//! Pattern 6: Nested Loop Minimal Lowering (Phase 188.1)
//!
//! Target: 1-level nested simple while loops
//! Example: `loop(i < 3) { loop(j < 2) { ... } }`
//!
//! # Transformation
//!
//! ```text
//! // Outer loop step function
//! fn outer_step(outer_i, k_outer_exit):
//!   exit_cond = !(outer_i < 3)
//!   Jump(k_outer_exit, [], cond=exit_cond)  // exit if condition false
//!
//!   // Initialize inner loop variables
//!   inner_j = 0
//!
//!   // Inner loop step function (nested)
//!   fn inner_step(inner_j, k_inner_exit):
//!     exit_cond = !(inner_j < 2)
//!     Jump(k_inner_exit, [], cond=exit_cond)
//!
//!     print(inner_j)
//!     inner_j_next = inner_j + 1
//!     Call(inner_step, [inner_j_next, k_inner_exit])  // tail recursion
//!
//!   // k_inner_exit continuation (resume outer loop)
//!   fn k_inner_exit():
//!     outer_i_next = outer_i + 1
//!     Call(outer_step, [outer_i_next, k_outer_exit])  // outer tail recursion
//!
//!   // Entry: call inner loop
//!   Call(inner_step, [inner_j, k_inner_exit])
//!
//! // Main entry
//! Call(outer_step, [0, k_main_exit])
//! ```
//!
//! # Key Design Points
//!
//! - Outer step function contains inner step function (nested structure)
//! - `k_inner_exit` resumes outer loop body after inner completes
//! - Outer/inner carriers are isolated (no shared carriers)
//! - Both loops use same tail-recursive pattern as Pattern 1
//!
//! # Supported Forms (Phase 188.1 Scope)
//!
//! - **Outer loop**: Pattern 1 (simple while, no break/continue)
//! - **Inner loop**: Pattern 1 (simple while, no break/continue)
//! - **Nesting depth**: EXACTLY 1 level (`max_loop_depth == 2`)
//! - **No control flow**: No break/continue in either loop
//! - **Sequential execution**: Inner loop completes before outer continues
//!
//! # Unsupported Forms (Explicit Error)
//!
//! - Deeper nesting (2+ levels): `max_loop_depth > 2`
//! - Inner loop with break/continue
//! - Outer loop with break/continue
//! - Multiple inner loops (siblings)
//!
//! # Reference
//!
//! See `docs/development/current/main/phases/phase-188.1/README.md` for complete specification.

use crate::mir::join_ir::lowering::loop_to_join::LoopToJoinLowerer;
use crate::mir::join_ir::JoinInst;
use crate::mir::loop_form::LoopForm;

/// Lower 1-level nested simple while loops to JoinIR
///
/// # Pattern 6 Transformation Steps
///
/// 1. **Detect Outer + Inner Loops**
///    - Validate outer loop is LoopForm
///    - Find inner loop within outer body
///    - Validate both are Pattern 1 (no break/continue)
///
/// 2. **Extract Outer Loop Variables**
///    - Analyze outer header PHI nodes
///    - Identify outer carriers (e.g., `outer_i`)
///
/// 3. **Extract Inner Loop Variables**
///    - Analyze inner header PHI nodes
///    - Identify inner carriers (e.g., `inner_j`)
///
/// 4. **Create Outer Step Function**
///    - Signature: `fn outer_step(outer_i, k_outer_exit)`
///    - Exit condition check: `!(outer_i < 3)`
///    - Contains inner loop initialization
///
/// 5. **Create Inner Step Function (Nested)**
///    - Signature: `fn inner_step(inner_j, k_inner_exit)`
///    - Exit condition check: `!(inner_j < 2)`
///    - Tail recursion: `Call(inner_step, [inner_j_next])`
///
/// 6. **Create k_inner_exit Continuation**
///    - Resumes outer loop after inner completes
///    - Updates outer carriers: `outer_i_next = outer_i + 1`
///    - Tail call to outer: `Call(outer_step, [outer_i_next])`
///
/// 7. **Wire Continuations**
///    - Inner exit → k_inner_exit
///    - Outer exit → k_outer_exit (parent continuation)
///
/// # Arguments
///
/// * `loop_form` - The outer loop structure (must contain inner loop)
/// * `lowerer` - The LoopToJoinLowerer builder (provides ValueId allocation)
///
/// # Returns
///
/// * `Some(JoinInst)` - Lowering succeeded, returns generated JoinIR instruction
/// * `None` - Lowering failed (pattern not matched or unsupported)
///
/// # Errors
///
/// Returns `None` if:
/// - Outer loop has break/continue (not Pattern 1)
/// - Inner loop has break/continue (not Pattern 1)
/// - Nesting depth > 2 (more than 1 level)
/// - Multiple inner loops detected (siblings)
/// - Inner loop not found in outer body
///
/// # Example Usage
///
/// ```rust,ignore
/// use crate::mir::loop_pattern_detection::LoopPatternKind;
///
/// if pattern == LoopPatternKind::Pattern6NestedLoopMinimal {
///     lower_nested_loop_minimal_to_joinir(&loop_form, &mut lowerer)?;
/// }
/// ```
pub fn lower_nested_loop_minimal_to_joinir(
    _loop_form: &LoopForm,
    _lowerer: &mut LoopToJoinLowerer,
) -> Option<JoinInst> {
    // TODO: Implement Pattern 6 lowering (Phase 188.1 Task 4)
    //
    // Step 1: Detect Outer + Inner Loops
    // ===================================
    // Validate loop_form is outer loop, find inner loop in body
    //
    // ```rust
    // let inner_loop = find_inner_loop_in_body(loop_form)?;
    // validate_nested_structure(loop_form, inner_loop)?;
    // ```
    //
    // Step 2: Extract Outer Loop Variables
    // =====================================
    // From outer header PHI: %2 = phi [%1, bb1], [%6, bb_outer_latch]
    //
    // ```rust
    // let outer_carriers = extract_carriers_from_header_phi(loop_form)?;
    // ```
    //
    // Step 3: Extract Inner Loop Variables
    // =====================================
    // From inner header PHI: %10 = phi [%9, bb_inner_preheader], [%14, bb_inner_latch]
    //
    // ```rust
    // let inner_carriers = extract_carriers_from_header_phi(inner_loop)?;
    // ```
    //
    // Step 4: Create Outer Step Function
    // ===================================
    // Signature: fn outer_step(outer_i: ValueId, k_outer_exit: JoinContId)
    //
    // ```rust
    // let outer_step_id = lowerer.allocate_join_func_id();
    // let k_outer_exit_id = lowerer.allocate_join_func_id();
    // let k_inner_exit_id = lowerer.allocate_join_func_id();
    //
    // // Outer step function body:
    // // 1. Exit condition check: exit_cond = !(outer_i < 3)
    // // 2. Jump(k_outer_exit, [], cond=exit_cond)
    // // 3. Initialize inner loop: inner_j = 0
    // // 4. Call(inner_step, [inner_j, k_inner_exit])
    // ```
    //
    // Step 5: Create Inner Step Function (Nested)
    // ============================================
    // Signature: fn inner_step(inner_j: ValueId, k_inner_exit: JoinContId)
    //
    // ```rust
    // let inner_step_id = lowerer.allocate_join_func_id();
    //
    // // Inner step function body:
    // // 1. Exit condition check: exit_cond = !(inner_j < 2)
    // // 2. Jump(k_inner_exit, [], cond=exit_cond)
    // // 3. Translate inner body instructions
    // // 4. Update inner carrier: inner_j_next = inner_j + 1
    // // 5. Tail call: Call(inner_step, [inner_j_next, k_inner_exit])
    // ```
    //
    // Step 6: Create k_inner_exit Continuation
    // =========================================
    // Resumes outer loop after inner completes
    //
    // ```rust
    // let k_inner_exit_func = JoinFunction {
    //     id: k_inner_exit_id,
    //     name: "k_inner_exit".to_string(),
    //     params: vec![],  // No values passed from inner to outer
    //     body: vec![
    //         // Update outer carrier: outer_i_next = outer_i + 1
    //         // Tail call to outer: Call(outer_step, [outer_i_next, k_outer_exit])
    //     ],
    //     exit_cont: Some(k_outer_exit_id),
    // };
    // lowerer.register_join_function(k_inner_exit_func);
    // ```
    //
    // Step 7: Wire Continuations
    // ===========================
    // Connect inner/outer exits to appropriate continuations
    //
    // ```rust
    // // Inner step exit → k_inner_exit
    // // Outer step exit → k_outer_exit (parent continuation)
    // ```

    // For now, return None (stub implementation)
    // Actual implementation will be added incrementally
    None
}

/// Validate nested loop structure meets Pattern 6 requirements
///
/// # Validation Rules (Phase 188.1)
///
/// 1. **Outer loop must be Pattern 1**
///    - No break statements
///    - No continue statements
///    - Simple while condition
///
/// 2. **Inner loop must be Pattern 1**
///    - No break statements
///    - No continue statements
///    - Simple while condition
///
/// 3. **Nesting depth must be exactly 2**
///    - Only 1 level of nesting (max_loop_depth == 2)
///    - Deeper nesting (3+ levels) is out-of-scope
///
/// 4. **No multiple inner loops**
///    - Only one inner loop allowed (no siblings)
///
/// # Arguments
///
/// * `outer_loop` - The outer loop structure
/// * `inner_loop` - The inner loop structure
///
/// # Returns
///
/// * `Ok(())` - Validation passed
/// * `Err(String)` - Validation failed with error message
///
/// # Errors
///
/// Uses `error_tags::freeze_with_hint()` format for explicit errors:
///
/// - `[joinir/nested_loop/outer_control_flow]` - Outer loop has break/continue
/// - `[joinir/nested_loop/inner_control_flow]` - Inner loop has break/continue
/// - `[joinir/nested_loop/depth_exceeded]` - Nesting depth > 2
/// - `[joinir/nested_loop/multiple_inner]` - Multiple inner loops detected
#[allow(dead_code)]
fn validate_nested_structure(
    _outer_loop: &LoopForm,
    _inner_loop: &LoopForm,
) -> Result<(), String> {
    // TODO: Implement validation (Phase 188.1 Task 4)
    //
    // Check 1: Outer loop must be Pattern 1
    // ======================================
    // if has_break_or_continue(outer_loop) {
    //     return Err(error_tags::freeze_with_hint(
    //         "nested_loop/outer_control_flow",
    //         "Outer loop has break/continue (not supported in Pattern 6)",
    //         "Only simple while loops supported as outer loops (Pattern 1 only)",
    //     ));
    // }
    //
    // Check 2: Inner loop must be Pattern 1
    // ======================================
    // if has_break_or_continue(inner_loop) {
    //     return Err(error_tags::freeze_with_hint(
    //         "nested_loop/inner_control_flow",
    //         "Inner loop has break/continue (not supported in Pattern 6)",
    //         "Only simple while loops supported as inner loops (Pattern 1 only)",
    //     ));
    // }
    //
    // Check 3: Nesting depth must be exactly 2
    // =========================================
    // (This is validated earlier in classify(), but double-check here)
    //
    // Check 4: No multiple inner loops
    // =================================
    // let inner_loop_count = count_inner_loops(outer_loop);
    // if inner_loop_count > 1 {
    //     return Err(error_tags::freeze_with_hint(
    //         "nested_loop/multiple_inner",
    //         &format!("Multiple inner loops detected ({} loops)", inner_loop_count),
    //         "Only one inner loop supported in Pattern 6 (Phase 188.1 scope)",
    //     ));
    // }

    // For now, return Ok (stub implementation)
    Ok(())
}

/// Check if loop has break or continue statements
///
/// # Arguments
///
/// * `loop_form` - The loop structure to check
///
/// # Returns
///
/// * `true` - Loop has break or continue
/// * `false` - Loop is Pattern 1 (no break/continue)
#[allow(dead_code)]
fn has_break_or_continue(_loop_form: &LoopForm) -> bool {
    // TODO: Check loop_form.break_targets, loop_form.continue_targets
    // For now, assume no break/continue (conservative)
    false
}

#[cfg(test)]
mod tests {
    #[test]
    #[ignore] // TODO: Implement test after lowering logic is complete
    fn test_pattern6_lowering_success() {
        // TODO: Add integration test for nested loop pattern lowering
        // Step 1: Create mock LoopForm for nested loop pattern
        // Step 2: Create mock LoopToJoinLowerer
        // Step 3: Call lower_nested_loop_minimal_to_joinir()
        // Step 4: Assert returns Some(JoinInst)
        // Step 5: Verify generated JoinIR structure (nested functions)
    }

    #[test]
    #[ignore] // TODO: Implement test after lowering logic is complete
    fn test_pattern6_rejects_outer_break() {
        // TODO: Add test that rejects outer loop with break
        // Step 1: Create mock LoopForm with break in outer loop
        // Step 2: Call lower_nested_loop_minimal_to_joinir()
        // Step 3: Assert returns None (unsupported pattern)
    }

    #[test]
    #[ignore] // TODO: Implement test after lowering logic is complete
    fn test_pattern6_rejects_inner_continue() {
        // TODO: Add test that rejects inner loop with continue
        // Step 1: Create mock LoopForm with continue in inner loop
        // Step 2: Call lower_nested_loop_minimal_to_joinir()
        // Step 3: Assert returns None (unsupported pattern)
    }

    #[test]
    #[ignore] // TODO: Implement test after lowering logic is complete
    fn test_pattern6_rejects_2level_nesting() {
        // TODO: Add test that rejects 2+ level nesting
        // Step 1: Create mock LoopForm with loop { loop { loop {} } }
        // Step 2: Call lower_nested_loop_minimal_to_joinir()
        // Step 3: Assert returns None (depth exceeded)
    }
}
