//! Symbolic execution engine.
//!
//! Explores all possible execution paths through a contract.

use serde::{Deserialize, Serialize};
use std::collections::HashMap;

use crate::ast::{BinaryOperator, Expression, Literal, Statement, UnaryOperator};
use crate::error::{VerifierError, VerifierResult};
use crate::prover::ProverConfig;
use crate::{VarType, VerificationContext};

/// Symbolic value representing an unknown or computed value.
#[derive(Clone, Debug, Serialize, Deserialize)]
pub enum SymbolicValue {
    /// Concrete literal value.
    Concrete(Literal),
    /// Symbolic variable.
    Symbol(String, VarType),
    /// Binary operation on symbolic values.
    BinaryOp {
        left: Box<SymbolicValue>,
        op: BinaryOperator,
        right: Box<SymbolicValue>,
    },
    /// Unary operation.
    UnaryOp {
        op: UnaryOperator,
        operand: Box<SymbolicValue>,
    },
    /// Conditional value (ITE = if-then-else).
    Ite {
        condition: Box<SymbolicValue>,
        then_val: Box<SymbolicValue>,
        else_val: Box<SymbolicValue>,
    },
    /// Array/mapping read.
    Select {
        array: Box<SymbolicValue>,
        index: Box<SymbolicValue>,
    },
    /// Array/mapping write.
    Store {
        array: Box<SymbolicValue>,
        index: Box<SymbolicValue>,
        value: Box<SymbolicValue>,
    },
}

impl SymbolicValue {
    /// Creates a new symbolic variable.
    pub fn symbol(name: &str, var_type: VarType) -> Self {
        SymbolicValue::Symbol(name.to_string(), var_type)
    }

    /// Creates a concrete boolean value.
    pub fn bool_const(v: bool) -> Self {
        SymbolicValue::Concrete(Literal::Bool(v))
    }

    /// Creates a concrete uint value.
    pub fn uint_const(v: u128) -> Self {
        SymbolicValue::Concrete(Literal::Uint(v))
    }

    /// Creates a binary operation.
    pub fn binary(left: SymbolicValue, op: BinaryOperator, right: SymbolicValue) -> Self {
        SymbolicValue::BinaryOp {
            left: Box::new(left),
            op,
            right: Box::new(right),
        }
    }

    /// Creates a conditional value.
    pub fn ite(condition: SymbolicValue, then_val: SymbolicValue, else_val: SymbolicValue) -> Self {
        SymbolicValue::Ite {
            condition: Box::new(condition),
            then_val: Box::new(then_val),
            else_val: Box::new(else_val),
        }
    }

    /// Attempts to evaluate to a concrete value.
    pub fn try_evaluate(&self) -> Option<Literal> {
        match self {
            SymbolicValue::Concrete(lit) => Some(lit.clone()),
            SymbolicValue::BinaryOp { left, op, right } => {
                let l = left.try_evaluate()?;
                let r = right.try_evaluate()?;
                Self::evaluate_binary(&l, *op, &r)
            }
            SymbolicValue::UnaryOp { op, operand } => {
                let v = operand.try_evaluate()?;
                Self::evaluate_unary(*op, &v)
            }
            _ => None,
        }
    }

    fn evaluate_binary(left: &Literal, op: BinaryOperator, right: &Literal) -> Option<Literal> {
        match (left, op, right) {
            (Literal::Uint(a), BinaryOperator::Add, Literal::Uint(b)) => {
                Some(Literal::Uint(a.wrapping_add(*b)))
            }
            (Literal::Uint(a), BinaryOperator::Sub, Literal::Uint(b)) => {
                Some(Literal::Uint(a.wrapping_sub(*b)))
            }
            (Literal::Uint(a), BinaryOperator::Mul, Literal::Uint(b)) => {
                Some(Literal::Uint(a.wrapping_mul(*b)))
            }
            (Literal::Uint(a), BinaryOperator::Div, Literal::Uint(b)) if *b != 0 => {
                Some(Literal::Uint(a / b))
            }
            (Literal::Uint(a), BinaryOperator::Eq, Literal::Uint(b)) => {
                Some(Literal::Bool(a == b))
            }
            (Literal::Uint(a), BinaryOperator::Lt, Literal::Uint(b)) => {
                Some(Literal::Bool(a < b))
            }
            (Literal::Uint(a), BinaryOperator::Le, Literal::Uint(b)) => {
                Some(Literal::Bool(a <= b))
            }
            (Literal::Uint(a), BinaryOperator::Gt, Literal::Uint(b)) => {
                Some(Literal::Bool(a > b))
            }
            (Literal::Uint(a), BinaryOperator::Ge, Literal::Uint(b)) => {
                Some(Literal::Bool(a >= b))
            }
            (Literal::Bool(a), BinaryOperator::And, Literal::Bool(b)) => {
                Some(Literal::Bool(*a && *b))
            }
            (Literal::Bool(a), BinaryOperator::Or, Literal::Bool(b)) => {
                Some(Literal::Bool(*a || *b))
            }
            _ => None,
        }
    }

    fn evaluate_unary(op: UnaryOperator, val: &Literal) -> Option<Literal> {
        match (op, val) {
            (UnaryOperator::Not, Literal::Bool(b)) => Some(Literal::Bool(!b)),
            (UnaryOperator::Neg, Literal::Int(i)) => Some(Literal::Int(-i)),
            _ => None,
        }
    }
}

/// Symbolic execution state.
#[derive(Clone, Debug)]
pub struct SymbolicState {
    /// Variable bindings.
    variables: HashMap<String, SymbolicValue>,
    /// Path condition (constraints on this path).
    path_condition: Vec<SymbolicValue>,
    /// Execution depth.
    depth: usize,
    /// Whether this path is still feasible.
    feasible: bool,
}

impl SymbolicState {
    /// Creates a new empty symbolic state.
    pub fn new() -> Self {
        Self {
            variables: HashMap::new(),
            path_condition: Vec::new(),
            depth: 0,
            feasible: true,
        }
    }

    /// Gets a variable's symbolic value.
    pub fn get(&self, name: &str) -> Option<&SymbolicValue> {
        self.variables.get(name)
    }

    /// Sets a variable's symbolic value.
    pub fn set(&mut self, name: &str, value: SymbolicValue) {
        self.variables.insert(name.to_string(), value);
    }

    /// Adds a path constraint.
    pub fn assume(&mut self, constraint: SymbolicValue) {
        self.path_condition.push(constraint);
    }

    /// Returns the path condition.
    pub fn path_condition(&self) -> &[SymbolicValue] {
        &self.path_condition
    }

    /// Returns all variable bindings.
    pub fn variables(&self) -> &HashMap<String, SymbolicValue> {
        &self.variables
    }

    /// Forks the state for branching.
    pub fn fork(&self) -> Self {
        Self {
            variables: self.variables.clone(),
            path_condition: self.path_condition.clone(),
            depth: self.depth,
            feasible: self.feasible,
        }
    }

    /// Increments depth.
    pub fn increment_depth(&mut self) {
        self.depth += 1;
    }

    /// Returns current depth.
    pub fn depth(&self) -> usize {
        self.depth
    }

    /// Marks path as infeasible.
    pub fn mark_infeasible(&mut self) {
        self.feasible = false;
    }

    /// Checks if path is still feasible.
    pub fn is_feasible(&self) -> bool {
        self.feasible
    }
}

impl Default for SymbolicState {
    fn default() -> Self {
        Self::new()
    }
}

/// Symbolic execution engine.
pub struct SymbolicExecutor {
    config: ProverConfig,
}

impl SymbolicExecutor {
    /// Creates a new symbolic executor.
    pub fn new(config: ProverConfig) -> Self {
        Self { config }
    }

    /// Creates an initial symbolic state from contract context.
    pub fn create_initial_state(
        &self,
        ctx: &VerificationContext,
    ) -> VerifierResult<SymbolicState> {
        let mut state = SymbolicState::new();

        // Create symbolic values for state variables
        for (name, var_type) in &ctx.state_vars {
            let sym = SymbolicValue::symbol(name, var_type.clone());
            state.set(name, sym);
        }

        // Add msg.sender, msg.value, etc.
        state.set("msg.sender", SymbolicValue::symbol("msg.sender", VarType::Address));
        state.set("msg.value", SymbolicValue::symbol("msg.value", VarType::Uint(256)));
        state.set("block.timestamp", SymbolicValue::symbol("block.timestamp", VarType::Uint(256)));
        state.set("block.number", SymbolicValue::symbol("block.number", VarType::Uint(256)));

        Ok(state)
    }

    /// Explores all possible execution paths.
    pub fn explore_all_paths(
        &self,
        ctx: &VerificationContext,
    ) -> VerifierResult<Vec<SymbolicState>> {
        let initial = self.create_initial_state(ctx)?;
        let mut completed = Vec::new();
        let mut worklist = vec![initial];

        while let Some(state) = worklist.pop() {
            if state.depth() >= self.config.max_depth {
                // Depth limit reached
                completed.push(state);
                continue;
            }

            if !state.is_feasible() {
                // Path is infeasible, skip
                continue;
            }

            // For now, just return the initial state
            // Full implementation would execute each function
            completed.push(state);
        }

        Ok(completed)
    }

    /// Symbolically executes a statement.
    pub fn execute_statement(
        &self,
        stmt: &Statement,
        state: &mut SymbolicState,
    ) -> VerifierResult<Vec<SymbolicState>> {
        state.increment_depth();

        if state.depth() >= self.config.max_depth {
            return Err(VerifierError::Internal("Max depth exceeded".to_string()));
        }

        match stmt {
            Statement::Assign { target, value } => {
                let sym_value = self.evaluate_expression(value, state)?;
                state.set(target, sym_value);
                Ok(vec![state.clone()])
            }

            Statement::If {
                condition,
                then_branch,
                else_branch,
            } => {
                let cond = self.evaluate_expression(condition, state)?;

                // Fork state for both branches
                let mut then_state = state.fork();
                then_state.assume(cond.clone());

                let mut else_state = state.fork();
                else_state.assume(SymbolicValue::UnaryOp {
                    op: UnaryOperator::Not,
                    operand: Box::new(cond),
                });

                // Execute branches
                let mut results = Vec::new();

                for stmt in then_branch {
                    let states = self.execute_statement(stmt, &mut then_state)?;
                    results.extend(states);
                }

                for stmt in else_branch {
                    let states = self.execute_statement(stmt, &mut else_state)?;
                    results.extend(states);
                }

                Ok(results)
            }

            Statement::While {
                condition,
                invariant: _,
                body,
            } => {
                // Bounded loop unrolling
                let mut current_states = vec![state.clone()];
                let mut exit_states = Vec::new();

                for _ in 0..self.config.max_loop_unroll {
                    let mut next_states = Vec::new();

                    for s in current_states.drain(..) {
                        let cond = self.evaluate_expression(condition, &s)?;

                        // Continue if condition might be true
                        let mut continue_state = s.fork();
                        continue_state.assume(cond.clone());

                        for stmt in body {
                            let results = self.execute_statement(stmt, &mut continue_state)?;
                            next_states.extend(results);
                        }

                        // Exit if condition might be false
                        let mut exit_state = s;
                        exit_state.assume(SymbolicValue::UnaryOp {
                            op: UnaryOperator::Not,
                            operand: Box::new(cond),
                        });
                        exit_states.push(exit_state);
                    }

                    if next_states.is_empty() {
                        break;
                    }
                    current_states = next_states;
                }

                // Combine remaining loop states with exit states
                exit_states.extend(current_states);
                Ok(exit_states)
            }

            Statement::Require(expr, _msg) => {
                let cond = self.evaluate_expression(expr, state)?;
                state.assume(cond);
                Ok(vec![state.clone()])
            }

            Statement::Assert(expr) => {
                let cond = self.evaluate_expression(expr, state)?;
                state.assume(cond);
                Ok(vec![state.clone()])
            }

            Statement::Assume(expr) => {
                let cond = self.evaluate_expression(expr, state)?;
                state.assume(cond);
                Ok(vec![state.clone()])
            }

            Statement::Return(_) => {
                // End of path
                Ok(vec![state.clone()])
            }

            Statement::Block(stmts) => {
                let mut current = state.clone();
                for stmt in stmts {
                    let results = self.execute_statement(stmt, &mut current)?;
                    if results.is_empty() {
                        return Ok(vec![]);
                    }
                    // Take first result (simplification)
                    current = results.into_iter().next().unwrap();
                }
                Ok(vec![current])
            }

            _ => {
                // Other statements not yet implemented
                Ok(vec![state.clone()])
            }
        }
    }

    /// Evaluates an expression to a symbolic value.
    pub fn evaluate_expression(
        &self,
        expr: &Expression,
        state: &SymbolicState,
    ) -> VerifierResult<SymbolicValue> {
        match expr {
            Expression::Literal(lit) => Ok(SymbolicValue::Concrete(lit.clone())),

            Expression::Variable(name) => {
                if let Some(val) = state.get(name) {
                    Ok(val.clone())
                } else {
                    // Unknown variable - create fresh symbol
                    Ok(SymbolicValue::symbol(name, VarType::Uint(256)))
                }
            }

            Expression::BinaryOp { left, op, right } => {
                let l = self.evaluate_expression(left, state)?;
                let r = self.evaluate_expression(right, state)?;
                Ok(SymbolicValue::binary(l, *op, r))
            }

            Expression::UnaryOp { op, operand } => {
                let v = self.evaluate_expression(operand, state)?;
                Ok(SymbolicValue::UnaryOp {
                    op: *op,
                    operand: Box::new(v),
                })
            }

            Expression::Ternary {
                condition,
                then_expr,
                else_expr,
            } => {
                let c = self.evaluate_expression(condition, state)?;
                let t = self.evaluate_expression(then_expr, state)?;
                let e = self.evaluate_expression(else_expr, state)?;
                Ok(SymbolicValue::ite(c, t, e))
            }

            Expression::MemberAccess { object, member } => {
                // Simplified: create compound symbol name
                let obj = self.evaluate_expression(object, state)?;
                let name = format!("{:?}.{}", obj, member);
                Ok(SymbolicValue::symbol(&name, VarType::Uint(256)))
            }

            Expression::Index { array, index } => {
                let arr = self.evaluate_expression(array, state)?;
                let idx = self.evaluate_expression(index, state)?;
                Ok(SymbolicValue::Select {
                    array: Box::new(arr),
                    index: Box::new(idx),
                })
            }

            Expression::Old(inner) => {
                // Old values are just the initial symbolic values
                self.evaluate_expression(inner, state)
            }

            Expression::Result => {
                // Result is a special symbol
                Ok(SymbolicValue::symbol("__result", VarType::Uint(256)))
            }

            Expression::FunctionCall { function, args: _ } => {
                // Model function call result as uninterpreted function
                let name = format!("{}(...)", function);
                Ok(SymbolicValue::symbol(&name, VarType::Uint(256)))
            }

            _ => {
                // Other expressions
                Ok(SymbolicValue::symbol("unknown", VarType::Uint(256)))
            }
        }
    }
}

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

    #[test]
    fn test_symbolic_value_concrete() {
        let v = SymbolicValue::uint_const(42);
        assert!(matches!(v.try_evaluate(), Some(Literal::Uint(42))));
    }

    #[test]
    fn test_symbolic_value_binary() {
        let a = SymbolicValue::uint_const(10);
        let b = SymbolicValue::uint_const(5);
        let sum = SymbolicValue::binary(a, BinaryOperator::Add, b);

        assert!(matches!(sum.try_evaluate(), Some(Literal::Uint(15))));
    }

    #[test]
    fn test_symbolic_state() {
        let mut state = SymbolicState::new();
        state.set("x", SymbolicValue::uint_const(100));

        assert!(state.get("x").is_some());
        assert!(state.get("y").is_none());
    }

    #[test]
    fn test_state_fork() {
        let mut state = SymbolicState::new();
        state.set("x", SymbolicValue::uint_const(1));

        let forked = state.fork();
        assert!(forked.get("x").is_some());
    }

    #[test]
    fn test_executor_creation() {
        let executor = SymbolicExecutor::new(ProverConfig::default());
        assert_eq!(executor.config.max_depth, 100);
    }
}
