// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package mathml

import (
	"strings"
	"unicode"
)

func isDigitByte(c byte) bool { return c >= '0' && c <= '9' }

// isIdentifierRune reports whether the rune names a variable: a letter in
// any script.
func isIdentifierRune(r rune) bool {
	return unicode.IsLetter(r)
}

func spaceNode(width string) *node {
	return &node{kind: "mspace", attrs: []attribute{{"width", width}}}
}

// fenced wraps content in stretchy fence delimiters.
func fenced(open string, content *node, close string) *node {
	row := el("mrow")
	row.children = []*node{fenceNode(open), content, fenceNode(close)}
	return row
}

func fenceNode(char string) *node {
	return text("mo", char, attribute{"fence", "true"}, attribute{"stretchy", "true"})
}

// wrapRow packs nodes into one row; an empty list becomes an empty row.
func wrapRow(nodes []*node) *node {
	if len(nodes) == 1 {
		return nodes[0]
	}
	row := el("mrow")
	row.children = nodes
	return row
}

// command parses one backslash command and whatever it takes with it. The
// cursor sits on the command token.
func (p *parser) command() *node {
	t := p.cur()
	name := t.text[1:]
	p.pos++
	// The single-character escapes: braces, the reserved characters and
	// the double bar.
	switch name {
	case "{", "}", "%", "$", "#", "&", "_", "|", "<", ">", ",", ":", ";", "!", " ":
		if w, ok := spaces[name]; ok {
			return spaceNode(w)
		}
		if name == "|" {
			return text("mo", "‖")
		}
		return p.withVariant(el("mo"), name)
	}
	switch name {
	case "frac", "dfrac", "tfrac", "cfrac":
		num := p.argument()
		den := p.argument()
		return el("mfrac", num, den)
	case "binom", "dbinom", "tbinom":
		num := p.argument()
		den := p.argument()
		return fenced("(", elA("mfrac", []attribute{{"linethickness", "0em"}}, num, den), ")")
	case "genfrac":
		return p.genfrac()
	case "sqrt":
		return p.sqrt()
	case "substack":
		raw, ok := p.rawBraced()
		if !ok {
			return errorNode(t.text)
		}
		return p.substack(raw)
	case "text", "textrm", "textnormal", "mbox", "textmd", "textsc", "textsl":
		return p.textArgument()
	case "textbf", "textup":
		return p.textArgument(attribute{"fontweight", "bold"})
	case "textit", "emph":
		return p.textArgument(attribute{"fontstyle", "italic"})
	case "textsf":
		return p.textArgument(attribute{"mathvariant", "sans-serif"})
	case "texttt":
		return p.textArgument(attribute{"mathvariant", "monospace"})
	case "operatorname", "operatornamewithlimits":
		raw, ok := p.rawBraced()
		if !ok {
			return errorNode(t.text)
		}
		return identifierMulti(raw, name == "operatornamewithlimits")
	case "operatorname*":
		raw, ok := p.rawBraced()
		if !ok {
			return errorNode(t.text)
		}
		return identifierMulti(raw, true)
	case "mathchoice":
		// four arguments, of which the parser renders the second, the
		// text style one; the display, script and scriptscript variants
		// of MathML Core are the renderer's business
		_, _ = p.argument(), p.argument()
		if n := p.argument(); n != nil {
			_, _ = p.argument(), p.argument()
			return n
		}
		return errorNode(t.text)
	case "phantom", "hphantom", "vphantom":
		arg := p.argument()
		inner := el("mphantom")
		inner.children = arg.children
		inner.text = arg.text
		return inner
	case "overset", "stackrel":
		over := p.argument()
		base := p.argument()
		return el("mover", base, over)
	case "underset":
		under := p.argument()
		base := p.argument()
		return el("munder", base, under)
	case "pmod":
		arg := p.argument()
		row := el("mrow")
		row.children = []*node{
			identifierMulti("mod", false), spaceNode("0.2778em"), arg,
		}
		return fenced("(", row, ")")
	case "left":
		return p.leftRight()
	case "big", "Big", "bigg", "Bigg",
		"bigl", "Bigl", "biggl", "Biggl",
		"bigr", "Bigr", "biggr", "Biggr",
		"bigm", "Bigm", "biggm", "Biggm":
		return p.bigDelimiter()
	case "middle":
		return text("mo", "", attribute{"fence", "true"}, attribute{"stretchy", "true"})
	case "pod":
		return fenced("(", p.argument(), ")")
	case "xrightarrow", "xleftarrow", "xleftrightarrow", "xhookrightarrow",
		"xhookleftarrow", "xRightarrow", "xLeftarrow", "xLeftrightarrow",
		"xrightleftharpoons", "xmapsto", "xtwoheadleftarrow", "xtwoheadrightarrow",
		"xleftharpoonup", "xrightharpoonup", "xleftharpoondown", "xrightharpoondown",
		"xleftrightharpoons", "xtofrom", "xlongequal":
		return p.xArrow(name)
	case "displaystyle", "textstyle", "scriptstyle", "scriptscriptstyle":
		return p.styleDeclaration(name)
	case "newcommand", "renewcommand", "providecommand", "def", "gdef", "DeclareMathOperator", "DeclareMathOperator*":
		return p.macroDefinition(name)
	case "begin":
		return p.environment()
	case "end":
		return errorNode(t.text)
	case "limits", "nolimits":
		return errorNode(t.text)
	case "not":
		return p.not()
	case "hspace", "mspace":
		return p.spaceArgument()
	case "color", "textcolor":
		return p.color(name)
	}
	if s, ok := symbols[name]; ok {
		return p.symbolNode(t.text, s)
	}
	if functions[name] {
		return identifierMulti(name, movableFunctions[name])
	}
	if v, ok := styles[name]; ok {
		return p.styleArgument(v)
	}
	if accent, ok := accents[name]; ok {
		return p.accent(accent)
	}
	if w, ok := spaces[name]; ok {
		return spaceNode(w)
	}
	return errorNode(t.text)
}

// symbolNode renders a table symbol, upright under a variant switch.
func (p *parser) symbolNode(source string, s symbol) *node {
	if s.mi {
		return p.identifier(s.char)
	}
	n := p.withVariant(el("mo"), s.char)
	if s.movable {
		n.attrs = append(n.attrs, attribute{"movablelimits", "true"})
	}
	return n
}

// identifierMulti renders a name as an upright identifier, which a
// multi-character mi is by default. The movable flag marks the limit
// operators.
func identifierMulti(name string, movable bool) *node {
	if movable {
		return text("mi", name, attribute{"movablelimits", "true"})
	}
	return text("mi", name)
}

// textArgument reads a text command's argument verbatim.
func (p *parser) textArgument(attrs ...attribute) *node {
	raw, ok := p.rawBraced()
	if !ok {
		return errorNode(`\` + "text")
	}
	return &node{kind: "mtext", text: raw, attrs: attrs}
}

// styleArgument parses the argument under a forced variant.
func (p *parser) styleArgument(variant string) *node {
	save := p.variant
	if variant == "italic" {
		p.variant = ""
	} else {
		p.variant = variant
	}
	arg := p.argument()
	p.variant = save
	return arg
}

// accent puts its mark over or under the argument.
func (p *parser) accent(a struct {
	char  string
	under bool
}) *node {
	base := p.argument()
	mark := text("mo", a.char, attribute{"stretchy", "false"})
	if a.under {
		return el("munder", base, mark)
	}
	if a.char == "\u203e" || a.char == "\u23de" || a.char == "_" || a.char == "\u23e1" || a.char == "\u23df" {
		mark = text("mo", a.char, attribute{"stretchy", "true"})
	}
	return el("mover", base, mark)
}

// sqrt parses a radical with its optional index.
func (p *parser) sqrt() *node {
	if p.at(tokChar) && p.cur().text == "[" {
		p.pos++
		var index []*node
		for !p.at(tokEOF) && !(p.at(tokChar) && p.cur().text == "]") {
			if n := p.atom(); n != nil {
				index = append(index, n)
			}
		}
		if p.at(tokChar) && p.cur().text == "]" {
			p.pos++
			return el("mroot", p.argument(), wrapRow(index))
		}
		return errorNode(`\sqrt`)
	}
	return el("msqrt", p.argument())
}

// genfrac parses the six arguments of \genfrac: the delimiters, the rule
// thickness, the style and the numerator and denominator.
func (p *parser) genfrac() *node {
	open := p.delimiterArg()
	close := p.delimiterArg()
	thick, hasThick := p.bracketArg()
	style, _ := p.bracketArg()
	num := p.argument()
	den := p.argument()
	frac := el("mfrac")
	if hasThick {
		frac.attrs = append(frac.attrs, attribute{"linethickness", thick})
	}
	frac.children = []*node{num, den}
	if style >= "2" {
		wrapped := elA("mstyle", []attribute{{"scriptlevel", "1"}})
		wrapped.children = []*node{frac}
		frac = wrapped
	}
	if open == "" && close == "" {
		return frac
	}
	if open == "" {
		open = "."
	}
	if close == "" {
		close = "."
	}
	row := el("mrow")
	row.children = []*node{p.fenceOf(open), frac, p.fenceOf(close)}
	return row
}

// bracketArg reads an optional bracketed argument, reporting whether one
// was there.
func (p *parser) bracketArg() (string, bool) {
	if !p.at(tokChar) || p.cur().text != "[" {
		return "", false
	}
	p.pos++
	var b strings.Builder
	for {
		t := p.cur()
		if t.kind == tokEOF {
			return "", true
		}
		if t.kind == tokChar && t.text == "]" {
			p.pos++
			return b.String(), true
		}
		b.WriteString(t.text)
		p.pos++
	}
}

// delimiterArg reads a \genfrac delimiter argument: a character, a
// command or an empty group.
func (p *parser) delimiterArg() string {
	t := p.cur()
	switch {
	case t.kind == tokLBrace:
		p.pos++
		if p.at(tokRBrace) {
			p.pos++
			return ""
		}
		d, _ := p.delimiter()
		return d
	case t.kind == tokChar || t.kind == tokCommand:
		d, _ := p.delimiter()
		return d
	}
	return ""
}

// leftRight parses a stretchy delimited row.
func (p *parser) leftRight() *node {
	start := p.pos
	open, ok := p.delimiter()
	if !ok {
		return errorNode(`\left`)
	}
	var content []*node
	for {
		t := p.cur()
		if t.kind == tokEOF {
			p.pos = len(p.toks) - 1
			return errorNode(string(p.src[start-1:]))
		}
		if t.kind == tokCommand && t.text == `\right` {
			p.pos++
			break
		}
		if n := p.atom(); n != nil {
			content = append(content, n)
		}
	}
	close, _ := p.delimiter()
	row := el("mrow")
	row.children = append([]*node{p.fenceOf(open)}, content...)
	row.children = append(row.children, p.fenceOf(close))
	return row
}

// delimiter reads one delimiter: a character or a table command, with the
// dot meaning invisible. The second result reports whether a delimiter
// was there at all.
func (p *parser) delimiter() (string, bool) {
	t := p.cur()
	switch t.kind {
	case tokChar:
		p.pos++
		if t.text == "." {
			return "", true
		}
		return t.text, true
	case tokCommand:
		name := t.text[1:]
		if name == "|" {
			p.pos++
			return "‖", true
		}
		if s, ok := symbols[name]; ok {
			p.pos++
			return s.char, true
		}
		if _, ok := spaces[name]; ok {
			p.pos++
			return "", true
		}
	}
	return "", false
}

// fenceOf makes the fence for a delimiter name; an empty name is the
// invisible fence.
func (p *parser) fenceOf(d string) *node {
	if d == "" {
		return text("mo", "", attribute{"fence", "true"})
	}
	if d == "." {
		return text("mo", "", attribute{"fence", "true"})
	}
	return fenceNode(d)
}

// bigDelimiter renders \big and its relatives around one delimiter.
func (p *parser) bigDelimiter() *node {
	t := p.cur()
	d, ok := p.delimiter()
	if !ok {
		return errorNode(t.text)
	}
	return text("mo", d, attribute{"stretchy", "true"})
}

// xArrowNames gives the shaft of every extensible arrow command.
var xArrowNames = map[string]string{
	"xrightarrow":        "→",
	"xleftarrow":         "←",
	"xleftrightarrow":    "↔",
	"xhookrightarrow":    "↪",
	"xhookleftarrow":     "↩",
	"xRightarrow":        "⇒",
	"xLeftarrow":         "⇐",
	"xLeftrightarrow":    "⇔",
	"xrightleftharpoons": "⇌",
	"xmapsto":            "↦",
	"xtwoheadleftarrow":  "↞",
	"xtwoheadrightarrow": "↠",
	"xleftharpoonup":     "↼",
	"xrightharpoonup":    "⇀",
	"xleftharpoondown":   "↽",
	"xrightharpoondown":  "⇁",
	"xleftrightharpoons": "⇋",
	"xtofrom":            "⇄",
	"xlongequal":         "=",
}

// xArrow parses an extensible arrow: an optional underscript in brackets,
// then the overscript in braces.
func (p *parser) xArrow(name string) *node {
	char := xArrowNames[name]
	arrow := text("mo", char, attribute{"stretchy", "true"})
	var under *node
	if c, ok := p.bracketArg(); ok && c != "" {
		under = &node{kind: "mrow", text: c}
	}
	over := p.argument()
	if under == nil {
		return el("mover", arrow, over)
	}
	return el("munderover", arrow, under, over)
}

// styleDeclaration wraps the rest of the current group in an mstyle.
func (p *parser) styleDeclaration(name string) *node {
	rest := p.sequence(false)
	switch name {
	case "displaystyle":
		n := elA("mstyle", []attribute{{"displaystyle", "true"}})
		n.children = rest
		return n
	case "textstyle":
		n := elA("mstyle", []attribute{{"displaystyle", "false"}})
		n.children = rest
		return n
	case "scriptstyle":
		n := elA("mstyle", []attribute{{"scriptlevel", "1"}})
		n.children = rest
		return n
	default:
		n := elA("mstyle", []attribute{{"scriptlevel", "2"}})
		n.children = rest
		return n
	}
}

// not combines the negation slash with the relation that follows.
func (p *parser) not() *node {
	t := p.cur()
	if t.kind == tokCommand {
		name := t.text[1:]
		if s, ok := symbols[name]; ok {
			p.pos++
			return text("mo", s.char+"̸")
		}
	}
	return text("mo", "¬")
}

// spaceArgument reads the argument of \hspace and \mspace.
func (p *parser) spaceArgument() *node {
	t := p.cur()
	if t.kind == tokLBrace {
		p.pos++
		var b strings.Builder
		for {
			c := p.cur()
			if c.kind == tokEOF || c.kind == tokRBrace {
				break
			}
			b.WriteString(c.text)
			p.pos++
		}
		if p.at(tokRBrace) {
			p.pos++
		}
		if validWidth(b.String()) {
			return spaceNode(b.String())
		}
		return errorNode(t.text)
	}
	return errorNode(t.text)
}

// validWidth accepts a number with a CSS length unit.
func validWidth(s string) bool {
	i := 0
	for i < len(s) && (isDigitByte(s[i]) || s[i] == '.' || s[i] == '-') {
		i++
	}
	if i == 0 {
		return false
	}
	switch s[i:] {
	case "em", "ex", "px", "pt", "cm", "mm", "in", "mu", "%":
		return true
	}
	return false
}

// color wraps its argument or the rest of the group in a coloured style.
func (p *parser) color(name string) *node {
	c, ok := p.rawBraced()
	if !ok || !validColour(c) {
		return errorNode(`\` + name)
	}
	n := elA("mstyle", []attribute{{"mathcolor", c}})
	if name == "textcolor" {
		n.children = []*node{p.argument()}
		return n
	}
	n.children = p.sequence(false)
	return n
}

// validColour accepts the colour names and the hex forms.
func validColour(s string) bool {
	switch s {
	case "red", "green", "blue", "cyan", "magenta", "yellow", "black",
		"white", "gray", "grey", "orange", "purple", "brown", "pink",
		"olive", "violet", "teal", "navy", "darkgray", "lightgray":
		return true
	}
	if len(s) == 7 && s[0] == '#' || len(s) == 4 && s[0] == '#' {
		for i := 1; i < len(s); i++ {
			c := s[i]
			if !isDigitByte(c) && !(c >= 'a' && c <= 'f') && !(c >= 'A' && c <= 'F') {
				return false
			}
		}
		return true
	}
	return false
}