// SPDX-License-Identifier: MPL-2.0
// SPDX-FileCopyrightText: 2024-2025 hyperpolymath

//! Effect Handling Runtime for AffineScript
//!
//! This module implements the runtime support for algebraic effects,
//! using evidence-passing compilation (Koka-style).
//!
//! # Architecture
//!
//! ```text
//! ┌─────────────────────────────────────────────────────────────┐
//! │                     Handler Stack                           │
//! ├─────────────────────────────────────────────────────────────┤
//! │  Handler N  │  Handler N-1  │  ...  │  Handler 1  │  Base  │
//! └─────────────────────────────────────────────────────────────┘
//!       │              │                      │
//!       ▼              ▼                      ▼
//! ┌──────────┐  ┌──────────┐           ┌──────────┐
//! │ Evidence │  │ Evidence │           │ Evidence │
//! │  (ops)   │  │  (ops)   │           │  (ops)   │
//! └──────────┘  └──────────┘           └──────────┘
//! ```
//!
//! # Evidence Passing
//!
//! Each effect operation receives an "evidence" parameter that contains:
//! - Pointer to the handler function for that operation
//! - Captured environment from the handler
//! - Continuation management data

#[cfg(not(feature = "std"))]
use core::ptr::NonNull;

#[cfg(feature = "std")]
use std::ptr::NonNull;

/// Evidence for an effect operation
///
/// This is passed to effectful functions and contains the handler
/// to invoke when an operation is performed.
#[repr(C)]
pub struct Evidence {
    /// Pointer to the handler function
    pub handler: *const (),
    /// Captured environment
    pub env: *mut (),
    /// Parent evidence (for nested handlers)
    pub parent: *mut Evidence,
    /// Handler frame pointer
    pub frame: *mut HandlerFrame,
}

impl Evidence {
    /// Create new evidence
    pub fn new(handler: *const (), env: *mut (), parent: *mut Evidence) -> Self {
        Evidence {
            handler,
            env,
            parent,
            frame: core::ptr::null_mut(),
        }
    }
}

/// Handler frame on the stack
///
/// Tracks the state needed to resume a continuation.
#[repr(C)]
pub struct HandlerFrame {
    /// Saved stack pointer
    pub sp: *mut u8,
    /// Saved frame pointer
    pub fp: *mut u8,
    /// Return address
    pub ra: *const (),
    /// Effect signature being handled
    pub effect_id: u32,
    /// Whether this is a one-shot or multi-shot handler
    pub is_linear: bool,
    /// Parent frame
    pub parent: *mut HandlerFrame,
}

/// Continuation representation
///
/// Captures the state needed to resume execution after an effect operation.
#[repr(C)]
pub struct Continuation {
    /// Saved execution state
    pub frame: HandlerFrame,
    /// Captured stack segment (for multi-shot)
    pub stack: Option<NonNull<u8>>,
    /// Stack segment size
    pub stack_size: usize,
    /// Whether this continuation has been used
    pub used: bool,
}

/// Handler for an effect
///
/// Contains the implementation of each operation.
#[repr(C)]
pub struct Handler {
    /// Operation implementations (function pointers)
    pub operations: *const *const (),
    /// Number of operations
    pub num_ops: usize,
    /// Return clause
    pub return_fn: *const (),
    /// Captured environment
    pub env: *mut (),
}

/// Global handler stack
struct HandlerStack {
    /// Top of stack
    top: *mut HandlerFrame,
    /// Stack of installed handlers
    handlers: [*mut Handler; 64],
    /// Number of installed handlers
    count: usize,
}

static mut HANDLER_STACK: HandlerStack = HandlerStack {
    top: core::ptr::null_mut(),
    handlers: [core::ptr::null_mut(); 64],
    count: 0,
};

/// Initialize the effect system
pub fn init() {
    // TODO: Phase 6 implementation
    // - [ ] Set up initial handler frame
    // - [ ] Install default handlers for built-in effects
    // - [ ] Initialize continuation pool
}

/// Install a handler
///
/// # Arguments
///
/// * `handler` - The handler to install
/// * `evidence` - Evidence to populate
///
/// # Returns
///
/// Opaque handle for uninstalling
#[no_mangle]
pub extern "C" fn install_handler(handler: *mut Handler, evidence: *mut Evidence) -> u32 {
    // TODO: Phase 6 implementation
    // - [ ] Push handler onto stack
    // - [ ] Set up evidence
    // - [ ] Return handle

    0
}

/// Uninstall a handler
///
/// # Arguments
///
/// * `handle` - Handle returned by `install_handler`
#[no_mangle]
pub extern "C" fn uninstall_handler(handle: u32) {
    // TODO: Phase 6 implementation
    // - [ ] Pop handler from stack
    // - [ ] Restore previous evidence
}

/// Perform an effect operation
///
/// # Arguments
///
/// * `evidence` - Evidence for the effect
/// * `op_index` - Index of the operation to perform
/// * `arg` - Argument to the operation
///
/// # Returns
///
/// Result of the operation
#[no_mangle]
pub extern "C" fn perform(evidence: *mut Evidence, op_index: u32, arg: *mut ()) -> *mut () {
    // TODO: Phase 6 implementation
    // - [ ] Look up handler in evidence
    // - [ ] Create continuation
    // - [ ] Call handler with continuation

    core::ptr::null_mut()
}

/// Resume a continuation
///
/// # Arguments
///
/// * `k` - The continuation to resume
/// * `value` - Value to pass to the continuation
///
/// # Returns
///
/// Result of resuming
#[no_mangle]
pub extern "C" fn resume(k: *mut Continuation, value: *mut ()) -> *mut () {
    // TODO: Phase 6 implementation
    // - [ ] Check if continuation is linear and already used
    // - [ ] Restore execution state
    // - [ ] Jump to continuation point

    core::ptr::null_mut()
}

/// Abort an effect (for one-shot handlers)
///
/// # Arguments
///
/// * `evidence` - Evidence for the effect
/// * `value` - Value to return
#[no_mangle]
pub extern "C" fn abort_effect(evidence: *mut Evidence, value: *mut ()) -> ! {
    // TODO: Phase 6 implementation
    // - [ ] Unwind to handler frame
    // - [ ] Call return clause

    loop {}
}

// TODO: Phase 6 implementation
// - [ ] Implement evidence passing transform in codegen
// - [ ] Implement handler frame management
// - [ ] Implement one-shot continuation optimization
// - [ ] Implement multi-shot continuation copying
// - [ ] Add effect row tracking at runtime (for debugging)
// - [ ] Implement tail-resumptive optimization
