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

package mathml

import "unicode/utf8"

type tokenKind uint8

const (
	tokEOF tokenKind = iota
	tokCommand
	tokChar
	tokLBrace
	tokRBrace
	tokCaret
	tokUnderscore
	tokAmpersand
	// tokDegraded stands for a span the parser gave up on: the parser
	// renders it as its verbatim source.
	tokDegraded
)

type token struct {
	kind  tokenKind
	text  string
	start int
	end   int
}

// tokenise splits source into TeX tokens. Whitespace between tokens is
// dropped: math mode ignores it, and the text commands read their argument
// from the raw source instead.
func tokenise(src []byte) []token {
	var toks []token
	i := 0
	for i < len(src) {
		c := src[i]
		start := i
		switch {
		case c == '\\' && i+1 < len(src):
			i++
			if isLetter(src[i]) {
				for i < len(src) && isLetter(src[i]) {
					i++
				}
				text := string(src[start:i])
				// A control word eats the spaces behind it, without them
				// becoming part of its name.
				for i < len(src) && (src[i] == ' ' || src[i] == '\t' || src[i] == '\n') {
					i++
				}
				toks = append(toks, token{kind: tokCommand, text: text, start: start, end: i})
			} else {
				i++
				toks = append(toks, token{kind: tokCommand, text: string(src[start:i]), start: start, end: i})
			}
		case c == '{':
			i++
			toks = append(toks, token{kind: tokLBrace, text: "{", start: start, end: i})
		case c == '}':
			i++
			toks = append(toks, token{kind: tokRBrace, text: "}", start: start, end: i})
		case c == '^':
			i++
			toks = append(toks, token{kind: tokCaret, text: "^", start: start, end: i})
		case c == '_':
			i++
			toks = append(toks, token{kind: tokUnderscore, text: "_", start: start, end: i})
		case c == '&':
			i++
			toks = append(toks, token{kind: tokAmpersand, text: "&", start: start, end: i})
		case c == ' ' || c == '\t' || c == '\n' || c == '\r':
			i++
		default:
			_, size := utf8.DecodeRune(src[i:])
			i += size
			toks = append(toks, token{kind: tokChar, text: string(src[start:i]), start: start, end: i})
		}
	}
	toks = append(toks, token{kind: tokEOF, start: len(src), end: len(src)})
	return toks
}

func isLetter(c byte) bool { return c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' }