// Package notes provides containment-checked read and surgical edit of brain
// notes on the live tree that brain-sync commits from.
//
// Everything here is defensive for one reason: brain content is ingested from
// the outside world, that content reaches a model's context via brain_query /
// brain_context, and these primitives are callable by that model. A path bug
// is therefore not a path bug — it is an outside author choosing a file.
package notes

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

// Scope is which part of the tree a caller is allowed to touch.
type Scope int

const (
	// ScopeNotes is the model-facing scope: markdown under knowledge/ and
	// wiki/ only.
	ScopeNotes Scope = iota
	// ScopeRoot admits the named governance files and NOTHING else. It exists so
	// there is an MCP path to CLAUDE.md at all — the absence of one is what
	// pushed a session to edit via the gitea REST API on 2026-06-16, which
	// never reached the live tree and wedged brain-sync for hours. It is NOT
	// reachable from the model-facing toolset; see tools_notes.go.
	ScopeRoot
)

// rootFiles is an exact-name allowlist, deliberately not a "root *.md" glob.
//
// Two reasons. log.md is gitignored, so an edit to it applies to the live tree
// and never reaches gitea — the exact shape of brain#2, admitted by a glob. And
// a glob auto-admits whatever governance file appears next (AGENTS.md, a
// bootstrap doc) with nobody reviewing that decision.
var rootFiles = map[string]bool{"CLAUDE.md": true}

// noteDirs are the two trees that hold notes.
var noteDirs = []string{"knowledge/", "wiki/"}

// deniedPrefixes are refused in every scope, independent of the allowlist.
//
// A static backstop rather than a consequence of the allowlist being right:
// .git/hooks/post-commit is executed by brain-sync inside a pod holding the
// Gitea push credentials, and archive/ + sessions/ hold verbatim session
// transcripts that BM25 never indexes and so nobody thinks of as reachable.
var deniedPrefixes = []string{".git/", "archive/", "sessions/", "raw/", "training-data/", ".obsidian/"}

// Errors are sentinels so callers can distinguish a refusal from an I/O fault.
var (
	ErrOutsideBrain  = errors.New("notes: path escapes the brain dir")
	ErrNotAllowed    = errors.New("notes: path is not editable through this tool")
	ErrNotSyncSource = errors.New("notes: brain dir is not the sync source")
)

// Target is a resolved, contained path.
type Target struct {
	Rel string // brain-relative, cleaned
	Abs string // absolute, symlink-checked
}

// Resolve canonicalises path, then checks it against the scope's allowlist,
// then re-checks containment after symlink evaluation.
//
// Order matters and is the point: a prefix test on the raw argument accepts
// "knowledge/../.git/hooks/post-commit", because Clean only collapses it
// afterwards.
func Resolve(brainDir, path string, scope Scope) (Target, error) {
	// Note: the sync-source assertion lives on the WRITE path (Edit), not
	// here. brain#2 was an edit landing where the sync could not commit it; a
	// read from such a tree is merely a read.

	// 1. Canonicalise. Rooting at "/" makes Clean collapse every ".." that
	//    would otherwise escape, and ToSlash normalises separators first.
	clean := strings.TrimPrefix(filepath.Clean("/"+filepath.ToSlash(path)), "/")
	if clean == "" || clean == "." {
		return Target{}, fmt.Errorf("%w: empty path", ErrNotAllowed)
	}

	// 2. Static denials, before any allowlist reasoning.
	for _, d := range deniedPrefixes {
		if strings.HasPrefix(clean+"/", d) {
			return Target{}, fmt.Errorf("%w: %q is not note content", ErrNotAllowed, clean)
		}
	}
	if strings.HasPrefix(filepath.Base(clean), ".") {
		return Target{}, fmt.Errorf("%w: dotfiles are not editable", ErrNotAllowed)
	}

	// 3. Scope allowlist.
	if err := allow(clean, scope); err != nil {
		return Target{}, err
	}

	// 4. Containment, lexically.
	abs := filepath.Join(brainDir, filepath.FromSlash(clean))
	if !within(brainDir, abs) {
		return Target{}, fmt.Errorf("%w: %q", ErrOutsideBrain, path)
	}

	// 5. Containment again, after resolving symlinks. The tree is Syncthing
	//    managed, so a link can appear without anyone adding one on purpose;
	//    a symlinked note or directory would otherwise read and write outside
	//    the brain entirely.
	if err := assertNoSymlinkEscape(brainDir, abs); err != nil {
		return Target{}, err
	}

	return Target{Rel: clean, Abs: abs}, nil
}

// allow enforces the scope's allowlist. The two scopes are DISJOINT, not
// nested: the privileged root scope reaches governance files and nothing else,
// so the tool that carries the higher privilege carries the smaller reach.
func allow(clean string, scope Scope) error {
	if scope == ScopeRoot {
		if !rootFiles[clean] {
			return fmt.Errorf("%w: %q is not a governance file", ErrNotAllowed, clean)
		}
		return nil
	}
	if !isNotePath(clean) {
		return fmt.Errorf("%w: %q is outside knowledge/ and wiki/", ErrNotAllowed, clean)
	}
	return nil
}

func isNotePath(clean string) bool {
	var inDir bool
	for _, d := range noteDirs {
		if strings.HasPrefix(clean, d) {
			inDir = true
			break
		}
	}
	if !inDir || filepath.Ext(clean) != ".md" {
		return false
	}
	// _index.md is generated per wing, and brain-sync's conflict resolver
	// deliberately resolves it in origin's favour — an edit here is reverted
	// by design, so accepting one would be a lie.
	return filepath.Base(clean) != "_index.md"
}

func within(root, abs string) bool {
	rel, err := filepath.Rel(root, abs)
	if err != nil {
		return false
	}
	return rel != ".." && !strings.HasPrefix(rel, ".."+string(filepath.Separator))
}

// assertNoSymlinkEscape resolves the deepest existing ancestor of abs and
// requires it to still be inside brainDir. The file itself need not exist
// (a create is legal); its directory chain must be real and contained.
func assertNoSymlinkEscape(brainDir, abs string) error {
	if fi, err := os.Lstat(abs); err == nil && fi.Mode()&os.ModeSymlink != 0 {
		return fmt.Errorf("%w: %q is a symlink", ErrNotAllowed, abs)
	}

	realRoot, err := filepath.EvalSymlinks(brainDir)
	if err != nil {
		return fmt.Errorf("notes: resolve brain dir: %w", err)
	}

	dir := filepath.Dir(abs)
	for {
		resolved, err := filepath.EvalSymlinks(dir)
		if err == nil {
			if !within(realRoot, resolved) {
				return fmt.Errorf("%w: %q resolves outside the brain dir", ErrOutsideBrain, dir)
			}
			return nil
		}
		if !os.IsNotExist(err) {
			return fmt.Errorf("notes: resolve %q: %w", dir, err)
		}
		parent := filepath.Dir(dir)
		if parent == dir {
			return fmt.Errorf("%w: no existing ancestor for %q", ErrOutsideBrain, abs)
		}
		dir = parent
	}
}

// assertSyncSource refuses a brain dir that is not a git working tree.
//
// brain#2 was an edit landing on a directory the sync does not commit from.
// That cannot be prevented by review — a wrong INGEST_BRAIN_DIR in a future
// manifest reintroduces it while every unit test still passes — so it is
// asserted at runtime, the same check brain-sync itself makes before running.
func assertSyncSource(brainDir string) error {
	if fi, err := os.Stat(filepath.Join(brainDir, ".git")); err != nil || !fi.IsDir() {
		return fmt.Errorf("%w: %q has no .git, so an edit there would never be committed",
			ErrNotSyncSource, brainDir)
	}
	return nil
}