package commandkit

import (
	"container/heap"
	"errors"
	"fmt"
)

// ErrFlagInput identifies a missing required captured flag selected for an input.
var ErrFlagInput = errors.New("required flag input is missing")

// ErrFlagWiring identifies an invalid captured binding or destination.
var ErrFlagWiring = errors.New("flag wiring is invalid")

// FlagValues stores invocation-local converted values and presence for captured
// flags. It does not retain an input instance; values can be applied to
// independently initialized destinations. Captured occurrence flags retain
// their relative command-line order across the selected bindings.
type FlagValues[I any] struct {
	values map[*bindingIdentity]preparedValue[I]
}

// CaptureFlags adapts flags declared for I into flags on Outer. During parsing,
// each adapted flag stores its prepared value in target, associated with the
// original inner binding identity. Copies of an original handle remain
// selectable; an independently constructed lookalike does not.
func CaptureFlags[Outer, I any](flags []Flag[I], target func(*Outer) *FlagValues[I]) []Flag[Outer] {
	captured := make([]Flag[Outer], len(flags))
	for index, original := range flags {
		inner := original
		identity := newBindingIdentity()
		binding := flagBinding[Outer]{
			identity:         identity,
			meta:             inner.binding.meta,
			deferredRequired: inner.binding.meta.Required,
			defaultText:      inner.binding.defaultText,
			repeated:         inner.binding.repeated,
			boolean:          inner.binding.boolean,
		}
		if target != nil && inner.binding.identity != nil && inner.binding.valid && inner.binding.prepare != nil {
			binding.valid = true
			binding.prepare = func(raw []string) (preparedValue[Outer], error) {
				prepared, err := inner.binding.prepare(raw)
				if err != nil {
					return preparedValue[Outer]{}, err
				}
				result := preparedValue[Outer]{
					identity: identity,
					supplied: prepared.supplied,
					apply: func(outer *Outer) {
						store := target(outer)
						if store == nil {
							return
						}
						if store.values == nil {
							store.values = make(map[*bindingIdentity]preparedValue[I])
						}
						store.values[inner.binding.identity] = prepared
					},
				}
				for _, occurrence := range prepared.ordered {
					result.ordered = append(result.ordered, preparedOccurrence[Outer]{ordinal: occurrence.ordinal})
				}
				return result, nil
			}
		}
		captured[index] = Flag[Outer]{binding: binding}
	}
	return captured
}

// ApplyTo applies selected original captured flags to an initialized
// destination. A missing captured binding or nil destination matches
// ErrFlagWiring. A required selected value that was not supplied matches
// ErrFlagInput. Ordinary selected values are applied first, followed by selected
// occurrence values in command-line order. Repeated core-owned values are copied
// for each application.
func (values FlagValues[I]) ApplyTo(target *I, selected ...Flag[I]) error {
	return values.applyTo(target, false, selected...)
}

// ApplySuppliedTo applies only explicitly supplied selected flags, preserving
// the destination's existing values for omitted flags. It checks selected
// bindings and required values like ApplyTo and preserves occurrence order.
func (values FlagValues[I]) ApplySuppliedTo(target *I, selected ...Flag[I]) error {
	return values.applyTo(target, true, selected...)
}

func (values FlagValues[I]) applyTo(target *I, suppliedOnly bool, selected ...Flag[I]) error {
	if target == nil {
		return fmt.Errorf("%w: destination is nil", ErrFlagWiring)
	}
	preparedValues := make([]preparedValue[I], 0, len(selected))
	for _, flag := range selected {
		binding := flag.binding
		if binding.identity == nil {
			return fmt.Errorf("%w: selected flag has no identity", ErrFlagWiring)
		}
		prepared, exists := values.values[binding.identity]
		if !exists {
			return fmt.Errorf("%w: flag --%s was not captured", ErrFlagWiring, binding.meta.Name)
		}
		if binding.meta.Required && !prepared.supplied {
			return fmt.Errorf("flag --%s: %w", binding.meta.Name, ErrFlagInput)
		}
		if suppliedOnly && !prepared.supplied {
			continue
		}
		preparedValues = append(preparedValues, prepared)
	}
	var ordered occurrenceHeap[I]
	for index, prepared := range preparedValues {
		if prepared.apply != nil {
			prepared.apply(target)
		}
		stream := occurrenceStream[I]{events: prepared.ordered, order: index}
		stream.skipUnapplied()
		if len(stream.events) > 0 {
			ordered = append(ordered, stream)
		}
	}
	// One selected stream already has the required order.
	if len(ordered) == 1 {
		for _, event := range ordered[0].events {
			if event.apply != nil {
				event.apply(target)
			}
		}
		return nil
	}
	heap.Init(&ordered)
	for len(ordered) > 0 {
		stream := &ordered[0]
		stream.events[0].apply(target)
		stream.events = stream.events[1:]
		stream.skipUnapplied()
		if len(stream.events) == 0 {
			heap.Pop(&ordered)
		} else {
			heap.Fix(&ordered, 0)
		}
	}
	return nil
}

// Each binding supplies a monotone stream. The selection index breaks ordinal
// ties in the same order as concatenation followed by a stable sort.
type occurrenceStream[I any] struct {
	events []preparedOccurrence[I]
	order  int
}

func (stream *occurrenceStream[I]) skipUnapplied() {
	for len(stream.events) > 0 && stream.events[0].apply == nil {
		stream.events = stream.events[1:]
	}
}

type occurrenceHeap[I any] []occurrenceStream[I]

func (h occurrenceHeap[I]) Len() int { return len(h) }
func (h occurrenceHeap[I]) Less(i, j int) bool {
	left, right := *h[i].events[0].ordinal, *h[j].events[0].ordinal
	if left == right {
		return h[i].order < h[j].order
	}
	return left < right
}
func (h occurrenceHeap[I]) Swap(i, j int)   { h[i], h[j] = h[j], h[i] }
func (h *occurrenceHeap[I]) Push(value any) { *h = append(*h, value.(occurrenceStream[I])) }
func (h *occurrenceHeap[I]) Pop() any {
	last := len(*h) - 1
	value := (*h)[last]
	(*h)[last] = occurrenceStream[I]{}
	*h = (*h)[:last]
	return value
}

func classifyInputError(err error) Outcome {
	if errors.Is(err, ErrFlagWiring) {
		return Failure(err.Error()).WithCause(err)
	}
	return Usage(err.Error()).WithCause(err)
}