//go:build result_goexit

// Optional runtime.Goexit implementation, enabled with -tags result_goexit.
//
// Expect and Expectf exit the goroutine via runtime.Goexit instead of panic.
// Goexit runs all deferred functions but is invisible to recover() — no
// recover() call anywhere in the call chain can accidentally swallow a result
// failure, making this build safe to use alongside frameworks or user code
// that uses bare recover() inside Go/Run closures.
//
// Cost: error path ≈ 5.5 µs / ~536 B per failure (goroutineID parse +
// sync.Map store/load + Goexit unwind) — about 4× the default panic build.
// Happy-path cost is identical to the default.

package result

import (
	"fmt"
	"runtime"
	"sync"
)

// gErrors stores errors set by Expect/Expectf before calling runtime.Goexit,
// keyed by goroutine ID. Entries are consumed by the enclosing Go or Run call.
var gErrors sync.Map

// goroutineID returns the current goroutine's numeric ID by parsing the first
// line of runtime.Stack output ("goroutine NNN [...]"). Called only on error
// paths so the runtime.Stack overhead is acceptable.
func goroutineID() uint64 {
	var buf [64]byte
	n := runtime.Stack(buf[:], false)
	var id uint64
	for _, b := range buf[10:n] { // skip "goroutine "
		if b < '0' || b > '9' {
			break
		}
		id = id*10 + uint64(b-'0')
	}
	return id
}

// Expect returns the value or exits the current goroutine via runtime.Goexit,
// storing the annotated error for collection by the enclosing [Go] or [Run]
// call. The stack trace is captured at this call site.
//
//	data := Parse(raw).Expect("parse user input")
func (r Expect[T]) Expect(msg string) T {
	if r.err != nil {
		gErrors.Store(goroutineID(), &stackError{
			err:   fmt.Errorf("%s: %w", msg, r.err),
			stack: callers(3),
		})
		runtime.Goexit()
	}
	return r.value
}

// Expectf is like [Expect.Expect] but accepts a fmt.Sprintf-style format string
// for the context message. The wrapped error is always appended as ": <err>".
//
//	data := Parse(raw).Expectf("parse user input id=%d", id)
func (r Expect[T]) Expectf(format string, args ...any) T {
	if r.err != nil {
		gErrors.Store(goroutineID(), &stackError{
			err:   fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), r.err),
			stack: callers(3),
		})
		runtime.Goexit()
	}
	return r.value
}

// Async runs fn in a new goroutine and returns a channel that will receive
// exactly one Expect[T] when the goroutine finishes.  The channel is buffered
// so the goroutine never blocks even if the caller is busy with other work.
//
// Use Async when you want to run several operations concurrently and collect
// their results later:
//
//	aCh := result.Async(func() *A { return buildA().Expect("build A") })
//	bCh := result.Async(func() *B { return buildB().Expect("build B") })
//	a := (<-aCh).Expect("A")
//	b := (<-bCh).Expect("B")
//
// Genuine runtime panics (nil-pointer dereferences, etc.) are not recovered —
// they still crash the program, as they should.
func Async[T any](fn func() T) <-chan Expect[T] {
	ch := make(chan Expect[T], 1)
	go func() {
		var (
			val      T
			finished bool
		)
		defer func() {
			if v := recover(); v != nil {
				if err, ok := v.(*stackError); ok {
					// Must() panic — treat as a collected failure.
					ch <- Err[T](err)
					return
				}
				panic(v) // genuine runtime panic — crash the program
			}
			if finished {
				ch <- Ok(val)
				return
			}
			// goroutineID is looked up here, on the error path only.
			if stored, ok := gErrors.LoadAndDelete(goroutineID()); ok {
				ch <- Err[T](stored.(error))
			} else {
				ch <- Errf[T]("goroutine exited unexpectedly")
			}
		}()
		val = fn()
		finished = true
	}()
	return ch
}