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// Copyright 2010 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 mime

import (
        "bytes"
        "errors"
        "fmt"
        "strings"
        "unicode"
)

// FormatMediaType serializes mediatype t and the parameters
// param as a media type conforming to RFC 2045 and RFC 2616.
// The type and parameter names are written in lower-case.
// When any of the arguments result in a standard violation then
// FormatMediaType returns the empty string.
func FormatMediaType(t string, param map[string]string) string {
        slash := strings.Index(t, "/")
        if slash == -1 {
                return ""
        }
        major, sub := t[:slash], t[slash+1:]
        if !IsToken(major) || !IsToken(sub) {
                return ""
        }
        var b bytes.Buffer
        b.WriteString(strings.ToLower(major))
        b.WriteByte('/')
        b.WriteString(strings.ToLower(sub))

        for attribute, value := range param {
                b.WriteByte(';')
                b.WriteByte(' ')
                if !IsToken(attribute) {
                        return ""
                }
                b.WriteString(strings.ToLower(attribute))
                b.WriteByte('=')
                if IsToken(value) {
                        b.WriteString(value)
                        continue
                }

                b.WriteByte('"')
                offset := 0
                for index, character := range value {
                        if character == '"' || character == '\r' {
                                b.WriteString(value[offset:index])
                                offset = index
                                b.WriteByte('\\')
                        }
                        if character&0x80 != 0 {
                                return ""
                        }
                }
                b.WriteString(value[offset:])
                b.WriteByte('"')
        }
        return b.String()
}

func checkMediaTypeDisposition(s string) error {
        typ, rest := consumeToken(s)
        if typ == "" {
                return errors.New("mime: no media type")
        }
        if rest == "" {
                return nil
        }
        if !strings.HasPrefix(rest, "/") {
                return errors.New("mime: expected slash after first token")
        }
        subtype, rest := consumeToken(rest[1:])
        if subtype == "" {
                return errors.New("mime: expected token after slash")
        }
        if rest != "" {
                return errors.New("mime: unexpected content after media subtype")
        }
        return nil
}

// ParseMediaType parses a media type value and any optional
// parameters, per RFC 1521.  Media types are the values in
// Content-Type and Content-Disposition headers (RFC 2183).
// On success, ParseMediaType returns the media type converted
// to lowercase and trimmed of white space and a non-nil map.
// The returned map, params, maps from the lowercase
// attribute to the attribute value with its case preserved.
func ParseMediaType(v string) (mediatype string, params map[string]string, err error) {
        i := strings.Index(v, ";")
        if i == -1 {
                i = len(v)
        }
        mediatype = strings.TrimSpace(strings.ToLower(v[0:i]))

        err = checkMediaTypeDisposition(mediatype)
        if err != nil {
                return
        }

        params = make(map[string]string)

        // Map of base parameter name -> parameter name -> value
        // for parameters containing a '*' character.
        // Lazily initialized.
        var continuation map[string]map[string]string

        v = v[i:]
        for len(v) > 0 {
                v = strings.TrimLeftFunc(v, unicode.IsSpace)
                if len(v) == 0 {
                        break
                }
                key, value, rest := consumeMediaParam(v)
                if key == "" {
                        if strings.TrimSpace(rest) == ";" {
                                // Ignore trailing semicolons.
                                // Not an error.
                                return
                        }
                        // Parse error.
                        return "", nil, errors.New("mime: invalid media parameter")
                }

                pmap := params
                if idx := strings.Index(key, "*"); idx != -1 {
                        baseName := key[:idx]
                        if continuation == nil {
                                continuation = make(map[string]map[string]string)
                        }
                        var ok bool
                        if pmap, ok = continuation[baseName]; !ok {
                                continuation[baseName] = make(map[string]string)
                                pmap = continuation[baseName]
                        }
                }
                if _, exists := pmap[key]; exists {
                        // Duplicate parameter name is bogus.
                        return "", nil, errors.New("mime: duplicate parameter name")
                }
                pmap[key] = value
                v = rest
        }

        // Stitch together any continuations or things with stars
        // (i.e. RFC 2231 things with stars: "foo*0" or "foo*")
        var buf bytes.Buffer
        for key, pieceMap := range continuation {
                singlePartKey := key + "*"
                if v, ok := pieceMap[singlePartKey]; ok {
                        decv := decode2231Enc(v)
                        params[key] = decv
                        continue
                }

                buf.Reset()
                valid := false
                for n := 0; ; n++ {
                        simplePart := fmt.Sprintf("%s*%d", key, n)
                        if v, ok := pieceMap[simplePart]; ok {
                                valid = true
                                buf.WriteString(v)
                                continue
                        }
                        encodedPart := simplePart + "*"
                        if v, ok := pieceMap[encodedPart]; ok {
                                valid = true
                                if n == 0 {
                                        buf.WriteString(decode2231Enc(v))
                                } else {
                                        decv, _ := percentHexUnescape(v)
                                        buf.WriteString(decv)
                                }
                        } else {
                                break
                        }
                }
                if valid {
                        params[key] = buf.String()
                }
        }

        return
}

func decode2231Enc(v string) string {
        sv := strings.SplitN(v, "'", 3)
        if len(sv) != 3 {
                return ""
        }
        // TODO: ignoring lang in sv[1] for now. If anybody needs it we'll
        // need to decide how to expose it in the API. But I'm not sure
        // anybody uses it in practice.
        charset := strings.ToLower(sv[0])
        if charset != "us-ascii" && charset != "utf-8" {
                // TODO: unsupported encoding
                return ""
        }
        encv, _ := percentHexUnescape(sv[2])
        return encv
}

func isNotTokenChar(r rune) bool {
        return !IsTokenChar(r)
}

// consumeToken consumes a token from the beginning of provided
// string, per RFC 2045 section 5.1 (referenced from 2183), and return
// the token consumed and the rest of the string.  Returns ("", v) on
// failure to consume at least one character.
func consumeToken(v string) (token, rest string) {
        notPos := strings.IndexFunc(v, isNotTokenChar)
        if notPos == -1 {
                return v, ""
        }
        if notPos == 0 {
                return "", v
        }
        return v[0:notPos], v[notPos:]
}

// consumeValue consumes a "value" per RFC 2045, where a value is
// either a 'token' or a 'quoted-string'.  On success, consumeValue
// returns the value consumed (and de-quoted/escaped, if a
// quoted-string) and the rest of the string.  On failure, returns
// ("", v).
func consumeValue(v string) (value, rest string) {
        if !strings.HasPrefix(v, `"`) && !strings.HasPrefix(v, `'`) {
                return consumeToken(v)
        }

        leadQuote := rune(v[0])

        // parse a quoted-string
        rest = v[1:] // consume the leading quote
        buffer := new(bytes.Buffer)
        var idx int
        var r rune
        var nextIsLiteral bool
        for idx, r = range rest {
                switch {
                case nextIsLiteral:
                        buffer.WriteRune(r)
                        nextIsLiteral = false
                case r == leadQuote:
                        return buffer.String(), rest[idx+1:]
                case r == '\\':
                        nextIsLiteral = true
                case r != '\r' && r != '\n':
                        buffer.WriteRune(r)
                default:
                        return "", v
                }
        }
        return "", v
}

func consumeMediaParam(v string) (param, value, rest string) {
        rest = strings.TrimLeftFunc(v, unicode.IsSpace)
        if !strings.HasPrefix(rest, ";") {
                return "", "", v
        }

        rest = rest[1:] // consume semicolon
        rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
        param, rest = consumeToken(rest)
        param = strings.ToLower(param)
        if param == "" {
                return "", "", v
        }

        rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
        if !strings.HasPrefix(rest, "=") {
                return "", "", v
        }
        rest = rest[1:] // consume equals sign
        rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
        value, rest = consumeValue(rest)
        if value == "" {
                return "", "", v
        }
        return param, value, rest
}

func percentHexUnescape(s string) (string, error) {
        // Count %, check that they're well-formed.
        percents := 0
        for i := 0; i < len(s); {
                if s[i] != '%' {
                        i++
                        continue
                }
                percents++
                if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
                        s = s[i:]
                        if len(s) > 3 {
                                s = s[0:3]
                        }
                        return "", fmt.Errorf("mime: bogus characters after %%: %q", s)
                }
                i += 3
        }
        if percents == 0 {
                return s, nil
        }

        t := make([]byte, len(s)-2*percents)
        j := 0
        for i := 0; i < len(s); {
                switch s[i] {
                case '%':
                        t[j] = unhex(s[i+1])<<4 | unhex(s[i+2])
                        j++
                        i += 3
                default:
                        t[j] = s[i]
                        j++
                        i++
                }
        }
        return string(t), nil
}

func ishex(c byte) bool {
        switch {
        case '0' <= c && c <= '9':
                return true
        case 'a' <= c && c <= 'f':
                return true
        case 'A' <= c && c <= 'F':
                return true
        }
        return false
}

func unhex(c byte) byte {
        switch {
        case '0' <= c && c <= '9':
                return c - '0'
        case 'a' <= c && c <= 'f':
                return c - 'a' + 10
        case 'A' <= c && c <= 'F':
                return c - 'A' + 10
        }
        return 0
}