// Package knowledge is the retrieval layer: ingest, permissioning and hybrid
// search over documents an agent may read.
//
// Two invariants shape every line of it, and they are not independent.
//
// **I1 — an agent reads exactly what its caller could read directly.** Not one
// chunk more. Retrieval is the easiest place in a platform to break this,
// because a retriever's natural signature is `retrieve(query, k)` and the
// caller is nowhere in it. §5 is blunt about the fix: the entry point is
// `retrieve(query, principal, scopes, k)` and there is no overload without a
// principal. This package has exactly one exported way to search and it will
// not run without one.
//
// **I2 — ACL filtering happens before scoring, never after.** The tempting
// implementation is to rank first and drop forbidden results afterwards; it is
// simpler, it is one line, and it leaks. Not through the text — the forbidden
// chunk is never printed — but through everything around it: a result count
// that is short, a top-3 that is missing its top-1, a summary whose confidence
// tracks documents the caller cannot see. So the permission predicate is pushed
// into BOTH the keyword query and the vector query as a pre-filter, and the
// fusion that follows only ever sees rows the caller was entitled to.
//
// This file is the permission half. It answers two questions and nothing else:
// what tags does a document carry, and what tags does this caller hold.
package knowledge

import (
	"fmt"
	"sort"
	"strings"

	"github.com/krow/krow-backend/go-api/internal/authctx"
	"github.com/krow/krow-backend/go-api/internal/domain"
)

// ACLVersion is the generation of the derivation below.
//
// §5: a reindex is required whenever ACL derivation logic changes. Bumping this
// constant is what makes that requirement enforceable — every document records
// the version that produced its tags, so "which documents predate the change"
// is a query rather than a guess, and a retriever can refuse stale rows instead
// of quietly serving tags that mean something different now.
//
// Bump it whenever GrantsFor or TagsFor changes what a tag MEANS. Adding a new
// tag kind that nothing yet emits does not need a bump; changing who `tenant`
// reaches does.
const ACLVersion = 1

/* ── The tag vocabulary ─────────────────────────────────────────────────── */

// Tag prefixes. A closed set, deliberately.
//
// The alternative — free-text tags supplied at ingest — makes the ACL a
// scripting surface: whoever writes the ingest call decides what "internal"
// means, and two callers can disagree. Here a tag is derived from a declared
// audience by code in this file, and a tag nobody can hold is refused at ingest
// rather than indexing a document into invisibility.
const (
	// TagTenant reaches everyone in the organization. The ordinary case for a
	// handbook or a policy: internal, but not restricted.
	TagTenant = "tenant"

	// TagRole reaches one role. `role:admin`, `role:employer`, `role:talent`.
	TagRole = "role:"

	// TagUser reaches one person by id. For a document about them.
	TagUser = "user:"

	// TagEmail reaches one person by email. The schema ties several resources
	// to a person by email rather than by foreign key (see the policy table's
	// note on ScopeEmail), so a document derived from one of those rows can
	// only name its subject this way.
	TagEmail = "email:"
)

// Audience is what an ingest call declares about who a document is for.
//
// Deliberately not tags. An ingester says "this is for the whole tenant" or
// "this is about this worker"; TagsFor turns that into the strings the index
// stores. Keeping the two apart is what lets ACLVersion mean anything — the
// declared audience is stable, the encoding of it is what changes.
type Audience struct {
	// Tenant makes the document readable by everyone in the organization.
	Tenant bool

	// Roles restricts it to specific roles.
	Roles []domain.Role

	// UserIDs and Emails restrict it to specific people.
	UserIDs []string
	Emails  []string
}

// TenantWide is the ordinary audience: everyone in the organization.
func TenantWide() Audience { return Audience{Tenant: true} }

// ForRoles restricts a document to specific roles.
func ForRoles(roles ...domain.Role) Audience { return Audience{Roles: roles} }

// ForPerson restricts a document to one person, by whichever identifiers are
// known. Both are accepted because the schema addresses people both ways.
func ForPerson(userID, email string) Audience {
	a := Audience{}
	if userID != "" {
		a.UserIDs = []string{userID}
	}
	if email != "" {
		a.Emails = []string{email}
	}
	return a
}

// TagsFor renders an audience as the tags a chunk row carries.
//
// Returns an error rather than an empty slice when an audience reaches nobody.
// §5 says a chunk without ACL metadata is rejected at ingest, and the reason is
// worth stating: an empty tag array is not "private", it is a row the `&&`
// operator can never match. A document that indexed to nothing looks ingested,
// reports a chunk count, and is silently unreachable — which is a support
// ticket that takes a week to diagnose.
func TagsFor(a Audience) ([]string, error) {
	seen := map[string]bool{}
	var tags []string
	add := func(t string) {
		if t == "" || seen[t] {
			return
		}
		seen[t] = true
		tags = append(tags, t)
	}

	if a.Tenant {
		add(TagTenant)
	}
	for _, r := range a.Roles {
		// Only the three the authorization table recognises. An unrecognised
		// role would produce a tag no principal can ever hold, which is the
		// invisible-document failure arriving by a different route.
		if _, ok := domain.ParseRole(string(r)); !ok {
			return nil, fmt.Errorf("knowledge: %q is not a role", r)
		}
		add(TagRole + string(r))
	}
	for _, id := range a.UserIDs {
		add(TagUser + strings.TrimSpace(id))
	}
	for _, email := range a.Emails {
		// Lower-cased at both ends. The column is citext so the database does
		// not care, but the tag is a plain text array element and `Maya@x` and
		// `maya@x` would be two different tags.
		add(TagEmail + strings.ToLower(strings.TrimSpace(email)))
	}

	if len(tags) == 0 {
		return nil, fmt.Errorf(
			"knowledge: this document declares no audience; a chunk with no ACL is not private, " +
				"it is unreachable, so ingest refuses it (§5)")
	}

	// Sorted so the same audience always produces the same array. Two documents
	// with identical permissions should compare equal, and a diff of a reindex
	// should show only what actually changed.
	sort.Strings(tags)
	return tags, nil
}

/* ── What a caller holds ────────────────────────────────────────────────── */

// GrantsFor is the tags a principal holds.
//
// The other side of TagsFor, and the whole of I1 as far as retrieval is
// concerned: a chunk is visible when `acl && grants` is true, so this function
// decides exactly what an agent can reach. It is small on purpose. Every line
// added here widens what every agent in the platform can see.
//
// Returns nil for a principal this platform does not recognise — no tenant, no
// role, an unlisted role. nil grants match nothing, because `acl && '{}'` is
// false for every row, so an unknown caller retrieves an empty result set
// rather than being special-cased somewhere downstream.
func GrantsFor(p authctx.Identity) []string {
	if strings.TrimSpace(p.OrgID) == "" {
		// I5. There is no cross-tenant reader and no "all organizations" mode.
		return nil
	}
	role, ok := domain.ParseRole(p.Role)
	if !ok {
		return nil
	}

	grants := []string{TagTenant, TagRole + string(role)}
	if id := strings.TrimSpace(p.UserID); id != "" {
		grants = append(grants, TagUser+id)
	}
	if email := strings.ToLower(strings.TrimSpace(p.Email)); email != "" {
		grants = append(grants, TagEmail+email)
	}

	sort.Strings(grants)
	return grants
}

// CanRead reports whether a set of grants reaches a set of tags.
//
// The Go mirror of the `&&` in the SQL, for tests and for the ingest-time
// sanity check. Retrieval does NOT call this: filtering in Go is exactly the
// post-filter I2 forbids, and having a Go implementation available is precisely
// the temptation worth naming here so nobody reaches for it.
func CanRead(grants, tags []string) bool {
	held := make(map[string]bool, len(grants))
	for _, g := range grants {
		held[g] = true
	}
	for _, t := range tags {
		if held[t] {
			return true
		}
	}
	return false
}