//! Phase 200-A: Function scope capture infrastructure
//!
//! This module provides types for capturing function-scoped variables
//! that are effectively immutable within a loop context.
//!
//! # Example
//!
//! For a function like JsonParser._atoi():
//!
//! ```nyash
//! method _atoi(s, pos, len) {
//!     local digits = "0123456789"  // <-- Captured variable
//!     local value = 0
//!     loop(pos < len) {
//!         local ch = s.charAt(pos)
//!         local digit = digits.indexOf(ch)  // Uses captured 'digits'
//!         if (digit < 0) { break }
//!         value = value * 10 + digit
//!         pos = pos + 1
//!     }
//!     return value
//! }
//! ```
//!
//! Here, `digits` is:
//! - Declared in function scope (before the loop)
//! - Never reassigned (effectively immutable)
//! - Referenced in loop body (digits.indexOf(ch))
//!
//! Phase 200-A creates the infrastructure to capture such variables.
//! Phase 200-B will implement the actual detection logic.

use crate::mir::ValueId;
use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
use crate::ast::ASTNode;

/// A variable captured from function scope for use in loop conditions/body.
///
/// Example: `local digits = "0123456789"` in JsonParser._atoi()
///
/// # Invariants
///
/// - `name`: Variable name as it appears in the source code
/// - `host_id`: MIR ValueId of the original definition in the host function
/// - `is_immutable`: True if the variable is never reassigned in the function
#[derive(Debug, Clone)]
pub struct CapturedVar {
    /// Variable name (e.g., "digits", "table")
    pub name: String,

    /// MIR ValueId of the original definition in the host function
    pub host_id: ValueId,

    /// Whether this variable is never reassigned in the function
    ///
    /// Phase 200-B will implement assignment analysis to determine this.
    /// For now, this is always set to true as a conservative default.
    pub is_immutable: bool,
}

/// Environment containing function-scoped captured variables.
///
/// Phase 200-A: Type definition only, not yet integrated with ConditionEnv.
/// Phase 200-B: Will be populated by FunctionScopeCaptureAnalyzer and
///               integrated into ConditionEnv via ConditionEnvBuilder v2.
#[derive(Debug, Clone, Default)]
pub struct CapturedEnv {
    /// List of captured variables
    pub vars: Vec<CapturedVar>,
}

impl CapturedEnv {
    /// Create a new empty environment
    pub fn new() -> Self {
        Self { vars: Vec::new() }
    }

    /// Check if the environment is empty
    pub fn is_empty(&self) -> bool {
        self.vars.is_empty()
    }

    /// Add a captured variable to the environment
    pub fn add_var(&mut self, var: CapturedVar) {
        self.vars.push(var);
    }

    /// Look up a captured variable by name
    ///
    /// Returns `Some(&CapturedVar)` if found, `None` otherwise.
    pub fn get(&self, name: &str) -> Option<&CapturedVar> {
        self.vars.iter().find(|v| v.name == name)
    }
}

/// Analyzes function-scoped variables that can be safely captured for loop conditions/body.
///
/// # Phase 200-A Status
///
/// Currently returns empty CapturedEnv (skeleton implementation).
/// Actual capture detection will be implemented in Phase 200-B.
///
/// # Future Detection Criteria (Phase 200-B+)
///
/// A variable is captured if ALL of the following conditions are met:
///
/// 1. **Declared before the loop**: Variable must be declared in function scope before the loop
/// 2. **Never reassigned**: Variable is never reassigned within the function (is_immutable = true)
/// 3. **Referenced in loop**: Variable is referenced in loop condition or body
/// 4. **Not a loop parameter**: Variable is not the loop iteration variable
/// 5. **Not a body-local**: Variable is not declared inside the loop body
///
/// # Example
///
/// ```nyash
/// method _atoi(s, pos, len) {
///     local digits = "0123456789"  // ✅ Captured (declared before loop, never reassigned)
///     local value = 0               // ❌ Not captured (reassigned in loop body)
///     loop(pos < len) {
///         local ch = s.charAt(pos)  // ❌ Not captured (body-local)
///         local digit = digits.indexOf(ch)
///         value = value * 10 + digit
///         pos = pos + 1
///     }
/// }
/// ```
///
/// # Arguments
///
/// * `_fn_body` - AST nodes of the function body (for analysis)
/// * `_loop_ast` - AST node of the loop statement
/// * `_scope` - LoopScopeShape (for excluding loop params and body-locals)
///
/// # Returns
///
/// `CapturedEnv` containing all captured variables (empty in Phase 200-A)
pub fn analyze_captured_vars(
    _fn_body: &[ASTNode],
    _loop_ast: &ASTNode,
    _scope: &LoopScopeShape,
) -> CapturedEnv {
    // Phase 200-A: Skeleton implementation
    // TODO(Phase 200-B): Implement actual capture detection
    //
    // Detection algorithm:
    // 1. Find all `local` declarations before the loop in fn_body
    // 2. For each declaration:
    //    a. Check if it's never reassigned in the function (is_immutable = true)
    //    b. Check if it's referenced in loop condition or body
    //    c. Exclude if it's in scope.pinned, scope.carriers, or scope.body_locals
    // 3. Collect matching variables into CapturedEnv
    // 4. Return the populated environment

    CapturedEnv::new()
}

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

    #[test]
    fn test_captured_env_empty() {
        let env = CapturedEnv::new();
        assert!(env.is_empty());
        assert!(env.get("digits").is_none());
    }

    #[test]
    fn test_captured_env_add_and_get() {
        let mut env = CapturedEnv::new();
        env.add_var(CapturedVar {
            name: "digits".to_string(),
            host_id: ValueId(42),
            is_immutable: true,
        });

        assert!(!env.is_empty());
        let var = env.get("digits").unwrap();
        assert_eq!(var.name, "digits");
        assert_eq!(var.host_id, ValueId(42));
        assert!(var.is_immutable);
    }

    #[test]
    fn test_captured_env_multiple_vars() {
        let mut env = CapturedEnv::new();
        env.add_var(CapturedVar {
            name: "digits".to_string(),
            host_id: ValueId(42),
            is_immutable: true,
        });
        env.add_var(CapturedVar {
            name: "table".to_string(),
            host_id: ValueId(100),
            is_immutable: true,
        });

        assert_eq!(env.vars.len(), 2);
        assert!(env.get("digits").is_some());
        assert!(env.get("table").is_some());
        assert!(env.get("nonexistent").is_none());
    }
}