package taskloop

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"

	"iop/packages/go/agentconfig"
	"iop/packages/go/agentguard"
	"iop/packages/go/agenttask"
)

const (
	planPrefix       = "PLAN-"
	reviewPrefix     = "CODE_REVIEW-"
	taskGroupPrefix  = "m-"
	completeFileName = "complete.log"
)

type MilestoneView struct {
	ID                 string
	Selected           bool
	WorkUnits          int
	CompletedWorkUnits int
}

// SelectionStore keeps the device-local explicit Milestone selection separate
// from the repository-owned runtime defaults.
type SelectionStore interface {
	SelectedMilestone(context.Context, string) (string, error)
}

// Workflow normalizes registered project artifacts for the shared manager.
// It never mutates a project checkout.
type Workflow struct {
	snapshot   agentconfig.RuntimeSnapshot
	selections SelectionStore
}

var _ agenttask.WorkflowAdapter = (*Workflow)(nil)

func NewWorkflow(snapshot agentconfig.RuntimeSnapshot, selections SelectionStore) (*Workflow, error) {
	if snapshot.Revision() == "" {
		return nil, errors.New("taskloop: runtime snapshot is required")
	}
	return &Workflow{snapshot: snapshot, selections: selections}, nil
}

func (workflow *Workflow) RegisteredProjects(ctx context.Context) ([]agenttask.ProjectID, error) {
	if err := ctx.Err(); err != nil {
		return nil, err
	}
	config := workflow.snapshot.Config()
	projects := make([]agenttask.ProjectID, 0, len(config.Projects))
	for id, registration := range config.Projects {
		if registration.Enabled {
			projects = append(projects, agenttask.ProjectID(id))
		}
	}
	sort.Slice(projects, func(left, right int) bool { return projects[left] < projects[right] })
	return projects, nil
}

func (workflow *Workflow) Snapshot(
	ctx context.Context,
	projectID agenttask.ProjectID,
) (agenttask.ProjectWorkflowSnapshot, error) {
	if err := ctx.Err(); err != nil {
		return agenttask.ProjectWorkflowSnapshot{}, err
	}
	registration, ok := workflow.snapshot.Project(string(projectID))
	if !ok || !registration.Enabled {
		return agenttask.ProjectWorkflowSnapshot{}, fmt.Errorf(
			"taskloop: project %q is not registered and enabled",
			projectID,
		)
	}
	milestone := registration.SelectedMilestone
	if workflow.selections != nil {
		selected, err := workflow.selections.SelectedMilestone(ctx, string(projectID))
		if err != nil {
			return agenttask.ProjectWorkflowSnapshot{}, err
		}
		if selected != "" {
			milestone = selected
		}
	}
	if strings.TrimSpace(milestone) == "" {
		return agenttask.ProjectWorkflowSnapshot{}, fmt.Errorf(
			"taskloop: project %q has no selected milestone",
			projectID,
		)
	}

	root, err := canonicalDirectory(registration.Workspace)
	if err != nil {
		return agenttask.ProjectWorkflowSnapshot{}, fmt.Errorf(
			"taskloop: project %q workspace: %w",
			projectID,
			err,
		)
	}
	units, evidence, err := scanWorkflow(root, milestone)
	if err != nil {
		return agenttask.ProjectWorkflowSnapshot{}, err
	}
	return agenttask.ProjectWorkflowSnapshot{
		ProjectID:   projectID,
		WorkspaceID: WorkspaceIdentity(root),
		Revision: agenttask.WorkflowRevision(
			digestStrings("workflow", string(projectID), milestone, evidence),
		),
		Units:      units,
		ObservedAt: time.Now().UTC(),
	}, nil
}

// WorkspaceIdentity is a stable, path-redacted identity for one canonical
// registered project root.
func WorkspaceIdentity(root string) agenttask.WorkspaceID {
	sum := sha256.Sum256([]byte(filepath.Clean(root)))
	return agenttask.WorkspaceID("workspace-" + hex.EncodeToString(sum[:]))
}

func parseMilestoneSlug(slug string) (string, error) {
	if slug == "" {
		return "", errors.New("taskloop: milestone slug cannot be empty")
	}
	for i := 0; i < len(slug); i++ {
		c := slug[i]
		if (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') {
			continue
		}
		if c == '-' && i > 0 {
			continue
		}
		return "", fmt.Errorf("taskloop: milestone slug %q contains invalid character %q", slug, string(c))
	}
	return slug, nil
}

func parseActiveGroupDir(dirName string) (string, error) {
	if !strings.HasPrefix(dirName, taskGroupPrefix) {
		return "", fmt.Errorf("taskloop: task group %q must have %q prefix", dirName, taskGroupPrefix)
	}
	rawSlug := strings.TrimPrefix(dirName, taskGroupPrefix)
	return parseMilestoneSlug(rawSlug)
}

func parseArchiveGroupDir(dirName string) (string, error) {
	if !strings.HasPrefix(dirName, taskGroupPrefix) {
		return "", fmt.Errorf("taskloop: task group %q must have %q prefix", dirName, taskGroupPrefix)
	}
	rest := strings.TrimPrefix(dirName, taskGroupPrefix)
	if idx := strings.IndexByte(rest, '_'); idx >= 0 {
		slugPart := rest[:idx]
		suffixPart := rest[idx+1:]
		if strings.IndexByte(suffixPart, '_') >= 0 || suffixPart == "" {
			return "", fmt.Errorf("taskloop: archive group directory %q has invalid collision suffix", dirName)
		}
		val, err := strconv.Atoi(suffixPart)
		if err != nil || val <= 0 || strconv.Itoa(val) != suffixPart {
			return "", fmt.Errorf("taskloop: archive group directory %q has invalid collision suffix %q", dirName, suffixPart)
		}
		return parseMilestoneSlug(slugPart)
	}
	return parseMilestoneSlug(rest)
}

func scanWorkflow(root, milestone string) ([]agenttask.WorkUnit, string, error) {
	slug := milestone
	if strings.HasPrefix(milestone, taskGroupPrefix) {
		slug = strings.TrimPrefix(milestone, taskGroupPrefix)
	}
	canonicalSlug, err := parseMilestoneSlug(slug)
	if err != nil {
		return nil, "", fmt.Errorf("taskloop: selected milestone %q: %w", milestone, err)
	}

	activeRoot := filepath.Join(root, "agent-task", taskGroupPrefix+canonicalSlug)
	entries, err := readOptionalTaskGroup(activeRoot)
	if err != nil {
		return nil, "", fmt.Errorf("taskloop: read active task group: %w", err)
	}

	units := make([]agenttask.WorkUnit, 0, len(entries)+1)
	evidence := make([]string, 0, len(entries)+1)
	seen := make(map[agenttask.WorkUnitID]string)
	single, singleEvidence, singleActive, err := readSinglePlanUnit(root, activeRoot, canonicalSlug)
	if err != nil {
		return nil, "", err
	}
	if singleActive {
		seen[single.ID] = taskGroupPrefix + canonicalSlug
		units = append(units, single)
		evidence = append(evidence, singleEvidence)
	}
	for _, entry := range entries {
		if !entry.IsDir() {
			continue
		}
		unit, unitEvidence, include, err := readActiveUnit(root, activeRoot, canonicalSlug, entry.Name())
		if err != nil {
			return nil, "", err
		}
		if !include {
			continue
		}
		if singleActive {
			return nil, "", fmt.Errorf(
				"taskloop: task group %q mixes a single-plan root pair with split-plan task directory %q",
				taskGroupPrefix+canonicalSlug,
				entry.Name(),
			)
		}
		if prior, duplicate := seen[unit.ID]; duplicate {
			return nil, "", fmt.Errorf(
				"taskloop: work identity %q is ambiguous between %q and %q",
				unit.ID,
				prior,
				entry.Name(),
			)
		}
		seen[unit.ID] = entry.Name()
		units = append(units, unit)
		evidence = append(evidence, unitEvidence)
	}
	archived, archivedEvidence, err := readArchivedUnits(root, canonicalSlug, seen)
	if err != nil {
		return nil, "", err
	}
	units = append(units, archived...)
	evidence = append(evidence, archivedEvidence...)
	if len(units) == 0 {
		return nil, "", fmt.Errorf(
			"taskloop: selected milestone %q has no active or archived task artifacts",
			canonicalSlug,
		)
	}
	sort.Slice(units, func(left, right int) bool { return units[left].ID < units[right].ID })
	sort.Strings(evidence)
	return units, strings.Join(evidence, "\n"), nil
}

func discoverMilestoneCandidates(root string) ([]string, error) {
	candidates := make(map[string]struct{})

	activeRoot := filepath.Join(root, "agent-task")
	activeEntries, err := readOptionalTaskGroup(activeRoot)
	if err != nil {
		return nil, fmt.Errorf("taskloop: read active task groups: %w", err)
	}
	for _, entry := range activeEntries {
		if !entry.IsDir() {
			continue
		}
		name := entry.Name()
		if name == "archive" {
			continue
		}
		if strings.HasPrefix(name, taskGroupPrefix) {
			slug, err := parseActiveGroupDir(name)
			if err != nil {
				return nil, err
			}
			candidates[slug] = struct{}{}
		}
	}

	archiveRoot := filepath.Join(root, "agent-task", "archive")
	years, err := os.ReadDir(archiveRoot)
	if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return nil, fmt.Errorf("taskloop: read archive root: %w", err)
	}
	if err == nil {
		for _, year := range years {
			if !year.IsDir() {
				continue
			}
			yearPath := filepath.Join(archiveRoot, year.Name())
			months, err := os.ReadDir(yearPath)
			if err != nil {
				return nil, fmt.Errorf("taskloop: read archive year %q: %w", year.Name(), err)
			}
			for _, month := range months {
				if !month.IsDir() {
					continue
				}
				monthPath := filepath.Join(yearPath, month.Name())
				groupDirs, err := os.ReadDir(monthPath)
				if err != nil {
					return nil, fmt.Errorf("taskloop: read archive month %q/%q: %w", year.Name(), month.Name(), err)
				}
				for _, groupDir := range groupDirs {
					if !groupDir.IsDir() {
						continue
					}
					name := groupDir.Name()
					if strings.HasPrefix(name, taskGroupPrefix) {
						slug, err := parseArchiveGroupDir(name)
						if err != nil {
							return nil, err
						}
						candidates[slug] = struct{}{}
					}
				}
			}
		}
	}

	slugs := make([]string, 0, len(candidates))
	for slug := range candidates {
		slugs = append(slugs, slug)
	}
	sort.Strings(slugs)
	return slugs, nil
}

func scanMilestones(root, selected string) ([]MilestoneView, error) {
	slugs, err := discoverMilestoneCandidates(root)
	if err != nil {
		return nil, err
	}
	views := make([]MilestoneView, 0, len(slugs))
	for _, slug := range slugs {
		units, _, err := scanWorkflow(root, slug)
		if err != nil {
			return nil, err
		}
		completed := 0
		for _, u := range units {
			if u.Completed {
				completed++
			}
		}
		isSelected := slug == selected || taskGroupPrefix+slug == selected
		views = append(views, MilestoneView{
			ID:                 slug,
			Selected:           isSelected,
			WorkUnits:          len(units),
			CompletedWorkUnits: completed,
		})
	}
	if len(views) == 0 {
		return nil, errors.New("taskloop: no workflow-backed milestones")
	}
	return views, nil
}

func readOptionalTaskGroup(root string) ([]os.DirEntry, error) {
	entries, err := os.ReadDir(root)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, nil
	}
	return entries, err
}

// InspectTaskGroup provides a configuration-free, read-only projection for an
// operator dry-run. It deliberately uses the same workflow parser as Runtime
// and never constructs a manager, provider, lease, or durable state store.
func InspectTaskGroup(root, group string) ([]agenttask.WorkUnit, error) {
	if !strings.HasPrefix(group, taskGroupPrefix) {
		return nil, fmt.Errorf("taskloop: task group must be an exact m- prefixed identifier")
	}
	slugCandidate := strings.TrimPrefix(group, taskGroupPrefix)
	slug, err := parseMilestoneSlug(slugCandidate)
	if err != nil {
		return nil, fmt.Errorf("taskloop: task group must be an exact m- prefixed identifier: %w", err)
	}
	canonicalRoot, err := canonicalDirectory(root)
	if err != nil {
		return nil, err
	}
	units, _, err := scanWorkflow(canonicalRoot, slug)
	return units, err
}

func readArchivedUnits(
	root, milestone string,
	seen map[agenttask.WorkUnitID]string,
) ([]agenttask.WorkUnit, []string, error) {
	archiveRoot := filepath.Join(root, "agent-task", "archive")
	years, err := os.ReadDir(archiveRoot)
	if errors.Is(err, fs.ErrNotExist) {
		return nil, nil, nil
	}
	if err != nil {
		return nil, nil, fmt.Errorf("taskloop: read archive root: %w", err)
	}

	type archiveMatch struct {
		completionPath string
		workID         agenttask.WorkUnitID
		aliases        []string
		taskDirectory  string
	}
	var matches []archiveMatch

	for _, year := range years {
		if !year.IsDir() {
			continue
		}
		yearPath := filepath.Join(archiveRoot, year.Name())
		months, err := os.ReadDir(yearPath)
		if err != nil {
			return nil, nil, fmt.Errorf("taskloop: read archive year %q: %w", year.Name(), err)
		}
		for _, month := range months {
			if !month.IsDir() {
				continue
			}
			monthPath := filepath.Join(yearPath, month.Name())
			groupDirs, err := os.ReadDir(monthPath)
			if err != nil {
				return nil, nil, fmt.Errorf("taskloop: read archive month %q/%q: %w", year.Name(), month.Name(), err)
			}
			for _, groupDir := range groupDirs {
				if !groupDir.IsDir() {
					continue
				}
				groupName := groupDir.Name()
				if !strings.HasPrefix(groupName, taskGroupPrefix) {
					continue
				}
				slug, err := parseArchiveGroupDir(groupName)
				if err != nil {
					return nil, nil, fmt.Errorf("taskloop: archived task group %q: %w", groupName, err)
				}
				if slug != milestone {
					continue
				}
				groupPath := filepath.Join(monthPath, groupName)

				directCompletion := filepath.Join(groupPath, completeFileName)
				if info, statErr := os.Stat(directCompletion); statErr == nil && info.Mode().IsRegular() {
					matches = append(matches, archiveMatch{
						completionPath: directCompletion,
						workID:         singlePlanWorkUnitID(milestone),
						aliases:        []string{taskGroupPrefix + milestone, milestone},
						taskDirectory:  taskGroupPrefix + milestone,
					})
				} else if statErr != nil && !errors.Is(statErr, fs.ErrNotExist) {
					return nil, nil, fmt.Errorf("taskloop: inspect archive completion %q: %w", directCompletion, statErr)
				}

				subEntries, err := os.ReadDir(groupPath)
				if err != nil {
					return nil, nil, fmt.Errorf("taskloop: read archive task group %q: %w", groupName, err)
				}
				for _, sub := range subEntries {
					if !sub.IsDir() {
						continue
					}
					taskDirName := sub.Name()
					subCompletion := filepath.Join(groupPath, taskDirName, completeFileName)
					info, statErr := os.Stat(subCompletion)
					if statErr == nil && info.Mode().IsRegular() {
						id, _, aliases, parseErr := parseTaskDirectory(taskDirName)
						if parseErr != nil {
							return nil, nil, fmt.Errorf("taskloop: archived task %q: %w", taskDirName, parseErr)
						}
						matches = append(matches, archiveMatch{
							completionPath: subCompletion,
							workID:         agenttask.WorkUnitID(id),
							aliases:        aliases,
							taskDirectory:  taskDirName,
						})
					} else if statErr != nil && !errors.Is(statErr, fs.ErrNotExist) {
						return nil, nil, fmt.Errorf("taskloop: inspect archive completion %q: %w", subCompletion, statErr)
					}
				}
			}
		}
	}

	sort.Slice(matches, func(i, j int) bool {
		return matches[i].completionPath < matches[j].completionPath
	})

	var units []agenttask.WorkUnit
	var evidence []string
	for _, m := range matches {
		unit, unitEvidence, include, err := readArchivedUnit(
			root,
			milestone,
			m.completionPath,
			m.workID,
			m.aliases,
			m.taskDirectory,
			seen,
		)
		if err != nil {
			return nil, nil, err
		}
		if include {
			units = append(units, unit)
			evidence = append(evidence, unitEvidence)
		}
	}
	return units, evidence, nil
}

func readArchivedUnit(
	root, milestone, completion string,
	workID agenttask.WorkUnitID,
	aliases []string,
	taskDirectory string,
	seen map[agenttask.WorkUnitID]string,
) (agenttask.WorkUnit, string, bool, error) {
	if _, exists := seen[workID]; exists {
		return agenttask.WorkUnit{}, "", false, nil
	}
	content, err := os.ReadFile(completion)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	seen[workID] = taskDirectory
	relative, err := filepath.Rel(root, completion)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	return agenttask.WorkUnit{
			ID:            workID,
			MilestoneID:   agenttask.MilestoneID(milestone),
			Aliases:       uniqueStrings(aliases),
			WriteSetKind:  agenttask.WriteSetUnknown,
			IsolationMode: agentguard.IsolationModeOverlay,
			Completed:     true,
			Metadata:      map[string]string{"task_directory": taskDirectory},
		},
		digestBytes(filepath.ToSlash(relative), content),
		true,
		nil
}

func readSinglePlanUnit(
	root, activeRoot, milestone string,
) (agenttask.WorkUnit, string, bool, error) {
	taskGroup := taskGroupPrefix + milestone
	plan, review, include, err := matchingActivePair(activeRoot, taskGroup)
	if err != nil || !include {
		return agenttask.WorkUnit{}, "", include, err
	}
	return normalizeActiveUnit(
		root,
		activeRoot,
		milestone,
		taskGroup,
		plan,
		review,
		string(singlePlanWorkUnitID(milestone)),
		nil,
		[]string{taskGroup, milestone},
	)
}

func readActiveUnit(
	root, activeRoot, milestone, directory string,
) (agenttask.WorkUnit, string, bool, error) {
	taskRoot := filepath.Join(activeRoot, directory)
	plan, review, include, err := matchingActivePair(taskRoot, directory)
	if err != nil || !include {
		return agenttask.WorkUnit{}, "", include, err
	}
	id, predecessors, aliases, err := parseTaskDirectory(directory)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	return normalizeActiveUnit(
		root,
		taskRoot,
		milestone,
		directory,
		plan,
		review,
		id,
		predecessors,
		aliases,
	)
}

func matchingActivePair(root, task string) (string, string, bool, error) {
	plans, err := matchingMarkdown(root, planPrefix)
	if errors.Is(err, fs.ErrNotExist) {
		return "", "", false, nil
	}
	if err != nil {
		return "", "", false, err
	}
	reviews, err := matchingMarkdown(root, reviewPrefix)
	if err != nil {
		return "", "", false, err
	}
	if len(plans) == 0 && len(reviews) == 0 {
		return "", "", false, nil
	}
	if len(plans) != 1 {
		return "", "", false, fmt.Errorf(
			"taskloop: task %q must contain exactly one active PLAN, found %d",
			task,
			len(plans),
		)
	}
	if len(reviews) != 1 {
		return "", "", false, fmt.Errorf(
			"taskloop: task %q must contain exactly one active CODE_REVIEW, found %d",
			task,
			len(reviews),
		)
	}
	return plans[0], reviews[0], true, nil
}

func normalizeActiveUnit(
	root, taskRoot, milestone, taskDirectory, plan, review, id string,
	predecessors, aliases []string,
) (agenttask.WorkUnit, string, bool, error) {
	planBytes, err := os.ReadFile(plan)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	writeSet, err := parseModifiedFilesSummary(root, planBytes)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, fmt.Errorf(
			"taskloop: task %q PLAN: %w",
			taskDirectory,
			err,
		)
	}
	planRelative, err := filepath.Rel(root, plan)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	reviewRelative, err := filepath.Rel(root, review)
	if err != nil {
		return agenttask.WorkUnit{}, "", false, err
	}
	completed := false
	if info, statErr := os.Stat(filepath.Join(taskRoot, completeFileName)); statErr == nil {
		completed = info.Mode().IsRegular()
	} else if !errors.Is(statErr, fs.ErrNotExist) {
		return agenttask.WorkUnit{}, "", false, statErr
	}
	dependencies := make([]agenttask.DependencyRef, len(predecessors))
	for index, predecessor := range predecessors {
		dependencies[index] = agenttask.DependencyRef{Ref: predecessor}
	}
	kind := agenttask.WriteSetUnknown
	if len(writeSet) > 0 {
		kind = agenttask.WriteSetDisjoint
	}
	unit := agenttask.WorkUnit{
		ID:                   agenttask.WorkUnitID(id),
		MilestoneID:          agenttask.MilestoneID(milestone),
		Aliases:              aliases,
		ExplicitPredecessors: dependencies,
		WriteSetKind:         kind,
		DeclaredWriteSet:     writeSet,
		IsolationMode:        agentguard.IsolationModeOverlay,
		Completed:            completed,
		Metadata: map[string]string{
			"task_directory": taskDirectory,
			"plan_path":      filepath.ToSlash(planRelative),
			"review_path":    filepath.ToSlash(reviewRelative),
		},
	}
	return unit, digestBytes(filepath.ToSlash(planRelative), planBytes), true, nil
}

func singlePlanWorkUnitID(milestone string) agenttask.WorkUnitID {
	return agenttask.WorkUnitID(taskGroupPrefix + milestone)
}

func matchingMarkdown(root, prefix string) ([]string, error) {
	entries, err := os.ReadDir(root)
	if err != nil {
		return nil, err
	}
	var matches []string
	for _, entry := range entries {
		if entry.IsDir() || !strings.HasPrefix(entry.Name(), prefix) ||
			!strings.HasSuffix(entry.Name(), ".md") {
			continue
		}
		matches = append(matches, filepath.Join(root, entry.Name()))
	}
	sort.Strings(matches)
	return matches, nil
}

func parseTaskDirectory(directory string) (string, []string, []string, error) {
	head, slug, found := strings.Cut(directory, "_")
	if !found || head == "" || slug == "" {
		return "", nil, nil, fmt.Errorf(
			"taskloop: task directory %q does not have <id>_<slug> form",
			directory,
		)
	}
	id := head
	var predecessors []string
	if primary, dependencyText, hasDependencies := strings.Cut(head, "+"); hasDependencies {
		id = primary
		if id == "" || dependencyText == "" {
			return "", nil, nil, fmt.Errorf(
				"taskloop: task directory %q has malformed explicit predecessors",
				directory,
			)
		}
		for _, predecessor := range strings.Split(dependencyText, ",") {
			if predecessor == "" {
				return "", nil, nil, fmt.Errorf(
					"taskloop: task directory %q has an empty predecessor",
					directory,
				)
			}
			predecessors = append(predecessors, predecessor)
		}
	}
	if strings.TrimSpace(id) != id || strings.ContainsAny(id, "/\\\x00\r\n") {
		return "", nil, nil, fmt.Errorf("taskloop: task directory %q has invalid identity", directory)
	}
	aliases := []string{directory, head, slug}
	if id != head {
		aliases = append(aliases, id)
	}
	return id, predecessors, uniqueStrings(aliases), nil
}

// ValidatePlanWriteSet validates a candidate PLAN without constructing a
// runtime, provider, lease, or durable store. The candidate may be outside
// workspace, but every declared write target must be a contained literal file.
func ValidatePlanWriteSet(workspace, planPath string) ([]string, error) {
	root, err := canonicalDirectory(workspace)
	if err != nil {
		return nil, fmt.Errorf("taskloop: workspace: %w", err)
	}
	if strings.TrimSpace(planPath) == "" {
		return nil, errors.New("taskloop: candidate PLAN path is required")
	}
	plan, err := os.ReadFile(planPath)
	if err != nil {
		return nil, fmt.Errorf("taskloop: read candidate PLAN: %w", err)
	}
	return parseModifiedFilesSummary(root, plan)
}

func parseModifiedFilesSummary(workspace string, plan []byte) ([]string, error) {
	lines := strings.Split(string(plan), "\n")
	heading := -1
	for index, line := range lines {
		if isModifiedFilesSummaryHeading(line) {
			if heading >= 0 {
				return nil, errors.New("Modified Files Summary appears more than once")
			}
			heading = index
		}
	}
	if heading < 0 {
		return nil, errors.New("Modified Files Summary is missing")
	}
	var files []string
	seen := make(map[string]struct{})
	for _, line := range lines[heading+1:] {
		trimmed := strings.TrimSpace(line)
		if strings.HasPrefix(trimmed, "## ") {
			break
		}
		if !strings.HasPrefix(trimmed, "|") {
			continue
		}
		columns := strings.Split(strings.Trim(trimmed, "|"), "|")
		if len(columns) == 0 {
			continue
		}
		cell := strings.TrimSpace(columns[0])
		if isModifiedFilesHeader(cell) || strings.Trim(cell, "-: ") == "" {
			continue
		}
		claims, err := backtickClaims(cell)
		if err != nil {
			return nil, err
		}
		for _, claim := range claims {
			path, err := canonicalWriteClaim(workspace, claim)
			if err != nil {
				return nil, err
			}
			if _, duplicate := seen[path]; duplicate {
				return nil, fmt.Errorf("file path %q is duplicated", path)
			}
			seen[path] = struct{}{}
			files = append(files, path)
		}
	}
	if len(files) == 0 {
		return nil, errors.New("Modified Files Summary has no file rows")
	}
	sort.Strings(files)
	return files, nil
}

func isModifiedFilesSummaryHeading(line string) bool {
	trimmed := strings.TrimSpace(line)
	if !strings.HasPrefix(trimmed, "##") || strings.HasPrefix(trimmed, "###") {
		return false
	}
	return strings.TrimSpace(strings.TrimPrefix(trimmed, "##")) == "Modified Files Summary" ||
		strings.TrimSpace(strings.TrimPrefix(trimmed, "##")) == "수정 파일 요약"
}

func isModifiedFilesHeader(cell string) bool {
	switch strings.ToLower(strings.TrimSpace(cell)) {
	case "file", "files", "path", "paths", "파일", "경로":
		return true
	default:
		return false
	}
}

func backtickClaims(cell string) ([]string, error) {
	if strings.Contains(cell, "``") {
		return nil, errors.New("empty file path claim is not allowed")
	}
	var claims []string
	for rest := cell; ; {
		start := strings.IndexByte(rest, '`')
		if start < 0 {
			break
		}
		rest = rest[start+1:]
		end := strings.IndexByte(rest, '`')
		if end < 0 {
			return nil, fmt.Errorf("file cell %q has an unmatched backtick", cell)
		}
		claim := strings.TrimSpace(rest[:end])
		if claim == "" {
			return nil, errors.New("empty file path claim is not allowed")
		}
		claims = append(claims, claim)
		rest = rest[end+1:]
	}
	if len(claims) == 0 {
		return nil, fmt.Errorf("file cell %q must contain a backtick path", cell)
	}
	return claims, nil
}

func canonicalWriteClaim(workspace, value string) (string, error) {
	claim := strings.TrimSpace(value)
	claim = trimOptionalLineSuffix(claim)
	if claim == "" {
		return "", errors.New("empty file path claim is not allowed")
	}
	lower := strings.ToLower(claim)
	if strings.HasPrefix(lower, "http://") || strings.HasPrefix(lower, "https://") {
		return "", fmt.Errorf("URL claim %q is not allowed", claim)
	}
	if strings.Contains(claim, "\\") || strings.ContainsAny(claim, "*?[]{}<>") || strings.Contains(claim, "...") ||
		containsPlaceholderPathToken(lower) {
		return "", fmt.Errorf("file path %q is not a contained literal file", claim)
	}
	if strings.HasSuffix(claim, "/") {
		return "", fmt.Errorf("directory claim %q is not allowed", claim)
	}
	candidate := filepath.FromSlash(claim)
	if !filepath.IsAbs(candidate) {
		candidate = filepath.Join(workspace, candidate)
	}
	candidate = filepath.Clean(candidate)
	resolved, err := canonicalizeCandidate(candidate)
	if err != nil {
		return "", fmt.Errorf("file path %q cannot be canonicalized: %w", claim, err)
	}
	relative, err := filepath.Rel(workspace, resolved)
	if err != nil || relative == "." || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
		if relative == "." {
			return "", errors.New("workspace root claim is not allowed")
		}
		return "", fmt.Errorf("file path %q is outside the workspace", claim)
	}
	if info, err := os.Stat(resolved); err == nil && info.IsDir() {
		return "", fmt.Errorf("directory claim %q is not allowed", claim)
	} else if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return "", fmt.Errorf("file path %q cannot be inspected: %w", claim, err)
	}
	return filepath.ToSlash(relative), nil
}

func trimOptionalLineSuffix(value string) string {
	last := strings.LastIndexByte(value, ':')
	if last < 0 || last == len(value)-1 || !allDigits(value[last+1:]) {
		return value
	}
	value = value[:last]
	last = strings.LastIndexByte(value, ':')
	if last >= 0 && last < len(value)-1 && allDigits(value[last+1:]) {
		return value[:last]
	}
	return value
}

func allDigits(value string) bool {
	return value != "" && strings.Trim(value, "0123456789") == ""
}

func containsPlaceholderPathToken(value string) bool {
	for _, token := range strings.FieldsFunc(value, func(r rune) bool {
		return r == '/' || r == '_' || r == '.' || r == '-'
	}) {
		if token == "tbd" || token == "todo" || token == "placeholder" {
			return true
		}
	}
	return false
}

func canonicalizeCandidate(candidate string) (string, error) {
	return canonicalizeCandidateSeen(filepath.Clean(candidate), make(map[string]struct{}), 0)
}

const maxCandidateSymlinkResolutions = 255

// canonicalizeCandidateSeen resolves existing path components one at a time so
// a dangling leaf symlink cannot be mistaken for a contained new file. A
// genuinely new suffix is retained after its nearest physical parent is
// resolved, which preserves normal PLAN entries for files that do not exist
// yet.
func canonicalizeCandidateSeen(candidate string, seen map[string]struct{}, depth int) (string, error) {
	if depth > maxCandidateSymlinkResolutions {
		return "", errors.New("too many symlink resolutions")
	}
	if resolved, err := filepath.EvalSymlinks(candidate); err == nil {
		return filepath.Clean(resolved), nil
	} else if !errors.Is(err, fs.ErrNotExist) {
		return "", err
	}

	info, err := os.Lstat(candidate)
	if err == nil && info.Mode()&os.ModeSymlink != 0 {
		if _, duplicate := seen[candidate]; duplicate {
			return "", errors.New("symlink loop")
		}
		seen[candidate] = struct{}{}
		target, err := os.Readlink(candidate)
		if err != nil {
			return "", err
		}
		if !filepath.IsAbs(target) {
			target = filepath.Join(filepath.Dir(candidate), target)
		}
		return canonicalizeCandidateSeen(filepath.Clean(target), seen, depth+1)
	}
	if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return "", err
	}

	parent, leaf := filepath.Dir(candidate), filepath.Base(candidate)
	// Walking ordinary missing path components does not resolve a symlink. Keep
	// the resolution budget for actual links so a valid, deeply nested new file
	// is not rejected merely because its parent directories do not exist yet.
	resolvedParent, err := canonicalizeCandidateSeen(parent, seen, depth)
	if err != nil {
		return "", err
	}
	return filepath.Join(resolvedParent, leaf), nil
}

func canonicalDirectory(path string) (string, error) {
	if strings.TrimSpace(path) == "" || !filepath.IsAbs(path) ||
		filepath.Clean(path) != path {
		return "", errors.New("path must be absolute and clean")
	}
	canonical, err := filepath.EvalSymlinks(path)
	if err != nil {
		return "", err
	}
	info, err := os.Stat(canonical)
	if err != nil {
		return "", err
	}
	if !info.IsDir() {
		return "", errors.New("path is not a directory")
	}
	return filepath.Clean(canonical), nil
}

func digestBytes(label string, data []byte) string {
	sum := sha256.Sum256(append(append([]byte(label), 0), data...))
	return "sha256:" + hex.EncodeToString(sum[:])
}

func digestStrings(parts ...string) string {
	hash := sha256.New()
	for _, part := range parts {
		_, _ = fmt.Fprintf(hash, "%d:", len(part))
		_, _ = hash.Write([]byte(part))
	}
	return "sha256:" + hex.EncodeToString(hash.Sum(nil))
}

func uniqueStrings(values []string) []string {
	seen := make(map[string]struct{}, len(values))
	out := make([]string, 0, len(values))
	for _, value := range values {
		if _, ok := seen[value]; ok {
			continue
		}
		seen[value] = struct{}{}
		out = append(out, value)
	}
	return out
}