chore: import upstream snapshot with attribution

internal/cli/mathnode.go

@@ -0,0 +1,66 @@
+package cli
+
+import (
+	"bytes"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+)
+
+var kindMath = ast.NewNodeKind("Math")
+
+type mathNode struct {
+	ast.BaseInline
+	value   string
+	display bool
+}
+
+func (n *mathNode) Kind() ast.NodeKind         { return kindMath }
+func (n *mathNode) Dump(src []byte, level int) { ast.DumpHelper(n, src, level, nil, nil) }
+
+type mathParser struct{}
+
+func (p *mathParser) Trigger() []byte { return []byte{'$'} }
+
+func (p *mathParser) Parse(parent ast.Node, block text.Reader, pc parser.Context) ast.Node {
+	line, _ := block.PeekLine()
+	if len(line) == 0 || line[0] != '$' {
+		return nil
+	}
+	display := len(line) >= 2 && line[1] == '$'
+	delim := 1
+	if display {
+		delim = 2
+	}
+
+	rest := line[delim:]
+	var closeAt int
+	if display {
+		closeAt = bytes.Index(rest, []byte("$$"))
+	} else {
+		closeAt = bytes.IndexByte(rest, '$')
+	}
+	if closeAt < 0 {
+		return nil
+	}
+	inner := rest[:closeAt]
+	if len(bytes.TrimSpace(inner)) == 0 {
+		return nil
+	}
+
+	// Currency guard (markdown-it-texmath rule): a single-$ span only counts as
+	// math when the open isn't followed by space, the close isn't preceded by
+	// space, and the char after the close isn't a digit — so "$5 and $10" stays
+	// prose. Display $$ is unambiguous and skips the check.
+	if !display {
+		after := closeAt + 1
+		if inner[0] == ' ' || inner[len(inner)-1] == ' ' ||
+			(after < len(rest) && rest[after] >= '0' && rest[after] <= '9') {
+			return nil
+		}
+	}
+
+	block.Advance(delim + closeAt + delim)
+	return &mathNode{value: latexToUnicode(string(bytes.TrimSpace(inner))), display: display}
+}