use std::str::FromStr;

use smartstring::{LazyCompact, SmartString};

use crate::{
	Label,
	objectpath::{PathParseError, PathSegment, tokenizer::Token},
};

/// Parse an ident token into a `PathSegment::Field`, handling optional args of
/// the form `name(args)`.  Parens inside args may be nested; `\(` and `\)` are
/// escaped and do not affect depth counting.
fn parse_field(ident: &str, position: usize) -> Result<PathSegment, PathParseError> {
	let bytes = ident.as_bytes();
	let mut i = 0;

	// Find the first unescaped '(' — everything before it is the name.
	let open_paren: Option<usize> = loop {
		if i >= bytes.len() {
			break None;
		}
		match bytes[i] {
			b'\\' => i += 2, // skip escaped character
			b'(' => break Some(i),
			_ => i += 1,
		}
	};

	let name_str = &ident[..open_paren.unwrap_or(bytes.len())];
	let name = Label::new(name_str).ok_or_else(|| PathParseError::InvalidField {
		position,
		str: name_str.into(),
	})?;

	let Some(open_pos) = open_paren else {
		return Ok(PathSegment::Field { name, args: None });
	};

	// Scan args, tracking paren depth.
	let args_start = open_pos + 1;
	let mut depth: usize = 1;
	let mut j = args_start;

	while j < bytes.len() {
		match bytes[j] {
			b'\\' => j += 2, // skip escaped character
			b'(' => {
				depth += 1;
				j += 1;
			}
			b')' => {
				depth -= 1;
				if depth == 0 {
					// Closing paren must be the last character.
					if j + 1 != bytes.len() {
						return Err(PathParseError::Syntax {
							position: position + j + 1,
						});
					}
					let args: SmartString<LazyCompact> = ident[args_start..j].into();
					return Ok(PathSegment::Field {
						name,
						args: Some(args),
					});
				}
				j += 1;
			}
			_ => j += 1,
		}
	}

	// Reached end of ident without finding the matching ')'.
	Err(PathParseError::Syntax {
		position: position + ident.len(),
	})
}

enum State {
	Start,

	/// We are holding a pointer to an object
	Selected,

	/// We are waiting for an identifier
	Dot,

	/// We are indexing an array, waiting for a number
	Index,

	/// We parsed the start index, waiting for `]` or the first `.` of `..`
	IndexAfterStart(i64),

	/// We saw one `.` after the start index, waiting for the second `.`
	IndexRangeDot1(i64),

	/// We saw `..`, waiting for the end index (optionally prefixed with `=`)
	IndexRangeDot2(i64),

	/// We are indexing an array, waiting for a close-bracket
	IndexClose,
}

pub struct Parser {
	state: State,
	segments: Vec<PathSegment>,
}

impl Parser {
	pub fn new() -> Self {
		Parser {
			state: State::Start,
			segments: Vec::new(),
		}
	}

	pub fn parse(
		mut self,
		source: &str,
		tokens: &[(usize, Token<'_>)],
	) -> Result<Vec<PathSegment>, PathParseError> {
		for t in tokens {
			match (self.state, t) {
				(State::Start, (_, Token::Root)) => {
					self.segments.push(PathSegment::Root);
					self.state = State::Selected
				}

				(State::Start, (p, Token::Ident(_))) => {
					return Err(PathParseError::MustStartWithRoot { position: *p });
				}

				(State::Start, (p, Token::Dot))
				| (State::Start, (p, Token::SqbOpen))
				| (State::Start, (p, Token::SqbClose)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				//
				// MARK: selected
				//
				(State::Selected, (_, Token::Dot)) => self.state = State::Dot,
				(State::Selected, (_, Token::SqbOpen)) => self.state = State::Index,

				(State::Selected, (p, Token::Root))
				| (State::Selected, (p, Token::Ident(_)))
				| (State::Selected, (p, Token::SqbClose)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				//
				// MARK: dot
				//
				(State::Dot, (p, Token::Ident(ident))) => {
					self.segments.push(parse_field(ident, *p)?);
					self.state = State::Selected;
				}

				(State::Dot, (p, Token::Root))
				| (State::Dot, (p, Token::Dot))
				| (State::Dot, (p, Token::SqbOpen))
				| (State::Dot, (p, Token::SqbClose)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				//
				// MARK: index
				//
				(State::Index, (p, Token::Ident(ident))) => {
					let idx: i64 = i64::from_str(ident).map_err(|_err| {
						PathParseError::InvalidIndexString {
							position: *p,
							str: (*ident).into(),
						}
					})?;

					self.state = State::IndexAfterStart(idx);
				}

				(State::Index, (p, Token::Root))
				| (State::Index, (p, Token::Dot))
				| (State::Index, (p, Token::SqbOpen))
				| (State::Index, (p, Token::SqbClose)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				(State::IndexAfterStart(idx), (_, Token::SqbClose)) => {
					self.segments.push(PathSegment::Index(idx));
					self.state = State::Selected;
				}
				(State::IndexAfterStart(idx), (_, Token::Dot)) => {
					self.state = State::IndexRangeDot1(idx);
				}
				(State::IndexAfterStart(_), (p, _)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				(State::IndexRangeDot1(idx), (_, Token::Dot)) => {
					self.state = State::IndexRangeDot2(idx);
				}
				(State::IndexRangeDot1(_), (p, _)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				(State::IndexRangeDot2(start), (p, Token::Ident(ident))) => {
					let (end_str, inclusive) = if let Some(stripped) = ident.strip_prefix('=') {
						(stripped, true)
					} else {
						(*ident, false)
					};

					let end: i64 = i64::from_str(end_str).map_err(|_err| {
						PathParseError::InvalidIndexString {
							position: *p,
							str: (*ident).into(),
						}
					})?;

					self.segments.push(PathSegment::Range {
						start,
						end,
						inclusive,
					});
					self.state = State::IndexClose;
				}
				(State::IndexRangeDot2(_), (p, _)) => {
					return Err(PathParseError::Syntax { position: *p });
				}

				(State::IndexClose, (_, Token::SqbClose)) => self.state = State::Selected,
				(State::IndexClose, (p, _)) => {
					return Err(PathParseError::Syntax { position: *p });
				}
			}
		}

		let position = source.len();
		match self.state {
			State::Start => Err(PathParseError::Syntax { position: 0 }),
			State::Dot => Err(PathParseError::Syntax { position }),
			State::Index => Err(PathParseError::Syntax { position }),
			State::IndexAfterStart(_) => Err(PathParseError::Syntax { position }),
			State::IndexRangeDot1(_) => Err(PathParseError::Syntax { position }),
			State::IndexRangeDot2(_) => Err(PathParseError::Syntax { position }),
			State::IndexClose => Err(PathParseError::Syntax { position }),
			State::Selected => Ok(()),
		}?;

		return Ok(self.segments);
	}
}

//
// MARK: tests
//

#[expect(clippy::unwrap_used)]
#[cfg(test)]
mod tests {
	use crate::objectpath::tokenizer::Tokenizer;

	use super::*;

	fn parse_test(source: &str, expected: Result<&[PathSegment], PathParseError>) {
		let parsed = Tokenizer::new()
			.tokenize(source)
			.and_then(|tokens| Parser::new().parse(source, &tokens[..]));

		match (parsed, expected) {
			(Ok(segments), Ok(segs)) => assert_eq!(segments, segs),
			(Err(e), Err(expected_err)) => assert_eq!(e, expected_err),
			(Ok(segments), Err(e)) => panic!("expected error {e}, got {:?}", segments),
			(Err(e), Ok(segs)) => panic!("expected {:?}, got error {e}", segs),
		}
	}

	#[test]
	fn root_only() {
		parse_test("$", Ok(&[PathSegment::Root]));
	}

	fn field(name: &str) -> PathSegment {
		PathSegment::Field {
			name: Label::new(name).unwrap(),
			args: None,
		}
	}

	fn field_args(name: &str, args: &str) -> PathSegment {
		PathSegment::Field {
			name: Label::new(name).unwrap(),
			args: Some(args.into()),
		}
	}

	#[test]
	fn single_field() {
		parse_test("$.foo", Ok(&[PathSegment::Root, field("foo")]));
	}

	#[test]
	fn nested_fields() {
		parse_test(
			"$.foo.bar.baz",
			Ok(&[PathSegment::Root, field("foo"), field("bar"), field("baz")]),
		);
	}

	#[test]
	fn array_index() {
		parse_test(
			"$.items[0]",
			Ok(&[PathSegment::Root, field("items"), PathSegment::Index(0)]),
		);
	}

	#[test]
	fn chained_indices() {
		parse_test(
			"$.a[1][2]",
			Ok(&[
				PathSegment::Root,
				field("a"),
				PathSegment::Index(1),
				PathSegment::Index(2),
			]),
		);
	}

	#[test]
	fn field_after_index() {
		parse_test(
			"$.a[0].b",
			Ok(&[
				PathSegment::Root,
				field("a"),
				PathSegment::Index(0),
				field("b"),
			]),
		);
	}

	#[test]
	fn negative_index() {
		parse_test(
			"$.a[-1]",
			Ok(&[PathSegment::Root, field("a"), PathSegment::Index(-1)]),
		);
	}

	// MARK: args

	#[test]
	fn field_with_simple_args() {
		parse_test(
			"$.foo(bar)",
			Ok(&[PathSegment::Root, field_args("foo", "bar")]),
		);
	}

	#[test]
	fn field_with_empty_args() {
		parse_test("$.foo()", Ok(&[PathSegment::Root, field_args("foo", "")]));
	}

	#[test]
	fn field_with_nested_parens_in_args() {
		parse_test(
			"$.foo(a(b)c)",
			Ok(&[PathSegment::Root, field_args("foo", "a(b)c")]),
		);
	}

	#[test]
	fn field_with_deeply_nested_parens_in_args() {
		parse_test(
			"$.foo(a(b(c))d)",
			Ok(&[PathSegment::Root, field_args("foo", "a(b(c))d")]),
		);
	}

	#[test]
	fn field_with_escaped_open_paren_in_args() {
		// "$.foo(a\(b)" — '\(' is escaped, so depth never rises above 1; ')' closes it
		parse_test(
			r"$.foo(a\(b)",
			Ok(&[PathSegment::Root, field_args("foo", r"a\(b")]),
		);
	}

	#[test]
	fn field_with_escaped_close_paren_in_args() {
		// "$.foo(a\)b)" — '\)' is escaped, the second ')' closes at depth 0
		parse_test(
			r"$.foo(a\)b)",
			Ok(&[PathSegment::Root, field_args("foo", r"a\)b")]),
		);
	}

	#[test]
	fn field_with_both_escaped_parens_in_args() {
		parse_test(
			r"$.foo(a\(b\)c)",
			Ok(&[PathSegment::Root, field_args("foo", r"a\(b\)c")]),
		);
	}

	#[test]
	fn field_args_with_multiple_segments() {
		parse_test(
			"$.foo(x).bar(y)",
			Ok(&[
				PathSegment::Root,
				field_args("foo", "x"),
				field_args("bar", "y"),
			]),
		);
	}

	#[test]
	fn field_args_unclosed_paren_error() {
		// Missing closing ')' → Syntax error at end of source
		parse_test("$.foo(bar", Err(PathParseError::Syntax { position: 9 }));
	}

	#[test]
	fn field_args_trailing_chars_after_close_error() {
		// Closing ')' is not the last char → Syntax error at the trailing char
		parse_test(
			"$.foo(bar)baz",
			Err(PathParseError::Syntax { position: 10 }),
		);
	}

	#[test]
	fn non_ascii_error() {
		parse_test(
			"$.fé",
			Err(PathParseError::NonAsciiChar {
				position: 3,
				char: 'é',
			}),
		);
	}

	// MARK: range

	fn range(start: i64, end: i64, inclusive: bool) -> PathSegment {
		PathSegment::Range {
			start,
			end,
			inclusive,
		}
	}

	#[test]
	fn exclusive_range() {
		parse_test(
			"$.a[0..5]",
			Ok(&[PathSegment::Root, field("a"), range(0, 5, false)]),
		);
	}

	#[test]
	fn inclusive_range() {
		parse_test(
			"$.a[1..=2]",
			Ok(&[PathSegment::Root, field("a"), range(1, 2, true)]),
		);
	}

	#[test]
	fn range_with_negative_end() {
		parse_test(
			"$.a[0..-1]",
			Ok(&[PathSegment::Root, field("a"), range(0, -1, false)]),
		);
	}

	#[test]
	fn range_with_negative_start() {
		parse_test(
			"$.a[-3..-1]",
			Ok(&[PathSegment::Root, field("a"), range(-3, -1, false)]),
		);
	}
}