module async

import context
import sync
import time

enum CancelSource {
    unknown
    parent
    timeout
    cancel
}

// AsyncContext is the small `context.Context` implementation owned by x.async.
//
// The public API still accepts and returns the standard `context.Context`
// interface. Internally, x.async needs a derived context whose cancellation is
// simple, idempotent, and non-blocking for Group and timeout lifecycles.
//
// This is not a scheduler or runtime. It is only the cancellation signal shared
// by jobs launched through this module.
@[heap]
struct AsyncContext {
mut:
    parent       context.Context
    done         chan int
    mutex        &sync.Mutex = sync.new_mutex()
    err          IError      = none
    cancel_src   CancelSource
    has_deadline bool
    deadline_at  time.Time
}

fn new_cancel_context(parent context.Context) (&AsyncContext, context.CancelFn) {
    mut ctx := &AsyncContext{
        parent: parent
        done:   chan int{cap: 1}
        mutex:  sync.new_mutex()
    }
    if !ctx.propagate_existing_parent_error() {
        spawn watch_parent_context(mut ctx)
    }
    cancel_fn := fn [mut ctx] () {
        ctx.cancel_with(error(context_canceled), .cancel)
    }
    return ctx, context.CancelFn(cancel_fn)
}

fn new_timeout_context(parent context.Context, timeout time.Duration) (&AsyncContext, context.CancelFn) {
    mut deadline_at := time.now().add(timeout)
    mut deadline_src := CancelSource.timeout
    if parent_deadline := parent.deadline() {
        if parent_deadline < deadline_at {
            deadline_at = parent_deadline
            deadline_src = .parent
        }
    }
    mut ctx := &AsyncContext{
        parent:       parent
        done:         chan int{cap: 1}
        mutex:        sync.new_mutex()
        has_deadline: true
        deadline_at:  deadline_at
    }
    if !ctx.propagate_existing_parent_error() {
        spawn watch_parent_context(mut ctx)
    }
    if deadline_src == .timeout {
        effective_timeout := deadline_at - time.now()
        if effective_timeout.nanoseconds() <= 0 {
            ctx.cancel_with(error(context_deadline_exceeded), .timeout)
        } else {
            spawn watch_timeout_context(mut ctx, effective_timeout)
        }
    }
    cancel_fn := fn [mut ctx] () {
        ctx.cancel_with(error(context_canceled), .cancel)
    }
    return ctx, context.CancelFn(cancel_fn)
}

// deadline returns x.async's own deadline when present, otherwise the parent's
// deadline. This preserves deadline metadata for callers that inspect it.
pub fn (ctx &AsyncContext) deadline() ?time.Time {
    if ctx.has_deadline {
        return ctx.deadline_at
    }
    return ctx.parent.deadline()
}

// value delegates to the parent context. x.async does not add request values.
pub fn (ctx &AsyncContext) value(key context.Key) ?context.Any {
    return ctx.parent.value(key)
}

// done returns the cancellation channel shared by jobs using this context.
pub fn (mut ctx AsyncContext) done() chan int {
    ctx.mutex.lock()
    done := ctx.done
    ctx.mutex.unlock()
    return done
}

// err returns the local cancellation reason, or the parent's reason if the
// parent is already canceled before propagation reaches this context.
pub fn (mut ctx AsyncContext) err() IError {
    ctx.mutex.lock()
    err := ctx.err
    ctx.mutex.unlock()
    if err !is none {
        return err
    }
    return ctx.parent.err()
}

fn (ctx &AsyncContext) was_canceled_by_timeout() bool {
    ctx.mutex.lock()
    cancel_src := ctx.cancel_src
    ctx.mutex.unlock()
    return cancel_src == .timeout
}

fn (mut ctx AsyncContext) cancel_with(err IError, cancel_src CancelSource) {
    if err is none {
        return
    }
    ctx.mutex.lock()
    if ctx.err !is none {
        ctx.mutex.unlock()
        return
    }
    ctx.err = err
    ctx.cancel_src = cancel_src
    if !ctx.done.closed {
        ctx.done <- 0
        ctx.done.close()
    }
    ctx.mutex.unlock()
}

fn (mut ctx AsyncContext) propagate_existing_parent_error() bool {
    mut parent := ctx.parent
    err := parent.err()
    if err !is none {
        ctx.cancel_with(err, .parent)
        return true
    }
    return false
}

fn watch_parent_context(mut ctx AsyncContext) {
    mut parent := ctx.parent
    parent_done := parent.done()
    local_done := ctx.done()
    select {
        _ := <-local_done {
            return
        }
        _ := <-parent_done {
            err := parent.err()
            if err !is none {
                ctx.cancel_with(err, .parent)
            }
        }
    }
}

fn watch_timeout_context(mut ctx AsyncContext, timeout time.Duration) {
    local_done := ctx.done()
    select {
        _ := <-local_done {
            return
        }
        timeout {
            ctx.cancel_with(error(context_deadline_exceeded), .timeout)
        }
    }
}