Synthesise meta style-entries on demand.

This includes line highlighting, numbers, etc.

Fixes #211.

formatters/html/html.go

@@ -99,34 +99,10 @@ func (f *Formatter) Format(w io.Writer, style *chroma.Style, iterator chroma.Ite
 	return f.writeHTML(w, style, iterator.Tokens())
 }
 
-// Ensure that style entries exist for highlighting, etc.
-func (f *Formatter) restyle(style *chroma.Style) (*chroma.Style, error) {
-	builder := style.Builder()
-	bg := builder.Get(chroma.Background)
-	// If we don't have a line highlight colour, make one that is 10% brighter/darker than the background.
-	if !style.Has(chroma.LineHighlight) {
-		highlight := chroma.StyleEntry{Background: bg.Background}
-		highlight.Background = highlight.Background.BrightenOrDarken(0.1)
-		builder.AddEntry(chroma.LineHighlight, highlight)
-	}
-	// If we don't have line numbers, use the text colour but 20% brighter/darker
-	if !style.Has(chroma.LineNumbers) {
-		text := chroma.StyleEntry{Colour: bg.Colour}
-		text.Colour = text.Colour.BrightenOrDarken(0.5)
-		builder.AddEntry(chroma.LineNumbers, text)
-		builder.AddEntry(chroma.LineNumbersTable, text)
-	}
-	return builder.Build()
-}
-
 // We deliberately don't use html/template here because it is two orders of magnitude slower (benchmarked).
 //
 // OTOH we need to be super careful about correct escaping...
 func (f *Formatter) writeHTML(w io.Writer, style *chroma.Style, tokens []chroma.Token) (err error) { // nolint: gocyclo
-	style, err = f.restyle(style)
-	if err != nil {
-		return err
-	}
 	css := f.styleToCSS(style)
 	if !f.Classes {
 		for t, style := range css {

style.go

@@ -247,15 +247,7 @@ func (s *Style) Builder() *StyleBuilder {
 //
 // This is distinct from Get() which will merge parent tokens.
 func (s *Style) Has(ttype TokenType) bool {
-	return !s.get(ttype).IsZero()
-}
-
-func (s *Style) get(ttype TokenType) StyleEntry {
-	out := s.entries[ttype]
-	if out.IsZero() && s.parent != nil {
-		return s.parent.get(ttype)
-	}
-	return out
+	return !s.get(ttype).IsZero() || s.synthesisable(ttype)
 }
 
 // Get a style entry. Will try sub-category or category if an exact match is not found, and
@@ -268,6 +260,38 @@ func (s *Style) Get(ttype TokenType) StyleEntry {
 		s.get(ttype.SubCategory()))
 }
 
+func (s *Style) get(ttype TokenType) StyleEntry {
+	out := s.entries[ttype]
+	if out.IsZero() && s.synthesisable(ttype) {
+		out = s.synthesise(ttype)
+	}
+	if out.IsZero() && s.parent != nil {
+		return s.parent.get(ttype)
+	}
+	return out
+}
+
+func (s *Style) synthesise(ttype TokenType) StyleEntry {
+	bg := s.get(Background)
+	text := StyleEntry{Colour: bg.Colour}
+	text.Colour = text.Colour.BrightenOrDarken(0.5)
+
+	switch ttype {
+	// If we don't have a line highlight colour, make one that is 10% brighter/darker than the background.
+	case LineHighlight:
+		return StyleEntry{Background: bg.Background.BrightenOrDarken(0.1)}
+
+	// If we don't have line numbers, use the text colour but 20% brighter/darker
+	case LineNumbers, LineNumbersTable:
+		return text
+	}
+	return StyleEntry{}
+}
+
+func (s *Style) synthesisable(ttype TokenType) bool {
+	return ttype == LineHighlight || ttype == LineNumbers || ttype == LineNumbersTable
+}
+
 // ParseStyleEntry parses a Pygments style entry.
 func ParseStyleEntry(entry string) (StyleEntry, error) { // nolint: gocyclo
 	out := StyleEntry{}

style_test.go

@@ -35,3 +35,16 @@ func TestStyleClone(t *testing.T) {
 	assert.Equal(t, "#00ff00 bg:#ffffff", clone.Get(Comment).String())
 	assert.Equal(t, "bg:#ffffff", parent.Get(Comment).String())
 }
+
+func TestSynthesisedStyleEntries(t *testing.T) {
+	style, err := NewStyle("test", StyleEntries{
+		Background: "bg:#ffffff",
+	})
+	assert.NoError(t, err)
+	assert.True(t, style.Has(LineHighlight))
+	assert.True(t, style.Has(LineNumbersTable))
+	assert.True(t, style.Has(LineNumbers))
+	assert.Equal(t, "bg:#e5e5e5", style.Get(LineHighlight).String())
+	assert.Equal(t, "#7f7f7f bg:#ffffff", style.Get(LineNumbers).String())
+	assert.Equal(t, "#7f7f7f bg:#ffffff", style.Get(LineNumbersTable).String())
+}