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

package mathml

import "strings"

// parser walks the token list. The variant field is the mathvariant the
// current style command imposes on the atoms it covers.
type parser struct {
	src        []byte
	toks       []token
	pos        int
	display    bool
	variant    string
	macros     map[string]macro
	expansions int
}

func newParser(src []byte, display bool) *parser {
	return &parser{src: src, toks: tokenise(src), display: display, macros: map[string]macro{}}
}

func (p *parser) cur() token {
	p.expandMacros()
	return p.toks[p.pos]
}

func (p *parser) at(kind tokenKind) bool { return p.cur().kind == kind }

func (p *parser) atCommand(name string) bool {
	t := p.cur()
	return t.kind == tokCommand && t.text == `\`+name
}

// parseAll parses the whole source into nodes. A boundary with nothing to
// bound degrades as the stray it is.
func (p *parser) parseAll() []*node {
	nodes := p.sequence(true)
	for p.cur().kind != tokEOF {
		if isBoundary(p.cur()) {
			t := p.cur()
			p.pos++
			nodes = append(nodes, errorNode(t.text))
			continue
		}
		nodes = append(nodes, p.sequence(true)...)
	}
	return nodes
}

// isBoundary reports whether the token ends a sequence: a cell separator,
// a row separator, an environment close or a closing brace.
func isBoundary(t token) bool {
	if t.kind == tokRBrace || t.kind == tokAmpersand || isRowEnd(t) {
		return true
	}
	return t.kind == tokCommand && t.text == `\end`
}

func isRowEnd(t token) bool {
	return t.kind == tokCommand && t.text == `\\`
}

// sequence parses atoms until a boundary or the end. When allowOver is
// set, the TeX infix constructs \over, \atop and \choose may appear and
// take everything parsed so far as their numerator.
func (p *parser) sequence(allowOver bool) []*node {
	var nodes []*node
	for {
		t := p.cur()
		if t.kind == tokEOF || isBoundary(t) {
			break
		}
		if t.kind == tokDegraded {
			nodes = append(nodes, errorNode(t.text))
			p.pos++
			continue
		}
		if t.kind == tokCommand && isOverCommand(t.text) {
			p.pos++
			if !allowOver {
				nodes = append(nodes, errorNode(t.text))
				continue
			}
			num := wrapRow(nodes)
			den := wrapRow(p.sequence(false))
			nodes = []*node{p.overNode(t.text, num, den)}
			continue
		}
		if n := p.atom(); n != nil {
			nodes = append(nodes, n)
		}
	}
	return nodes
}

func isOverCommand(text string) bool {
	switch text {
	case `\over`, `\atop`, `\choose`, `\overwithdelims`, `\atopwithdelims`, `\abovewithdelims`:
		return true
	}
	return false
}

func (p *parser) overNode(cmd string, num, den *node) *node {
	switch cmd {
	case `\choose`:
		return fenced("(", elA("mfrac", []attribute{{"linethickness", "0em"}}, num, den), ")")
	case `\atop`:
		return elA("mfrac", []attribute{{"linethickness", "0em"}}, num, den)
	default:
		return el("mfrac", num, den)
	}
}

// atom parses one unit with its scripts, or a whole command construct.
func (p *parser) atom() *node {
	t := p.cur()
	switch t.kind {
	case tokLBrace:
		return p.braceGroup()
	case tokChar:
		return p.scripts(p.charAtom(t))
	case tokCommand:
		n := p.command()
		if n == nil {
			return nil
		}
		return p.scripts(n)
	case tokCaret, tokUnderscore:
		// A script without a base, the isotope notation of ^3He, takes
		// an empty base: the script renders over nothing, the way the
		// notation is printed, and degrading it would refuse notation
		// the mathematics carries.
		return p.scripts(el("mrow"))
	}
	p.pos++
	return errorNode(t.text)
}

// braceGroup parses a braced group, degrading the whole span when the
// closing brace never comes. A group of one node is that node: the braces
// only grouped.
func (p *parser) braceGroup() *node {
	open := p.cur()
	p.pos++
	nodes := p.sequence(true)
	if p.at(tokRBrace) {
		p.pos++
		if len(nodes) == 1 {
			return nodes[0]
		}
		if len(nodes) == 0 {
			return el("mrow")
		}
		row := el("mrow")
		row.children = nodes
		return row
	}
	p.pos = len(p.toks) - 1
	return errorNode(string(p.src[open.start:]))
}

// charAtom maps one character token to its element. The reserved TeX
// characters degrade rather than pass as content.
func (p *parser) charAtom(t token) *node {
	c := t.text
	switch c {
	case "#", "$", "%":
		p.pos++
		return errorNode(c)
	case "~":
		p.pos++
		return spaceNode("0.25em")
	case "'":
		p.pos++
		return text("mo", "′")
	}
	r := []rune(c)[0]
	switch {
	case isDigitByte(c[0]) && len(c) == 1:
		p.pos++
		return p.numberRun(c)
	case isIdentifierRune(r):
		p.pos++
		return p.identifier(c)
	default:
		p.pos++
		return p.withVariant(el("mo"), c)
	}
}

// numberRun collects the digits and decimal points that follow.
func (p *parser) numberRun(first string) *node {
	var b strings.Builder
	b.WriteString(first)
	for {
		t := p.cur()
		if t.kind != tokChar || len(t.text) != 1 || !isDigitByte(t.text[0]) && t.text != "." {
			break
		}
		b.WriteString(t.text)
		p.pos++
	}
	return p.withVariant(el("mn"), b.String())
}

// identifier renders one letter, upright when the variant says so.
func (p *parser) identifier(c string) *node {
	return p.withVariant(el("mi"), c)
}

// withVariant fills a leaf node with text, applying the active variant.
func (p *parser) withVariant(n *node, c string) *node {
	n.text = c
	if p.variant != "" && (n.kind == "mi" || n.kind == "mn" || n.kind == "mo") {
		if !(n.kind == "mi" && p.variant == "italic") {
			n.attrs = append(n.attrs, attribute{"mathvariant", p.variant})
		}
	}
	return n
}

// scripts attaches the sub and superscript runs that follow a base. A
// movable base puts its scripts under and over in display style; inline,
// the movablelimits attribute lets the renderer decide.
func (p *parser) scripts(base *node) *node {
	movable := baseMovable(base)
	if p.atCommand("limits") {
		p.pos++
		movable = true
	} else if p.atCommand("nolimits") {
		p.pos++
		movable = false
	}
	var sub, sup *node
	for {
		t := p.cur()
		if t.kind == tokUnderscore && sub == nil {
			p.pos++
			sub = p.argument()
			continue
		}
		if t.kind == tokCaret && sup == nil {
			p.pos++
			sup = p.argument()
			continue
		}
		break
	}
	under := movable && p.display
	switch {
	case sub == nil && sup == nil:
		return base
	case under && sub != nil && sup != nil:
		return el("munderover", base, sub, sup)
	case under && sub != nil:
		return el("munder", base, sub)
	case under && sup != nil:
		return el("mover", base, sup)
	case sub != nil && sup != nil:
		return el("msubsup", base, sub, sup)
	case sub != nil:
		return el("msub", base, sub)
	default:
		return el("msup", base, sup)
	}
}

// baseMovable reports whether the base carries movable limits.
func baseMovable(base *node) bool {
	if base.kind != "mo" {
		return false
	}
	for _, a := range base.attrs {
		if a.key == "movablelimits" {
			return a.value == "true"
		}
	}
	return false
}

// argument reads one macro argument or script: a braced group or a single
// token. A single digit stays one digit, the way TeX reads x^10.
func (p *parser) argument() *node {
	t := p.cur()
	switch {
	case t.kind == tokLBrace:
		return p.braceGroup()
	case t.kind == tokChar && len(t.text) == 1 && isDigitByte(t.text[0]):
		p.pos++
		return p.withVariant(el("mn"), t.text)
	case t.kind == tokChar || t.kind == tokCommand:
		n := p.atom()
		if n == nil {
			return el("mrow")
		}
		return n
	}
	return errorNode(t.text)
}

// rawBraced reads a braced group from the source as verbatim text,
// keeping the spaces. It fails when the closing brace is missing.
func (p *parser) rawBraced() (string, bool) {
	t := p.cur()
	if t.kind != tokLBrace {
		return "", false
	}
	depth := 0
	for i := t.start; i < len(p.src); {
		switch p.src[i] {
		case '{':
			depth++
		case '}':
			depth--
			if depth == 0 {
				text := string(p.src[t.start+1 : i])
				// consume the tokens the span covers
				for p.pos < len(p.toks) && p.toks[p.pos].start <= i {
					p.pos++
				}
				return text, true
			}
		case '\\':
			i++ // an escaped character never opens or closes
		}
		i++
	}
	return "", false
}