package notes

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"syscall"
	"unicode/utf8"
)

// maxNoteBytes caps what Read will hand back. Generous for knowledge/ and
// wiki/ (the largest note today is far under it) and small enough that a
// pathological file cannot flood a caller's context.
const maxNoteBytes = 256 << 10

// Errors callers distinguish.
var (
	ErrNoMatch    = errors.New("notes: old_str not found")
	ErrAmbiguous  = errors.New("notes: old_str matches more than once")
	ErrNotUTF8    = errors.New("notes: content is not valid UTF-8")
	ErrGitBusy    = errors.New("notes: a git operation is in progress")
	ErrTooLarge   = errors.New("notes: file exceeds the readable size limit")
	ErrDegenerate = errors.New("notes: edit is degenerate")
	ErrInConflict = errors.New("notes: file contains unresolved conflict markers")
)

// pathLocks serialises edits per file within this process. The flock below
// covers the cross-process case (two pods share the hostPath during a rolling
// update, and brain-fix/brain-enrich write the same tree).
var pathLocks sync.Map // string -> *sync.Mutex

// Result is what a successful edit reports.
type Result struct {
	Path string `json:"path"`
	// PrevContentHash is the sha256 of the file as it was before this edit.
	// Paired with ContentHash it identifies a rewrite in an audit trail or an
	// alert without carrying any of the bytes (hyperguild#97).
	PrevContentHash string `json:"prev_content_hash"`
	ContentHash     string `json:"content_hash"`
	BytesBefore     int    `json:"bytes_before"`
	BytesAfter      int    `json:"bytes_after"`
	ReindexError    string `json:"reindex_error,omitempty"`
}

// Read returns a note's bytes verbatim.
//
// No frontmatter parsing, no excerpting, no normalisation: the entire purpose
// is to give a caller an exact basis for an edit. An excerpt from brain_query
// would be truncated and normalised, and every old_str built from one would
// miss.
func Read(brainDir, path string) ([]byte, error) {
	t, err := Resolve(brainDir, path, ScopeNotes)
	if err != nil {
		return nil, err
	}
	return readContained(t)
}

// ReadScoped is Read for a caller already holding the privileged scope.
func ReadScoped(brainDir, path string, scope Scope) ([]byte, error) {
	t, err := Resolve(brainDir, path, scope)
	if err != nil {
		return nil, err
	}
	return readContained(t)
}

func readContained(t Target) ([]byte, error) {
	fi, err := os.Stat(t.Abs)
	if err != nil {
		return nil, fmt.Errorf("notes: stat %s: %w", t.Rel, err)
	}
	if fi.Size() > maxNoteBytes {
		return nil, fmt.Errorf("%w: %s is %d bytes (limit %d)", ErrTooLarge, t.Rel, fi.Size(), maxNoteBytes)
	}
	b, err := os.ReadFile(t.Abs)
	if err != nil {
		return nil, fmt.Errorf("notes: read %s: %w", t.Rel, err)
	}
	// The MCP response marshals this into a JSON string, and encoding/json
	// substitutes U+FFFD for invalid UTF-8 — silently returning different
	// bytes than the file holds, which defeats "verbatim" entirely.
	if !utf8.Valid(b) {
		return nil, fmt.Errorf("%w: %s", ErrNotUTF8, t.Rel)
	}
	return b, nil
}

// Edit replaces exactly one occurrence of oldStr with newStr, in place.
//
// Fails loudly and writes nothing on: zero matches, more than one match
// (counted with overlap), degenerate arguments, invalid UTF-8, a git operation
// in flight, or unresolved conflict markers in the target.
func Edit(brainDir, path, oldStr, newStr string, scope Scope) (Result, error) {
	t, err := Resolve(brainDir, path, scope)
	if err != nil {
		return Result{}, err
	}
	if oldStr == "" {
		return Result{}, fmt.Errorf("%w: old_str is empty", ErrDegenerate)
	}
	if oldStr == newStr {
		return Result{}, fmt.Errorf("%w: old_str equals new_str", ErrDegenerate)
	}
	if !utf8.ValidString(newStr) {
		return Result{}, fmt.Errorf("%w: new_str", ErrNotUTF8)
	}
	// brain#2 guard: refuse to write to a tree the sync does not commit from.
	if err := assertSyncSource(brainDir); err != nil {
		return Result{}, err
	}
	if err := assertGitIdle(brainDir); err != nil {
		return Result{}, err
	}

	mu := lockFor(t.Abs)
	mu.Lock()
	defer mu.Unlock()

	unlock, err := flockFile(t.Abs)
	if err != nil {
		return Result{}, err
	}
	defer unlock()

	// Read INSIDE the lock: the match count must be evaluated against the
	// bytes we are about to overwrite, or a concurrent edit is silently lost
	// while both callers are told they succeeded.
	before, err := readContained(t)
	if err != nil {
		return Result{}, err
	}
	if hasConflictMarkers(before) {
		return Result{}, fmt.Errorf("%w: %s", ErrInConflict, t.Rel)
	}

	switch n := countOverlapping(string(before), oldStr); {
	case n == 0:
		return Result{}, fmt.Errorf("%w in %s (0 matches)%s", ErrNoMatch, t.Rel, crlfHint(string(before), oldStr))
	case n > 1:
		return Result{}, fmt.Errorf("%w in %s (%d matches) — extend old_str until it is unique",
			ErrAmbiguous, t.Rel, n)
	}

	after := strings.Replace(string(before), oldStr, newStr, 1)
	if err := writeAtomic(t.Abs, []byte(after)); err != nil {
		return Result{}, err
	}

	sum := sha256.Sum256([]byte(after))
	prev := sha256.Sum256(before)
	return Result{
		Path:            t.Rel,
		PrevContentHash: hex.EncodeToString(prev[:]),
		ContentHash:     hex.EncodeToString(sum[:]),
		BytesBefore:     len(before),
		BytesAfter:      len(after),
	}, nil
}

// countOverlapping counts occurrences allowing overlap. bytes.Count does not:
// "aa" occurs once in "aaa" by its reckoning, so a Count-based uniqueness
// check accepts an ambiguous edit as unique.
func countOverlapping(s, sub string) int {
	var n, from int
	for {
		i := strings.Index(s[from:], sub)
		if i < 0 {
			return n
		}
		n++
		from += i + 1
	}
}

// crlfHint reports whether the caller's old_str would have matched if line
// endings agreed. It never normalises — that would break byte-identity — but
// an opaque "no match" is unfixable from the caller's side, and a stripped \r
// is the likeliest cause.
func crlfHint(content, oldStr string) string {
	if strings.Contains(content, "\r\n") && !strings.Contains(oldStr, "\r\n") {
		if strings.Contains(strings.ReplaceAll(content, "\r\n", "\n"), oldStr) {
			return " — the file uses CRLF line endings and old_str does not; include the \\r"
		}
	}
	return ""
}

func hasConflictMarkers(b []byte) bool {
	s := string(b)
	return strings.Contains(s, "\n<<<<<<< ") || strings.HasPrefix(s, "<<<<<<< ")
}

// assertGitIdle refuses while brain-sync (or brain-fix, or a human) has a git
// operation in flight. An edit landing between `git merge` and its abort is
// either hard-reset away or committed as a wrongly resolved conflict — and the
// caller was told it succeeded.
func assertGitIdle(brainDir string) error {
	for _, marker := range []string{"MERGE_HEAD", "REBASE_HEAD", "CHERRY_PICK_HEAD", "index.lock"} {
		if _, err := os.Stat(filepath.Join(brainDir, ".git", marker)); err == nil {
			return fmt.Errorf("%w (.git/%s present) — retry after the sync settles", ErrGitBusy, marker)
		}
	}
	return nil
}

func lockFor(abs string) *sync.Mutex {
	v, _ := pathLocks.LoadOrStore(abs, &sync.Mutex{})
	return v.(*sync.Mutex)
}

// flockFile takes an advisory lock covering other processes sharing the
// hostPath — during a RollingUpdate two ingestion pods do.
func flockFile(abs string) (func(), error) {
	f, err := os.OpenFile(abs+".lock", os.O_CREATE|os.O_RDWR, 0o644)
	if err != nil {
		return nil, fmt.Errorf("notes: lock %s: %w", filepath.Base(abs), err)
	}
	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
		_ = f.Close()
		return nil, fmt.Errorf("notes: flock %s: %w", filepath.Base(abs), err)
	}
	return func() {
		_ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
		_ = f.Close()
		_ = os.Remove(abs + ".lock")
	}, nil
}

// writeAtomic replaces the file via a same-directory temp + rename, then
// fsyncs the directory so the rename survives a node crash.
//
// Same directory because a rename across the hostPath boundary fails with
// EXDEV; the ".tmp" suffix because the tree's .gitignore already covers it, so
// a leaked temp is not committed by `git add -A`.
func writeAtomic(abs string, content []byte) error {
	dir := filepath.Dir(abs)
	tmp, err := os.CreateTemp(dir, filepath.Base(abs)+".*.tmp")
	if err != nil {
		return fmt.Errorf("notes: temp file: %w", err)
	}
	tmpName := tmp.Name()
	defer func() { _ = os.Remove(tmpName) }() // no-op once renamed

	if _, err := tmp.Write(content); err != nil {
		_ = tmp.Close()
		return fmt.Errorf("notes: write temp: %w", err)
	}
	if err := tmp.Sync(); err != nil {
		_ = tmp.Close()
		return fmt.Errorf("notes: sync temp: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("notes: close temp: %w", err)
	}
	// CreateTemp makes 0600; every other writer of this tree uses 0644 and
	// other uids read it.
	if err := os.Chmod(tmpName, 0o644); err != nil {
		return fmt.Errorf("notes: chmod temp: %w", err)
	}
	if err := os.Rename(tmpName, abs); err != nil {
		return fmt.Errorf("notes: rename: %w", err)
	}
	return fsyncDir(dir)
}

func fsyncDir(dir string) error {
	d, err := os.Open(dir)
	if err != nil {
		return fmt.Errorf("notes: open dir: %w", err)
	}
	defer func() { _ = d.Close() }()
	if err := d.Sync(); err != nil {
		return fmt.Errorf("notes: fsync dir: %w", err)
	}
	return nil
}