		if curNode.endpoint[0] == '*' {
			var p Params = Params{}
			p[curNode.endpoint[1:]] = strings.Join(path[i:], "/")
			return curNode.handler, p
		}

		// If this is a parametrised endpoint, then add its name to
		// a path's part. It will be used further to parse parameters.
		if curNode.endpoint[0] == ':' {
			path[i] = curNode.endpoint + ":" + path[i]
		}

		pathNextIdx := i + 1

		if pathLen == pathNextIdx {
			var params Params = make(Params)

			for _, part := range path {
				if part[0] == ':' {
					param := strings.Split(part[1:], ":")
					params[param[0]] = param[1]
				}
			}

			return curNode.handler, params
		}

		if pathLen > pathNextIdx {
			if len(curNode.children) == 0 {
				break outer
			}

			var paramNode *node

			for _, next := range curNode.children {
				if next.endpoint == path[pathNextIdx] {
					curNode = next
					continue outer
				}

				if next.endpoint[0] == ':' || next.endpoint[0] == '*' {
					paramNode = next
				}
			}

			if paramNode != nil {
				curNode = paramNode
				continue outer
			}

			break outer
		}
	}

	return nil, nil
}

func (n *node) add(path path, handler http.HandlerFunc) error {
	pathLastIdx := len(path) - 1
	curNode := n

outer:
	for i := range path {
		if pathLastIdx == i {
			if curNode.handler != nil {
				return errors.New("attempt to redefine a handler for already existing path")
			}
			curNode.endpoint = path[i]
			curNode.handler = handler
			return nil
		}

		pathNextIdx := i + 1

		for _, child := range curNode.children {
			firstChar := path[pathNextIdx][0]
			if (firstChar == ':' || firstChar == '*') && firstChar == child.endpoint[0] {
				// Do not allow different param names, because only the first one
				// is saved, so a param won't be available by a new name.
				// Therefore, it is good to return an error because in this case
				// you're doing something wrong.
				if path[pathNextIdx] != child.endpoint {
					return errors.New("param names " + path[pathNextIdx] + " and " + child.endpoint + " are differ")
				}
				curNode = child
				continue outer
			}

			if child.endpoint == path[pathNextIdx] {
				curNode = child
				continue outer
			}
		}

		newChild := &node{endpoint: path[pathNextIdx]}
		curNode.children = append(curNode.children, newChild)
		curNode = newChild
	}

	return nil
}

type Router struct {
	tree            map[string]*node
	NotFoundHandler http.HandlerFunc
}

func New() *Router {
	return &Router{tree: make(map[string]*node)}
}

func (rr *Router) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	tree, ok := rr.tree[r.Method]
	if !ok {
		http.Error(w, http.StatusText(http.StatusNotImplemented), http.StatusNotImplemented)
		return
	}

	path, err := newServePath(r.URL.Path)
	if err != nil {
		http.Error(w, err.Error(), http.StatusNotAcceptable)
		return
	}

	if handler, params := tree.get(path); handler != nil {
		if params != nil {
			r = r.WithContext(context.WithValue(r.Context(), ParamsKey, params))
		}
		handler(w, r)
	} else {
		if rr.NotFoundHandler != nil {
			rr.NotFoundHandler(w, r)
		} else {
			http.Error(w, "Not Found", http.StatusNotFound)
		}
	}
}

// Handler registers a handler for provided pattern for a given HTTP method.
// Pattern must start with a slash (/) symbol.
func (rr *Router) Handler(method, pattern string, handler http.HandlerFunc) error {
	path, err := newPath(pattern)
	if err != nil {
		return err
	}

	if rr.tree[method] == nil {
		rr.tree[method] = &node{endpoint: "/"}
	}

	return rr.tree[method].add(path, handler)
}

// ServeStatic serves a given file system.
//
// Path should end with /*filepath to work.
func (rr *Router) ServeStatic(path string, root http.FileSystem) error {
	fileServer := http.FileServer(root)

	return rr.Handler(http.MethodGet, path, func(w http.ResponseWriter, r *http.Request) {
		r.URL.Path = Param(r, "filepath")
		fileServer.ServeHTTP(w, r)
	})
}

// subPath attaches a base path in front of a pattern.
//
// It is not a sub-router, it just passes a resulted pattern down to
// a router instance.
type subPath struct {
	r    *Router
	base string
}

// Sub creates a group of handlers with the same base path.
//
// How to use:
//
//	r := httpr.New()
//	...
//	s := r.Sub("/api/v1")
//	s.Handler(http.MethodGet, "/", func(w, r) {...})
//	s.Handler(http.MethodGet, "/section", func(w, r) {...})
func (rr *Router) Sub(base string) *subPath {
	if base[len(base)-1] == '/' {
		base = base[:len(base)-1]
	}

	return &subPath{
		r:    rr,
		base: base,
	}
}

// Handler registers a handler for a sub-path.
func (sp *subPath) Handler(method, pattern string, handler http.HandlerFunc) error {
	return sp.r.Handler(method, sp.base+pattern, handler)
}

// Param returns a URL parameter set with :key, or an empty string if not found.
func Param(r *http.Request, key string) string {
	if params := r.Context().Value(ParamsKey).(Params); params != nil {
		return params[key]
	}

	return ""
}