// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// HTTP file system request handler

package httputil

import (
	"net/http"
	"net/textproto"
	"strings"
	"time"
)

// scanETag determines if a syntactically valid ETag is present at s. If so,
// the ETag and remaining text after consuming ETag is returned. Otherwise,
// it returns "", "".
func scanETag(s string) (etag string, remain string) {
	s = textproto.TrimString(s)
	start := 0
	if strings.HasPrefix(s, "W/") {
		start = 2
	}
	if len(s[start:]) < 2 || s[start] != '"' {
		return "", ""
	}
	// ETag is either W/"text" or "text".
	// See RFC 7232 2.3.
	for i := start + 1; i < len(s); i++ {
		c := s[i]
		switch {
		// Character values allowed in ETags.
		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
		case c == '"':
			return s[:i+1], s[i+1:]
		default:
			return "", ""
		}
	}
	return "", ""
}

// etagStrongMatch reports whether a and b match using strong ETag comparison.
// Assumes a and b are valid ETags.
func etagStrongMatch(a, b string) bool {
	return a == b && a != "" && a[0] == '"'
}

// etagWeakMatch reports whether a and b match using weak ETag comparison.
// Assumes a and b are valid ETags.
func etagWeakMatch(a, b string) bool {
	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
}

// condResult is the result of an HTTP request precondition check.
// See https://tools.ietf.org/html/rfc7232 section 3.
type condResult int

const (
	condNone condResult = iota
	condTrue
	condFalse
)

func checkIfMatch(w http.ResponseWriter, r *http.Request) condResult {
	im := r.Header.Get("If-Match")
	if im == "" {
		return condNone
	}
	for {
		im = textproto.TrimString(im)
		if len(im) == 0 {
			break
		}
		if im[0] == ',' {
			im = im[1:]
			continue
		}
		if im[0] == '*' {
			return condTrue
		}
		etag, remain := scanETag(im)
		if etag == "" {
			break
		}
		if etagStrongMatch(etag, w.Header().Get("Etag")) {
			return condTrue
		}
		im = remain
	}

	return condFalse
}

func checkIfUnmodifiedSince(r *http.Request, modtime time.Time) condResult {
	ius := r.Header.Get("If-Unmodified-Since")
	if ius == "" || isZeroTime(modtime) {
		return condNone
	}
	if t, err := http.ParseTime(ius); err == nil {
		// The Date-Modified header truncates sub-second precision, so
		// use mtime < t+1s instead of mtime <= t to check for unmodified.
		if modtime.Before(t.Add(1 * time.Second)) {
			return condTrue
		}
		return condFalse
	}
	return condNone
}

func checkIfNoneMatch(w http.ResponseWriter, r *http.Request) condResult {
	inm := r.Header.Get("If-None-Match")
	if inm == "" {
		return condNone
	}
	buf := inm
	for {
		buf = textproto.TrimString(buf)
		if len(buf) == 0 {
			break
		}
		if buf[0] == ',' {
			buf = buf[1:]
		}
		if buf[0] == '*' {
			return condFalse
		}
		etag, remain := scanETag(buf)
		if etag == "" {
			break
		}
		if etagWeakMatch(etag, w.Header().Get("Etag")) {
			return condFalse
		}
		buf = remain
	}
	return condTrue
}

func checkIfModifiedSince(r *http.Request, modtime time.Time) condResult {
	if r.Method != "GET" && r.Method != "HEAD" {
		return condNone
	}
	ims := r.Header.Get("If-Modified-Since")
	if ims == "" || isZeroTime(modtime) {
		return condNone
	}
	t, err := http.ParseTime(ims)
	if err != nil {
		return condNone
	}
	// The Date-Modified header truncates sub-second precision, so
	// use mtime < t+1s instead of mtime <= t to check for unmodified.
	if modtime.Before(t.Add(1 * time.Second)) {
		return condFalse
	}
	return condTrue
}

var unixEpochTime = time.Unix(0, 0)

// isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
func isZeroTime(t time.Time) bool {
	return t.IsZero() || t.Equal(unixEpochTime)
}

func setLastModified(w http.ResponseWriter, modtime time.Time) {
	if !isZeroTime(modtime) {
		w.Header().Set("Last-Modified", modtime.UTC().Format(http.TimeFormat))
	}
}

func writeNotModified(w http.ResponseWriter) {
	// RFC 7232 section 4.1:
	// a sender SHOULD NOT generate representation metadata other than the
	// above listed fields unless said metadata exists for the purpose of
	// guiding cache updates (e.g., Last-Modified might be useful if the
	// response does not have an ETag field).
	h := w.Header()
	delete(h, "Content-Type")
	delete(h, "Content-Length")
	if h.Get("Etag") != "" {
		delete(h, "Last-Modified")
	}
	w.WriteHeader(http.StatusNotModified)
}

// CheckPreconditions evaluates request preconditions and reports whether a precondition
// resulted in sending StatusNotModified or StatusPreconditionFailed.
func CheckPreconditions(w http.ResponseWriter, r *http.Request, modtime time.Time) (done bool) {
	// This function carefully follows RFC 7232 section 6.
	ch := checkIfMatch(w, r)
	if ch == condNone {
		ch = checkIfUnmodifiedSince(r, modtime)
	}
	if ch == condFalse {
		w.WriteHeader(http.StatusPreconditionFailed)
		return true
	}
	switch checkIfNoneMatch(w, r) {
	case condFalse:
		if r.Method == "GET" || r.Method == "HEAD" {
			writeNotModified(w)
			return true
		} else {
			w.WriteHeader(http.StatusPreconditionFailed)
			return true
		}
	case condNone:
		if checkIfModifiedSince(r, modtime) == condFalse {
			writeNotModified(w)
			return true
		}
	}

	return false
}
