Implement passthrough extension

Implement passthrough extension

Closes #1


Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>

passthrough/go.mod

@@ -5,6 +5,7 @@ go 1.20
 require github.com/frankban/quicktest v1.14.6
 
 require (
+	github.com/yuin/goldmark v1.6.0
 	github.com/google/go-cmp v0.5.9 // indirect
 	github.com/kr/pretty v0.3.1 // indirect
 	github.com/kr/text v0.2.0 // indirect

passthrough/go.sum

@@ -10,3 +10,5 @@ github.com/kr/text v0.2.0/go.mod h1:eLer722TekiGuMkidMxC/pM04lWEeraHUUmBw8l2grE=
 github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 github.com/rogpeppe/go-internal v1.9.0 h1:73kH8U+JUqXU8lRuOHeVHaa/SZPifC7BkcraZVejAe8=
 github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
+github.com/yuin/goldmark v1.6.0 h1:boZcn2GTjpsynOsC0iJHnBWa4Bi0qzfJjthwauItG68=
+github.com/yuin/goldmark v1.6.0/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=

passthrough/passthrough.go

@@ -1,6 +1,382 @@
 package passthrough
 
-// Foo returns a string.
-func Foo() string {
-	return "foo"
+import (
+	"bytes"
+	"fmt"
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+	"strings"
+)
+
+type Delimiters struct {
+	Open  string
+	Close string
+}
+
+// Determine if a byte array starts with a given string
+func startsWith(b []byte, s string) bool {
+	if len(b) < len(s) {
+		return false
+	}
+
+	return string(b[:len(s)]) == s
+}
+
+type PassthroughInline struct {
+	ast.BaseInline
+
+	// The segment of text that this inline passthrough represents.
+	Segment text.Segment
+
+	// The matched delimiters
+	Delimiters *Delimiters
+}
+
+func NewPassthroughInline(segment text.Segment, delimiters *Delimiters) *PassthroughInline {
+	return &PassthroughInline{
+		Segment:    segment,
+		Delimiters: delimiters,
+	}
+}
+
+// Text implements Node.Text.
+func (n *PassthroughInline) Text(source []byte) []byte {
+	return n.Segment.Value(source)
+}
+
+// Dump implements Node.Dump.
+func (n *PassthroughInline) Dump(source []byte, level int) {
+	indent := strings.Repeat("    ", level)
+	fmt.Printf("%sPassthroughInline {\n", indent)
+	indent2 := strings.Repeat("    ", level+1)
+	fmt.Printf("%sSegment: \"%s\"\n", indent2, n.Text(source))
+	fmt.Printf("%s}\n", indent)
+}
+
+// KindPassthroughInline is a NodeKind of the PassthroughInline node.
+var KindPassthroughInline = ast.NewNodeKind("PassthroughInline")
+
+// Kind implements Node.Kind.
+func (n *PassthroughInline) Kind() ast.NodeKind {
+	return KindPassthroughInline
+}
+
+type inlinePassthroughParser struct {
+	PassthroughDelimiters []Delimiters
+}
+
+func NewInlinePassthroughParser(ds []Delimiters) parser.InlineParser {
+	return &inlinePassthroughParser{
+		PassthroughDelimiters: ds,
+	}
+}
+
+// Determine if the input slice starts with a full valid opening delimiter.
+// If so, returns the delimiter struct, otherwise returns nil.
+func GetFullOpeningDelimiter(delims []Delimiters, line []byte) *Delimiters {
+	for _, d := range delims {
+		if startsWith(line, d.Open) {
+			return &d
+		}
+	}
+
+	return nil
+}
+
+// Return an array of bytes containing the first byte of each opening
+// delimiter. Used to populate the trigger list for inline and block parsers.
+// `Parse` will be executed once for each character that is in this list of
+// allowed trigger characters. Our parse function needs to do some additional
+// checks because Trigger only works for single-byte delimiters.
+func OpenersFirstByte(delims []Delimiters) []byte {
+	var firstBytes []byte
+	containsBackslash := false
+	for _, d := range delims {
+		if d.Open[0] == '\\' {
+			containsBackslash = true
+		}
+		firstBytes = append(firstBytes, d.Open[0])
+	}
+
+	if !containsBackslash {
+		// always trigger on backslash because it can be used to escape the opening
+		// delimiter.
+		firstBytes = append(firstBytes, '\\')
+	}
+	return firstBytes
+}
+
+// Determine if the input list of delimiters contains the given delimiter pair
+func ContainsDelimiters(delims []Delimiters, toFind *Delimiters) bool {
+	for _, d := range delims {
+		if d.Open == toFind.Open && d.Close == toFind.Close {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (s *inlinePassthroughParser) Trigger() []byte {
+	return OpenersFirstByte(s.PassthroughDelimiters)
+}
+
+func (s *inlinePassthroughParser) Parse(parent ast.Node, block text.Reader, pc parser.Context) ast.Node {
+	// In order to prevent other parser extensions from operating on the text
+	// between passthrough delimiters, we must process the entire inline
+	// passthrough in one execution of Parse. This means we can't use the style
+	// of multiple triggers with parser.Context state saved between calls.
+	line, startSegment := block.PeekLine()
+
+	fencePair := GetFullOpeningDelimiter(s.PassthroughDelimiters, line)
+	// fencePair == nil can happen if only the first byte of an opening delimiter
+	// matches, but it is not the complete opening delimiter. The trigger causes
+	// this Parse function to execute, but the trigger interface is limited to
+	// matching single bytes.
+	// It can also be because the opening delimiter is escaped with a
+	// double-backslash. In this case, we advance and return nil.
+	if fencePair == nil {
+		if len(line) > 2 && line[0] == '\\' && line[1] == '\\' {
+			fencePair = GetFullOpeningDelimiter(s.PassthroughDelimiters, line[2:])
+			if fencePair != nil {
+				// Opening delimiter is escaped, return the escaped opener as plain text
+				// So that the characters are not processed again.
+				block.Advance(2 + len(fencePair.Open))
+				return ast.NewTextSegment(startSegment.WithStop(startSegment.Start + len(fencePair.Open) + 2))
+			}
+		}
+		return nil
+	}
+
+	// This roughly follows goldmark/parser/code_span.go
+	block.Advance(len(fencePair.Open))
+	openerSize := len(fencePair.Open)
+	l, pos := block.Position()
+
+	for {
+		line, lineSegment := block.PeekLine()
+		if line == nil {
+			block.SetPosition(l, pos)
+			return ast.NewTextSegment(startSegment.WithStop(startSegment.Start + openerSize))
+		}
+
+		closingDelimiterPos := bytes.Index(line, []byte(fencePair.Close))
+		if closingDelimiterPos == -1 { // no closer on this line
+			block.AdvanceLine()
+			continue
+		}
+
+		// This segment spans from the original starting trigger (including the delimiter)
+		// up to and including the closing delimiter.
+		seg := startSegment.WithStop(lineSegment.Start + closingDelimiterPos + len(fencePair.Close))
+		if seg.Len() == len(fencePair.Open)+len(fencePair.Close) {
+			return nil
+		}
+
+		block.Advance(closingDelimiterPos + len(fencePair.Close))
+		return NewPassthroughInline(seg, fencePair)
+	}
+}
+
+type passthroughInlineRenderer struct {
+}
+
+func (r *passthroughInlineRenderer) renderRawInline(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		w.WriteString(string(n.Text(source)))
+	}
+	return ast.WalkContinue, nil
+}
+
+// A PassthroughBlock struct represents a fenced block of raw text to pass
+// through unchanged. This is not parsed directly, but emitted by an
+// ASTTransformer that splits a paragraph at the point of an inline passthrough
+// with the matching block delimiters.
+type PassthroughBlock struct {
+	ast.BaseBlock
+}
+
+// Dump implements Node.Dump.
+func (n *PassthroughBlock) Dump(source []byte, level int) {
+	ast.DumpHelper(n, source, level, nil, nil)
+}
+
+// KindPassthroughBlock is a NodeKind of the PassthroughBlock node.
+var KindPassthroughBlock = ast.NewNodeKind("PassthroughBlock")
+
+// Kind implements Node.Kind.
+func (n *PassthroughBlock) Kind() ast.NodeKind {
+	return KindPassthroughBlock
+}
+
+// NewPassthroughBlock return a new PassthroughBlock node.
+func NewPassthroughBlock() *PassthroughBlock {
+	return &PassthroughBlock{
+		BaseBlock: ast.BaseBlock{},
+	}
+}
+
+type passthroughBlockRenderer struct {
+}
+
+func (r *passthroughBlockRenderer) renderRawBlock(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		l := n.Lines().Len()
+		for i := 0; i < l; i++ {
+			line := n.Lines().At(i)
+			w.WriteString(string(line.Value(source)))
+		}
+		w.WriteString("\n")
+	}
+	return ast.WalkSkipChildren, nil
+}
+
+// To support the use of passthrough block delimiters in inline contexts, I
+// wasn't able to get the normal block parser to work. Goldmark seems to only
+// trigger the inline parser when the trigger is not the first characters in a
+// block. So instead we hook into the transformer interface, and process an
+// inline passthrough after it's parsed, looking for nodes whose delimiters
+// match the block delimiters, and splitting the paragraph at that point.
+type passthroughInlineTransformer struct {
+	BlockDelimiters []Delimiters
+}
+
+var PassthroughInlineTransformer = &passthroughInlineTransformer{}
+
+// Note, this transformer destroys the RawText attributes of the paragraph
+// nodes that it transforms. However, this does not seem to have an impact on
+// rendering.
+func (p *passthroughInlineTransformer) Transform(
+	doc *ast.Document, reader text.Reader, pc parser.Context) {
+
+	ast.Walk(doc, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
+		// Anchor on paragraphs
+		if n.Kind() != ast.KindParagraph || !entering {
+			return ast.WalkContinue, nil
+		}
+
+		// If no direct children are passthroughs, skip it.
+		foundInlinePassthrough := false
+		for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+			if c.Kind() == KindPassthroughInline {
+				foundInlinePassthrough = true
+				break
+			}
+		}
+		if !foundInlinePassthrough {
+			return ast.WalkContinue, nil
+		}
+
+		parent := n.Parent()
+		currentParagraph := ast.NewParagraph()
+		// AppendChild breaks the link between the node and its siblings, so we
+		// need to manually track the current and next node.
+		currentNode := n.FirstChild()
+		insertionPoint := n
+
+		for currentNode != nil {
+			nextNode := currentNode.NextSibling()
+			if currentNode.Kind() != KindPassthroughInline {
+				currentParagraph.AppendChild(currentParagraph, currentNode)
+				currentNode = nextNode
+			} else if currentNode.Kind() == KindPassthroughInline {
+				inline := currentNode.(*PassthroughInline)
+
+				// Only split into a new block if the delimiters are block delimiters
+				if !ContainsDelimiters(p.BlockDelimiters, inline.Delimiters) {
+					currentParagraph.AppendChild(currentParagraph, currentNode)
+					currentNode = nextNode
+					continue
+				}
+
+				newBlock := NewPassthroughBlock()
+				newBlock.Lines().Append(inline.Segment)
+				if len(currentParagraph.Text(reader.Source())) > 0 {
+					parent.InsertAfter(parent, insertionPoint, currentParagraph)
+					insertionPoint = currentParagraph
+				}
+				parent.InsertAfter(parent, insertionPoint, newBlock)
+				insertionPoint = newBlock
+				currentParagraph = ast.NewParagraph()
+				currentNode = nextNode
+			}
+		}
+
+		if currentParagraph.ChildCount() > 0 {
+			parent.InsertAfter(parent, insertionPoint, currentParagraph)
+		}
+
+		parent.RemoveChild(parent, n)
+		return ast.WalkContinue, nil
+	})
+}
+
+func NewPassthroughInlineTransformer(ds []Delimiters) parser.ASTTransformer {
+	return &passthroughInlineTransformer{
+		BlockDelimiters: ds,
+	}
+}
+
+// RegisterFuncs implements renderer.NodeRenderer.RegisterFuncs.
+func (r *passthroughInlineRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	reg.Register(KindPassthroughInline, r.renderRawInline)
+}
+
+// RegisterFuncs implements renderer.NodeRenderer.RegisterFuncs.
+func (r *passthroughBlockRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	reg.Register(KindPassthroughBlock, r.renderRawBlock)
+}
+
+func NewPassthroughInlineRenderer() renderer.NodeRenderer {
+	return &passthroughInlineRenderer{}
+}
+
+func NewPassthroughBlockRenderer() renderer.NodeRenderer {
+	return &passthroughBlockRenderer{}
+}
+
+// ---- Extension and config ----
+
+type passthrough struct {
+	InlineDelimiters []Delimiters
+	BlockDelimiters  []Delimiters
+}
+
+func NewPassthroughWithDelimiters(
+	InlineDelimiters []Delimiters,
+	BlockDelimiters []Delimiters) goldmark.Extender {
+	// The parser executes in two phases:
+	//
+	// Phase 1: parse the input with all delimiters treated as inline, and block delimiters
+	// taking precedence over inline delimiters.
+	//
+	// Phase 2: transform the parsed AST to split paragraphs at the point of
+	// inline passthroughs with matching block delimiters.
+	combinedDelimiters := make([]Delimiters, len(InlineDelimiters)+len(BlockDelimiters))
+	copy(combinedDelimiters, BlockDelimiters)
+	copy(combinedDelimiters[len(BlockDelimiters):], InlineDelimiters)
+	return &passthrough{
+		InlineDelimiters: combinedDelimiters,
+		BlockDelimiters:  BlockDelimiters,
+	}
+}
+
+func (e *passthrough) Extend(m goldmark.Markdown) {
+	m.Parser().AddOptions(
+		parser.WithInlineParsers(
+			util.Prioritized(NewInlinePassthroughParser(e.InlineDelimiters), 201),
+		),
+		parser.WithASTTransformers(
+			util.Prioritized(NewPassthroughInlineTransformer(e.BlockDelimiters), 0),
+		),
+	)
+
+	m.Renderer().AddOptions(renderer.WithNodeRenderers(
+		util.Prioritized(NewPassthroughInlineRenderer(), 101),
+		util.Prioritized(NewPassthroughBlockRenderer(), 99),
+	))
 }