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// Copyright 2011 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.

package template

import (
        "bytes"
        "fmt"
        "strings"
)

// urlFilter returns its input unless it contains an unsafe protocol in which
// case it defangs the entire URL.
func urlFilter(args ...interface{}) string {
        s, t := stringify(args...)
        if t == contentTypeURL {
                return s
        }
        if i := strings.IndexRune(s, ':'); i >= 0 && strings.IndexRune(s[:i], '/') < 0 {
                protocol := strings.ToLower(s[:i])
                if protocol != "http" && protocol != "https" && protocol != "mailto" {
                        return "#" + filterFailsafe
                }
        }
        return s
}

// urlEscaper produces an output that can be embedded in a URL query.
// The output can be embedded in an HTML attribute without further escaping.
func urlEscaper(args ...interface{}) string {
        return urlProcessor(false, args...)
}

// urlEscaper normalizes URL content so it can be embedded in a quote-delimited
// string or parenthesis delimited url(...).
// The normalizer does not encode all HTML specials. Specifically, it does not
// encode '&' so correct embedding in an HTML attribute requires escaping of
// '&' to '&amp;'.
func urlNormalizer(args ...interface{}) string {
        return urlProcessor(true, args...)
}

// urlProcessor normalizes (when norm is true) or escapes its input to produce
// a valid hierarchical or opaque URL part.
func urlProcessor(norm bool, args ...interface{}) string {
        s, t := stringify(args...)
        if t == contentTypeURL {
                norm = true
        }
        var b bytes.Buffer
        written := 0
        // The byte loop below assumes that all URLs use UTF-8 as the
        // content-encoding. This is similar to the URI to IRI encoding scheme
        // defined in section 3.1 of  RFC 3987, and behaves the same as the
        // EcmaScript builtin encodeURIComponent.
        // It should not cause any misencoding of URLs in pages with
        // Content-type: text/html;charset=UTF-8.
        for i, n := 0, len(s); i < n; i++ {
                c := s[i]
                switch c {
                // Single quote and parens are sub-delims in RFC 3986, but we
                // escape them so the output can be embedded in in single
                // quoted attributes and unquoted CSS url(...) constructs.
                // Single quotes are reserved in URLs, but are only used in
                // the obsolete "mark" rule in an appendix in RFC 3986
                // so can be safely encoded.
                case '!', '#', '$', '&', '*', '+', ',', '/', ':', ';', '=', '?', '@', '[', ']':
                        if norm {
                                continue
                        }
                // Unreserved according to RFC 3986 sec 2.3
                // "For consistency, percent-encoded octets in the ranges of
                // ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D),
                // period (%2E), underscore (%5F), or tilde (%7E) should not be
                // created by URI producers
                case '-', '.', '_', '~':
                        continue
                case '%':
                        // When normalizing do not re-encode valid escapes.
                        if norm && i+2 < len(s) && isHex(s[i+1]) && isHex(s[i+2]) {
                                continue
                        }
                default:
                        // Unreserved according to RFC 3986 sec 2.3
                        if 'a' <= c && c <= 'z' {
                                continue
                        }
                        if 'A' <= c && c <= 'Z' {
                                continue
                        }
                        if '0' <= c && c <= '9' {
                                continue
                        }
                }
                b.WriteString(s[written:i])
                fmt.Fprintf(&b, "%%%02x", c)
                written = i + 1
        }
        if written == 0 {
                return s
        }
        b.WriteString(s[written:])
        return b.String()
}