feat: render Markdown, mathematics and Mermaid diagrams server-side

internal/markdown/node.go

@@ -0,0 +1,116 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+// nodeKind identifies a block node of the document tree.
+type nodeKind uint8
+
+const (
+	kindDocument nodeKind = iota
+	kindParagraph
+	kindHeading
+	kindCodeBlock
+	kindHTMLBlock
+	kindBlockquote
+	kindList
+	kindListItem
+	kindThematicBreak
+	kindTable
+	kindFootnoteDef
+	kindDefList
+	kindDefTerm
+	kindDefItem
+)
+
+// listKind distinguishes bullet lists from ordered lists.
+type listKind uint8
+
+const (
+	bulletList listKind = iota
+	orderedList
+)
+
+// Column alignment of a table, carried per column.
+const (
+	alignNone uint8 = iota
+	alignLeft
+	alignCentre
+	alignRight
+)
+
+// Node is one block of the parsed document. The zero value is a document
+// root; every other kind is created by the parser.
+type Node struct {
+	kind     nodeKind
+	parent   *Node
+	children []*Node
+
+	// content holds the raw text of a leaf block: paragraph or heading
+	// inline text, code source, or the lines of an HTML block.
+	content []byte
+
+	level int // heading level, 1 to 6
+
+	fenced      bool // code block opened by a fence
+	fenceChar   byte // fence character, '`' or '~'
+	fenceLength int  // length of the opening fence
+	fenceOffset int  // columns of indentation before the opening fence
+	info        string
+
+	htmlType int // HTML block start condition, 1 to 7
+
+	listKind     listKind
+	bulletChar   byte // bullet list: the marker character
+	delimiter    byte // ordered list: '.' or ')'
+	start        int  // ordered list: number of the first item
+	tight        bool
+	markerOffset int // item: indentation of the marker inside its container
+	padding      int // item: columns from the marker start to the content
+
+	// refs collects the link reference definitions of the document; it is
+	// carried by the root node only.
+	refs map[string]reference
+
+	// footnotes collects the footnote definitions of the document, in
+	// document order; it is carried by the root node only. The renderer
+	// orders the rendered section by reference.
+	footnotes []*Node
+
+	// label names a footnote definition.
+	label string
+
+	// table columns and rows, cells as raw inline content.
+	align  []uint8
+	header [][]byte
+	rows   [][][]byte
+
+	task     bool // item: the first paragraph begins with a task marker
+	taskDone bool
+
+	startLine int
+
+	lastLineBlank   bool
+	lastLineChecked bool
+	finalised       bool
+}
+
+// reference is one link reference definition of the document.
+type reference struct {
+	destination string
+	title       string
+	hasTitle    bool
+}
+
+// canContain reports whether parent accepts child blocks of the given kind.
+func canContain(parent, child nodeKind) bool {
+	switch parent {
+	case kindDocument, kindBlockquote, kindListItem, kindDefItem, kindFootnoteDef:
+		return child != kindDocument
+	case kindList:
+		return child == kindListItem
+	case kindDefList:
+		return child == kindDefTerm || child == kindDefItem || child == kindParagraph
+	}
+	return false
+}