package configkit

import (
	"bytes"
	"errors"
	"io"
	"slices"
	"strconv"
	"strings"

	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
)

const historyNodeLimit = 4_000_000

var (
	errExplanationUnavailable = errors.New("configkit: configuration explanation is unavailable")
	errExplanationUnknownPath = errors.New("configkit: configuration explanation path is unknown")
)

// ValueSnapshot is a detached, deterministic YAML representation of one value.
// Present distinguishes absence or deletion from an explicit null value.
type ValueSnapshot struct {
	Present  bool
	Redacted bool
	YAML     string
}

// ValueChange describes one explicit definition or audited domain operation.
type ValueChange struct {
	Before       ValueSnapshot
	After        ValueSnapshot
	Origin       Origin
	Reasons      []string
	Dependencies []Path
}

// SecretReferenceInfo describes a successfully resolved effective reference.
// Name is the environment-variable name, never its value.
type SecretReferenceInfo struct {
	Name string
}

// Explanation is a detached safe account of one effective or historical path.
type Explanation struct {
	Path            Path
	Effective       ValueSnapshot
	Origin          Origin
	History         []ValueChange
	SecretReference *SecretReferenceInfo
}

type historyApplication struct {
	layer  Layer
	path   Path
	value  *document.Node
	origin Origin
}

func cloneHistoryApplications(values []historyApplication) []historyApplication {
	if values == nil {
		return nil
	}
	cloned := make([]historyApplication, len(values))
	for index, value := range values {
		cloned[index] = historyApplication{
			layer: value.layer, path: value.path.clone(), value: value.value,
			origin: cloneOrigin(value.origin),
		}
	}
	return cloned
}

func cloneSecretReferences(values map[string]string) map[string]string {
	if values == nil {
		return nil
	}
	cloned := make(map[string]string, len(values))
	for path, name := range values {
		cloned[path] = name
	}
	return cloned
}

// Explain returns safe effective and chronological values for a known final or
// historical path. Unknown and malformed paths return false.
func (r *Result[T]) Explain(path Path) (Explanation, bool) {
	if r == nil || r.final == nil || r.baseline == nil || !path.valid() {
		return Explanation{}, false
	}
	explanation := Explanation{Path: path.clone(), Effective: valueSnapshot(r.final, path, r.secrets)}
	if info, ok := r.metadata.Lookup(path); ok {
		explanation.Origin = cloneOrigin(info.Origin)
	}

	working := document.Clone(r.baseline)
	if _, present := documentAtPath(working, path); present {
		explanation.History = append(explanation.History, ValueChange{
			After: valueSnapshot(working, path, r.secrets), Origin: Origin{Layer: LayerDefault},
		})
	}
	for _, application := range r.applications {
		operationPath, relevant := applicationOperationPath(application, path)
		var before ValueSnapshot
		if relevant {
			before = valueSnapshot(working, path, r.secrets)
		}
		applyHistoryApplication(&working, application)
		if !relevant {
			continue
		}
		explanation.History = append(explanation.History, ValueChange{
			Before: before,
			After:  valueSnapshot(working, path, r.secrets),
			Origin: historyApplicationOrigin(application, operationPath),
		})
	}

	working = document.Clone(r.input)
	for _, snapshot := range r.resolution {
		relevant := pathsIntersect(snapshot.change.Path, path)
		var before ValueSnapshot
		if relevant {
			before = valueSnapshot(working, path, r.secrets)
		}
		replaceHistoryPath(&working, snapshot.change.Path, snapshot.after)
		if !relevant {
			continue
		}
		explanation.History = append(explanation.History, ValueChange{
			Before:       before,
			After:        valueSnapshot(working, path, r.secrets),
			Origin:       cloneOrigin(snapshot.change.Origin),
			Reasons:      append([]string(nil), snapshot.change.Reasons...),
			Dependencies: clonePaths(snapshot.change.Dependencies),
		})
	}

	if !explanation.Effective.Present && len(explanation.History) == 0 {
		return Explanation{}, false
	}
	if name, ok := r.secretReferences[path.String()]; ok && !pathIsSecret(r.secrets, path) {
		explanation.SecretReference = &SecretReferenceInfo{Name: name}
	}
	return explanation, true
}

func applicationOperationPath(application historyApplication, query Path) (Path, bool) {
	if !application.path.valid() || application.value == nil {
		return Path{}, false
	}
	if application.path.String() != "" {
		if pathsIntersect(application.path, query) {
			return application.path.clone(), true
		}
		return Path{}, false
	}
	if len(query.segments) == 0 {
		return Path{}, true
	}
	for length := 1; length <= len(query.segments); length++ {
		prefix := Path{segments: clonePathSegments(query.segments[:length], 0)}
		node, present := documentAtPath(application.value, prefix)
		if !present {
			return Path{}, false
		}
		if length == len(query.segments) || node.Kind != document.Mapping {
			return prefix, true
		}
	}
	return Path{}, false
}

func historyApplicationOrigin(application historyApplication, operationPath Path) Origin {
	if application.origin.Layer != "" {
		return cloneOrigin(application.origin)
	}
	origin := Origin{Layer: application.layer}
	relative, ok := relativeHistoryPath(application.path, operationPath)
	if !ok {
		return origin
	}
	if node, present := documentAtPath(application.value, relative); present {
		origin.Locations = []Location{publicLocation(node.Location)}
	}
	return origin
}

func relativeHistoryPath(root, path Path) (Path, bool) {
	if !pathContains(root, path) {
		return Path{}, false
	}
	return Path{segments: clonePathSegments(path.segments[len(root.segments):], 0)}, true
}

func pathsIntersect(left, right Path) bool {
	return pathContains(left, right) || pathContains(right, left)
}

func applyHistoryApplication(root **document.Node, application historyApplication) {
	if application.path.String() == "" {
		mergeHistoryDocument(root, application.value)
		return
	}
	mergeHistoryAtPath(root, application.path, application.value)
}

func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
	if incoming == nil {
		return
	}
	if *target == nil || (*target).Kind != document.Mapping || incoming.Kind != document.Mapping {
		*target = document.Clone(incoming)
		return
	}
	for _, entry := range incoming.Mapping {
		position := documentEntryIndex((*target).Mapping, entry.Key)
		if position < 0 {
			cloned := entry
			cloned.Value = document.Clone(entry.Value)
			(*target).Mapping = append((*target).Mapping, cloned)
			continue
		}
		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value)
	}
}

func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node) {
	if root == nil || *root == nil || len(path.segments) == 0 {
		mergeHistoryDocument(root, incoming)
		return
	}
	current := *root
	for index, segment := range path.segments {
		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
			return
		}
		position := documentEntryIndex(current.Mapping, segment.key)
		if index == len(path.segments)-1 {
			if position < 0 {
				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
			} else {
				mergeHistoryDocument(&current.Mapping[position].Value, incoming)
			}
			return
		}
		if position < 0 {
			child := &document.Node{Kind: document.Mapping}
			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
			current = child
		} else {
			current = current.Mapping[position].Value
		}
	}
}

func replaceHistoryPath(root **document.Node, path Path, replacement *document.Node) {
	if root == nil || *root == nil {
		return
	}
	if len(path.segments) == 0 {
		*root = document.Clone(replacement)
		return
	}
	current := *root
	for index, segment := range path.segments {
		last := index == len(path.segments)-1
		switch current.Kind {
		case document.Mapping:
			if segment.kind == pathSegmentIndex {
				return
			}
			position := documentEntryIndex(current.Mapping, segment.key)
			if position < 0 {
				return
			}
			if last {
				if replacement == nil {
					current.Mapping = append(current.Mapping[:position], current.Mapping[position+1:]...)
				} else {
					current.Mapping[position].Value = document.Clone(replacement)
				}
				return
			}
			current = current.Mapping[position].Value
		case document.Sequence:
			position, ok := schemaIndex(segment)
			if !ok || position >= len(current.Sequence) {
				return
			}
			if last {
				if replacement != nil {
					current.Sequence[position] = document.Clone(replacement)
				}
				return
			}
			current = current.Sequence[position]
		default:
			return
		}
	}
}

func valueSnapshot(root *document.Node, path Path, secrets map[string]bool) ValueSnapshot {
	node, present := documentAtPath(root, path)
	if !present {
		return ValueSnapshot{}
	}
	safe, redacted := safeSnapshotDocument(node, secrets, path)
	var output bytes.Buffer
	if err := yamladapter.Write(&output, safe); err != nil {
		return ValueSnapshot{Present: true, Redacted: true, YAML: "'[REDACTED]'\n"}
	}
	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}
}

func safeSnapshotDocument(node *document.Node, secrets map[string]bool, path Path) (*document.Node, bool) {
	if node == nil {
		return nil, false
	}
	if pathIsSecret(secrets, path) {
		return &document.Node{Kind: document.String, Text: "[REDACTED]"}, true
	}
	result := *node
	redacted := false
	switch node.Kind {
	case document.Mapping:
		result.Mapping = make([]document.Entry, len(node.Mapping))
		copy(result.Mapping, node.Mapping)
		sortDocumentMapping(result.Mapping)
		for index := range result.Mapping {
			entry := &result.Mapping[index]
			entry.Value, redacted = safeSnapshotChild(entry.Value, secrets, path.Key(entry.Key), redacted)
		}
	case document.Sequence:
		result.Sequence = make([]*document.Node, len(node.Sequence))
		for index, child := range node.Sequence {
			result.Sequence[index], redacted = safeSnapshotChild(child, secrets, path.Index(index), redacted)
		}
	}
	return &result, redacted
}

func safeSnapshotChild(node *document.Node, secrets map[string]bool, path Path, prior bool) (*document.Node, bool) {
	safe, redacted := safeSnapshotDocument(node, secrets, path)
	return safe, prior || redacted
}

func sortDocumentMapping(entries []document.Entry) {
	slices.SortFunc(entries, func(left, right document.Entry) int {
		return strings.Compare(left.Key, right.Key)
	})
}

func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions)
}

func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
	seen := make(map[*document.Node]bool)
	count := 0
	var visit func(*document.Node) bool
	visit = func(node *document.Node) bool {
		if node == nil || seen[node] {
			return true
		}
		seen[node] = true
		count++
		if count > limit {
			return false
		}
		for _, entry := range node.Mapping {
			if !visit(entry.Value) {
				return false
			}
		}
		for _, child := range node.Sequence {
			if !visit(child) {
				return false
			}
		}
		return true
	}
	if !visit(baseline) || !visit(input) || !visit(final) {
		return false
	}
	for _, application := range applications {
		if !visit(application.value) {
			return false
		}
	}
	for _, resolution := range resolutions {
		if !visit(resolution.before) || !visit(resolution.after) {
			return false
		}
	}
	return true
}

// WriteExplanation writes a deterministic human-readable rendering of Explain.
// Unknown paths produce no output. Writer failures may leave partial output.
func (r *Result[T]) WriteExplanation(output io.Writer, path Path) error {
	if r == nil || output == nil {
		return errExplanationUnavailable
	}
	explanation, ok := r.Explain(path)
	if !ok {
		return errExplanationUnknownPath
	}
	var rendered strings.Builder
	rendered.WriteString("path: ")
	rendered.WriteString(strconv.Quote(explanation.Path.String()))
	rendered.WriteByte('\n')
	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, r.sources)
	writeExplanationSnapshot(&rendered, "effective (canonical typed)", explanation.Effective, 0)
	if explanation.SecretReference != nil {
		rendered.WriteString("secret reference: ")
		rendered.WriteString(strconv.Quote(explanation.SecretReference.Name))
		rendered.WriteString(" (resolved from process environment)\n")
	}
	rendered.WriteString("history:\n")
	for _, change := range explanation.History {
		rendered.WriteString("  - representation: ")
		if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerProfile || change.Origin.Layer == LayerCLI {
			rendered.WriteString("policy-validated input\n")
		} else {
			rendered.WriteString("canonical typed\n")
		}
		writeExplanationOrigin(&rendered, "    source", change.Origin, r.sources)
		writeExplanationSnapshot(&rendered, "    before", change.Before, 4)
		writeExplanationSnapshot(&rendered, "    after", change.After, 4)
		for _, reason := range change.Reasons {
			rendered.WriteString("    reason: ")
			rendered.WriteString(strconv.Quote(reason))
			rendered.WriteByte('\n')
		}
		for _, dependency := range change.Dependencies {
			rendered.WriteString("    dependency: ")
			rendered.WriteString(strconv.Quote(dependency.String()))
			rendered.WriteByte('\n')
		}
	}
	data := []byte(rendered.String())
	written, err := output.Write(data)
	if err == nil && written != len(data) {
		return io.ErrShortWrite
	}
	return err
}

func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, sources []SourceInfo) {
	output.WriteString(label)
	output.WriteString(": layer=")
	output.WriteString(strconv.Quote(string(origin.Layer)))
	if origin.Operation != "" {
		output.WriteString(" operation=")
		output.WriteString(strconv.Quote(string(origin.Operation)))
	}
	output.WriteByte('\n')
	for _, location := range origin.Locations {
		output.WriteString("      location: source=")
		output.WriteString(strconv.Quote(location.SourceID))
		if label := sourceLabel(sources, location.SourceID); label != "" {
			output.WriteString(" label=")
			output.WriteString(strconv.Quote(label))
		}
		output.WriteString(" line=")
		output.WriteString(strconv.Itoa(location.Line))
		output.WriteString(" column=")
		output.WriteString(strconv.Itoa(location.Column))
		output.WriteByte('\n')
	}
}

func sourceLabel(sources []SourceInfo, sourceID string) string {
	for _, source := range sources {
		if source.ID == sourceID {
			return source.Label
		}
	}
	return ""
}

func writeExplanationSnapshot(output *strings.Builder, label string, snapshot ValueSnapshot, indent int) {
	if !snapshot.Present {
		output.WriteString(label)
		output.WriteString(": absent\n")
		return
	}
	output.WriteString(label)
	if snapshot.Redacted {
		output.WriteString(" (redacted)")
	}
	output.WriteString(":\n")
	prefix := strings.Repeat(" ", indent+2)
	for _, line := range strings.SplitAfter(snapshot.YAML, "\n") {
		if line == "" {
			continue
		}
		output.WriteString(prefix)
		output.WriteString(line)
		if !strings.HasSuffix(line, "\n") {
			output.WriteByte('\n')
		}
	}
}