//! Loop Update Expression Analyzer
//!
//! Phase 197: Extracts update expressions from loop body to generate
//! correct carrier update semantics.
//!
//! # Purpose
//!
//! The Pattern 4 lowerer needs to know how each carrier variable is updated
//! in the loop body. Instead of hardcoding "count uses +1, sum uses +i",
//! we extract the actual update expressions from the AST.
//!
//! # Example
//!
//! ```nyash
//! loop(i < 10) {
//!   i = i + 1         // UpdateExpr::BinOp { lhs: "i", op: Add, rhs: Const(1) }
//!   sum = sum + i     // UpdateExpr::BinOp { lhs: "sum", op: Add, rhs: "i" }
//!   count = count + 1 // UpdateExpr::BinOp { lhs: "count", op: Add, rhs: Const(1) }
//! }
//! ```

use crate::ast::{ASTNode, BinaryOperator, LiteralValue};
use crate::mir::join_ir::lowering::carrier_info::CarrierVar;
use crate::mir::join_ir::BinOpKind;
use std::collections::HashMap;

/// Update expression for a carrier variable
#[derive(Debug, Clone)]
pub enum UpdateExpr {
    /// Constant increment: carrier = carrier + N
    Const(i64),
    /// Binary operation: carrier = carrier op rhs
    BinOp {
        lhs: String,
        op: BinOpKind,
        rhs: UpdateRhs,
    },
}

/// Right-hand side of update expression
#[derive(Debug, Clone)]
pub enum UpdateRhs {
    Const(i64),
    Variable(String),
}

pub struct LoopUpdateAnalyzer;

impl LoopUpdateAnalyzer {
    /// Analyze carrier update expressions from loop body
    ///
    /// Extracts update patterns like:
    /// - `sum = sum + i` → BinOp { lhs: "sum", op: Add, rhs: Variable("i") }
    /// - `count = count + 1` → BinOp { lhs: "count", op: Add, rhs: Const(1) }
    ///
    /// # Parameters
    /// - `body`: Loop body AST nodes
    /// - `carriers`: Carrier variables to analyze
    ///
    /// # Returns
    /// Map from carrier name to UpdateExpr
    pub fn analyze_carrier_updates(
        body: &[ASTNode],
        carriers: &[CarrierVar],
    ) -> HashMap<String, UpdateExpr> {
        let mut updates = HashMap::new();

        // Extract carrier names for quick lookup
        let carrier_names: Vec<&str> = carriers.iter().map(|c| c.name.as_str()).collect();

        // Scan all statements in the loop body
        for node in body {
            if let ASTNode::Assignment { target, value, .. } = node {
                // Check if this is a carrier update (e.g., sum = sum + i)
                if let Some(target_name) = Self::extract_variable_name(target) {
                    if carrier_names.contains(&target_name.as_str()) {
                        // This is a carrier update, analyze the RHS
                        if let Some(update_expr) = Self::analyze_update_value(&target_name, value) {
                            updates.insert(target_name, update_expr);
                        }
                    }
                }
            }
        }

        updates
    }

    /// Extract variable name from AST node (for assignment target)
    fn extract_variable_name(node: &ASTNode) -> Option<String> {
        match node {
            ASTNode::Variable { name, .. } => Some(name.clone()),
            _ => None,
        }
    }

    /// Analyze update value expression
    ///
    /// Recognizes patterns like:
    /// - `sum + i` → BinOp { lhs: "sum", op: Add, rhs: Variable("i") }
    /// - `count + 1` → BinOp { lhs: "count", op: Add, rhs: Const(1) }
    fn analyze_update_value(carrier_name: &str, value: &ASTNode) -> Option<UpdateExpr> {
        match value {
            ASTNode::BinaryOp {
                operator,
                left,
                right,
                ..
            } => {
                // Check if LHS is the carrier itself (e.g., sum in "sum + i")
                if let Some(lhs_name) = Self::extract_variable_name(left) {
                    if lhs_name == carrier_name {
                        // Convert operator
                        let op = Self::convert_operator(operator)?;

                        // Analyze RHS
                        let rhs = Self::analyze_rhs(right)?;

                        return Some(UpdateExpr::BinOp {
                            lhs: lhs_name,
                            op,
                            rhs,
                        });
                    }
                }
                None
            }
            _ => None,
        }
    }

    /// Analyze right-hand side of update expression
    fn analyze_rhs(node: &ASTNode) -> Option<UpdateRhs> {
        match node {
            // Constant: count + 1
            ASTNode::Literal {
                value: LiteralValue::Integer(n),
                ..
            } => Some(UpdateRhs::Const(*n)),

            // Variable: sum + i
            ASTNode::Variable { name, .. } => {
                Some(UpdateRhs::Variable(name.clone()))
            }

            _ => None,
        }
    }

    /// Convert AST operator to MIR BinOpKind
    fn convert_operator(op: &BinaryOperator) -> Option<BinOpKind> {
        match op {
            BinaryOperator::Add => Some(BinOpKind::Add),
            BinaryOperator::Subtract => Some(BinOpKind::Sub),
            BinaryOperator::Multiply => Some(BinOpKind::Mul),
            BinaryOperator::Divide => Some(BinOpKind::Div),
            _ => None, // Only support arithmetic operators for now
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_analyze_simple_increment() {
        // Test case: count = count + 1
        use crate::ast::Span;

        let body = vec![ASTNode::Assignment {
            target: Box::new(ASTNode::Variable {
                name: "count".to_string(),
                span: Span::default(),
            }),
            value: Box::new(ASTNode::BinaryOp {
                operator: BinaryOperator::Add,
                left: Box::new(ASTNode::Variable {
                    name: "count".to_string(),
                    span: Span::default(),
                }),
                right: Box::new(ASTNode::Literal {
                    value: LiteralValue::Integer(1),
                    span: Span::default(),
                }),
                span: Span::default(),
            }),
            span: Span::default(),
        }];

        let carriers = vec![CarrierVar {
            name: "count".to_string(),
            host_id: crate::mir::ValueId(0),
        }];

        let updates = LoopUpdateAnalyzer::analyze_carrier_updates(&body, &carriers);

        assert_eq!(updates.len(), 1);
        assert!(updates.contains_key("count"));

        if let Some(UpdateExpr::BinOp { lhs, op, rhs }) = updates.get("count") {
            assert_eq!(lhs, "count");
            assert_eq!(*op, BinOpKind::Add);
            if let UpdateRhs::Const(n) = rhs {
                assert_eq!(*n, 1);
            } else {
                panic!("Expected Const(1), got {:?}", rhs);
            }
        } else {
            panic!("Expected BinOp, got {:?}", updates.get("count"));
        }
    }
}