// 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 rewrite

import (
	"net/http"
	"net/url"
	"strconv"
	"strings"

	"github.com/caddyserver/caddy/v2"
	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
	"go.uber.org/zap"
)

func init() {
	caddy.RegisterModule(Rewrite{})
}

// Rewrite is a middleware which can rewrite HTTP requests.
//
// These rewrite properties are applied to a request in this order:
// Method, URI, StripPrefix, StripSuffix, URISubstring.
//
// TODO: This module is still a WIP and may experience breaking changes.
type Rewrite struct {
	// Changes the request's HTTP verb.
	Method string `json:"method,omitempty"`

	// Changes the request's URI (path, query string, and fragment if present).
	// Only components of the URI that are specified will be changed.
	URI string `json:"uri,omitempty"`

	// Strips the given prefix from the beginning of the URI path.
	StripPrefix string `json:"strip_prefix,omitempty"`

	// Strips the given suffix from the end of the URI path.
	StripSuffix string `json:"strip_suffix,omitempty"`

	// Performs substring replacements on the URI.
	URISubstring []replacer `json:"uri_substring,omitempty"`

	// If set to a 3xx HTTP status code and if the URI was rewritten (changed),
	// the handler will issue a simple HTTP redirect to the new URI using the
	// given status code.
	HTTPRedirect caddyhttp.WeakString `json:"http_redirect,omitempty"`

	logger *zap.Logger
}

// CaddyModule returns the Caddy module information.
func (Rewrite) CaddyModule() caddy.ModuleInfo {
	return caddy.ModuleInfo{
		ID:  "http.handlers.rewrite",
		New: func() caddy.Module { return new(Rewrite) },
	}
}

// Provision sets up rewr.
func (rewr *Rewrite) Provision(ctx caddy.Context) error {
	rewr.logger = ctx.Logger(rewr)
	return nil
}

func (rewr Rewrite) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)

	logger := rewr.logger.With(
		zap.Object("request", caddyhttp.LoggableHTTPRequest{Request: r}),
	)

	changed := rewr.rewrite(r, repl, logger)

	if changed {
		logger.Debug("rewrote request",
			zap.String("method", r.Method),
			zap.String("uri", r.RequestURI),
		)
		if rewr.HTTPRedirect != "" {
			statusCode, err := strconv.Atoi(repl.ReplaceAll(rewr.HTTPRedirect.String(), ""))
			if err != nil {
				return caddyhttp.Error(http.StatusInternalServerError, err)
			}
			w.Header().Set("Location", r.RequestURI)
			w.WriteHeader(statusCode)
			return nil
		}
	}

	return next.ServeHTTP(w, r)
}

// rewrite performs the rewrites on r using repl, which should
// have been obtained from r, but is passed in for efficiency.
// It returns true if any changes were made to r.
func (rewr Rewrite) rewrite(r *http.Request, repl *caddy.Replacer, logger *zap.Logger) bool {
	oldMethod := r.Method
	oldURI := r.RequestURI

	// method
	if rewr.Method != "" {
		r.Method = strings.ToUpper(repl.ReplaceAll(rewr.Method, ""))
	}

	// uri (path, query string, and fragment just because)
	if uri := rewr.URI; uri != "" {
		// find the bounds of each part of the URI that exist
		pathStart, qsStart, fragStart := -1, -1, -1
		pathEnd, qsEnd := -1, -1
		for i, ch := range uri {
			switch {
			case ch == '?' && qsStart < 0:
				pathEnd, qsStart = i, i+1
			case ch == '#' && fragStart < 0:
				qsEnd, fragStart = i, i+1
			case pathStart < 0 && qsStart < 0 && fragStart < 0:
				pathStart = i
			}
		}
		if pathStart >= 0 && pathEnd < 0 {
			pathEnd = len(uri)
		}
		if qsStart >= 0 && qsEnd < 0 {
			qsEnd = len(uri)
		}

		if pathStart >= 0 {
			r.URL.Path = repl.ReplaceAll(uri[pathStart:pathEnd], "")
		}
		if qsStart >= 0 {
			r.URL.RawQuery = buildQueryString(uri[qsStart:qsEnd], repl)
		}
		if fragStart >= 0 {
			r.URL.Fragment = repl.ReplaceAll(uri[fragStart:], "")
		}
	}

	// strip path prefix or suffix
	if rewr.StripPrefix != "" {
		prefix := repl.ReplaceAll(rewr.StripPrefix, "")
		r.URL.Path = strings.TrimPrefix(r.URL.Path, prefix)
	}
	if rewr.StripSuffix != "" {
		suffix := repl.ReplaceAll(rewr.StripSuffix, "")
		r.URL.Path = strings.TrimSuffix(r.URL.Path, suffix)
	}

	// substring replacements in URI
	for _, rep := range rewr.URISubstring {
		rep.do(r, repl)
	}

	// update the encoded copy of the URI
	r.RequestURI = r.URL.RequestURI()

	// return true if anything changed
	return r.Method != oldMethod || r.RequestURI != oldURI
}

// buildQueryString takes an input query string and
// performs replacements on each component, returning
// the resulting query string.
func buildQueryString(qs string, repl *caddy.Replacer) string {
	var sb strings.Builder
	var wroteKey bool

	for len(qs) > 0 {
		// determine the end of this component
		nextEq, nextAmp := strings.Index(qs, "="), strings.Index(qs, "&")
		end := min(nextEq, nextAmp)
		if end == -1 {
			end = len(qs) // if there is nothing left, go to end of string
		}

		// consume the component and write the result
		comp := qs[:end]
		comp, _ = repl.ReplaceFunc(comp, func(name, val string) (string, error) {
			if name == "http.request.uri.query" {
				return val, nil // already escaped
			}
			return url.QueryEscape(val), nil
		})
		if end < len(qs) {
			end++ // consume delimiter
		}
		qs = qs[end:]

		if wroteKey {
			sb.WriteRune('=')
		} else if sb.Len() > 0 {
			sb.WriteRune('&')
		}

		// remember that we just wrote a key, which is if the next
		// delimiter is an equals sign or if there is no ampersand
		wroteKey = nextEq < nextAmp || nextAmp < 0

		sb.WriteString(comp)
	}

	return sb.String()
}

func min(a, b int) int {
	if b < a {
		return b
	}
	return a
}

// replacer describes a simple and fast substring replacement.
type replacer struct {
	// The substring to find. Supports placeholders.
	Find string `json:"find,omitempty"`

	// The substring to replace. Supports placeholders.
	Replace string `json:"replace,omitempty"`

	// Maximum number of replacements per string.
	// Set to <= 0 for no limit (default).
	Limit int `json:"limit,omitempty"`
}

// do performs the replacement on r and returns true if any changes were made.
func (rep replacer) do(r *http.Request, repl *caddy.Replacer) bool {
	if rep.Find == "" || rep.Replace == "" {
		return false
	}

	lim := rep.Limit
	if lim == 0 {
		lim = -1
	}

	find := repl.ReplaceAll(rep.Find, "")
	replace := repl.ReplaceAll(rep.Replace, "")

	oldPath := r.URL.Path
	oldQuery := r.URL.RawQuery

	r.URL.Path = strings.Replace(oldPath, find, replace, lim)
	r.URL.RawQuery = strings.Replace(oldQuery, find, replace, lim)

	return r.URL.Path != oldPath && r.URL.RawQuery != oldQuery
}

// Interface guard
var _ caddyhttp.MiddlewareHandler = (*Rewrite)(nil)