Pad/internal/editor/session.go

// State restoration on relaunch (spec §7).
//
// The restorable state is a small snapshot (SessionState) the logic
// goroutine takes on demand: the last opened file, the cursor byte offset,
// the editor scroll offset, the live selection, and the find bar (query,
// open/closed, current match). The search RESULTS themselves are not part
// of the snapshot: on relaunch they are regenerated by re-scanning the
// restored query, and the current match is re-selected by byte offset
// (FindState.Restoring/RestoreMatch), without re-scrolling the restored
// viewport.
//
// Ownership (architecture.md §1): the logic goroutine owns the snapshot
// content. The cmd layer (cmd/pad/main.go) owns the tiny JSON file that
// stores it and registers the writer (Logic.SetSessionSaver); the logic
// calls it rate-limited from emitFrame and unconditionally at Shutdown, so
// the file is current even if the process is killed shortly after a change.
// On startup the cmd layer reads the file and hands it to Logic.BeginRestore
// (before Run, single-threaded), which re-opens the file and shows the
// editor immediately; the cursor/selection/find state land as the file's
// stat and content arrive (handleWorkerResult). A restore whose file is
// gone falls back to the browser page.

package editor

import (
	"log"
	"time"

	"pad/internal/ui"
)

// sessionSaveInterval rate-limits the periodic session save. The file is
// tiny (a few hundred bytes), so a few writes a second at most is
// negligible. The window bounds how stale the persisted state can be if
// the process is killed (recents-wipe) right after a change: at 1s, a
// kill less than a second after the last edit lost the cursor, selection
// and final scroll. See saveSessionIfChanged for the urgent path.
const sessionSaveInterval = 250 * time.Millisecond

// SessionState is the relaunch snapshot (see the file doc above). All
// positions are absolute file byte offsets; Scroll is in Dp. It is a plain
// comparable value (used with == for change detection).
type SessionState struct {
	File        string  // last opened file ("" = nothing to restore)
	Cursor      int     // cursor byte offset
	Scroll      float64 // editor scroll offset, Dp
	SelStart    int     // selection start byte (-1 = no selection)
	SelEnd      int     // selection end byte, exclusive
	FindQuery   string  // find bar query ("" = none)
	FindVisible bool    // find bar was open
	FindCurByte int     // start byte of the current find match (-1 = none)
}

// SetSessionSaver registers the callback that persists snapshots (the cmd
// layer's JSON file writer). It is invoked from the logic goroutine:
// rate-limited from emitFrame, and unconditionally at Shutdown. The
// callback must be fast (a tiny file write).
func (l *Logic) SetSessionSaver(f func(SessionState)) {
	l.sessionSaver = f
}

// SnapshotSession captures the current relaunch snapshot. The cursor is
// clamped to the file length; the selection and current find match are -1/
// -1 when inactive. Must be called on the logic goroutine (or after it has
// fully exited, as Shutdown does).
func (l *Logic) SnapshotSession() SessionState {
	s := l.state
	e := &s.Editor
	cur := e.CursorPosition
	if cb := e.ChunkedBuffer; cb != nil {
		if fl := int(cb.FileLen()); cur > fl {
			cur = fl
		}
	}
	f := &e.Find
	curByte := -1
	if f.Cur >= 0 && f.Cur < len(f.Matches) {
		curByte = f.Matches[f.Cur][0]
	}
	return SessionState{
		File:        e.Filename,
		Cursor:      cur,
		Scroll:      float64(s.ScrollOffset),
		SelStart:    e.SelectionStart,
		SelEnd:      e.SelectionEnd,
		FindQuery:   f.Query,
		FindVisible: f.Visible,
		FindCurByte: curByte,
	}
}

// saveSessionIfChanged persists the snapshot when it differs from the last
// saved one and the rate limit has elapsed. Must be called on the logic
// goroutine (it is called from emitFrame).
//
// While a restore is still pending (l.session set, not yet landed or
// abandoned) saves are suppressed: the pre-land snapshot has zeroed
// cursor/selection, and persisting it would clobber the positions being
// restored if the process is killed during startup.
func (l *Logic) saveSessionIfChanged() {
	if l.sessionSaver == nil {
		return
	}
	if l.session.File != "" {
		return
	}
	now := time.Now()
	s := l.SnapshotSession()
	if s == l.lastSession {
		return
	}
	// Save immediately when the file changes or a selection appears or
	// vanishes — low-frequency, high-value changes (the user just opened a
	// file or highlighted/cleared text). Everything else (scroll ticks,
	// cursor moves, selection-handle drags) is high-frequency and stays
	// rate-limited; a kill within the window then loses at most a fraction
	// of a second of motion, not the edit state.
	urgent := s.File != l.lastSession.File ||
		(s.SelStart >= 0) != (l.lastSession.SelStart >= 0)
	if !urgent && now.Sub(l.lastSessionSave) < sessionSaveInterval {
		return
	}
	l.sessionSaver(s)
	l.lastSession = s
	l.lastSessionSave = now
}

// BeginRestore prepares the state for relaunch restoration: the last file
// is re-opened and the editor page shown immediately. The cursor,
// selection, find state and scroll land as the file's stat/content arrive
// (handleWorkerResult); the scroll offset is applied now and clamped by
// EditorLayout once the line index sets the real MaxScroll. Must be called
// after NewLogic and before Run (the cmd layer does both before starting
// the logic goroutine), so it touches state single-threaded.
func (l *Logic) BeginRestore(s SessionState) {
	if s.File == "" {
		return
	}
	l.session = s
	e := &l.state.Editor
	e.Filename = s.File
	e.CursorPosition = 0 // the restored cursor lands with the content
	e.TooLarge = false
	e.TooLargeSize = 0
	e.SelectionAnchor = -1
	e.SelectionStart = -1
	e.SelectionEnd = -1
	// Fresh per-file find state; the snapshot's query and visibility land
	// with the stat (the file must exist first).
	e.findReset()
	// The scroll offset is armed, not applied: it lands in the emitFrame
	// hook, once the layout pass has a trustworthy viewport (see the Logic
	// struct).
	l.restoreScroll = ui.Dp(s.Scroll)
	l.restoreScrollArmed = s.Scroll > 0
	l.state.page = EditorPage
	l.state.FocusedElementID = "editor_text"
	l.state.justOpenedAt = time.Now()
	l.emitFrame() // show the editor page now; the content loads async
}

// abortRestore ends a restore without landing it (the user opened another
// file, the read failed, or the file is too large): clears the pending
// snapshot so session saving resumes (see saveSessionIfChanged). The editor
// state itself is left alone. Must be called on the logic goroutine.
func (l *Logic) abortRestore() {
	l.restoreFile = ""
	l.restoreScrollArmed = false
	l.restoreContentLanded = false
	l.session = SessionState{}
}

// maybeApplyRestoreScroll lands the armed restore scroll offset once the
// viewport is trustworthy, clamped to the current MaxScroll (the layout
// clamps again if the line index shrinks it later). It is called from
// emitFrame AFTER the layout pass, because that pass is where MaxScroll is
// computed for the current viewport — applying the offset before it (or in a
// pass with an unknown viewport) would hit the one-way clamp and lose the
// offset. On device the first ScaleEvent can precede the size ConfigEvent
// (and both can precede the restored content), so the offset stays armed
// until scale, size and editor content are all known; it is applied on the
// first such emitFrame. restoreContentLanded (not FileLen, which the stat
// result sets before the content arrives) marks that the buffer holds the
// restored file; clearing restoreFile any earlier would make the read
// result's path guard miss the just-loaded file. Returns true if it
// applied. Must be called on the logic goroutine.
func (l *Logic) maybeApplyRestoreScroll() bool {
	if !l.restoreScrollArmed {
		return false
	}
	if !l.scaleSeen {
		return false
	}
	if l.state.PixelWidth <= 0 || l.state.scale <= 0 {
		return false
	}
	if l.state.Page() != EditorPage || !l.restoreContentLanded {
		return false
	}
	if l.restoreScroll > l.state.MaxScroll {
		l.restoreScroll = l.state.MaxScroll
	}
	l.state.ScrollOffset = l.restoreScroll
	l.restoreScrollArmed = false
	// The snapshot has fully landed (cursor/selection/find with the
	// content, scroll now): lift the save suppression.
	l.session = SessionState{}
	return true
}

// applyRestorePositions clamps the restored snapshot's cursor and selection
// to a file of n bytes and applies them. Must be called on the logic
// goroutine.
func (l *Logic) applyRestorePositions(n int) {
	s := l.session
	e := &l.state.Editor
	if c := s.Cursor; c >= 0 {
		if c > n {
			c = n
		}
		e.CursorPosition = c
	}
	if ss, se := s.SelStart, s.SelEnd; ss >= 0 && se > ss {
		if se > n {
			se = n
		}
		if ss < se {
			e.SelectionAnchor = ss
			e.SelectionStart = ss
			e.SelectionEnd = se
		}
	}
}

// abandonRestore gives up on the relaunch restore (the file's stat failed:
// it was deleted or moved since the last session): reset the editor and
// land on the browser page, as if nothing had been restored. Must be called
// on the logic goroutine.
func (l *Logic) abandonRestore() {
	l.abortRestore()
	e := &l.state.Editor
	log.Printf("Logic: cannot restore %q; starting in the browser", e.Filename)
	e.Filename = ""
	e.ChunkedBuffer = nil
	e.CursorPosition = 0
	e.SelectionAnchor = -1
	e.SelectionStart = -1
	e.SelectionEnd = -1
	e.Find = FindState{SettleByte: -1}
	l.state.page = BrowserPage
	l.state.ScrollOffset = 0
	l.state.FocusedElementID = ""
}