/
splash.go
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/
splash.go
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// Package splash adds a dash of color to embedded source code in HTML
package splash
import (
"bytes"
"errors"
"html"
"regexp"
"unicode"
"github.com/alecthomas/chroma/v2"
chromaHTML "github.com/alecthomas/chroma/v2/formatters/html"
"github.com/alecthomas/chroma/v2/lexers"
"github.com/alecthomas/chroma/v2/styles"
)
var (
errHEAD = errors.New("HTML should contain <head> or <html> when adding CSS")
defaultLanguage = "shell"
)
// Splash takes HTML code as bytes and tries to syntax highlight code between
// <pre> and </pre> tags.
//
// "style" is a syntax highlight style, like "monokai".
//
// Full style list here: https://github.com/alecthomas/chroma/tree/master/styles
//
// Returns the modified HTML source code with embedded CSS as a <style> tag.
// Requires the given HTML to contain </head> or <html>.
//
// language specifiers like <code class="language-c"> are supported.
func Splash(htmlData []byte, styleName string) ([]byte, error) {
return highlightPre(htmlData, styleName, false)
}
// UnescapeSplash does the same as Splash, but unescapes the HTML in the source
// code before highlighting. Use this if "&" appears in the highlighted code
// instead of "&", and that is not what you wanted.
// Useful when highlighting source code after having rendered Markdown.
func UnescapeSplash(htmlData []byte, styleName string) ([]byte, error) {
return highlightPre(htmlData, styleName, true)
}
// SetDefaultLanguage changes the default language from "shell" to something else. Must be supported by chroma.
func SetDefaultLanguage(languageName string) {
defaultLanguage = languageName
}
// Highlight takes HTML code as bytes and tries to syntax highlight code between
// <pre> and </pre> tags.
//
// "style" is a syntax highlight style, like "monokai".
//
// Full style list here: https://github.com/alecthomas/chroma/tree/master/styles
//
// Returns the modified HTML source code and CSS style.
//
// unescape can be set to true for unescaping already escaped code in <pre> tags,
// which can be useful when highlighting code in newly rendered markdown.
func Highlight(htmlData []byte, styleName string, unescape bool) ([]byte, []byte, error) {
// Try to use the given style name
style := styles.Get(styleName)
if style == nil {
// Could not use the given style name
style = styles.Fallback
}
// Create a HTML formatter
formatter := chromaHTML.New(chromaHTML.WithClasses(true))
if formatter == nil {
return []byte{}, []byte{}, errors.New("unable to instanciate the Chroma HTML formatter")
}
var (
cssBuf bytes.Buffer // buffer for generated CSS
mutableBytes = htmlData[:]
outerErr error
)
// Replace the non-highlighted code with highlighted code
re := regexp.MustCompile(`(?m)(?s)(<pre>|<pre .*?chroma.*?>)(.*?)(</pre>)`)
mutableBytes = re.ReplaceAllFunc(mutableBytes, func(preSource []byte) []byte {
strippedPreTag1 := false
if bytes.HasPrefix(preSource, []byte("<pre>")) && bytes.HasSuffix(preSource, []byte("</pre>")) {
// Remove leading and trailing pre tags
preSource = preSource[5 : len(preSource)-6]
strippedPreTag1 = true
}
strippedCodeTag := false
if bytes.HasPrefix(preSource, []byte("<code>")) && bytes.HasSuffix(preSource, []byte("</code>")) {
// Remove leading and trailing pre tags
preSource = preSource[6 : len(preSource)-7]
strippedCodeTag = true
}
strippedPreTag2 := false
if bytes.HasPrefix(preSource, []byte("<pre>")) && bytes.HasSuffix(preSource, []byte("</pre>")) {
// Remove leading and trailing pre tags
preSource = preSource[5 : len(preSource)-6]
strippedPreTag2 = true
}
// Check if something like <code class="language-c"> has been specified
language := ""
strippedLongerCodeTag := false
if bytes.HasPrefix(preSource, []byte(`<code class="language-`)) && bytes.Count(preSource, []byte(`"`)) >= 2 {
language = string(bytes.SplitN((bytes.SplitN(preSource, []byte(`"`), 3)[1]), []byte("-"), 2)[1])
// Then strip the longer tag, if possible
if bytes.HasPrefix(preSource, []byte(`<code class="language-`+language+`">`)) && bytes.HasSuffix(preSource, []byte("</code>")) {
// Remove leading and trailing pre tags
preSource = preSource[len(language)+24 : len(preSource)-7]
strippedLongerCodeTag = true
}
}
// From bytes to string, while trimming away whitespace from only the end of the string.
// There may be wanted indentation at the beginning of the string.
preSourceString := string(bytes.TrimRightFunc(preSource, unicode.IsSpace))
// Unescape HTML, like &, if this has already been done by ie. a Markdown renderer
if unescape {
preSourceString = html.UnescapeString(preSourceString)
}
// Try to find a suitable lexer
var lexer chroma.Lexer
if language != "" {
// Try to use the specified language
lexer = lexers.Get(language)
}
if lexer == nil {
// Try to identify the language based on the source code that is to be highlighted
lexer = lexers.Analyse(preSourceString)
}
if lexer == nil {
// Could not identify the language, use the default language
lexer = lexers.Get(defaultLanguage)
}
if lexer == nil {
// Could not use the default language, use the fallback
lexer = lexers.Fallback
}
// Combine token runs
lexer = chroma.Coalesce(lexer)
// Prepare to iterate over the tokens in the source code
iterator, err := lexer.Tokenise(nil, preSourceString)
if err != nil {
outerErr = err
return []byte{}
}
// Write the needed CSS to cssBuf
err = formatter.WriteCSS(&cssBuf, style)
if err != nil {
outerErr = err
return []byte{}
}
// Write the highlightet HTML to the hiBuf buffer
var hiBuf bytes.Buffer
err = formatter.Format(&hiBuf, style, iterator)
if err != nil {
outerErr = err
return []byte{}
}
// Check that the highlighted bytes have a minimum of information
hiBytes := hiBuf.Bytes()
if !strippedPreTag2 {
// Remove the <pre> tag that was added by chroma
hlen := len(hiBytes)
if bytes.HasPrefix(hiBytes, []byte(`<pre class="chroma">`)) && bytes.HasSuffix(hiBytes, []byte("</pre>")) {
// Remove the leading <pre class="chroma"> and the trailing </pre> tag
hiBytes = hiBytes[len(`<pre class="chroma">`) : hlen-len("</pre>")]
} else if bytes.HasPrefix(hiBytes, []byte(`<pre tabindex="0" class="chroma">`)) && bytes.HasSuffix(hiBytes, []byte("</pre>")) {
// Remove the leading <pre class="chroma"> and the trailing </pre> tag
hiBytes = hiBytes[len(`<pre tabindex="0" class="chroma">`) : hlen-len("</pre>")]
}
}
if strippedCodeTag || strippedLongerCodeTag {
// Add the <code> tag again
hiBytes = []byte("<code>" + string(hiBytes) + "</code>")
}
if strippedPreTag1 {
// Add the <pre> tag
hiBytes = []byte(`<pre class="chroma">` + string(hiBytes) + "</pre>")
}
// TODO: This is a hack! Find a cleaner way.
to := []byte("<pre tabindex=\"0\" class=\"chroma\"><code>")
from := []byte(`<code><pre class="chroma"><code><pre tabindex="0" class="chroma"><code>`)
hiBytes = bytes.ReplaceAll(hiBytes, from, to)
from = []byte(`<code><pre style="background-color: #ffffff;" class="chroma"><pre style="background-color: #ffffff;" tabindex="0" class="chroma"><code>`)
hiBytes = bytes.ReplaceAll(hiBytes, from, to)
from = []byte(`<code><pre class="chroma"><pre tabindex="0" class="chroma"><code>`)
hiBytes = bytes.ReplaceAll(hiBytes, from, to)
from = []byte(`<code><code>`)
to = []byte(`<code>`)
hiBytes = bytes.ReplaceAll(hiBytes, from, to)
hiBytes = bytes.ReplaceAll(hiBytes, []byte("</code></pre></code></pre>"), []byte("</code></pre>"))
return hiBytes
})
if outerErr != nil {
return []byte{}, []byte{}, outerErr
}
re = regexp.MustCompile(`(?s)/\*.*?\*/|\n`) // Strip comments and newlines
stripped := []byte(re.ReplaceAllString(cssBuf.String(), "$1"))
return mutableBytes, stripped, nil
}
// highlightPre takes HTML code as bytes and tries to syntax highlight code between
// <pre> and </pre> tags.
//
// "style" is a syntax highlight style, like "monokai".
//
// Full style list here: https://github.com/alecthomas/chroma/tree/master/styles
//
// Returns the modified HTML source code with embedded CSS as a <style> tag.
// Requires the given HTML to contain </head> or <html>.
//
// unescape can be set to true for unescaping already escaped code in <pre> tags,
// which can be useful when highlighting code in newly rendered markdown.
func highlightPre(htmlData []byte, styleName string, unescape bool) ([]byte, error) {
HTML, CSS, err := Highlight(htmlData, styleName, unescape)
if err != nil {
return []byte{}, err
}
// Add all the generated CSS to a <style> tag in the generated HTML, without newlines
htmlBytes, err := AddCSSToHTML(HTML, CSS)
if err != nil {
return []byte{}, err
}
return htmlBytes, nil
}
// AddCSSToHTML takes htmlData and adds cssData in a <style> tag.
// Returns an error if </head> or <html> does not already exists.
// Tries to add CSS as late in <head> as possible.
func AddCSSToHTML(htmlData, cssData []byte) ([]byte, error) {
if bytes.Contains(htmlData, []byte("<head>")) {
var buf bytes.Buffer
buf.WriteString("<head><style>")
buf.Write(cssData)
buf.WriteString("</style>\n")
return bytes.Replace(htmlData, []byte("<head>"), buf.Bytes(), 1), nil
} else if bytes.Contains(htmlData, []byte("<html>")) {
var buf bytes.Buffer
buf.WriteString("<html><head><style>")
buf.Write(cssData)
buf.WriteString("</style></head>\n")
return bytes.Replace(htmlData, []byte("<html>"), buf.Bytes(), 1), nil
} else {
return []byte{}, errHEAD
}
}