package starssh

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"
	"reflect"
	"sync"
	"unsafe"
)

// TerminalInputSourceProvider lets wrapper readers expose a closer-friendly source reader.
// Implementations that buffer data should return a source that already includes any prefetched bytes.
type TerminalInputSourceProvider interface {
	TerminalInputSource() io.Reader
}

// TerminalInputCanceler lets wrapper readers expose an explicit cancellation hook.
// It is useful for line editors or custom buffered readers that cannot safely expose a raw io.ReadCloser.
type TerminalInputCanceler interface {
	TerminalInputCancel() error
}

// TerminalInputAdapter adapts wrapper readers into a cancelable terminal input source.
// Reader is what TerminalSession consumes, Source is the closer-friendly underlying reader when available.
type TerminalInputAdapter struct {
	Reader io.Reader
	Source io.Reader
	Cancel func() error
}

func (a TerminalInputAdapter) Read(p []byte) (int, error) {
	if a.Reader == nil {
		return 0, io.EOF
	}
	return a.Reader.Read(p)
}

func (a TerminalInputAdapter) TerminalInputSource() io.Reader {
	if a.Source != nil {
		return a.Source
	}
	return a.Reader
}

func (a TerminalInputAdapter) TerminalInputCancel() error {
	if a.Cancel != nil {
		return a.Cancel()
	}
	if closer, ok := a.Source.(io.Closer); ok && closer != nil {
		return closer.Close()
	}
	if closer, ok := a.Reader.(io.Closer); ok && closer != nil {
		return closer.Close()
	}
	return nil
}

func prepareTerminalInputReader(in io.Reader) (io.Reader, func(), bool, error) {
	if in == nil {
		return nil, func() {}, false, nil
	}

	var cancelOnce sync.Once
	wrapCancel := func(fn func()) func() {
		return func() {
			cancelOnce.Do(fn)
		}
	}

	if provider, ok := in.(TerminalInputSourceProvider); ok {
		source := provider.TerminalInputSource()
		if source == nil || sameReader(source, in) {
			return prepareDirectTerminalInputReader(in, wrapCancel)
		}

		prepared, cancel, cancelable, err := prepareTerminalInputReader(source)
		if err != nil {
			return nil, nil, false, err
		}
		if canceler, ok := in.(TerminalInputCanceler); ok {
			return prepared, wrapCancel(func() {
				cancel()
				_ = canceler.TerminalInputCancel()
			}), true, nil
		}
		return prepared, cancel, cancelable, nil
	}

	return prepareDirectTerminalInputReader(in, wrapCancel)
}

func prepareDirectTerminalInputReader(in io.Reader, wrapCancel func(func()) func()) (io.Reader, func(), bool, error) {
	if in == nil {
		return nil, func() {}, false, nil
	}

	switch typed := in.(type) {
	case *bufio.Reader:
		return prepareBufferedTerminalInputReader(typed)
	case *bufio.ReadWriter:
		if typed.Reader == nil {
			return in, func() {}, false, nil
		}
		return prepareBufferedTerminalInputReader(typed.Reader)
	}

	if canceler, ok := in.(TerminalInputCanceler); ok {
		return in, wrapCancel(func() {
			_ = canceler.TerminalInputCancel()
		}), true, nil
	}

	if file, ok := in.(*os.File); ok {
		dup, err := duplicateTerminalInputFile(file)
		if err != nil {
			return nil, nil, false, fmt.Errorf("duplicate terminal input: %w", err)
		}
		return dup, wrapCancel(func() {
			_ = dup.Close()
		}), true, nil
	}

	if closer, ok := in.(io.ReadCloser); ok {
		return closer, wrapCancel(func() {
			_ = closer.Close()
		}), true, nil
	}

	return in, func() {}, false, nil
}

func prepareBufferedTerminalInputReader(reader *bufio.Reader) (io.Reader, func(), bool, error) {
	if reader == nil {
		return nil, func() {}, false, nil
	}

	bufferedPrefix, err := snapshotBufferedPrefix(reader)
	if err != nil {
		return nil, nil, false, err
	}

	underlying := unwrapBufioReader(reader)
	if underlying == nil {
		if len(bufferedPrefix) == 0 {
			return reader, func() {}, false, nil
		}
		return io.MultiReader(bytes.NewReader(bufferedPrefix), reader), func() {}, false, nil
	}

	prepared, cancel, cancelable, err := prepareTerminalInputReader(underlying)
	if err != nil {
		return nil, nil, false, err
	}
	if len(bufferedPrefix) == 0 {
		return prepared, cancel, cancelable, nil
	}
	if prepared == nil {
		return bytes.NewReader(bufferedPrefix), cancel, cancelable, nil
	}
	return io.MultiReader(bytes.NewReader(bufferedPrefix), prepared), cancel, cancelable, nil
}

func snapshotBufferedPrefix(reader *bufio.Reader) ([]byte, error) {
	if reader == nil {
		return nil, nil
	}

	buffered := reader.Buffered()
	if buffered == 0 {
		return nil, nil
	}

	chunk, err := reader.Peek(buffered)
	if err != nil && !errors.Is(err, io.EOF) {
		return nil, fmt.Errorf("peek terminal input buffer: %w", err)
	}
	prefix := append([]byte(nil), chunk...)
	if _, err := reader.Discard(len(prefix)); err != nil {
		return nil, fmt.Errorf("discard terminal input buffer: %w", err)
	}
	return prefix, nil
}

func unwrapBufioReader(reader *bufio.Reader) io.Reader {
	if reader == nil {
		return nil
	}

	value := reflect.ValueOf(reader)
	if value.Kind() != reflect.Pointer || value.IsNil() {
		return nil
	}

	field := value.Elem().FieldByName("rd")
	if !field.IsValid() {
		return nil
	}

	underlyingValue := reflect.NewAt(field.Type(), unsafe.Pointer(field.UnsafeAddr())).Elem()
	underlying, ok := underlyingValue.Interface().(io.Reader)
	if !ok || underlying == nil || sameReader(underlying, reader) {
		return nil
	}
	return underlying
}

func sameReader(left io.Reader, right io.Reader) bool {
	if left == nil || right == nil {
		return false
	}

	leftValue := reflect.ValueOf(left)
	rightValue := reflect.ValueOf(right)
	if !leftValue.IsValid() || !rightValue.IsValid() {
		return false
	}
	if leftValue.Kind() != reflect.Pointer || rightValue.Kind() != reflect.Pointer {
		return false
	}
	return leftValue.Pointer() == rightValue.Pointer()
}