package handler

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"strings"
	"time"

	"codeberg.org/VARASYS/ZDDC/zddc/internal/config"
	"codeberg.org/VARASYS/ZDDC/zddc/internal/policy"
	"codeberg.org/VARASYS/ZDDC/zddc/internal/zddc"
)

// File API — authenticated CRUD over the served tree.
//
//	PUT    /<path>             write or overwrite. Body = file bytes (capped
//	                           by cfg.MaxWriteBytes). Auto-creates parent
//	                           directories. Optional If-Match for optimistic
//	                           concurrency. Returns 201 Created (new) or 200
//	                           OK (overwrite) with ETag.
//	DELETE /<path>             remove a file. Optional If-Match. Refuses to
//	                           delete directories or hidden paths.
//	POST   /<path>             control verb dispatched by X-ZDDC-Op header:
//	                             move:  X-ZDDC-Destination is the new path.
//	                                    Atomic os.Rename. Optional If-Match.
//	                             mkdir: create directory at /<path>/. Idempotent.
//
// All operations route through the same ACL chain as GET — but with
// policy action="write" so external Rego can split read from write.
// The internal decider treats both identically.
//
// Path posture matches the rest of the dispatch:
//   - hidden segments (./_-prefixed) are 404'd
//   - the apps cache directory _app is 404'd
//   - traversal that escapes Root is 404'd
//
// Audit: every successful write logs a structured `file_write` event
// (op, path, email, status, bytes) at INFO. Failed writes log at WARN.
const (
	headerOp          = "X-ZDDC-Op"
	headerDestination = "X-ZDDC-Destination"

	opMove  = "move"
	opMkdir = "mkdir"
	// opSSRRename / opPlanReview / opAcceptTransmittal are declared
	// alongside their handler files. Listed in the dispatch switch
	// below so they're discoverable from a single place.
)

// IsWriteMethod reports whether this method is handled by the file API.
// Used by the dispatcher to gate writes through ServeFileAPI before the
// read-path tree of static / app / directory handling.
func IsWriteMethod(method string) bool {
	switch method {
	case http.MethodPut, http.MethodDelete, http.MethodPost:
		return true
	}
	return false
}

// ServeFileAPI is the entry point for write methods. The dispatcher
// has already verified the path doesn't contain reserved segments.
// Caller must have already enforced the dot-prefix / _app guards
// (these match dispatch's existing ones, but we re-check defensively).
func ServeFileAPI(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	switch r.Method {
	case http.MethodPut:
		serveFilePut(cfg, w, r)
	case http.MethodDelete:
		serveFileDelete(cfg, w, r)
	case http.MethodPost:
		serveFilePost(cfg, w, r)
	default:
		w.Header().Set("Allow", "GET, HEAD, PUT, DELETE, POST, OPTIONS")
		http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
	}
}

// resolveTargetPath validates urlPath, joins it onto cfg.Root, and
// rejects traversal/hidden segments. Returns absolute path + the
// cleaned URL path (with one leading "/").
func resolveTargetPath(cfg config.Config, urlPath string) (absPath, cleanURL string, ok bool, status int, msg string) {
	if urlPath == "" || urlPath == "/" {
		return "", "", false, http.StatusBadRequest, "empty path"
	}
	cleanURL = "/" + strings.Trim(urlPath, "/")
	if strings.HasSuffix(urlPath, "/") {
		cleanURL += "/"
	}

	// Reject hidden / reserved segments. Mirrors dispatch's guard,
	// applied here too because external callers reach ServeFileAPI
	// only via dispatch — but defense in depth costs nothing.
	// Carve-out: `.zddc` as a leaf segment is writable (admin-gated)
	// via the file API. Other dot/underscore segments stay reserved.
	segs := strings.Split(strings.Trim(cleanURL, "/"), "/")
	for i, seg := range segs {
		if seg == "" {
			continue
		}
		if seg == ZddcFileBasename && i == len(segs)-1 {
			continue
		}
		if seg == "_app" || strings.HasPrefix(seg, ".") || strings.HasPrefix(seg, "_") {
			return "", "", false, http.StatusNotFound, "reserved path segment"
		}
	}

	rel := filepath.FromSlash(strings.TrimPrefix(strings.TrimSuffix(cleanURL, "/"), "/"))
	abs := filepath.Join(cfg.Root, rel)
	if !strings.HasPrefix(abs, cfg.Root+string(filepath.Separator)) && abs != cfg.Root {
		return "", "", false, http.StatusNotFound, "path traversal"
	}
	return abs, cleanURL, true, 0, ""
}

// authorizeAction runs the ACL chain for a verb-tagged write to absPath.
// The chain is computed from the closest existing ancestor (so writes
// that create a brand-new file inherit the parent directory's chain).
// Returns allowed=false with the response status already written on deny.
//
// All admin / WORM / ACL logic lives downstream in the decider's single
// bypass site (policy.InternalDecider.Allow). AllowActionFromChainP
// computes IsActiveAdmin from the chain and Principal.Elevated, with
// the strict-ancestor rule applied when action == ActionAdmin (the
// caller tags .zddc writes that way). The handler does NOT make
// admin/elevation decisions of its own — one bypass site, one helper.
func authorizeAction(cfg config.Config, w http.ResponseWriter, r *http.Request, absPath, urlPath, action string) bool {
	probe := filepath.Dir(absPath)
	for {
		info, err := os.Stat(probe)
		if err == nil && info.IsDir() {
			break
		}
		if probe == cfg.Root || !strings.HasPrefix(probe, cfg.Root+string(filepath.Separator)) {
			probe = cfg.Root
			break
		}
		probe = filepath.Dir(probe)
	}

	p := PrincipalFromContext(r)
	chain, err := zddc.EffectivePolicy(cfg.Root, probe)
	if err != nil {
		slog.Warn("file API ACL chain error", "path", absPath, "err", err)
	}

	decider := DeciderFromContext(r)
	allowed, _ := policy.AllowActionFromChainP(r.Context(), decider, chain, p, urlPath, action)
	if !allowed {
		http.Error(w, "Forbidden", http.StatusForbidden)
		return false
	}
	return true
}

// readBodyCapped consumes r.Body up to cfg.MaxWriteBytes. Returns 413
// on overflow. The body is read fully into memory — small / medium files
// are the dominant traffic and atomic write needs the whole payload before
// the rename. Streaming PUTs (chunked uploads, multi-part resumable)
// are out of scope for this iteration.
func readBodyCapped(cfg config.Config, w http.ResponseWriter, r *http.Request) ([]byte, bool) {
	limit := cfg.MaxWriteBytes
	if limit <= 0 {
		limit = 256 * 1024 * 1024
	}
	// http.MaxBytesReader writes a 413 itself when the limit is hit
	// during read, but its error message is not always recognizable —
	// we wrap it to surface a clean status code from the wrapped error.
	r.Body = http.MaxBytesReader(w, r.Body, limit)
	body, err := io.ReadAll(r.Body)
	if err != nil {
		var maxErr *http.MaxBytesError
		if errors.As(err, &maxErr) {
			http.Error(w, "Request Entity Too Large", http.StatusRequestEntityTooLarge)
			return nil, false
		}
		http.Error(w, "Bad Request — could not read body: "+err.Error(), http.StatusBadRequest)
		return nil, false
	}
	return body, true
}

// fileETag returns the SHA-256 first-32-hex of bytes — the same scheme
// the static file serve handler uses, so PUT response ETags match what
// a subsequent GET would compute.
func fileETag(body []byte) string {
	sum := sha256.Sum256(body)
	return hex.EncodeToString(sum[:])[:32]
}

// fileETagOnDisk returns the ETag of the file at absPath (or "" if it
// doesn't exist). Used to evaluate If-Match on PUT/DELETE/MOVE.
func fileETagOnDisk(absPath string) (string, error) {
	body, err := os.ReadFile(absPath)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return "", nil
		}
		return "", err
	}
	return fileETag(body), nil
}

// checkIfMatch returns true if the request's If-Match header (when
// present) matches the current ETag for absPath. Empty header always
// passes. Wildcard ("*") passes iff the file exists. On precondition
// failure the response is written as 412 and false is returned.
//
// Special case for PUT: when allowMissing is true and the file doesn't
// exist, the wildcard "*" form fails (per RFC) but a specific ETag is
// treated as a no-current-file hit (412). This distinguishes
// create-new from update-existing semantically.
//
// Also honors If-Unmodified-Since (RFC 7232 §3.4): the request fails
// with 412 if the current file's mtime is strictly later than the
// header value. Used by the cache layer's offline-write outbox to
// detect concurrent modifications without ETag round-trips — the
// cached file's mtime (set from upstream's Last-Modified) becomes the
// base for the precondition. Either header (or both) can be present;
// both must pass.
func checkIfMatch(w http.ResponseWriter, r *http.Request, absPath string) bool {
	if !checkIfUnmodifiedSince(w, r, absPath) {
		return false
	}
	header := strings.TrimSpace(r.Header.Get("If-Match"))
	if header == "" {
		return true
	}
	current, err := fileETagOnDisk(absPath)
	if err != nil {
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return false
	}
	if header == "*" {
		if current == "" {
			http.Error(w, "Precondition Failed — target does not exist", http.StatusPreconditionFailed)
			return false
		}
		return true
	}
	want := strings.Trim(header, `"`)
	if want != current {
		http.Error(w, "Precondition Failed — ETag mismatch", http.StatusPreconditionFailed)
		return false
	}
	return true
}

// checkIfUnmodifiedSince evaluates the RFC 7232 §3.4 precondition.
// Returns true (pass) when the header is absent or unparseable, when
// the target file does not exist, or when the file's current mtime
// is at or before the header value. Returns false (fail) and writes a
// 412 response when the file has been modified after the header time.
//
// mtime comparison uses the file's mod time truncated to whole
// seconds — HTTP-Date format has 1-second resolution, so a finer
// comparison would spuriously fail on filesystems that retain ns
// precision. "After" therefore means strictly greater than the
// header value at second resolution.
func checkIfUnmodifiedSince(w http.ResponseWriter, r *http.Request, absPath string) bool {
	header := strings.TrimSpace(r.Header.Get("If-Unmodified-Since"))
	if header == "" {
		return true
	}
	since, err := http.ParseTime(header)
	if err != nil {
		// Per RFC 7232: if the header value is unparseable, ignore.
		return true
	}
	info, err := os.Stat(absPath)
	if err != nil {
		// Missing file → no resource to compare against. Pass.
		return true
	}
	current := info.ModTime().Truncate(time.Second)
	if current.After(since.Truncate(time.Second)) {
		http.Error(w, "Precondition Failed — If-Unmodified-Since: file modified at "+current.UTC().Format(http.TimeFormat), http.StatusPreconditionFailed)
		return false
	}
	return true
}

func serveFilePut(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	abs, cleanURL, ok, status, msg := resolveTargetPath(cfg, r.URL.Path)
	if !ok {
		http.Error(w, msg, status)
		return
	}
	if strings.HasSuffix(cleanURL, "/") {
		http.Error(w, "PUT must target a file, not a directory", http.StatusBadRequest)
		return
	}

	// Virtual project-level table views — SSR / MDL rollup / RSK
	// rollup. The PUT URL lives in <project>/{ssr,mdl,rsk}/...; the
	// underlying bytes belong inside <project>/archive/<party>/. We
	// rewrite abs + cleanURL to the canonical path so the rest of
	// this function (ACL gate, ETag, audit, conversion-cache purge)
	// operates on the real file location.
	//
	// SSR row PUTs land at archive/<party>/ssr.yaml; MDL/RSK rollup
	// row PUTs land at archive/<party>/<slot>/<file>.yaml. Same
	// shape as the virtual-received rewrite below.
	if vv := zddc.ResolveVirtualView(cfg.Root, cleanURL); vv.Resolved && vv.Kind.IsRowKind() {
		abs = vv.CanonicalAbs
		cleanURL = vv.CanonicalURL
		w.Header().Set("X-ZDDC-Resolved-Path", cleanURL)
	}

	// Virtual received/ rewrite. When the PUT targets a file under the
	// synthetic <workflow>/received/<file> URL, the canonical record is
	// WORM — we can't write there. Convention: treat the drop as a
	// review comment, write it into the workflow folder as
	// <base>+C<n><suffix> where n increments past any existing comments
	// on the same target. The target filename comes from the URL's
	// final segment.
	if vr := zddc.ResolveVirtualReceived(cfg.Root, cleanURL); vr.Resolved && !vr.IsRoot {
		targetName := filepath.Base(vr.SuffixURL)
		commentName, cerr := zddc.CommentResolvedName(vr.WorkflowAbs, targetName)
		if cerr != nil {
			http.Error(w, "Bad Request — comment upload requires a ZDDC-parseable target filename: "+cerr.Error(), http.StatusBadRequest)
			return
		}
		// Race-fix: if the computed filename already exists (concurrent
		// upload), step the counter forward until we find a free slot.
		abs = filepath.Join(vr.WorkflowAbs, commentName)
		for i := 0; i < 32; i++ {
			if _, err := os.Stat(abs); errors.Is(err, os.ErrNotExist) {
				break
			} else if err != nil {
				http.Error(w, "Internal Server Error", http.StatusInternalServerError)
				return
			}
			// Bump: recompute with one more existing sibling.
			commentName, cerr = zddc.CommentResolvedName(vr.WorkflowAbs, targetName)
			if cerr != nil {
				http.Error(w, "Internal Server Error — comment counter: "+cerr.Error(), http.StatusInternalServerError)
				return
			}
			abs = filepath.Join(vr.WorkflowAbs, commentName)
		}
		// Rewrite cleanURL so audit logs + response headers reflect
		// the actual destination, not the virtual one. Surface to the
		// client via X-ZDDC-Resolved-Path so the status line can show
		// "Saved as <resolved name>".
		cleanURL = vr.WorkflowURL + commentName
		w.Header().Set("X-ZDDC-Resolved-Path", cleanURL)
		// Continue with normal write flow — ACL on the workflow folder
		// gates the write, and existed=false (new file) selects
		// ActionCreate.
	}

	// Resolve canonical-folder casing on the way in (no side effects): a
	// request for /Project/working/foo.md when the on-disk folder is
	// Working/ should land in Working/, not create a duplicate sibling.
	// The actual MkdirAll for missing canonical ancestors and the
	// auto-own .zddc seeding happen after authorisation, below.
	if r2, err := zddc.ResolveCanonicalPath(cfg.Root, abs); err == nil {
		abs = r2
	}

	// Stat first so we can choose action=create vs action=write before the
	// ACL gate runs — this matters because role grants may include `c` but
	// not `w` (or vice versa), and the gate must check the right verb.
	existed := false
	if info, err := os.Stat(abs); err == nil {
		if info.IsDir() {
			http.Error(w, "Conflict — a directory exists at this path", http.StatusConflict)
			return
		}
		existed = true
	}

	action := policy.ActionCreate
	if existed {
		action = policy.ActionWrite
	}
	// .zddc writes always require `a` (admin) regardless of create/overwrite.
	if filepath.Base(abs) == ".zddc" {
		action = policy.ActionAdmin
	}

	if !authorizeAction(cfg, w, r, abs, cleanURL, action) {
		return
	}
	if !checkIfMatch(w, r, abs) {
		return
	}

	body, ok := readBodyCapped(cfg, w, r)
	if !ok {
		return
	}

	// Now that the write is authorized, materialise any missing canonical
	// ancestors and seed auto-own .zddc files for them.
	if email := EmailFromContext(r); email != "" {
		if _, err := zddc.EnsureCanonicalAncestors(cfg.Root, abs, email, 0o755); err != nil {
			slog.Warn("ensure canonical ancestors", "path", abs, "err", err)
		}
	}

	if err := zddc.WriteAtomic(abs, body); err != nil {
		auditFile(r, "put", cleanURL, http.StatusInternalServerError, len(body), err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	// Invalidate ETag cache (static.go memoizes by mtime; rename produces
	// a fresh mtime so a stale entry is harmless, but clearing is cheap).
	etagCacheM.Delete(abs)
	// Invalidate any cached MD→{docx,html,pdf} conversions sitting in
	// the sibling .converted/ dir for this source.
	purgeConverted(abs)

	etag := fileETag(body)
	w.Header().Set("ETag", `"`+etag+`"`)
	w.Header().Set("X-ZDDC-Source", "fileapi:put")
	respStatus := http.StatusCreated
	if existed {
		respStatus = http.StatusOK
	}
	w.WriteHeader(respStatus)
	auditFile(r, "put", cleanURL, respStatus, len(body), nil)
}

func serveFileDelete(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	abs, cleanURL, ok, status, msg := resolveTargetPath(cfg, r.URL.Path)
	if !ok {
		http.Error(w, msg, status)
		return
	}
	if strings.HasSuffix(cleanURL, "/") {
		http.Error(w, "DELETE must target a file, not a directory", http.StatusBadRequest)
		return
	}

	// Virtual project-level table views. SSR row deletes are refused
	// (would orphan the party folder and its mdl/rsk contents) — use
	// the archive view to delete a party. MDL/RSK rollup row deletes
	// pass through to the canonical archive/<party>/<slot>/<file>.yaml
	// path with the normal ACL gate.
	if vv := zddc.ResolveVirtualView(cfg.Root, cleanURL); vv.Resolved && vv.Kind.IsRowKind() {
		if vv.Kind == zddc.VirtualViewSSRRow {
			http.Error(w, "Method Not Allowed — delete the party folder via the archive view, not the SSR table", http.StatusMethodNotAllowed)
			return
		}
		abs = vv.CanonicalAbs
		cleanURL = vv.CanonicalURL
		w.Header().Set("X-ZDDC-Resolved-Path", cleanURL)
	}

	info, err := os.Stat(abs)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			http.Error(w, "Not Found", http.StatusNotFound)
			return
		}
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	if info.IsDir() {
		http.Error(w, "Conflict — DELETE of directories is not supported", http.StatusConflict)
		return
	}

	action := policy.ActionDelete
	if filepath.Base(abs) == ".zddc" {
		action = policy.ActionAdmin
	}
	if !authorizeAction(cfg, w, r, abs, cleanURL, action) {
		return
	}
	if !checkIfMatch(w, r, abs) {
		return
	}

	if err := os.Remove(abs); err != nil {
		auditFile(r, "delete", cleanURL, http.StatusInternalServerError, 0, err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	etagCacheM.Delete(abs)
	purgeConverted(abs)

	w.Header().Set("X-ZDDC-Source", "fileapi:delete")
	w.WriteHeader(http.StatusNoContent)
	auditFile(r, "delete", cleanURL, http.StatusNoContent, 0, nil)
}

func serveFilePost(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	op := strings.ToLower(strings.TrimSpace(r.Header.Get(headerOp)))
	switch op {
	case opMove:
		serveFileMove(cfg, w, r)
	case opMkdir:
		serveFileMkdir(cfg, w, r)
	case opPlanReview:
		servePlanReview(cfg, w, r)
	case opAcceptTransmittal:
		serveAcceptTransmittal(cfg, w, r)
	case opSSRRename:
		serveSSRRename(cfg, w, r)
	case "":
		http.Error(w, "Bad Request — missing "+headerOp+" header", http.StatusBadRequest)
	default:
		http.Error(w, "Bad Request — unknown "+headerOp+" value: "+op, http.StatusBadRequest)
	}
}

func serveFileMove(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	srcAbs, srcURL, ok, status, msg := resolveTargetPath(cfg, r.URL.Path)
	if !ok {
		http.Error(w, msg, status)
		return
	}
	if strings.HasSuffix(srcURL, "/") {
		http.Error(w, "MOVE source must be a file path", http.StatusBadRequest)
		return
	}

	dstHeader := r.Header.Get(headerDestination)
	if dstHeader == "" {
		http.Error(w, "Bad Request — missing "+headerDestination+" header", http.StatusBadRequest)
		return
	}
	// Destination is sent as a URL path; decode percent-encoding.
	if dec, err := url.PathUnescape(dstHeader); err == nil {
		dstHeader = dec
	}
	dstAbs, dstURL, ok, status, msg := resolveTargetPath(cfg, dstHeader)
	if !ok {
		http.Error(w, "destination: "+msg, status)
		return
	}
	if strings.HasSuffix(dstURL, "/") {
		http.Error(w, "MOVE destination must be a file path", http.StatusBadRequest)
		return
	}

	// Resolve canonical-folder casing on src + dst (no side effects).
	if r2, err := zddc.ResolveCanonicalPath(cfg.Root, srcAbs); err == nil {
		srcAbs = r2
	}
	if r2, err := zddc.ResolveCanonicalPath(cfg.Root, dstAbs); err == nil {
		dstAbs = r2
	}

	// Source must exist as a regular file.
	srcInfo, err := os.Stat(srcAbs)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			http.Error(w, "Not Found", http.StatusNotFound)
		} else {
			http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		}
		return
	}
	if srcInfo.IsDir() {
		http.Error(w, "Conflict — MOVE of directories is not supported", http.StatusConflict)
		return
	}

	// Destination must not exist (no implicit overwrite). If-Match on the
	// SOURCE is still respected for concurrency on the source bytes.
	if _, err := os.Stat(dstAbs); err == nil {
		http.Error(w, "Conflict — destination already exists", http.StatusConflict)
		return
	} else if !errors.Is(err, os.ErrNotExist) {
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}

	// ACL: source side requires `w` (rename mutates the source); dest
	// side requires `c` (creates a new path). Cross-folder moves run
	// both gates against potentially different chains.
	if !authorizeAction(cfg, w, r, srcAbs, srcURL, policy.ActionWrite) {
		return
	}
	if !authorizeAction(cfg, w, r, dstAbs, dstURL, policy.ActionCreate) {
		return
	}
	if !checkIfMatch(w, r, srcAbs) {
		return
	}

	// Ensure destination's canonical ancestors are created (with auto-own
	// .zddc seeding) before the rename. This lets a MOVE from working/foo
	// → archive/<party>/issued/foo materialise the per-party folders on
	// the way in.
	if email := EmailFromContext(r); email != "" {
		if _, err := zddc.EnsureCanonicalAncestors(cfg.Root, dstAbs, email, 0o755); err != nil {
			slog.Warn("ensure canonical ancestors (move dst)", "path", dstAbs, "err", err)
		}
	}
	if err := os.MkdirAll(filepath.Dir(dstAbs), 0o755); err != nil {
		auditFile(r, "move", srcURL, http.StatusInternalServerError, 0, err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}

	if err := os.Rename(srcAbs, dstAbs); err != nil {
		// Cross-device or permission errors: report 500 — the client
		// will retry or surface the failure to the user.
		auditFile(r, "move", srcURL, http.StatusInternalServerError, 0, err)
		http.Error(w, "Internal Server Error — rename failed: "+err.Error(), http.StatusInternalServerError)
		return
	}
	etagCacheM.Delete(srcAbs)
	etagCacheM.Delete(dstAbs)
	purgeConverted(srcAbs)
	purgeConverted(dstAbs)

	// Compute new ETag from the moved bytes for the response — clients
	// that want to keep tracking should pin to this ETag.
	if etag, err := fileETagOnDisk(dstAbs); err == nil && etag != "" {
		w.Header().Set("ETag", `"`+etag+`"`)
	}
	w.Header().Set("X-ZDDC-Source", "fileapi:move")
	w.Header().Set("X-ZDDC-Destination", dstURL)
	w.WriteHeader(http.StatusOK)
	auditFile(r, "move", srcURL+" -> "+dstURL, http.StatusOK, 0, nil)
}

func serveFileMkdir(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	abs, cleanURL, ok, status, msg := resolveTargetPath(cfg, r.URL.Path)
	if !ok {
		http.Error(w, msg, status)
		return
	}

	// Resolve canonical-folder casing on the way in (no side effects).
	if r2, err := zddc.ResolveCanonicalPath(cfg.Root, abs); err == nil {
		abs = r2
	}

	if !authorizeAction(cfg, w, r, abs, cleanURL, policy.ActionCreate) {
		return
	}

	// Idempotent: if the dir already exists, treat it as success;
	// if a file is at the path, conflict.
	if info, err := os.Stat(abs); err == nil {
		if info.IsDir() {
			w.Header().Set("X-ZDDC-Source", "fileapi:mkdir")
			w.WriteHeader(http.StatusOK)
			auditFile(r, "mkdir", cleanURL, http.StatusOK, 0, nil)
			return
		}
		http.Error(w, "Conflict — a file exists at this path", http.StatusConflict)
		return
	}

	// Materialise any missing canonical ancestors (working/, staging/,
	// archive/<party>/incoming/) before creating the target itself. This
	// also seeds auto-own .zddc on each newly-created canonical ancestor.
	email := EmailFromContext(r)
	if email != "" {
		if _, err := zddc.EnsureCanonicalAncestors(cfg.Root, abs, email, 0o755); err != nil {
			slog.Warn("ensure canonical ancestors", "path", abs, "err", err)
		}
	}

	if err := os.MkdirAll(abs, 0o755); err != nil {
		auditFile(r, "mkdir", cleanURL, http.StatusInternalServerError, 0, err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}

	// Auto-ownership for the newly-created directory. The .zddc
	// cascade's `auto_own:` flag (see defaults.zddc.yaml) drives this,
	// same as EnsureCanonicalAncestors. A creator-owned .zddc lands
	// inside abs when:
	//   - abs itself is declared auto_own (e.g. an explicit mkdir of
	//     /Project/working), or
	//   - abs's parent is declared auto_own — every child mkdir under
	//     an auto-own folder (working/, staging/, archive/<party>/,
	//     archive/<party>/incoming/, …) gets the creator's grant.
	// The fence (inherit:false) follows abs's own cascade level:
	// per-user homes under working/ declare auto_own_fenced, so the
	// generated .zddc is private; other auto-own positions are
	// unfenced so ancestor grants still cascade through.
	if email != "" {
		if zddc.AutoOwnAt(cfg.Root, abs) || zddc.AutoOwnAt(cfg.Root, filepath.Dir(abs)) {
			var werr error
			if zddc.AutoOwnFencedAt(cfg.Root, abs) {
				werr = zddc.WriteAutoOwnZddcFenced(abs, email)
			} else {
				werr = zddc.WriteAutoOwnZddc(abs, email)
			}
			if werr != nil {
				slog.Warn("auto-own .zddc write failed", "path", abs, "err", werr)
			}
		}
	}

	// Staging↔working mirror: when a folder created under staging/ matches
	// the ZDDC transmittal-folder grammar AND its tracking number contains
	// -SUB- or -TRN-, also create the same-named folder under working/ as
	// a drafting space for staff. The mirror is one-way and one-shot —
	// renames or deletions of either side are not propagated.
	if email != "" {
		mirrorStagingToWorking(cfg, abs, email)
	}

	w.Header().Set("X-ZDDC-Source", "fileapi:mkdir")
	w.WriteHeader(http.StatusCreated)
	auditFile(r, "mkdir", cleanURL, http.StatusCreated, 0, nil)
}

// mirrorStagingToWorking creates a paired drafting folder under working/
// when newAbs is a transmittal-named folder under <project>/staging/. Best
// effort — failures are logged but do not affect the staging mkdir result.
//
// Eligibility:
//   - newAbs's parent is exactly <project>/staging/ (case-fold)
//   - filepath.Base(newAbs) parses as a transmittal folder
//     (YYYY-MM-DD_<tracking> (<status>) - <title>)
//   - tracking contains -SUB- or -TRN- (case-fold)
//
// Side effects on success:
//   - <project>/working/ created if missing, with auto-own .zddc seeded
//     (via EnsureCanonicalAncestors)
//   - <project>/working/<sameName>/ created if missing, with its own
//     auto-own .zddc (it's a child of the working/ canonical folder)
func mirrorStagingToWorking(cfg config.Config, newAbs, email string) {
	rel, err := filepath.Rel(cfg.Root, newAbs)
	if err != nil {
		return
	}
	rel = filepath.ToSlash(rel)
	parts := strings.Split(rel, "/")
	if len(parts) != 3 {
		// Mirror only fires for direct children of staging/. Deeper paths
		// (staging/<name>/sub/) are user-managed.
		return
	}
	if !strings.EqualFold(parts[1], "staging") {
		return
	}

	name := parts[2]
	_, tracking, _, _, ok := zddc.ParseTransmittalFolder(name)
	if !ok || !zddc.IsTrnOrSubTracking(tracking) {
		return
	}

	mirrorPath := filepath.Join(cfg.Root, parts[0], "working", name)
	// Idempotent: skip if the working sibling already exists.
	if info, err := os.Stat(mirrorPath); err == nil && info.IsDir() {
		return
	}

	// EnsureCanonicalAncestors creates working/ (with its own auto-own .zddc)
	// if missing; we then MkdirAll the mirror folder itself and seed its
	// auto-own grant.
	if _, err := zddc.EnsureCanonicalAncestors(cfg.Root, mirrorPath, email, 0o755); err != nil {
		slog.Warn("mirror: ensure ancestors", "path", mirrorPath, "err", err)
		return
	}
	if err := os.MkdirAll(mirrorPath, 0o755); err != nil {
		slog.Warn("mirror: mkdir", "path", mirrorPath, "err", err)
		return
	}
	if err := zddc.WriteAutoOwnZddc(mirrorPath, email); err != nil {
		slog.Warn("mirror: auto-own .zddc", "path", mirrorPath, "err", err)
	}
}

// auditFile emits a structured log line for each file API operation.
// AccessLogMiddleware already logs every request — this adds an
// op-tagged line so audit consumers can filter by operation without
// pattern-matching on method + path.
func auditFile(r *http.Request, op, path string, status any, bytes int, err error) {
	email := EmailFromContext(r)
	if email == "" {
		email = "anonymous"
	}
	args := []any{
		"op", op,
		"path", path,
		"email", email,
		"status", fmt.Sprint(status),
		"bytes", bytes,
	}
	if err != nil {
		args = append(args, "err", err.Error())
		slog.Warn("file_write", args...)
		return
	}
	slog.Info("file_write", args...)
}