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// 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.

// Godoc comment extraction and comment -> HTML formatting.

package doc

import (
        "io"
        "regexp"
        "strings"
        "text/template" // for HTMLEscape
        "unicode"
        "unicode/utf8"
)

var (
        ldquo = []byte("“")
        rdquo = []byte("”")
)

// Escape comment text for HTML. If nice is set,
// also turn `` into “ and '' into ”.
func commentEscape(w io.Writer, text string, nice bool) {
        last := 0
        if nice {
                for i := 0; i < len(text)-1; i++ {
                        ch := text[i]
                        if ch == text[i+1] && (ch == '`' || ch == '\'') {
                                template.HTMLEscape(w, []byte(text[last:i]))
                                last = i + 2
                                switch ch {
                                case '`':
                                        w.Write(ldquo)
                                case '\'':
                                        w.Write(rdquo)
                                }
                                i++ // loop will add one more
                        }
                }
        }
        template.HTMLEscape(w, []byte(text[last:]))
}

const (
        // Regexp for Go identifiers
        identRx = `[a-zA-Z_][a-zA-Z_0-9]*` // TODO(gri) ASCII only for now - fix this

        // Regexp for URLs
        protocol = `(https?|ftp|file|gopher|mailto|news|nntp|telnet|wais|prospero):`
        hostPart = `[a-zA-Z0-9_@\-]+`
        filePart = `[a-zA-Z0-9_?%#~&/\-+=]+`
        urlRx    = protocol + `//` + // http://
                hostPart + `([.:]` + hostPart + `)*/?` + // //www.google.com:8080/
                filePart + `([:.,]` + filePart + `)*`
)

var matchRx = regexp.MustCompile(`(` + urlRx + `)|(` + identRx + `)`)

var (
        html_a      = []byte(`<a href="`)
        html_aq     = []byte(`">`)
        html_enda   = []byte("</a>")
        html_i      = []byte("<i>")
        html_endi   = []byte("</i>")
        html_p      = []byte("<p>\n")
        html_endp   = []byte("</p>\n")
        html_pre    = []byte("<pre>")
        html_endpre = []byte("</pre>\n")
        html_h      = []byte(`<h3 id="`)
        html_hq     = []byte(`">`)
        html_endh   = []byte("</h3>\n")
)

// Emphasize and escape a line of text for HTML. URLs are converted into links;
// if the URL also appears in the words map, the link is taken from the map (if
// the corresponding map value is the empty string, the URL is not converted
// into a link). Go identifiers that appear in the words map are italicized; if
// the corresponding map value is not the empty string, it is considered a URL
// and the word is converted into a link. If nice is set, the remaining text's
// appearance is improved where it makes sense (e.g., `` is turned into &ldquo;
// and '' into &rdquo;).
func emphasize(w io.Writer, line string, words map[string]string, nice bool) {
        for {
                m := matchRx.FindStringSubmatchIndex(line)
                if m == nil {
                        break
                }
                // m >= 6 (two parenthesized sub-regexps in matchRx, 1st one is urlRx)

                // write text before match
                commentEscape(w, line[0:m[0]], nice)

                // analyze match
                match := line[m[0]:m[1]]
                url := ""
                italics := false
                if words != nil {
                        url, italics = words[string(match)]
                }
                if m[2] >= 0 {
                        // match against first parenthesized sub-regexp; must be match against urlRx
                        if !italics {
                                // no alternative URL in words list, use match instead
                                url = string(match)
                        }
                        italics = false // don't italicize URLs
                }

                // write match
                if len(url) > 0 {
                        w.Write(html_a)
                        template.HTMLEscape(w, []byte(url))
                        w.Write(html_aq)
                }
                if italics {
                        w.Write(html_i)
                }
                commentEscape(w, match, nice)
                if italics {
                        w.Write(html_endi)
                }
                if len(url) > 0 {
                        w.Write(html_enda)
                }

                // advance
                line = line[m[1]:]
        }
        commentEscape(w, line, nice)
}

func indentLen(s string) int {
        i := 0
        for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
                i++
        }
        return i
}

func isBlank(s string) bool {
        return len(s) == 0 || (len(s) == 1 && s[0] == '\n')
}

func commonPrefix(a, b string) string {
        i := 0
        for i < len(a) && i < len(b) && a[i] == b[i] {
                i++
        }
        return a[0:i]
}

func unindent(block []string) {
        if len(block) == 0 {
                return
        }

        // compute maximum common white prefix
        prefix := block[0][0:indentLen(block[0])]
        for _, line := range block {
                if !isBlank(line) {
                        prefix = commonPrefix(prefix, line[0:indentLen(line)])
                }
        }
        n := len(prefix)

        // remove
        for i, line := range block {
                if !isBlank(line) {
                        block[i] = line[n:]
                }
        }
}

// heading returns the trimmed line if it passes as a section heading;
// otherwise it returns the empty string. 
func heading(line string) string {
        line = strings.TrimSpace(line)
        if len(line) == 0 {
                return ""
        }

        // a heading must start with an uppercase letter
        r, _ := utf8.DecodeRuneInString(line)
        if !unicode.IsLetter(r) || !unicode.IsUpper(r) {
                return ""
        }

        // it must end in a letter or digit:
        r, _ = utf8.DecodeLastRuneInString(line)
        if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
                return ""
        }

        // exclude lines with illegal characters
        if strings.IndexAny(line, ",.;:!?+*/=()[]{}_^°&§~%#@<\">\\") >= 0 {
                return ""
        }

        // allow "'" for possessive "'s" only
        for b := line; ; {
                i := strings.IndexRune(b, '\'')
                if i < 0 {
                        break
                }
                if i+1 >= len(b) || b[i+1] != 's' || (i+2 < len(b) && b[i+2] != ' ') {
                        return "" // not followed by "s "
                }
                b = b[i+2:]
        }

        return line
}

type op int

const (
        opPara op = iota
        opHead
        opPre
)

type block struct {
        op    op
        lines []string
}

var nonAlphaNumRx = regexp.MustCompile(`[^a-zA-Z0-9]`)

func anchorID(line string) string {
        return nonAlphaNumRx.ReplaceAllString(line, "_")
}

// ToHTML converts comment text to formatted HTML.
// The comment was prepared by DocReader,
// so it is known not to have leading, trailing blank lines
// nor to have trailing spaces at the end of lines.
// The comment markers have already been removed.
//
// Turn each run of multiple \n into </p><p>.
// Turn each run of indented lines into a <pre> block without indent.
// Enclose headings with header tags.
//
// URLs in the comment text are converted into links; if the URL also appears
// in the words map, the link is taken from the map (if the corresponding map
// value is the empty string, the URL is not converted into a link).
//
// Go identifiers that appear in the words map are italicized; if the corresponding
// map value is not the empty string, it is considered a URL and the word is converted
// into a link.
func ToHTML(w io.Writer, text string, words map[string]string) {
        for _, b := range blocks(text) {
                switch b.op {
                case opPara:
                        w.Write(html_p)
                        for _, line := range b.lines {
                                emphasize(w, line, words, true)
                        }
                        w.Write(html_endp)
                case opHead:
                        w.Write(html_h)
                        id := ""
                        for _, line := range b.lines {
                                if id == "" {
                                        id = anchorID(line)
                                        w.Write([]byte(id))
                                        w.Write(html_hq)
                                }
                                commentEscape(w, line, true)
                        }
                        if id == "" {
                                w.Write(html_hq)
                        }
                        w.Write(html_endh)
                case opPre:
                        w.Write(html_pre)
                        for _, line := range b.lines {
                                emphasize(w, line, nil, false)
                        }
                        w.Write(html_endpre)
                }
        }
}

func blocks(text string) []block {
        var (
                out  []block
                para []string

                lastWasBlank   = false
                lastWasHeading = false
        )

        close := func() {
                if para != nil {
                        out = append(out, block{opPara, para})
                        para = nil
                }
        }

        lines := strings.SplitAfter(text, "\n")
        unindent(lines)
        for i := 0; i < len(lines); {
                line := lines[i]
                if isBlank(line) {
                        // close paragraph
                        close()
                        i++
                        lastWasBlank = true
                        continue
                }
                if indentLen(line) > 0 {
                        // close paragraph
                        close()

                        // count indented or blank lines
                        j := i + 1
                        for j < len(lines) && (isBlank(lines[j]) || indentLen(lines[j]) > 0) {
                                j++
                        }
                        // but not trailing blank lines
                        for j > i && isBlank(lines[j-1]) {
                                j--
                        }
                        pre := lines[i:j]
                        i = j

                        unindent(pre)

                        // put those lines in a pre block
                        out = append(out, block{opPre, pre})
                        lastWasHeading = false
                        continue
                }

                if lastWasBlank && !lastWasHeading && i+2 < len(lines) &&
                        isBlank(lines[i+1]) && !isBlank(lines[i+2]) && indentLen(lines[i+2]) == 0 {
                        // current line is non-blank, surrounded by blank lines
                        // and the next non-blank line is not indented: this
                        // might be a heading.
                        if head := heading(line); head != "" {
                                close()
                                out = append(out, block{opHead, []string{head}})
                                i += 2
                                lastWasHeading = true
                                continue
                        }
                }

                // open paragraph
                lastWasBlank = false
                lastWasHeading = false
                para = append(para, lines[i])
                i++
        }
        close()

        return out
}

// ToText prepares comment text for presentation in textual output.
// It wraps paragraphs of text to width or fewer Unicode code points
// and then prefixes each line with the indent.  In preformatted sections
// (such as program text), it prefixes each non-blank line with preIndent.
func ToText(w io.Writer, text string, indent, preIndent string, width int) {
        l := lineWrapper{
                out:    w,
                width:  width,
                indent: indent,
        }
        for _, b := range blocks(text) {
                switch b.op {
                case opPara:
                        // l.write will add leading newline if required
                        for _, line := range b.lines {
                                l.write(line)
                        }
                        l.flush()
                case opHead:
                        w.Write(nl)
                        for _, line := range b.lines {
                                l.write(line + "\n")
                        }
                        l.flush()
                case opPre:
                        w.Write(nl)
                        for _, line := range b.lines {
                                if !isBlank(line) {
                                        w.Write([]byte(preIndent))
                                        w.Write([]byte(line))
                                }
                        }
                }
        }
}

type lineWrapper struct {
        out       io.Writer
        printed   bool
        width     int
        indent    string
        n         int
        pendSpace int
}

var nl = []byte("\n")
var space = []byte(" ")

func (l *lineWrapper) write(text string) {
        if l.n == 0 && l.printed {
                l.out.Write(nl) // blank line before new paragraph
        }
        l.printed = true

        for _, f := range strings.Fields(text) {
                w := utf8.RuneCountInString(f)
                // wrap if line is too long
                if l.n > 0 && l.n+l.pendSpace+w > l.width {
                        l.out.Write(nl)
                        l.n = 0
                        l.pendSpace = 0
                }
                if l.n == 0 {
                        l.out.Write([]byte(l.indent))
                }
                l.out.Write(space[:l.pendSpace])
                l.out.Write([]byte(f))
                l.n += l.pendSpace + w
                l.pendSpace = 1
        }
}

func (l *lineWrapper) flush() {
        if l.n == 0 {
                return
        }
        l.out.Write(nl)
        l.pendSpace = 0
        l.n = 0
}