blackfriday/v2/markdown.go

// Blackfriday Markdown Processor
// Available at http://github.com/russross/blackfriday
//
// Copyright © 2011 Russ Ross <russ@russross.com>.
// Distributed under the Simplified BSD License.
// See README.md for details.

package blackfriday

import (
        "bytes"
        "fmt"
        "io"
        "strings"
        "unicode/utf8"
)

//
// Markdown parsing and processing
//

// Version string of the package. Appears in the rendered document when
// CompletePage flag is on.
const Version = "2.0"

// Extensions is a bitwise or'ed collection of enabled Blackfriday's
// extensions.
type Extensions int

// These are the supported markdown parsing extensions.
// OR these values together to select multiple extensions.
const (
        NoExtensions           Extensions = 0
        NoIntraEmphasis        Extensions = 1 << iota // Ignore emphasis markers inside words
        Tables                                        // Render tables
        FencedCode                                    // Render fenced code blocks
        Autolink                                      // Detect embedded URLs that are not explicitly marked
        Strikethrough                                 // Strikethrough text using ~~test~~
        LaxHTMLBlocks                                 // Loosen up HTML block parsing rules
        SpaceHeadings                                 // Be strict about prefix heading rules
        HardLineBreak                                 // Translate newlines into line breaks
        TabSizeEight                                  // Expand tabs to eight spaces instead of four
        Footnotes                                     // Pandoc-style footnotes
        NoEmptyLineBeforeBlock                        // No need to insert an empty line to start a (code, quote, ordered list, unordered list) block
        HeadingIDs                                    // specify heading IDs  with {#id}
        Titleblock                                    // Titleblock ala pandoc
        AutoHeadingIDs                                // Create the heading ID from the text
        BackslashLineBreak                            // Translate trailing backslashes into line breaks
        DefinitionLists                               // Render definition lists

        CommonHTMLFlags HTMLFlags = UseXHTML | Smartypants |
                SmartypantsFractions | SmartypantsDashes | SmartypantsLatexDashes

        CommonExtensions Extensions = NoIntraEmphasis | Tables | FencedCode |
                Autolink | Strikethrough | SpaceHeadings | HeadingIDs |
                BackslashLineBreak | DefinitionLists
)

// ListType contains bitwise or'ed flags for list and list item objects.
type ListType int

// These are the possible flag values for the ListItem renderer.
// Multiple flag values may be ORed together.
// These are mostly of interest if you are writing a new output format.
const (
        ListTypeOrdered ListType = 1 << iota
        ListTypeDefinition
        ListTypeTerm

        ListItemContainsBlock
        ListItemBeginningOfList // TODO: figure out if this is of any use now
        ListItemEndOfList
)

// CellAlignFlags holds a type of alignment in a table cell.
type CellAlignFlags int

// These are the possible flag values for the table cell renderer.
// Only a single one of these values will be used; they are not ORed together.
// These are mostly of interest if you are writing a new output format.
const (
        TableAlignmentLeft CellAlignFlags = 1 << iota
        TableAlignmentRight
        TableAlignmentCenter = (TableAlignmentLeft | TableAlignmentRight)
)

// The size of a tab stop.
const (
        TabSizeDefault = 4
        TabSizeDouble  = 8
)

// blockTags is a set of tags that are recognized as HTML block tags.
// Any of these can be included in markdown text without special escaping.
var blockTags = map[string]struct{}{
        "blockquote": {},
        "del":        {},
        "div":        {},
        "dl":         {},
        "fieldset":   {},
        "form":       {},
        "h1":         {},
        "h2":         {},
        "h3":         {},
        "h4":         {},
        "h5":         {},
        "h6":         {},
        "iframe":     {},
        "ins":        {},
        "math":       {},
        "noscript":   {},
        "ol":         {},
        "pre":        {},
        "p":          {},
        "script":     {},
        "style":      {},
        "table":      {},
        "ul":         {},

        // HTML5
        "address":    {},
        "article":    {},
        "aside":      {},
        "canvas":     {},
        "figcaption": {},
        "figure":     {},
        "footer":     {},
        "header":     {},
        "hgroup":     {},
        "main":       {},
        "nav":        {},
        "output":     {},
        "progress":   {},
        "section":    {},
        "video":      {},
}

// Renderer is the rendering interface. This is mostly of interest if you are
// implementing a new rendering format.
//
// Only an HTML implementation is provided in this repository, see the README
// for external implementations.
type Renderer interface {
        // RenderNode is the main rendering method. It will be called once for
        // every leaf node and twice for every non-leaf node (first with
        // entering=true, then with entering=false). The method should write its
        // rendition of the node to the supplied writer w.
        RenderNode(w io.Writer, node *Node, entering bool) WalkStatus

        // RenderHeader is a method that allows the renderer to produce some
        // content preceding the main body of the output document. The header is
        // understood in the broad sense here. For example, the default HTML
        // renderer will write not only the HTML document preamble, but also the
        // table of contents if it was requested.
        //
        // The method will be passed an entire document tree, in case a particular
        // implementation needs to inspect it to produce output.
        //
        // The output should be written to the supplied writer w. If your
        // implementation has no header to write, supply an empty implementation.
        RenderHeader(w io.Writer, ast *Node)

        // RenderFooter is a symmetric counterpart of RenderHeader.
        RenderFooter(w io.Writer, ast *Node)
}

// Callback functions for inline parsing. One such function is defined
// for each character that triggers a response when parsing inline data.
type inlineParser func(p *Markdown, data []byte, offset int) (int, *Node)

// Markdown is a type that holds extensions and the runtime state used by
// Parse, and the renderer. You can not use it directly, construct it with New.
type Markdown struct {
        renderer          Renderer
        referenceOverride ReferenceOverrideFunc
        refs              map[string]*reference
        inlineCallback    [256]inlineParser
        extensions        Extensions
        nesting           int
        maxNesting        int
        insideLink        bool

        // Footnotes need to be ordered as well as available to quickly check for
        // presence. If a ref is also a footnote, it's stored both in refs and here
        // in notes. Slice is nil if footnotes not enabled.
        notes []*reference

        doc                  *Node
        tip                  *Node // = doc
        oldTip               *Node
        lastMatchedContainer *Node // = doc
        allClosed            bool
}

func (p *Markdown) getRef(refid string) (ref *reference, found bool) {
        if p.referenceOverride != nil {
                r, overridden := p.referenceOverride(refid)
                if overridden {
                        if r == nil {
                                return nil, false
                        }
                        return &reference{
                                link:     []byte(r.Link),
                                title:    []byte(r.Title),
                                noteID:   0,
                                hasBlock: false,
                                text:     []byte(r.Text)}, true
                }
        }
        // refs are case insensitive
        ref, found = p.refs[strings.ToLower(refid)]
        return ref, found
}

func (p *Markdown) finalize(block *Node) {
        above := block.Parent
        block.open = false
        p.tip = above
}

func (p *Markdown) addChild(node NodeType, offset uint32) *Node {
        return p.addExistingChild(NewNode(node), offset)
}

func (p *Markdown) addExistingChild(node *Node, offset uint32) *Node {
        for !p.tip.canContain(node.Type) {
                p.finalize(p.tip)
        }
        p.tip.AppendChild(node)
        p.tip = node
        return node
}

func (p *Markdown) closeUnmatchedBlocks() {
        if !p.allClosed {
                for p.oldTip != p.lastMatchedContainer {
                        parent := p.oldTip.Parent
                        p.finalize(p.oldTip)
                        p.oldTip = parent
                }
                p.allClosed = true
        }
}

//
//
// Public interface
//
//

// Reference represents the details of a link.
// See the documentation in Options for more details on use-case.
type Reference struct {
        // Link is usually the URL the reference points to.
        Link string
        // Title is the alternate text describing the link in more detail.
        Title string
        // Text is the optional text to override the ref with if the syntax used was
        // [refid][]
        Text string
}

// ReferenceOverrideFunc is expected to be called with a reference string and
// return either a valid Reference type that the reference string maps to or
// nil. If overridden is false, the default reference logic will be executed.
// See the documentation in Options for more details on use-case.
type ReferenceOverrideFunc func(reference string) (ref *Reference, overridden bool)

// New constructs a Markdown processor. You can use the same With* functions as
// for Run() to customize parser's behavior and the renderer.
func New(opts ...Option) *Markdown {
        var p Markdown
        for _, opt := range opts {
                opt(&p)
        }
        p.refs = make(map[string]*reference)
        p.maxNesting = 16
        p.insideLink = false
        docNode := NewNode(Document)
        p.doc = docNode
        p.tip = docNode
        p.oldTip = docNode
        p.lastMatchedContainer = docNode
        p.allClosed = true
        // register inline parsers
        p.inlineCallback[' '] = maybeLineBreak
        p.inlineCallback['*'] = emphasis
        p.inlineCallback['_'] = emphasis
        if p.extensions&Strikethrough != 0 {
                p.inlineCallback['~'] = emphasis
        }
        p.inlineCallback['`'] = codeSpan
        p.inlineCallback['\n'] = lineBreak
        p.inlineCallback['['] = link
        p.inlineCallback['<'] = leftAngle
        p.inlineCallback['\\'] = escape
        p.inlineCallback['&'] = entity
        p.inlineCallback['!'] = maybeImage
        p.inlineCallback['^'] = maybeInlineFootnote
        if p.extensions&Autolink != 0 {
                p.inlineCallback['h'] = maybeAutoLink
                p.inlineCallback['m'] = maybeAutoLink
                p.inlineCallback['f'] = maybeAutoLink
                p.inlineCallback['H'] = maybeAutoLink
                p.inlineCallback['M'] = maybeAutoLink
                p.inlineCallback['F'] = maybeAutoLink
        }
        if p.extensions&Footnotes != 0 {
                p.notes = make([]*reference, 0)
        }
        return &p
}

// Option customizes the Markdown processor's default behavior.
type Option func(*Markdown)

// WithRenderer allows you to override the default renderer.
func WithRenderer(r Renderer) Option {
        return func(p *Markdown) {
                p.renderer = r
        }
}

// WithExtensions allows you to pick some of the many extensions provided by
// Blackfriday. You can bitwise OR them.
func WithExtensions(e Extensions) Option {
        return func(p *Markdown) {
                p.extensions = e
        }
}

// WithNoExtensions turns off all extensions and custom behavior.
func WithNoExtensions() Option {
        return func(p *Markdown) {
                p.extensions = NoExtensions
                p.renderer = NewHTMLRenderer(HTMLRendererParameters{
                        Flags: HTMLFlagsNone,
                })
        }
}

// WithRefOverride sets an optional function callback that is called every
// time a reference is resolved.
//
// In Markdown, the link reference syntax can be made to resolve a link to
// a reference instead of an inline URL, in one of the following ways:
//
//  * [link text][refid]
//  * [refid][]
//
// Usually, the refid is defined at the bottom of the Markdown document. If
// this override function is provided, the refid is passed to the override
// function first, before consulting the defined refids at the bottom. If
// the override function indicates an override did not occur, the refids at
// the bottom will be used to fill in the link details.
func WithRefOverride(o ReferenceOverrideFunc) Option {
        return func(p *Markdown) {
                p.referenceOverride = o
        }
}

// Run is the main entry point to Blackfriday. It parses and renders a
// block of markdown-encoded text.
//
// The simplest invocation of Run takes one argument, input:
//     output := Run(input)
// This will parse the input with CommonExtensions enabled and render it with
// the default HTMLRenderer (with CommonHTMLFlags).
//
// Variadic arguments opts can customize the default behavior. Since Markdown
// type does not contain exported fields, you can not use it directly. Instead,
// use the With* functions. For example, this will call the most basic
// functionality, with no extensions:
//     output := Run(input, WithNoExtensions())
//
// You can use any number of With* arguments, even contradicting ones. They
// will be applied in order of appearance and the latter will override the
// former:
//     output := Run(input, WithNoExtensions(), WithExtensions(exts),
//         WithRenderer(yourRenderer))
func Run(input []byte, opts ...Option) []byte {
        r := NewHTMLRenderer(HTMLRendererParameters{
                Flags: CommonHTMLFlags,
        })
        optList := []Option{WithRenderer(r), WithExtensions(CommonExtensions)}
        optList = append(optList, opts...)
        parser := New(optList...)
        ast := parser.Parse(input)
        var buf bytes.Buffer
        parser.renderer.RenderHeader(&buf, ast)
        ast.Walk(func(node *Node, entering bool) WalkStatus {
                return parser.renderer.RenderNode(&buf, node, entering)
        })
        parser.renderer.RenderFooter(&buf, ast)
        return buf.Bytes()
}

// Parse is an entry point to the parsing part of Blackfriday. It takes an
// input markdown document and produces a syntax tree for its contents. This
// tree can then be rendered with a default or custom renderer, or
// analyzed/transformed by the caller to whatever non-standard needs they have.
// The return value is the root node of the syntax tree.
func (p *Markdown) Parse(input []byte) *Node {
        p.block(input)
        // Walk the tree and finish up some of unfinished blocks
        for p.tip != nil {
                p.finalize(p.tip)
        }
        // Walk the tree again and process inline markdown in each block
        p.doc.Walk(func(node *Node, entering bool) WalkStatus {
                if node.Type == Paragraph || node.Type == Heading || node.Type == TableCell {
                        p.inline(node, node.content)
                        node.content = nil
                }
                return GoToNext
        })
        p.parseRefsToAST()
        return p.doc
}

func (p *Markdown) parseRefsToAST() {
        if p.extensions&Footnotes == 0 || len(p.notes) == 0 {
                return
        }
        p.tip = p.doc
        block := p.addBlock(List, nil)
        block.IsFootnotesList = true
        block.ListFlags = ListTypeOrdered
        flags := ListItemBeginningOfList
        // Note: this loop is intentionally explicit, not range-form. This is
        // because the body of the loop will append nested footnotes to p.notes and
        // we need to process those late additions. Range form would only walk over
        // the fixed initial set.
        for i := 0; i < len(p.notes); i++ {
                ref := p.notes[i]
                p.addExistingChild(ref.footnote, 0)
                block := ref.footnote
                block.ListFlags = flags | ListTypeOrdered
                block.RefLink = ref.link
                if ref.hasBlock {
                        flags |= ListItemContainsBlock
                        p.block(ref.title)
                } else {
                        p.inline(block, ref.title)
                }
                flags &^= ListItemBeginningOfList | ListItemContainsBlock
        }
        above := block.Parent
        finalizeList(block)
        p.tip = above
        block.Walk(func(node *Node, entering bool) WalkStatus {
                if node.Type == Paragraph || node.Type == Heading {
                        p.inline(node, node.content)
                        node.content = nil
                }
                return GoToNext
        })
}

//
// Link references
//
// This section implements support for references that (usually) appear
// as footnotes in a document, and can be referenced anywhere in the document.
// The basic format is:
//
//    [1]: http://www.google.com/ "Google"
//    [2]: http://www.github.com/ "Github"
//
// Anywhere in the document, the reference can be linked by referring to its
// label, i.e., 1 and 2 in this example, as in:
//
//    This library is hosted on [Github][2], a git hosting site.
//
// Actual footnotes as specified in Pandoc and supported by some other Markdown
// libraries such as php-markdown are also taken care of. They look like this:
//
//    This sentence needs a bit of further explanation.[^note]
//
//    [^note]: This is the explanation.
//
// Footnotes should be placed at the end of the document in an ordered list.
// Finally, there are inline footnotes such as:
//
//    Inline footnotes^[Also supported.] provide a quick inline explanation,
//    but are rendered at the bottom of the document.
//

// reference holds all information necessary for a reference-style links or
// footnotes.
//
// Consider this markdown with reference-style links:
//
//     [link][ref]
//
//     [ref]: /url/ "tooltip title"
//
// It will be ultimately converted to this HTML:
//
//     <p><a href=\"/url/\" title=\"title\">link</a></p>
//
// And a reference structure will be populated as follows:
//
//     p.refs["ref"] = &reference{
//         link: "/url/",
//         title: "tooltip title",
//     }
//
// Alternatively, reference can contain information about a footnote. Consider
// this markdown:
//
//     Text needing a footnote.[^a]
//
//     [^a]: This is the note
//
// A reference structure will be populated as follows:
//
//     p.refs["a"] = &reference{
//         link: "a",
//         title: "This is the note",
//         noteID: <some positive int>,
//     }
//
// TODO: As you can see, it begs for splitting into two dedicated structures
// for refs and for footnotes.
type reference struct {
        link     []byte
        title    []byte
        noteID   int // 0 if not a footnote ref
        hasBlock bool
        footnote *Node // a link to the Item node within a list of footnotes

        text []byte // only gets populated by refOverride feature with Reference.Text
}

func (r *reference) String() string {
        return fmt.Sprintf("{link: %q, title: %q, text: %q, noteID: %d, hasBlock: %v}",
                r.link, r.title, r.text, r.noteID, r.hasBlock)
}

// Check whether or not data starts with a reference link.
// If so, it is parsed and stored in the list of references
// (in the render struct).
// Returns the number of bytes to skip to move past it,
// or zero if the first line is not a reference.
func isReference(p *Markdown, data []byte, tabSize int) int {
        // up to 3 optional leading spaces
        if len(data) < 4 {
                return 0
        }
        i := 0
        for i < 3 && data[i] == ' ' {
                i++
        }

        noteID := 0

        // id part: anything but a newline between brackets
        if data[i] != '[' {
                return 0
        }
        i++
        if p.extensions&Footnotes != 0 {
                if i < len(data) && data[i] == '^' {
                        // we can set it to anything here because the proper noteIds will
                        // be assigned later during the second pass. It just has to be != 0
                        noteID = 1
                        i++
                }
        }
        idOffset := i
        for i < len(data) && data[i] != '\n' && data[i] != '\r' && data[i] != ']' {
                i++
        }
        if i >= len(data) || data[i] != ']' {
                return 0
        }
        idEnd := i
        // footnotes can have empty ID, like this: [^], but a reference can not be
        // empty like this: []. Break early if it's not a footnote and there's no ID
        if noteID == 0 && idOffset == idEnd {
                return 0
        }
        // spacer: colon (space | tab)* newline? (space | tab)*
        i++
        if i >= len(data) || data[i] != ':' {
                return 0
        }
        i++
        for i < len(data) && (data[i] == ' ' || data[i] == '\t') {
                i++
        }
        if i < len(data) && (data[i] == '\n' || data[i] == '\r') {
                i++
                if i < len(data) && data[i] == '\n' && data[i-1] == '\r' {
                        i++
                }
        }
        for i < len(data) && (data[i] == ' ' || data[i] == '\t') {
                i++
        }
        if i >= len(data) {
                return 0
        }

        var (
                linkOffset, linkEnd   int
                titleOffset, titleEnd int
                lineEnd               int
                raw                   []byte
                hasBlock              bool
        )

        if p.extensions&Footnotes != 0 && noteID != 0 {
                linkOffset, linkEnd, raw, hasBlock = scanFootnote(p, data, i, tabSize)
                lineEnd = linkEnd
        } else {
                linkOffset, linkEnd, titleOffset, titleEnd, lineEnd = scanLinkRef(p, data, i)
        }
        if lineEnd == 0 {
                return 0
        }

        // a valid ref has been found

        ref := &reference{
                noteID:   noteID,
                hasBlock: hasBlock,
        }

        if noteID > 0 {
                // reusing the link field for the id since footnotes don't have links
                ref.link = data[idOffset:idEnd]
                // if footnote, it's not really a title, it's the contained text
                ref.title = raw
        } else {
                ref.link = data[linkOffset:linkEnd]
                ref.title = data[titleOffset:titleEnd]
        }

        // id matches are case-insensitive
        id := string(bytes.ToLower(data[idOffset:idEnd]))

        p.refs[id] = ref

        return lineEnd
}

func scanLinkRef(p *Markdown, data []byte, i int) (linkOffset, linkEnd, titleOffset, titleEnd, lineEnd int) {
        // link: whitespace-free sequence, optionally between angle brackets
        if data[i] == '<' {
                i++
        }
        linkOffset = i
        for i < len(data) && data[i] != ' ' && data[i] != '\t' && data[i] != '\n' && data[i] != '\r' {
                i++
        }
        linkEnd = i
        if data[linkOffset] == '<' && data[linkEnd-1] == '>' {
                linkOffset++
                linkEnd--
        }

        // optional spacer: (space | tab)* (newline | '\'' | '"' | '(' )
        for i < len(data) && (data[i] == ' ' || data[i] == '\t') {
                i++
        }
        if i < len(data) && data[i] != '\n' && data[i] != '\r' && data[i] != '\'' && data[i] != '"' && data[i] != '(' {
                return
        }

        // compute end-of-line
        if i >= len(data) || data[i] == '\r' || data[i] == '\n' {
                lineEnd = i
        }
        if i+1 < len(data) && data[i] == '\r' && data[i+1] == '\n' {
                lineEnd++
        }

        // optional (space|tab)* spacer after a newline
        if lineEnd > 0 {
                i = lineEnd + 1
                for i < len(data) && (data[i] == ' ' || data[i] == '\t') {
                        i++
                }
        }

        // optional title: any non-newline sequence enclosed in '"() alone on its line
        if i+1 < len(data) && (data[i] == '\'' || data[i] == '"' || data[i] == '(') {
                i++
                titleOffset = i

                // look for EOL
                for i < len(data) && data[i] != '\n' && data[i] != '\r' {
                        i++
                }
                if i+1 < len(data) && data[i] == '\n' && data[i+1] == '\r' {
                        titleEnd = i + 1
                } else {
                        titleEnd = i
                }

                // step back
                i--
                for i > titleOffset && (data[i] == ' ' || data[i] == '\t') {
                        i--
                }
                if i > titleOffset && (data[i] == '\'' || data[i] == '"' || data[i] == ')') {
                        lineEnd = titleEnd
                        titleEnd = i
                }
        }

        return
}

// The first bit of this logic is the same as Parser.listItem, but the rest
// is much simpler. This function simply finds the entire block and shifts it
// over by one tab if it is indeed a block (just returns the line if it's not).
// blockEnd is the end of the section in the input buffer, and contents is the
// extracted text that was shifted over one tab. It will need to be rendered at
// the end of the document.
func scanFootnote(p *Markdown, data []byte, i, indentSize int) (blockStart, blockEnd int, contents []byte, hasBlock bool) {
        if i == 0 || len(data) == 0 {
                return
        }

        // skip leading whitespace on first line
        for i < len(data) && data[i] == ' ' {
                i++
        }

        blockStart = i

        // find the end of the line
        blockEnd = i
        for i < len(data) && data[i-1] != '\n' {
                i++
        }

        // get working buffer
        var raw bytes.Buffer

        // put the first line into the working buffer
        raw.Write(data[blockEnd:i])
        blockEnd = i

        // process the following lines
        containsBlankLine := false

gatherLines:
        for blockEnd < len(data) {
                i++

                // find the end of this line
                for i < len(data) && data[i-1] != '\n' {
                        i++
                }

                // if it is an empty line, guess that it is part of this item
                // and move on to the next line
                if p.isEmpty(data[blockEnd:i]) > 0 {
                        containsBlankLine = true
                        blockEnd = i
                        continue
                }

                n := 0
                if n = isIndented(data[blockEnd:i], indentSize); n == 0 {
                        // this is the end of the block.
                        // we don't want to include this last line in the index.
                        break gatherLines
                }

                // if there were blank lines before this one, insert a new one now
                if containsBlankLine {
                        raw.WriteByte('\n')
                        containsBlankLine = false
                }

                // get rid of that first tab, write to buffer
                raw.Write(data[blockEnd+n : i])
                hasBlock = true

                blockEnd = i
        }

        if data[blockEnd-1] != '\n' {
                raw.WriteByte('\n')
        }

        contents = raw.Bytes()

        return
}

//
//
// Miscellaneous helper functions
//
//

// Test if a character is a punctuation symbol.
// Taken from a private function in regexp in the stdlib.
func ispunct(c byte) bool {
        for _, r := range []byte("!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~") {
                if c == r {
                        return true
                }
        }
        return false
}

// Test if a character is a whitespace character.
func isspace(c byte) bool {
        return ishorizontalspace(c) || isverticalspace(c)
}

// Test if a character is a horizontal whitespace character.
func ishorizontalspace(c byte) bool {
        return c == ' ' || c == '\t'
}

// Test if a character is a vertical character.
func isverticalspace(c byte) bool {
        return c == '\n' || c == '\r' || c == '\f' || c == '\v'
}

// Test if a character is letter.
func isletter(c byte) bool {
        return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
}

// Test if a character is a letter or a digit.
// TODO: check when this is looking for ASCII alnum and when it should use unicode
func isalnum(c byte) bool {
        return (c >= '0' && c <= '9') || isletter(c)
}

// Replace tab characters with spaces, aligning to the next TAB_SIZE column.
// always ends output with a newline
func expandTabs(out *bytes.Buffer, line []byte, tabSize int) {
        // first, check for common cases: no tabs, or only tabs at beginning of line
        i, prefix := 0, 0
        slowcase := false
        for i = 0; i < len(line); i++ {
                if line[i] == '\t' {
                        if prefix == i {
                                prefix++
                        } else {
                                slowcase = true
                                break
                        }
                }
        }

        // no need to decode runes if all tabs are at the beginning of the line
        if !slowcase {
                for i = 0; i < prefix*tabSize; i++ {
                        out.WriteByte(' ')
                }
                out.Write(line[prefix:])
                return
        }

        // the slow case: we need to count runes to figure out how
        // many spaces to insert for each tab
        column := 0
        i = 0
        for i < len(line) {
                start := i
                for i < len(line) && line[i] != '\t' {
                        _, size := utf8.DecodeRune(line[i:])
                        i += size
                        column++
                }

                if i > start {
                        out.Write(line[start:i])
                }

                if i >= len(line) {
                        break
                }

                for {
                        out.WriteByte(' ')
                        column++
                        if column%tabSize == 0 {
                                break
                        }
                }

                i++
        }
}

// Find if a line counts as indented or not.
// Returns number of characters the indent is (0 = not indented).
func isIndented(data []byte, indentSize int) int {
        if len(data) == 0 {
                return 0
        }
        if data[0] == '\t' {
                return 1
        }
        if len(data) < indentSize {
                return 0
        }
        for i := 0; i < indentSize; i++ {
                if data[i] != ' ' {
                        return 0
                }
        }
        return indentSize
}

// Create a url-safe slug for fragments
func slugify(in []byte) []byte {
        if len(in) == 0 {
                return in
        }
        out := make([]byte, 0, len(in))
        sym := false

        for _, ch := range in {
                if isalnum(ch) {
                        sym = false
                        out = append(out, ch)
                } else if sym {
                        continue
                } else {
                        out = append(out, '-')
                        sym = true
                }
        }
        var a, b int
        var ch byte
        for a, ch = range out {
                if ch != '-' {
                        break
                }
        }
        for b = len(out) - 1; b > 0; b-- {
                if out[b] != '-' {
                        break
                }
        }
        return out[a : b+1]
}
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