module textscanner

// TextScanner simplifies writing small scanners/parsers
// by providing safe methods to scan texts character by
// character, peek for the next characters, go back, etc.
pub struct TextScanner {
pub:
	input string
	ilen  int
mut:
	pos int // current position; pos is *always* kept in [0,ilen]
}

// new returns a stack allocated instance of TextScanner.
pub fn new(input string) TextScanner {
	return TextScanner{
		input: input
		ilen: input.len
	}
}

// free frees all allocated resources.
[unsafe]
pub fn (mut ss TextScanner) free() {
	unsafe {
		ss.input.free()
	}
}

// remaining returns how many characters remain from current position.
[inline]
pub fn (ss &TextScanner) remaining() int {
	return ss.ilen - ss.pos
}

// next returns the next character code from the input text.
// next returns `-1` if it can't reach the next character.
[direct_array_access; inline]
pub fn (mut ss TextScanner) next() int {
	if ss.pos < ss.ilen {
		opos := ss.pos
		ss.pos++
		return ss.input[opos]
	}
	return -1
}

// skip skips one character ahead; `skip()` is slightly faster than `.next()`.
// `skip()` does not return a result.
[inline]
pub fn (mut ss TextScanner) skip() {
	if ss.pos + 1 < ss.ilen {
		ss.pos++
	}
}

// skip_n skips ahead `n` characters, stopping at the end of the input.
[inline]
pub fn (mut ss TextScanner) skip_n(n int) {
	ss.pos += n
	if ss.pos > ss.ilen {
		ss.pos = ss.ilen
	}
}

// peek returns the *next* character code from the input text.
// peek returns `-1` if it can't peek the next character.
// unlike `next()`, `peek()` does not change the state of the scanner.
[direct_array_access; inline]
pub fn (ss &TextScanner) peek() int {
	if ss.pos < ss.ilen {
		return ss.input[ss.pos]
	}
	return -1
}

// peek_n returns the character code from the input text at position + `n`.
// peek_n returns `-1` if it can't peek `n` characters ahead.
[direct_array_access; inline]
pub fn (ss &TextScanner) peek_n(n int) int {
	if ss.pos + n < ss.ilen {
		return ss.input[ss.pos + n]
	}
	return -1
}

// back goes back one character from the current scanner position.
[inline]
pub fn (mut ss TextScanner) back() {
	if ss.pos > 0 {
		ss.pos--
	}
}

// back_n goes back `n` characters from the current scanner position.
pub fn (mut ss TextScanner) back_n(n int) {
	ss.pos -= n
	if ss.pos < 0 {
		ss.pos = 0
	}
	if ss.pos > ss.ilen {
		ss.pos = ss.ilen
	}
}

// reset resets the internal state of the scanner.
pub fn (mut ss TextScanner) reset() {
	ss.pos = 0
}