// Package rewrite is middleware for rewriting requests internally to
// a different path.
package rewrite

import (
	"net/http"

	"fmt"
	"github.com/mholt/caddy/middleware"
	"net/url"
	"path"
	"path/filepath"
	"regexp"
	"strings"
)

// Rewrite is middleware to rewrite request locations internally before being handled.
type Rewrite struct {
	Next  middleware.Handler
	Rules []Rule
}

// ServeHTTP implements the middleware.Handler interface.
func (rw Rewrite) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
	for _, rule := range rw.Rules {
		if ok := rule.Rewrite(r); ok {
			break
		}
	}
	return rw.Next.ServeHTTP(w, r)
}

// Rule describes an internal location rewrite rule.
type Rule interface {
	// Rewrite rewrites the internal location of the current request.
	Rewrite(*http.Request) bool
}

// SimpleRule is a simple rewrite rule.
type SimpleRule struct {
	From, To string
}

// NewSimpleRule creates a new Simple Rule
func NewSimpleRule(from, to string) SimpleRule {
	return SimpleRule{from, to}
}

// Rewrite rewrites the internal location of the current request.
func (s SimpleRule) Rewrite(r *http.Request) bool {
	if s.From == r.URL.Path {
		r.URL.Path = s.To
		return true
	}
	return false
}

// RegexpRule is a rewrite rule based on a regular expression
type RegexpRule struct {
	// Path base. Request to this path and subpaths will be rewritten
	Base string

	// Path to rewrite to
	To string

	// Extensions to filter by
	Exts []string

	*regexp.Regexp
}

// NewRegexpRule creates a new RegexpRule. It returns an error if regexp
// pattern (pattern) or extensions (ext) are invalid.
func NewRegexpRule(base, pattern, to string, ext []string) (*RegexpRule, error) {
	r, err := regexp.Compile(pattern)
	if err != nil {
		return nil, err
	}

	// validate extensions
	for _, v := range ext {
		if len(v) < 2 || (len(v) < 3 && v[0] == '!') {
			// check if no extension is specified
			if v != "/" && v != "!/" {
				return nil, fmt.Errorf("Invalid extension %v", v)
			}
		}
	}

	return &RegexpRule{
		base,
		to,
		ext,
		r,
	}, nil
}

// regexpVars are variables that can be used for To (rewrite destination path).
var regexpVars []string = []string{
	"{path}",
	"{query}",
	"{file}",
	"{dir}",
	"{frag}",
}

// Rewrite rewrites the internal location of the current request.
func (r *RegexpRule) Rewrite(req *http.Request) bool {
	rPath := req.URL.Path

	// validate base
	if !strings.HasPrefix(rPath, r.Base) {
		return false
	}

	// validate extensions
	if !r.matchExt(rPath) {
		return false
	}

	// validate regexp
	if !r.MatchString(rPath[len(r.Base):]) {
		return false
	}

	to := r.To

	// check variables
	for _, v := range regexpVars {
		if strings.Contains(r.To, v) {
			switch v {
			case "{path}":
				to = strings.Replace(to, v, req.URL.Path[1:], -1)
			case "{query}":
				to = strings.Replace(to, v, req.URL.RawQuery, -1)
			case "{frag}":
				to = strings.Replace(to, v, req.URL.Fragment, -1)
			case "{file}":
				_, file := path.Split(req.URL.Path)
				to = strings.Replace(to, v, file, -1)
			case "{dir}":
				dir, _ := path.Split(req.URL.Path)
				to = path.Clean(strings.Replace(to, v, dir, -1))
			}
		}
	}

	// validate resulting path
	url, err := url.Parse(to)
	if err != nil {
		return false
	}

	// perform rewrite
	req.URL.Path = url.Path
	if url.RawQuery != "" {
		// overwrite query string if present
		req.URL.RawQuery = url.RawQuery
	}
	return true
}

// matchExt matches rPath against registered file extensions.
// Returns true if a match is found and false otherwise.
func (r *RegexpRule) matchExt(rPath string) bool {
	f := filepath.Base(rPath)
	ext := path.Ext(f)
	if ext == "" {
		ext = "/"
	}

	mustUse := false
	for _, v := range r.Exts {
		use := true
		if v[0] == '!' {
			use = false
			v = v[1:]
		}

		if use {
			mustUse = true
		}

		if ext == v {
			return use
		}
	}

	if mustUse {
		return false
	}
	return true
}