// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.

module main

import (
	os
	strings
)

const (
	single_quote = `\'`
	double_quote = `"`
	error_context_before = 2 // how many lines of source context to print before the pointer line
	error_context_after = 2  // ^^^ same, but after
)

struct Scanner {
mut:
	file_path      string
	text           string
	pos            int
	line_nr        int
	last_nl_pos    int // for calculating column
	inside_string  bool
	inter_start   bool // for hacky string interpolation TODO simplify
	inter_end     bool
	debug          bool
	line_comment   string
	started        bool
	// vfmt fields
	fmt_out        strings.Builder
	fmt_indent     int
	fmt_line_empty bool
	prev_tok Token
	fn_name string // needed for @FN
	should_print_line_on_error bool
	should_print_errors_in_color bool
	quote byte // which quote is used to denote current string: ' or "
	file_lines   []string // filled *only on error* by rescanning the source till the error (and several lines more)
}

// new scanner from file.
fn new_scanner_file(file_path string) &Scanner {
	if !os.file_exists(file_path) {
		verror("$file_path doesn't exist")
	}

	mut raw_text := os.read_file(file_path) or {
		verror('scanner: failed to open $file_path')
		return 0
	}
		
	// BOM check
	if raw_text.len >= 3 {
		c_text := raw_text.str

		if c_text[0] == 0xEF && c_text[1] == 0xBB && c_text[2] == 0xBF {
			// skip three BOM bytes
			offset_from_begin := 3
			raw_text = tos(c_text[offset_from_begin], vstrlen(c_text) - offset_from_begin)
		}
	}

	mut s := new_scanner(raw_text)
	s.file_path = file_path

	return s
}

// new scanner from string.
fn new_scanner(text string) &Scanner {
	return &Scanner {
		text: text
		fmt_out: strings.new_builder(1000)
		should_print_line_on_error: true
		should_print_errors_in_color: true
	}
}


// TODO remove once multiple return values are implemented
struct ScanRes {
	tok Token
	lit string
}

fn scan_res(tok Token, lit string) ScanRes {
	return ScanRes{tok, lit}
}

fn (s mut Scanner) ident_name() string {
	start := s.pos
	for {
		s.pos++
		if s.pos >= s.text.len {
			break
		}
		c := s.text[s.pos]
		if !is_name_char(c) && !c.is_digit() {
			break
		}
	}
	name := s.text.substr(start, s.pos)
	s.pos--
	return name
}

fn (s mut Scanner) ident_hex_number() string {
	start_pos := s.pos
	s.pos += 2 // skip '0x'
	for {
		if s.pos >= s.text.len {
			break
		}
		c := s.text[s.pos]
		if !c.is_hex_digit() {
			break
		}
		s.pos++
	}
	number := s.text.substr(start_pos, s.pos)
	s.pos--
	return number
}

fn (s mut Scanner) ident_oct_number() string {
	start_pos := s.pos
	for {
		if s.pos >= s.text.len {
			break
		}
		c := s.text[s.pos]
		if c.is_digit() {
			if !c.is_oct_digit() {
				s.error('malformed octal constant')
			}
		} else {
			break
		}
		s.pos++
	}
	number := s.text.substr(start_pos, s.pos)
	s.pos--
	return number
}

fn (s mut Scanner) ident_dec_number() string {
	start_pos := s.pos

	// scan integer part
	for s.pos < s.text.len && s.text[s.pos].is_digit() {
		s.pos++
	}

	// e.g. 1..9
	// we just return '1' and don't scan '..9'
	if s.expect('..', s.pos) {
		number := s.text.substr(start_pos, s.pos)
		s.pos--
		return number
	}

	// scan fractional part
	if s.pos < s.text.len && s.text[s.pos] == `.` {
		s.pos++
		for s.pos < s.text.len && s.text[s.pos].is_digit() {
			s.pos++
		}
		if !s.inside_string && s.pos < s.text.len && s.text[s.pos] == `f` {
			s.error('no `f` is needed for floats')
		}
	}

	// scan exponential part
	mut has_exponential_part := false
	if s.expect('e+', s.pos) || s.expect('e-', s.pos) {
		exp_start_pos := s.pos += 2
		for s.pos < s.text.len && s.text[s.pos].is_digit() {
			s.pos++
		}
		if exp_start_pos == s.pos {
			s.error('exponent has no digits')
		}
		has_exponential_part = true
	}

	// error check: 1.23.4, 123.e+3.4
	if s.pos < s.text.len && s.text[s.pos] == `.` {
		if has_exponential_part {
			s.error('exponential part should be integer')
		}
		else {
			s.error('too many decimal points in number')
		}
	}

	number := s.text.substr(start_pos, s.pos)
	s.pos--
	return number
}

fn (s mut Scanner) ident_number() string {
	if s.expect('0x', s.pos) {
		return s.ident_hex_number()
	}

	if s.expect('0.', s.pos) || s.expect('0e', s.pos) {
		return s.ident_dec_number()
	}

	if s.text[s.pos] == `0` {
		return s.ident_oct_number()
	}

	return s.ident_dec_number()
}

fn (s mut Scanner) skip_whitespace() {
	for s.pos < s.text.len && s.text[s.pos].is_white() {
		// Count \r\n as one line
		if is_nl(s.text[s.pos]) && !s.expect('\r\n', s.pos-1) {
			s.inc_line_number()
		}
		s.pos++
	}
}

fn (s mut Scanner) scan() ScanRes {
	if s.line_comment != '' {
		//s.fgenln('// LOL "$s.line_comment"')
		//s.line_comment = ''
	}
	if s.started {
		s.pos++
	}
	s.started = true
	if s.pos >= s.text.len {
		return scan_res(.eof, '')
	}
	// skip whitespace
	if !s.inside_string {
		s.skip_whitespace()
	}
	// End of $var, start next string
	if s.inter_end {
		if s.text[s.pos] == `\'` {
			s.inter_end = false
			return scan_res(.str, '')
		}
		s.inter_end = false
		return scan_res(.str, s.ident_string())
	}
	s.skip_whitespace()
	// end of file
	if s.pos >= s.text.len {
		return scan_res(.eof, '')
	}
	// handle each char
	c := s.text[s.pos]
	mut nextc := `\0`
	if s.pos + 1 < s.text.len {
		nextc = s.text[s.pos + 1]
	}
	// name or keyword
	if is_name_char(c) {
		name := s.ident_name()
		// tmp hack to detect . in ${}
		// Check if not .eof to prevent panic
		next_char := if s.pos + 1 < s.text.len { s.text[s.pos + 1] } else { `\0` }
		if is_key(name) {
			return scan_res(key_to_token(name), '')
		}
		// 'asdf $b' => "b" is the last name in the string, dont start parsing string
		// at the next ', skip it
		if s.inside_string {
			if next_char == `\'` {
				s.inter_end = true
				s.inter_start = false
				s.inside_string = false
			}
		}
		if s.inter_start && next_char != `.` {
			s.inter_end = true
			s.inter_start = false
		}
		if s.pos == 0 && next_char == ` ` {
			s.pos++
			//If a single letter name at the start of the file, increment
			//Otherwise the scanner would be stuck at s.pos = 0
		}
		return scan_res(.name, name)
	}
	// `123`, `.123`
	else if c.is_digit() || (c == `.` && nextc.is_digit()) {
		num := s.ident_number()
		return scan_res(.number, num)
	}
	// all other tokens
	switch c {
	case `+`:
		if nextc == `+` {
			s.pos++
			return scan_res(.inc, '')
		}
		else if nextc == `=` {
			s.pos++
			return scan_res(.plus_assign, '')
		}
		return scan_res(.plus, '')
	case `-`:
		if nextc == `-` {
			s.pos++
			return scan_res(.dec, '')
		}
		else if nextc == `=` {
			s.pos++
			return scan_res(.minus_assign, '')
		}
		return scan_res(.minus, '')
	case `*`:
		if nextc == `=` {
			s.pos++
			return scan_res(.mult_assign, '')
		}
		return scan_res(.mul, '')
	case `^`:
		if nextc == `=` {
			s.pos++
			return scan_res(.xor_assign, '')
		}
		return scan_res(.xor, '')
	case `%`:
		if nextc == `=` {
			s.pos++
			return scan_res(.mod_assign, '')
		}
		return scan_res(.mod, '')
	case `?`:
		return scan_res(.question, '')
	case single_quote, double_quote:
		return scan_res(.str, s.ident_string())
	case `\``: // ` // apostrophe balance comment. do not remove
		return scan_res(.chartoken, s.ident_char())
	case `(`:
		return scan_res(.lpar, '')
	case `)`:
		return scan_res(.rpar, '')
	case `[`:
		return scan_res(.lsbr, '')
	case `]`:
		return scan_res(.rsbr, '')
	case `{`:
		// Skip { in ${ in strings
		if s.inside_string {
			return s.scan()
		}
		return scan_res(.lcbr, '')
	case `$`:
		return scan_res(.dollar, '')
	case `}`:
		// s = `hello $name !`
		// s = `hello ${name} !`
		if s.inside_string {
			s.pos++
			// TODO UNNEEDED?
			if s.text[s.pos] == `\'` {
				s.inside_string = false
				return scan_res(.str, '')
			}
			return scan_res(.str, s.ident_string())
		}
		else {
			return scan_res(.rcbr, '')
		}
	case `&`:
		if nextc == `=` {
			s.pos++
			return scan_res(.and_assign, '')
		}
		if nextc == `&` {
			s.pos++
			return scan_res(.and, '')
		}
		return scan_res(.amp, '')
	case `|`:
		if nextc == `|` {
			s.pos++
			return scan_res(.logical_or, '')
		}
		if nextc == `=` {
			s.pos++
			return scan_res(.or_assign, '')
		}
		return scan_res(.pipe, '')
	case `,`:
		return scan_res(.comma, '')
	case `@`:
		s.pos++
		name := s.ident_name()
		// @FN => will be substituted with the name of the current V function
		// @FILE => will be substituted with the path of the V source file
		// @LINE => will be substituted with the V line number where it appears (as a string).
		// @COLUMN => will be substituted with the column where it appears (as a string).
		// @VHASH  => will be substituted with the shortened commit hash of the V compiler (as a string).
		// This allows things like this:
		// println( 'file: ' + @FILE + ' | line: ' + @LINE + ' | fn: ' + @FN)
		// ... which is useful while debugging/tracing
		if name == 'FN' { return scan_res(.str, s.fn_name) }
		if name == 'FILE' { return scan_res(.str, os.realpath(s.file_path).replace('\\', '\\\\')) } // escape \
		if name == 'LINE' { return scan_res(.str, (s.line_nr+1).str()) }
		if name == 'COLUMN' { return scan_res(.str, (s.current_column()).str()) }
		if name == 'VHASH' { return scan_res(.str, vhash()) }
		if !is_key(name) {
			s.error('@ must be used before keywords (e.g. `@type string`)')
		}
		return scan_res(.name, name)
	case `\r`:
		if nextc == `\n` {
			s.pos++
			s.last_nl_pos = s.pos
			return scan_res(.nl, '')
		}
	case `\n`:
		s.last_nl_pos = s.pos
		return scan_res(.nl, '')
	case `.`:
		if nextc == `.` {
			s.pos++
			if s.text[s.pos+1] == `.` {
				s.pos++
				return scan_res(.ellipsis, '')
			}
			return scan_res(.dotdot, '')
		}
		return scan_res(.dot, '')
	case `#`:
		start := s.pos + 1
		s.ignore_line()
		if nextc == `!` {
			// treat shebang line (#!) as a comment
			s.line_comment = s.text.substr(start + 1, s.pos).trim_space()
			s.fgenln('// shebang line "$s.line_comment"')
			return s.scan()
		}
		hash := s.text.substr(start, s.pos)
		return scan_res(.hash, hash.trim_space())
	case `>`:
		if nextc == `=` {
			s.pos++
			return scan_res(.ge, '')
		}
		else if nextc == `>` {
			if s.pos + 2 < s.text.len && s.text[s.pos + 2] == `=` {
				s.pos += 2
				return scan_res(.righ_shift_assign, '')
			}
			s.pos++
			return scan_res(.righ_shift, '')
		}
		else {
			return scan_res(.gt, '')
		}
	case 0xE2:
		//case `≠`:
		if nextc == 0x89 && s.text[s.pos + 2] == 0xA0 {
			s.pos += 2
			return scan_res(.ne, '')
		}
		// ⩽
		else if nextc == 0x89 && s.text[s.pos + 2] == 0xBD {
			s.pos += 2
			return scan_res(.le, '')
		}
		// ⩾
		else if nextc == 0xA9 && s.text[s.pos + 2] == 0xBE {
			s.pos += 2
			return scan_res(.ge, '')
		}
	case `<`:
		if nextc == `=` {
			s.pos++
			return scan_res(.le, '')
		}
		else if nextc == `<` {
			if s.pos + 2 < s.text.len && s.text[s.pos + 2] == `=` {
				s.pos += 2
				return scan_res(.left_shift_assign, '')
			}
			s.pos++
			return scan_res(.left_shift, '')
		}
		else {
			return scan_res(.lt, '')
		}
	case `=`:
		if nextc == `=` {
			s.pos++
			return scan_res(.eq, '')
		}
		else if nextc == `>` {
			s.pos++
			return scan_res(.arrow, '')
		}
		else {
			return scan_res(.assign, '')
		}
	case `:`:
		if nextc == `=` {
			s.pos++
			return scan_res(.decl_assign, '')
		}
		else {
			return scan_res(.colon, '')
		}
	case `;`:
		return scan_res(.semicolon, '')
	case `!`:
		if nextc == `=` {
			s.pos++
			return scan_res(.ne, '')
		}
		else {
			return scan_res(.not, '')
		}
	case `~`:
		return scan_res(.bit_not, '')
	case `/`:
		if nextc == `=` {
			s.pos++
			return scan_res(.div_assign, '')
		}
		if nextc == `/` {
			start := s.pos + 1
			s.ignore_line()
			s.line_comment = s.text.substr(start + 1, s.pos)
			s.line_comment = s.line_comment.trim_space()
			s.fgenln('// ${s.prev_tok.str()} "$s.line_comment"')
			// Skip the comment (return the next token)
			return s.scan()
		}
		// Multiline comments
		if nextc == `*` {
			start := s.pos
			mut nest_count := 1
			// Skip comment
			for nest_count > 0 {
				s.pos++
				if s.pos >= s.text.len {
					s.line_nr--
					s.error('comment not terminated')
				}
				if s.text[s.pos] == `\n` {
					s.inc_line_number()
					continue
				}
				if s.expect('/*', s.pos) {
					nest_count++
					continue
				}
				if s.expect('*/', s.pos) {
					nest_count--
				}
			}
			s.pos++
			end := s.pos + 1
			comm := s.text.substr(start, end)
			s.fgenln(comm)
			// Skip if not in fmt mode
			return s.scan()
		}
		return scan_res(.div, '')
	}
	$if windows {
		if c == `\0` {
			return scan_res(.eof, '')
		}
	}
	mut msg := 'invalid character `${c.str()}`'
	if c == `"` {
		msg += ', use \' to denote strings'
	}
	s.error(msg)
	return scan_res(.eof, '')
}

fn (s &Scanner) current_column() int {
	return s.pos - s.last_nl_pos
}

fn (s Scanner) count_symbol_before(p int, sym byte) int {
  mut count := 0
  for i:=p; i>=0; i-- {
	if s.text[i] != sym {
	  break
	}
	count++
  }
  return count
}

// println('array out of bounds $idx len=$a.len')
// This is really bad. It needs a major clean up
fn (s mut Scanner) ident_string() string {
	q := s.text[s.pos]
	if (q == single_quote || q == double_quote) &&	!s.inside_string{
		s.quote = q
	}
	//if s.file_path.contains('string_test') {
	//println('\nident_string() at char=${s.text[s.pos].str()}')
	//println('linenr=$s.line_nr quote=  $qquote ${qquote.str()}')
	//}
	mut start := s.pos
	s.inside_string = false
	slash := `\\`
	for {
		s.pos++
		if s.pos >= s.text.len {
			break
		}
		c := s.text[s.pos]
		prevc := s.text[s.pos - 1]
		// end of string
		if c == s.quote && (prevc != slash || (prevc == slash && s.text[s.pos - 2] == slash)) {
			// handle '123\\'  slash at the end
			break
		}
		if c == `\n` {
			s.inc_line_number()
		}
		// Don't allow \0
		if c == `0` && s.pos > 2 && s.text[s.pos - 1] == `\\` {
			s.error('0 character in a string literal')
		}
		// Don't allow \x00
		if c == `0` && s.pos > 5 && s.expect('\\x0', s.pos - 3) {
			s.error('0 character in a string literal')
		}
		// ${var}
		if c == `{` && prevc == `$` && s.count_symbol_before(s.pos-2, `\\`) % 2 == 0 {
			s.inside_string = true
			// so that s.pos points to $ at the next step
			s.pos -= 2
			break
		}
		// $var
		if (c.is_letter() || c == `_`) && prevc == `$` && s.count_symbol_before(s.pos-2, `\\`) % 2 == 0 {
			s.inside_string = true
			s.inter_start = true
			s.pos -= 2
			break
		}
	}
	mut lit := ''
	if s.text[start] == s.quote {
		start++
	}
	mut end := s.pos
	if s.inside_string {
		end++
	}
	if start > s.pos{}
	else {
		lit = s.text.substr(start, end)
	}
	return lit
}

fn (s mut Scanner) ident_char() string {
	start := s.pos
	slash := `\\`
	mut len := 0
	for {
		s.pos++
		if s.pos >= s.text.len {
			break
		}
		if s.text[s.pos] != slash {
			len++
		}
		double_slash := s.expect('\\\\', s.pos - 2)
		if s.text[s.pos] == `\`` && (s.text[s.pos - 1] != slash || double_slash) { // ` // apostrophe balance comment. do not remove
			if double_slash {
				len++
			}
			break
		}
	}
	len--
	c := s.text.substr(start + 1, s.pos)
	if len != 1 {
		u := c.ustring()
		if u.len != 1 {
			s.error('invalid character literal (more than one character: $len)')
		}
	}
	if c == '\\`' {
		return '`'
	}	
	// Escapes a `'` character
	return if c == '\'' { '\\' + c } else { c }
}

fn (s &Scanner) expect(want string, start_pos int) bool {
	end_pos := start_pos + want.len
	if start_pos < 0 || start_pos >= s.text.len {
		return false
	}
	if end_pos < 0 || end_pos > s.text.len {
		return false
	}
	for pos in start_pos..end_pos {
		if s.text[pos] != want[pos-start_pos] {
			return false
		}
	}
	return true
}

fn (s mut Scanner) debug_tokens() {
	s.pos = 0
	s.debug = true

	fname := s.file_path.all_after('/')
	println('\n===DEBUG TOKENS $fname===')

	for {
		res := s.scan()
		tok := res.tok
		lit := res.lit
		print(tok.str())
		if lit != '' {
			println(' `$lit`')
		}
		else {
			println('')
		}
		if tok == .eof {
			println('============ END OF DEBUG TOKENS ==================')
			break
		}
	}
}


fn (s mut Scanner) ignore_line() {
	s.eat_to_end_of_line()
	s.inc_line_number()
}

fn (s mut Scanner) eat_to_end_of_line(){
	for s.pos < s.text.len && s.text[s.pos] != `\n` {
		s.pos++
	}
}

fn (s mut Scanner) inc_line_number() {
	s.last_nl_pos = s.pos
	s.line_nr++
}

fn is_name_char(c byte) bool {
	return c.is_letter() || c == `_`
}

fn is_nl(c byte) bool {
	return c == `\r` || c == `\n`
}

fn contains_capital(s string) bool {
	for c in s {
		if c >= `A` && c <= `Z` {
			return true
		}
	}
	return false
}

// HTTPRequest  bad
// HttpRequest  good
fn good_type_name(s string) bool {
	if s.len < 4 {
		return true
	}
	for i in 2 .. s.len {
		if s[i].is_capital() && s[i-1].is_capital() && s[i-2].is_capital() {
			return false
		}
	}
	return true
}

// registration_date good
// registrationdate  bad
fn (s &Scanner) validate_var_name(name string) {
	if name.len > 11 && !name.contains('_') {
		s.error('bad variable name `$name`\n' +
'looks like you have a multi-word name without separating them with `_`' +
'\nfor example, use `registration_date` instead of `registrationdate` ')
		
	}	
}