use anyhow::{anyhow, Result};

use super::ast::{AggFunc, BinOp, Expr, Filter, Formula};

/// Parse a formula string like "Profit = Revenue - Cost"
/// or "Tax = Revenue * 0.08 WHERE Region = \"East\""
pub fn parse_formula(raw: &str, target_category: &str) -> Result<Formula> {
    let raw = raw.trim();

    // Split on first `=` to get target = expression
    let eq_pos = raw
        .find('=')
        .ok_or_else(|| anyhow!("Formula must contain '=': {raw}"))?;
    let target = raw[..eq_pos].trim().to_string();
    let rest = raw[eq_pos + 1..].trim();

    // Check for WHERE clause at top level
    let (expr_str, filter) = split_where(rest);
    let filter = filter.map(parse_where).transpose()?;

    let expr = parse_expr(expr_str.trim())?;

    Ok(Formula::new(raw, target, target_category, expr, filter))
}

fn split_where(s: &str) -> (&str, Option<&str>) {
    // Find WHERE not inside parens or quotes
    let bytes = s.as_bytes();
    let mut depth = 0i32;
    let mut i = 0;
    while i < bytes.len() {
        match bytes[i] {
            b'(' => depth += 1,
            b')' => depth -= 1,
            b'"' => {
                i += 1;
                while i < bytes.len() && bytes[i] != b'"' {
                    i += 1;
                }
            }
            _ if depth == 0 => {
                if s[i..].to_ascii_uppercase().starts_with("WHERE") {
                    let before = &s[..i];
                    let after = &s[i + 5..];
                    if before.ends_with(char::is_whitespace) || i == 0 {
                        return (before.trim(), Some(after.trim()));
                    }
                }
            }
            _ => {}
        }
        i += 1;
    }
    (s, None)
}

fn parse_where(s: &str) -> Result<Filter> {
    // Format: Category = "Item" or Category = Item
    let eq_pos = s
        .find('=')
        .ok_or_else(|| anyhow!("WHERE clause must contain '=': {s}"))?;
    let category = s[..eq_pos].trim().to_string();
    let item_raw = s[eq_pos + 1..].trim();
    let item = item_raw.trim_matches('"').to_string();
    Ok(Filter { category, item })
}

/// Parse an expression using recursive descent
pub fn parse_expr(s: &str) -> Result<Expr> {
    let tokens = tokenize(s)?;
    let mut pos = 0;
    let expr = parse_add_sub(&tokens, &mut pos)?;
    if pos < tokens.len() {
        return Err(anyhow!(
            "Unexpected token at position {pos}: {:?}",
            tokens[pos]
        ));
    }
    Ok(expr)
}

#[derive(Debug, Clone, PartialEq)]
enum Token {
    Number(f64),
    Ident(String),
    Str(String),
    Plus,
    Minus,
    Star,
    Slash,
    Caret,
    LParen,
    RParen,
    Comma,
    Eq,
    Ne,
    Lt,
    Gt,
    Le,
    Ge,
}

fn tokenize(s: &str) -> Result<Vec<Token>> {
    let mut tokens = Vec::new();
    let chars: Vec<char> = s.chars().collect();
    let mut i = 0;

    while i < chars.len() {
        match chars[i] {
            ' ' | '\t' | '\n' => i += 1,
            '+' => {
                tokens.push(Token::Plus);
                i += 1;
            }
            '-' => {
                tokens.push(Token::Minus);
                i += 1;
            }
            '*' => {
                tokens.push(Token::Star);
                i += 1;
            }
            '/' => {
                tokens.push(Token::Slash);
                i += 1;
            }
            '^' => {
                tokens.push(Token::Caret);
                i += 1;
            }
            '(' => {
                tokens.push(Token::LParen);
                i += 1;
            }
            ')' => {
                tokens.push(Token::RParen);
                i += 1;
            }
            ',' => {
                tokens.push(Token::Comma);
                i += 1;
            }
            '!' if chars.get(i + 1) == Some(&'=') => {
                tokens.push(Token::Ne);
                i += 2;
            }
            '<' if chars.get(i + 1) == Some(&'=') => {
                tokens.push(Token::Le);
                i += 2;
            }
            '>' if chars.get(i + 1) == Some(&'=') => {
                tokens.push(Token::Ge);
                i += 2;
            }
            '<' => {
                tokens.push(Token::Lt);
                i += 1;
            }
            '>' => {
                tokens.push(Token::Gt);
                i += 1;
            }
            '=' => {
                tokens.push(Token::Eq);
                i += 1;
            }
            '"' => {
                i += 1;
                let mut s = String::new();
                while i < chars.len() && chars[i] != '"' {
                    s.push(chars[i]);
                    i += 1;
                }
                if i < chars.len() {
                    i += 1;
                }
                tokens.push(Token::Str(s));
            }
            c if c.is_ascii_digit() || c == '.' => {
                let mut num = String::new();
                while i < chars.len() && (chars[i].is_ascii_digit() || chars[i] == '.') {
                    num.push(chars[i]);
                    i += 1;
                }
                tokens.push(Token::Number(num.parse()?));
            }
            c if c.is_alphabetic() || c == '_' => {
                let mut ident = String::new();
                while i < chars.len()
                    && (chars[i].is_alphanumeric() || chars[i] == '_' || chars[i] == ' ')
                {
                    // Don't consume trailing spaces if next non-space is operator
                    if chars[i] == ' ' {
                        // Peek ahead
                        let j = i + 1;
                        let next_nonspace = chars[j..].iter().find(|&&c| c != ' ');
                        if matches!(
                            next_nonspace,
                            Some('+')
                                | Some('-')
                                | Some('*')
                                | Some('/')
                                | Some('^')
                                | Some(')')
                                | Some(',')
                                | None
                        ) {
                            break;
                        }
                    }
                    ident.push(chars[i]);
                    i += 1;
                }
                let ident = ident.trim_end().to_string();
                tokens.push(Token::Ident(ident));
            }
            c => return Err(anyhow!("Unexpected character '{c}' in expression")),
        }
    }
    Ok(tokens)
}

fn parse_add_sub(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    let mut left = parse_mul_div(tokens, pos)?;
    while *pos < tokens.len() {
        let op = match &tokens[*pos] {
            Token::Plus => BinOp::Add,
            Token::Minus => BinOp::Sub,
            _ => break,
        };
        *pos += 1;
        let right = parse_mul_div(tokens, pos)?;
        left = Expr::BinOp(op, Box::new(left), Box::new(right));
    }
    Ok(left)
}

fn parse_mul_div(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    let mut left = parse_pow(tokens, pos)?;
    while *pos < tokens.len() {
        let op = match &tokens[*pos] {
            Token::Star => BinOp::Mul,
            Token::Slash => BinOp::Div,
            _ => break,
        };
        *pos += 1;
        let right = parse_pow(tokens, pos)?;
        left = Expr::BinOp(op, Box::new(left), Box::new(right));
    }
    Ok(left)
}

fn parse_pow(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    let base = parse_unary(tokens, pos)?;
    if *pos < tokens.len() && tokens[*pos] == Token::Caret {
        *pos += 1;
        let exp = parse_unary(tokens, pos)?;
        return Ok(Expr::BinOp(BinOp::Pow, Box::new(base), Box::new(exp)));
    }
    Ok(base)
}

fn parse_unary(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    if *pos < tokens.len() && tokens[*pos] == Token::Minus {
        *pos += 1;
        let e = parse_primary(tokens, pos)?;
        return Ok(Expr::UnaryMinus(Box::new(e)));
    }
    parse_primary(tokens, pos)
}

fn parse_primary(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    if *pos >= tokens.len() {
        return Err(anyhow!("Unexpected end of expression"));
    }
    match &tokens[*pos].clone() {
        Token::Number(n) => {
            *pos += 1;
            Ok(Expr::Number(*n))
        }
        Token::Ident(name) => {
            let name = name.clone();
            *pos += 1;
            // Check for function call
            let upper = name.to_ascii_uppercase();
            match upper.as_str() {
                "SUM" | "AVG" | "MIN" | "MAX" | "COUNT" => {
                    let func = match upper.as_str() {
                        "SUM" => AggFunc::Sum,
                        "AVG" => AggFunc::Avg,
                        "MIN" => AggFunc::Min,
                        "MAX" => AggFunc::Max,
                        "COUNT" => AggFunc::Count,
                        _ => unreachable!(),
                    };
                    if *pos < tokens.len() && tokens[*pos] == Token::LParen {
                        *pos += 1;
                        let inner = parse_add_sub(tokens, pos)?;
                        // Optional WHERE filter
                        let filter = if *pos < tokens.len() {
                            if let Token::Ident(kw) = &tokens[*pos] {
                                if kw.eq_ignore_ascii_case("WHERE") {
                                    *pos += 1;
                                    let cat = match &tokens[*pos] {
                                        Token::Ident(s) => {
                                            let s = s.clone();
                                            *pos += 1;
                                            s
                                        }
                                        t => {
                                            return Err(anyhow!(
                                                "Expected category name, got {t:?}"
                                            ))
                                        }
                                    };
                                    // expect =
                                    if *pos < tokens.len() && tokens[*pos] == Token::Eq {
                                        *pos += 1;
                                    }
                                    let item = match &tokens[*pos] {
                                        Token::Str(s) | Token::Ident(s) => {
                                            let s = s.clone();
                                            *pos += 1;
                                            s
                                        }
                                        t => return Err(anyhow!("Expected item name, got {t:?}")),
                                    };
                                    Some(Filter {
                                        category: cat,
                                        item,
                                    })
                                } else {
                                    None
                                }
                            } else {
                                None
                            }
                        } else {
                            None
                        };
                        // expect )
                        if *pos < tokens.len() && tokens[*pos] == Token::RParen {
                            *pos += 1;
                        } else {
                            return Err(anyhow!("Expected ')' to close aggregate function"));
                        }
                        return Ok(Expr::Agg(func, Box::new(inner), filter));
                    }
                    Ok(Expr::Ref(name))
                }
                "IF" => {
                    if *pos < tokens.len() && tokens[*pos] == Token::LParen {
                        *pos += 1;
                        let cond = parse_comparison(tokens, pos)?;
                        if *pos < tokens.len() && tokens[*pos] == Token::Comma {
                            *pos += 1;
                        }
                        let then = parse_add_sub(tokens, pos)?;
                        if *pos < tokens.len() && tokens[*pos] == Token::Comma {
                            *pos += 1;
                        }
                        let else_ = parse_add_sub(tokens, pos)?;
                        if *pos < tokens.len() && tokens[*pos] == Token::RParen {
                            *pos += 1;
                        } else {
                            return Err(anyhow!("Expected ')' to close IF(...)"));
                        }
                        return Ok(Expr::If(Box::new(cond), Box::new(then), Box::new(else_)));
                    }
                    Ok(Expr::Ref(name))
                }
                _ => Ok(Expr::Ref(name)),
            }
        }
        Token::LParen => {
            *pos += 1;
            let e = parse_add_sub(tokens, pos)?;
            if *pos < tokens.len() && tokens[*pos] == Token::RParen {
                *pos += 1;
            }
            Ok(e)
        }
        t => Err(anyhow!("Unexpected token in expression: {t:?}")),
    }
}

fn parse_comparison(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
    let left = parse_add_sub(tokens, pos)?;
    if *pos >= tokens.len() {
        return Ok(left);
    }
    let op = match &tokens[*pos] {
        Token::Eq => BinOp::Eq,
        Token::Ne => BinOp::Ne,
        Token::Lt => BinOp::Lt,
        Token::Gt => BinOp::Gt,
        Token::Le => BinOp::Le,
        Token::Ge => BinOp::Ge,
        _ => return Ok(left),
    };
    *pos += 1;
    let right = parse_add_sub(tokens, pos)?;
    Ok(Expr::BinOp(op, Box::new(left), Box::new(right)))
}

#[cfg(test)]
mod tests {
    use super::parse_formula;
    use crate::formula::{AggFunc, BinOp, Expr};

    #[test]
    fn parse_simple_subtraction() {
        let f = parse_formula("Profit = Revenue - Cost", "Measure").unwrap();
        assert_eq!(f.target, "Profit");
        assert_eq!(f.target_category, "Measure");
        assert!(matches!(f.expr, Expr::BinOp(BinOp::Sub, _, _)));
    }

    #[test]
    fn parse_where_clause() {
        let f = parse_formula("EastRev = Revenue WHERE Region = \"East\"", "Measure").unwrap();
        assert_eq!(f.target, "EastRev");
        let filter = f.filter.as_ref().unwrap();
        assert_eq!(filter.category, "Region");
        assert_eq!(filter.item, "East");
    }

    #[test]
    fn parse_sum_aggregation() {
        let f = parse_formula("Total = SUM(Revenue)", "Measure").unwrap();
        assert!(matches!(f.expr, Expr::Agg(AggFunc::Sum, _, _)));
    }

    #[test]
    fn parse_avg_aggregation() {
        let f = parse_formula("Avg = AVG(Revenue)", "Measure").unwrap();
        assert!(matches!(f.expr, Expr::Agg(AggFunc::Avg, _, _)));
    }

    #[test]
    fn parse_if_expression() {
        let f = parse_formula("Capped = IF(Revenue > 1000, 1000, Revenue)", "Measure").unwrap();
        assert!(matches!(f.expr, Expr::If(_, _, _)));
    }

    #[test]
    fn parse_numeric_literal() {
        let f = parse_formula("Fixed = 42", "Measure").unwrap();
        assert!(matches!(f.expr, Expr::Number(n) if (n - 42.0).abs() < 1e-10));
    }

    #[test]
    fn parse_chained_arithmetic() {
        parse_formula("X = (A + B) * (C - D)", "Cat").unwrap();
    }

    #[test]
    fn parse_missing_equals_returns_error() {
        assert!(parse_formula("BadFormula Revenue Cost", "Cat").is_err());
    }
}