package httpapi

import (
	"context"
	"encoding/json"
	"errors"
	"log/slog"
	"net/http"
	"net/url"
	"regexp"
	"strings"

	"github.com/go-chi/chi/v5"
	"github.com/google/uuid"
	"github.com/jackc/pgx/v5"
)

// Operator-facing write path for the knowledge base. Until this file, the
// only way anything reached knowledge_entities was the MCP tool
// upsert_knowledge (internal/mcp/server.go) — an agent-only surface. The web
// UI could search and read but never create, correct, or remove a note, so
// the operator's own knowledge had nowhere to go and an agent mistake had no
// fix short of psql.
//
// All routes here are non-OpenAPI custom routes, consistent with the existing
// knowledge read routes (see the carve-out block in server.go): they trade in
// raw markdown and ad-hoc aggregates rather than generated schema types.
//
// Deletion is soft (deleted_at) — see migrations/022_knowledge_revisions.up.sql
// for why — so every read path in this file filters on `ke.deleted_at IS NULL`.

// knowledgeSlugSegmentRe strips a title down to a single slug segment.
// Mirrors knowledgeSlugRe in internal/mcp/server.go; duplicated rather than
// exported across the package boundary because the two callers namespace
// their output differently (see knowledgeSlugFor).
var knowledgeSlugSegmentRe = regexp.MustCompile(`[^a-z0-9]+`)

// knowledgeSlugFor builds `<kind>:<folder>/<title-slug>`. The MCP tool's
// equivalent hardcodes the `nomos/` folder; operator-created notes need to
// land somewhere else so the navigator tree can tell at a glance who wrote
// what, and so an operator note can never collide with an agent note that
// happens to share a title.
func knowledgeSlugFor(kind, folder, title string) string {
	s := strings.ToLower(strings.TrimSpace(title))
	s = knowledgeSlugSegmentRe.ReplaceAllString(s, "-")
	s = strings.Trim(s, "-")
	if s == "" {
		s = "note"
	}
	if len(s) > 80 {
		s = s[:80]
	}
	folder = strings.Trim(strings.ToLower(strings.TrimSpace(folder)), "/")
	folder = knowledgeSlugSegmentRe.ReplaceAllString(folder, "-")
	folder = strings.Trim(folder, "-")
	if folder == "" {
		folder = "operator"
	}
	return kind + ":" + folder + "/" + s
}

// validKnowledgeKind mirrors the three entity types that knowledge_entities
// rows are allowed to hang off (see upsert_knowledge's own check).
func validKnowledgeKind(kind string) bool {
	switch kind {
	case "document", "investigation", "runbook":
		return true
	}
	return false
}

// resolveKnowledgeEntity maps an id-or-slug path segment to the entity id of
// a live (non-deleted) knowledge note. Returns pgx.ErrNoRows when there's no
// such note, which callers turn into a 404.
func (s *Server) resolveKnowledgeEntity(ctx context.Context, idOrSlug string) (uuid.UUID, error) {
	var id uuid.UUID
	err := s.pool.QueryRow(ctx, `
		SELECT ke.entity_id
		FROM knowledge_entities ke
		JOIN entities e ON e.id = ke.entity_id
		WHERE (e.slug = $1 OR e.id::text = $1)
		  AND ke.deleted_at IS NULL`, idOrSlug).Scan(&id)
	return id, err
}

// resolveKnowledgeEntityAny is resolveKnowledgeEntity without the
// deleted_at filter — for the one read path (revisions) that must still work
// on a deleted note. The whole point of soft-delete is that a note's history
// stays inspectable after removal (e.g. to confirm what was lost before
// restoring it); requiring the note to be live first would defeat that.
func (s *Server) resolveKnowledgeEntityAny(ctx context.Context, idOrSlug string) (uuid.UUID, error) {
	var id uuid.UUID
	err := s.pool.QueryRow(ctx, `
		SELECT ke.entity_id
		FROM knowledge_entities ke
		JOIN entities e ON e.id = ke.entity_id
		WHERE e.slug = $1 OR e.id::text = $1`, idOrSlug).Scan(&id)
	return id, err
}

// pathParam pulls a chi URL param and percent-decodes it. Knowledge slugs
// contain both ':' and '/' (e.g. "document:containers/101-jellyfin"), so they
// reach the handler still encoded — chi.URLParam does no decoding of its own
// on manually-registered routes (unlike the OpenAPI-generated ones, which
// decode via runtime.BindStyledParameterWithOptions).
func pathParam(req *http.Request, name string) (string, error) {
	return url.PathUnescape(chi.URLParam(req, name))
}

// serveKnowledgeList returns every live note without its body — the backing
// data for the wiki navigator tree. Distinct from /knowledge/recent, which
// caps at 200 and exists to answer "what changed lately" for the stats view:
// the tree needs the complete set, and needs the linked-entity slugs so it
// can offer a group-by-entity arrangement without N+1 fetches.
//
// Body text is deliberately excluded — with ~100 notes averaging ~1 KB the
// full payload would be ~100 KB per app open, to render a list that shows
// only titles.
func (s *Server) serveKnowledgeList(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	type item struct {
		ID        string   `json:"id"`
		Slug      string   `json:"slug"`
		Title     string   `json:"title"`
		Kind      string   `json:"kind"`
		Source    string   `json:"source"`
		EditedBy  string   `json:"edited_by"`
		Tags      []string `json:"tags"`
		About     []string `json:"about"`
		Size      int      `json:"size"`
		UpdatedAt string   `json:"updated_at"`
		CreatedAt string   `json:"created_at"`
		Revisions int      `json:"revisions"`
	}

	// The `about` aggregate mirrors GetEntityKnowledge's first UNION branch
	// (documents/about edges) — the 'procedure-for' branch is left out here
	// because it joins against entity *types* rather than entities and can't
	// produce a per-note slug list.
	rows, err := s.pool.Query(ctx, `
		SELECT e.id::text, e.slug, ke.title, e.type, COALESCE(ke.source,''),
		       COALESCE(ke.edited_by,''), COALESCE(ke.tags, '{}'),
		       COALESCE((
		         SELECT array_agg(DISTINCT t.slug)
		         FROM relationships r
		         JOIN entities t ON t.id = r.target_id
		         WHERE r.source_id = ke.entity_id
		           AND r.valid_to IS NULL
		           AND r.type IN ('documents', 'about')
		       ), '{}'),
		       length(ke.content),
		       ke.updated_at::text, ke.created_at::text,
		       (SELECT count(*) FROM knowledge_revisions kr WHERE kr.entity_id = ke.entity_id)
		FROM knowledge_entities ke
		JOIN entities e ON e.id = ke.entity_id
		WHERE ke.deleted_at IS NULL
		ORDER BY ke.updated_at DESC`)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "query failed", err.Error())
		return
	}
	defer rows.Close()

	items := []item{}
	for rows.Next() {
		var it item
		if err := rows.Scan(&it.ID, &it.Slug, &it.Title, &it.Kind, &it.Source,
			&it.EditedBy, &it.Tags, &it.About, &it.Size,
			&it.UpdatedAt, &it.CreatedAt, &it.Revisions); err != nil {
			slog.Error("httpapi: knowledge/list row scan failed", "error", err)
			continue
		}
		items = append(items, it)
	}

	writeJSON(w, map[string]any{"items": items})
}

// serveKnowledgeTrash lists soft-deleted notes — the counterpart to
// serveKnowledgeList, and what the "restore" affordance in the UI browses.
// Without this, a deleted note is invisible from every list endpoint
// (correctly — they all filter deleted_at) with no way to even discover it
// exists to restore.
func (s *Server) serveKnowledgeTrash(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	rows, err := s.pool.Query(ctx, `
		SELECT e.slug, ke.title, e.type, COALESCE(ke.edited_by,''), ke.deleted_at::text
		FROM knowledge_entities ke
		JOIN entities e ON e.id = ke.entity_id
		WHERE ke.deleted_at IS NOT NULL
		ORDER BY ke.deleted_at DESC`)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "query failed", err.Error())
		return
	}
	defer rows.Close()

	type item struct {
		Slug      string `json:"slug"`
		Title     string `json:"title"`
		Kind      string `json:"kind"`
		DeletedBy string `json:"deleted_by"`
		DeletedAt string `json:"deleted_at"`
	}
	items := []item{}
	for rows.Next() {
		var it item
		if err := rows.Scan(&it.Slug, &it.Title, &it.Kind, &it.DeletedBy, &it.DeletedAt); err != nil {
			slog.Error("httpapi: knowledge/trash row scan failed", "error", err)
			continue
		}
		items = append(items, it)
	}

	writeJSON(w, map[string]any{"items": items})
}

// knowledgeWriteBody is the shared request shape for create and update.
// Every field is a pointer so update can distinguish "not supplied" (leave
// alone) from "supplied empty" (clear it) — a PUT that only changes tags
// must not blank the body.
type knowledgeWriteBody struct {
	Title   *string   `json:"title"`
	Content *string   `json:"content"`
	Kind    *string   `json:"kind"`
	Tags    *[]string `json:"tags"`
	Folder  *string   `json:"folder"`
	About   *[]string `json:"about"`
}

// serveCreateKnowledge creates a note plus its backing entity, and links it
// to whatever entities it's about.
func (s *Server) serveCreateKnowledge(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	var body knowledgeWriteBody
	if err := json.NewDecoder(req.Body).Decode(&body); err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid request body", err.Error())
		return
	}

	title := strings.TrimSpace(deref(body.Title))
	content := strings.TrimSpace(deref(body.Content))
	if title == "" || content == "" {
		writeProblem(w, req, http.StatusBadRequest, "title and content are required", "")
		return
	}
	kind := deref(body.Kind)
	if kind == "" {
		kind = "document"
	}
	if !validKnowledgeKind(kind) {
		writeProblem(w, req, http.StatusBadRequest, "invalid kind",
			"kind must be document, investigation, or runbook")
		return
	}
	tags := normalizeTags(derefSlice(body.Tags))
	slug := knowledgeSlugFor(kind, deref(body.Folder), title)
	_, actorLabel := actorInfo(ctx)

	tx, err := s.pool.Begin(ctx)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "begin failed", err.Error())
		return
	}
	defer tx.Rollback(ctx)

	docID, _ := uuid.NewV7()
	// ON CONFLICT covers the soft-deleted case: the entity row survives a
	// delete, so recreating a note under the same slug must reuse it rather
	// than fail the unique constraint.
	if err := tx.QueryRow(ctx, `
		INSERT INTO entities (id, slug, type, name, attributes)
		VALUES ($1, $2, $3, $4, '{}')
		ON CONFLICT (slug) DO UPDATE SET name = EXCLUDED.name, updated_at = now()
		RETURNING id`, docID, slug, kind, title).Scan(&docID); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "create entity failed", err.Error())
		return
	}

	// Refuse to silently overwrite an existing LIVE note — upsert_knowledge
	// (the MCP tool) deliberately upserts by title (the agent re-records the
	// same finding as it learns more), but an operator hitting "create" with
	// a colliding title almost certainly means to write something new.
	//
	// The `WHERE knowledge_entities.deleted_at IS NOT NULL` guard makes this
	// check atomic with the write, rather than a separate SELECT before it:
	// a plain pre-check has a TOCTOU race where two concurrent creates of
	// the same title can both pass the check and then both proceed to
	// INSERT ON CONFLICT DO UPDATE, silently clobbering each other. Here,
	// the UPDATE branch only actually applies when the conflicting row is
	// soft-deleted (a legitimate "resurrect" case). When it isn't, the row
	// is left untouched, RETURNING yields no row, and pgx.ErrNoRows below
	// becomes the 409 — the collision can never be missed, no matter how
	// the two writers interleave.
	var wroteID uuid.UUID
	err = tx.QueryRow(ctx, `
		INSERT INTO knowledge_entities
			(entity_id, title, content, source, tags, edited_by, updated_at, deleted_at)
		VALUES ($1, $2, $3, $4, $5, $4, now(), NULL)
		ON CONFLICT (entity_id) DO UPDATE
		  SET title = EXCLUDED.title, content = EXCLUDED.content,
		      tags = EXCLUDED.tags, edited_by = EXCLUDED.edited_by,
		      updated_at = now(), deleted_at = NULL
		  WHERE knowledge_entities.deleted_at IS NOT NULL
		RETURNING entity_id`,
		docID, title, content, actorLabel, tags).Scan(&wroteID)
	if errors.Is(err, pgx.ErrNoRows) {
		writeProblem(w, req, http.StatusConflict, "a note with this title already exists", slug)
		return
	} else if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "write knowledge failed", err.Error())
		return
	}

	linked := s.linkKnowledgeAbout(ctx, tx, docID, derefSlice(body.About))

	if err := tx.Commit(ctx); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "commit failed", err.Error())
		return
	}

	slog.Info("knowledge created", "slug", slug, "kind", kind, "actor", actorLabel, "linked", linked)
	// Content-Type before WriteHeader — setting it after is a no-op, the
	// status line is already on the wire.
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusCreated)
	if err := json.NewEncoder(w).Encode(map[string]any{
		"slug": slug, "id": docID.String(), "linked": linked,
	}); err != nil {
		slog.Error("httpapi: json encode failed", "error", err)
	}
}

// serveUpdateKnowledge edits a live note in place. The prior version is
// captured by the trg_knowledge_revision trigger, not by this handler — see
// the migration for why that lives in the database.
//
// Note the slug is intentionally NOT recomputed when the title changes:
// slugs are the wiki's stable link target ([[slug]] references, relationship
// rows, bookmarked window ids), and silently re-slugging on a typo fix would
// break every inbound link.
func (s *Server) serveUpdateKnowledge(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	idOrSlug, err := pathParam(req, "id")
	if err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid id", err.Error())
		return
	}

	var body knowledgeWriteBody
	if err := json.NewDecoder(req.Body).Decode(&body); err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid request body", err.Error())
		return
	}
	if body.Title == nil && body.Content == nil && body.Tags == nil && body.About == nil {
		writeProblem(w, req, http.StatusBadRequest, "nothing to update",
			"supply at least one of title, content, tags, about")
		return
	}
	if body.Title != nil && strings.TrimSpace(*body.Title) == "" {
		writeProblem(w, req, http.StatusBadRequest, "title cannot be empty", "")
		return
	}
	if body.Content != nil && strings.TrimSpace(*body.Content) == "" {
		writeProblem(w, req, http.StatusBadRequest, "content cannot be empty", "")
		return
	}

	entityID, err := s.resolveKnowledgeEntity(ctx, idOrSlug)
	if err != nil {
		writeProblem(w, req, http.StatusNotFound, "no such knowledge note", "")
		return
	}
	_, actorLabel := actorInfo(ctx)

	tx, err := s.pool.Begin(ctx)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "begin failed", err.Error())
		return
	}
	defer tx.Rollback(ctx)

	// COALESCE keeps unsupplied fields untouched; edited_by and updated_at
	// always move so the UI can show who last touched it. The trigger only
	// snapshots when title/content/tags actually differ, so a no-op save
	// doesn't manufacture a revision.
	var newTitle *string
	if body.Title != nil {
		t := strings.TrimSpace(*body.Title)
		newTitle = &t
	}
	var newContent *string
	if body.Content != nil {
		c := strings.TrimSpace(*body.Content)
		newContent = &c
	}
	var newTags *[]string
	if body.Tags != nil {
		t := normalizeTags(*body.Tags)
		newTags = &t
	}

	if _, err := tx.Exec(ctx, `
		UPDATE knowledge_entities
		SET title      = COALESCE($2, title),
		    content    = COALESCE($3, content),
		    tags       = COALESCE($4, tags),
		    edited_by  = $5,
		    updated_at = now()
		WHERE entity_id = $1`,
		entityID, newTitle, newContent, newTags, actorLabel); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "update failed", err.Error())
		return
	}

	// Keep the entity's display name in step with the note title — the graph
	// and the fleet table read entities.name, and leaving it stale is exactly
	// the drift this app exists to fight.
	if newTitle != nil {
		if _, err := tx.Exec(ctx,
			`UPDATE entities SET name = $2, updated_at = now() WHERE id = $1`,
			entityID, *newTitle); err != nil {
			writeProblem(w, req, http.StatusInternalServerError, "rename entity failed", err.Error())
			return
		}
	}

	// About is replace-semantics, not merge: the editor presents the full
	// link set, so an absent slug means the operator removed it. Existing
	// edges are closed (valid_to) rather than deleted, preserving history.
	var linked []string
	if body.About != nil {
		if _, err := tx.Exec(ctx, `
			UPDATE relationships SET valid_to = now()
			WHERE source_id = $1 AND valid_to IS NULL AND type IN ('documents', 'about')`,
			entityID); err != nil {
			writeProblem(w, req, http.StatusInternalServerError, "unlink failed", err.Error())
			return
		}
		linked = s.linkKnowledgeAbout(ctx, tx, entityID, *body.About)
	}

	if err := tx.Commit(ctx); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "commit failed", err.Error())
		return
	}

	slog.Info("knowledge updated", "entity_id", entityID, "actor", actorLabel)
	// `linked` lets the caller diff against what it submitted and warn about
	// any slug that didn't resolve — see linkKnowledgeAbout: a typo'd entity
	// slug otherwise fails with nothing but a server-side slog.Warn, so the
	// operator gets no feedback that one of their About links didn't take.
	writeJSON(w, map[string]any{"ok": true, "linked": linked})
}

// serveDeleteKnowledge soft-deletes a note. The row, its revision trail and
// its entity all survive; only the deleted_at stamp changes, and every read
// path filters on it.
func (s *Server) serveDeleteKnowledge(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	idOrSlug, err := pathParam(req, "id")
	if err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid id", err.Error())
		return
	}
	entityID, err := s.resolveKnowledgeEntity(ctx, idOrSlug)
	if err != nil {
		writeProblem(w, req, http.StatusNotFound, "no such knowledge note", "")
		return
	}
	_, actorLabel := actorInfo(ctx)

	// Snapshot the live version before tombstoning. The trigger fires on
	// title/content/tags changes only, and a delete changes none of them —
	// without this the most recent version would be the one version missing
	// from the history if the note is later restored.
	if _, err := s.pool.Exec(ctx, `
		INSERT INTO knowledge_revisions
			(entity_id, title, content, source, tags, edited_by, version_at)
		SELECT entity_id, title, content, source, tags, COALESCE(edited_by,''), updated_at
		FROM knowledge_entities WHERE entity_id = $1`, entityID); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "snapshot failed", err.Error())
		return
	}
	if _, err := s.pool.Exec(ctx, `
		UPDATE knowledge_entities SET deleted_at = now(), edited_by = $2
		WHERE entity_id = $1`, entityID, actorLabel); err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "delete failed", err.Error())
		return
	}

	slog.Info("knowledge deleted", "entity_id", entityID, "actor", actorLabel)
	writeJSON(w, map[string]any{"ok": true})
}

// serveRestoreKnowledge undoes a soft delete. The counterpart to
// serveDeleteKnowledge — without it, "recoverable by clearing the column"
// (see the migration) would only be true via psql, which isn't a real
// recovery path for an operator using the wiki.
func (s *Server) serveRestoreKnowledge(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	idOrSlug, err := pathParam(req, "id")
	if err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid id", err.Error())
		return
	}
	entityID, err := s.resolveKnowledgeEntityAny(ctx, idOrSlug)
	if err != nil {
		writeProblem(w, req, http.StatusNotFound, "no such knowledge note", "")
		return
	}
	_, actorLabel := actorInfo(ctx)

	ct, err := s.pool.Exec(ctx, `
		UPDATE knowledge_entities SET deleted_at = NULL, edited_by = $2
		WHERE entity_id = $1 AND deleted_at IS NOT NULL`, entityID, actorLabel)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "restore failed", err.Error())
		return
	}
	if ct.RowsAffected() == 0 {
		writeProblem(w, req, http.StatusConflict, "note is not deleted", "")
		return
	}

	slog.Info("knowledge restored", "entity_id", entityID, "actor", actorLabel)
	writeJSON(w, map[string]any{"ok": true})
}

// serveKnowledgeRevisions returns the note's superseded versions, newest
// first. Bodies are included: revisions are small (~1 KB) and few, and the
// diff view needs both sides anyway — paginating would cost a round trip per
// comparison to save nothing.
func (s *Server) serveKnowledgeRevisions(w http.ResponseWriter, req *http.Request) {
	ctx := req.Context()

	idOrSlug, err := pathParam(req, "id")
	if err != nil {
		writeProblem(w, req, http.StatusBadRequest, "invalid id", err.Error())
		return
	}
	entityID, err := s.resolveKnowledgeEntityAny(ctx, idOrSlug)
	if err != nil {
		writeProblem(w, req, http.StatusNotFound, "no such knowledge note", "")
		return
	}

	rows, err := s.pool.Query(ctx, `
		SELECT id, title, content, COALESCE(edited_by,''), COALESCE(tags,'{}'),
		       version_at::text, revised_at::text
		FROM knowledge_revisions
		WHERE entity_id = $1
		ORDER BY version_at DESC`, entityID)
	if err != nil {
		writeProblem(w, req, http.StatusInternalServerError, "query failed", err.Error())
		return
	}
	defer rows.Close()

	type revision struct {
		ID        int64    `json:"id"`
		Title     string   `json:"title"`
		Content   string   `json:"content"`
		EditedBy  string   `json:"edited_by"`
		Tags      []string `json:"tags"`
		VersionAt string   `json:"version_at"`
		RevisedAt string   `json:"revised_at"`
	}
	items := []revision{}
	for rows.Next() {
		var r revision
		if err := rows.Scan(&r.ID, &r.Title, &r.Content, &r.EditedBy, &r.Tags,
			&r.VersionAt, &r.RevisedAt); err != nil {
			slog.Error("httpapi: knowledge/revisions row scan failed", "error", err)
			continue
		}
		items = append(items, r)
	}

	writeJSON(w, map[string]any{"items": items})
}

// linkKnowledgeAbout points a note at the entities it concerns, skipping
// slugs that don't resolve and edges that already exist. Returns the slugs
// actually linked so the caller can report what stuck — a typo'd slug is a
// silent no-op otherwise.
func (s *Server) linkKnowledgeAbout(ctx context.Context, tx pgx.Tx, docID uuid.UUID, slugs []string) []string {
	linked := []string{}
	for _, raw := range slugs {
		slug := strings.TrimSpace(raw)
		if slug == "" {
			continue
		}
		var targetID uuid.UUID
		if err := tx.QueryRow(ctx, `SELECT id FROM entities WHERE slug = $1`, slug).Scan(&targetID); err != nil {
			slog.Warn("knowledge: about slug not found, skipping", "slug", slug)
			continue
		}
		if _, err := tx.Exec(ctx, `
			INSERT INTO relationships (source_id, target_id, type, attributes, valid_from)
			SELECT $1, $2, 'about', '{"by":"operator"}'::jsonb, now()
			WHERE NOT EXISTS (
				SELECT 1 FROM relationships
				WHERE source_id = $1 AND target_id = $2 AND type = 'about' AND valid_to IS NULL)`,
			docID, targetID); err != nil {
			slog.Warn("knowledge: link failed", "slug", slug, "error", err)
			continue
		}
		linked = append(linked, slug)
	}
	return linked
}

// normalizeTags trims, lowercases and de-duplicates while preserving order.
// Lowercasing is the fix for the casing drift already in the data — `oom`
// and `OOM` were separate tags on separate notes, so neither tag page showed
// the full set. Applied on every write so the split can't reopen.
func normalizeTags(in []string) []string {
	seen := map[string]bool{}
	out := []string{}
	for _, t := range in {
		t = strings.ToLower(strings.TrimSpace(t))
		if t == "" || seen[t] {
			continue
		}
		seen[t] = true
		out = append(out, t)
	}
	return out
}

func deref(p *string) string {
	if p == nil {
		return ""
	}
	return *p
}

func derefSlice(p *[]string) []string {
	if p == nil {
		return nil
	}
	return *p
}

// writeJSON is the success-path counterpart to writeProblem, so the handlers
// in this file don't each repeat the header/encode dance.
func writeJSON(w http.ResponseWriter, v any) {
	w.Header().Set("Content-Type", "application/json")
	if err := json.NewEncoder(w).Encode(v); err != nil {
		slog.Error("httpapi: json encode failed", "error", err)
	}
}