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"
	"math"
	"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).
//
// Scroll is backed by ScrollLine: the pixel offset lives in VISUAL-line
// space (a wrapped line occupies several visual lines, and the scroll-to-
// line mapping runs through the WrapIndex), but on relaunch the counts of
// every line above the restored viewport are still the estimate (1) until
// the line is shaped, and lines above the visible window are never shaped.
// Mapping the saved offset through that fresh index would land a logical
// line DEEPER by all the wrapped continuations above the viewport, with no
// self-correction. The logical line at the viewport top and the sub-line
// remainder are wrap-independent, so they are the restorable unit; Scroll
// is kept as the pre-line-coordinate value (legacy session files).
type SessionState struct {
	File        string  // last opened file ("" = nothing to restore)
	Cursor      int     // cursor byte offset
	Scroll      float64 // editor scroll offset, Dp
	ScrollLine  int     // logical line at the viewport top (-1 = unknown)
	ScrollSub   float64 // Scroll's sub-line remainder as a FRACTION of the line height (0 <= f < 1); font-independent. Snapshots written before the pinch feature stored Dp instead; restore converts values > 1.
	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)
	// AppFontScale is the app-local pinch font scale (0 = default 1.0).
	AppFontScale float64
}

// 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]
	}
	// Persist the scroll in wrap-independent coordinates (see the
	// SessionState doc). The derivation mirrors visibleByteRangePrecise
	// (scrollDecompose, then LineForVisual against the CURRENT index), so
	// the line is exactly the one the layout is showing at this offset.
	scrollLine := -1
	var scrollSub float64
	if cb := e.ChunkedBuffer; cb != nil && !e.TooLarge {
		lh := EffectiveLineHeight()
		if gl := e.GlyphLayout; gl.LineHeight > 0 {
			lh = gl.LineHeight
		}
		v0, r0 := scrollDecompose(s.ScrollOffset, lh)
		scrollLine = int(v0)
		if w := cb.WrapIndex; w != nil {
			scrollLine = w.LineForVisual(int32(v0))
		}
		// Store the sub-line remainder as a fraction of the line height so
		// it stays valid if the font scale changes between save and restore
		// (r0 < lh always, so the fraction is < 1 — that is also the legacy
		// Dp discriminator used on restore).
		scrollSub = r0 / float64(lh)
	}
	return SessionState{
		File:         e.Filename,
		Cursor:       cur,
		Scroll:       float64(s.ScrollOffset),
		ScrollLine:   scrollLine,
		ScrollSub:    scrollSub,
		SelStart:     e.SelectionStart,
		SelEnd:       e.SelectionEnd,
		FindQuery:    f.Query,
		FindVisible:  f.Visible,
		FindCurByte:  curByte,
		AppFontScale: float64(s.appFontScale),
	}
}

// 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.writeSession(s, now)
}

// flushSessionSave persists the current snapshot now, bypassing the rate
// limit (still honoring the restore-pending suppression: a pre-land
// snapshot would clobber the positions being restored). Must be called on
// the logic goroutine.
func (l *Logic) flushSessionSave() {
	if l.sessionSaver == nil || l.session.File != "" {
		return
	}
	s := l.SnapshotSession()
	if s == l.lastSession {
		return
	}
	l.writeSession(s, time.Now())
}

// writeSession is the shared persist tail: hand the snapshot to the cmd
// layer's saver and record it as the new baseline. Must be called on the
// logic goroutine.
func (l *Logic) writeSession(s SessionState, now time.Time) {
	l.sessionSaver(s)
	l.lastSession = s
	l.lastSessionSave = now
}

// FlushSession requests an immediate session persist from any goroutine.
// The Android activity's onStop (recents-wipe, app switch) calls it via
// JNI: the OS gives no user-space hook for the subsequent process kill,
// so onStop is the last reliable moment to flush. Buffered delivery means
// the caller never blocks; if an earlier flush is still queued, one
// snapshot is coalesced into it (the state is the same frame's anyway).
func (l *Logic) FlushSession() {
	select {
	case l.flushSession <- struct{}{}:
	default:
	}
}

// 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
	}
	// The zero-value ScrollLine (0) is ambiguous: a genuine top-of-file
	// snapshot always has Scroll < lineHeight (no line above line 0
	// contributes visual lines), so line 0 with a deep offset is an unset
	// field (an in-process caller or test built the snapshot without it) —
	// fall back to the pixel offset rather than snapping to the top.
	if s.ScrollLine == 0 && s.Scroll >= 2*float64(EffectiveLineHeight()) {
		s.ScrollLine = -1
	}
	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
	// The app-local font scale lands immediately, BEFORE any layout: the
	// restore's line-based offset math reads it through EffectiveLineHeight.
	if as := s.AppFontScale; as > 0 {
		if as < MinAppFontScale {
			as = MinAppFontScale
		}
		if as > MaxAppFontScale {
			as = MaxAppFontScale
		}
		l.state.appFontScale = float32(as)
	}
	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.restoreScrollLine = -1
	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
	}
	// Re-derive the offset from the persisted logical line (see the
	// SessionState doc): the snapshot's pixel offset mapped to its position
	// only while the saving session's WrapIndex was valid. On relaunch the
	// counts above the restored viewport are all the estimate until shaped,
	// and those lines never are (shaping covers the visible window only),
	// so the raw offset would land a line deeper by every wrapped
	// continuation above the viewport. The line-based offset lands on the
	// saved line regardless of wrap state.
	s := l.session
	if s.ScrollLine >= 0 {
		lh := EffectiveLineHeight()
		if gl := l.state.Editor.GlyphLayout; gl.LineHeight > 0 {
			lh = gl.LineHeight
		}
		// The offset for a logical line is V(L)*lh + sub under the CURRENT
		// WrapIndex (V(L) = L while the index is all estimates). Wrapping
		// below the line was already corrected before the offset could be
		// applied (pre-apply frames shaped the top-of-file window), so
		// mapping through V(L) lands on the line under either state.
		base := float64(s.ScrollLine)
		if cb := l.state.Editor.ChunkedBuffer; cb != nil && cb.WrapIndex != nil && s.ScrollLine < cb.WrapIndex.Len() {
			base = float64(cb.WrapIndex.VisualsBefore(s.ScrollLine))
		}
		sub := s.ScrollSub
		if sub > 1 {
			// Legacy snapshot: the sub-line remainder was stored in Dp, not
			// as a fraction. Convert with the current line height (the
			// error is a fraction of one sub-line line, at most).
			sub = sub / float64(lh)
		}
		l.restoreScroll = ui.Dp(base*float64(lh) + sub*float64(lh))
		// Arm the line-pin (see refreshRestorePin): wrap-count corrections
		// landing below this line would shift the mapping and drag the
		// viewport off the restored line while the index settles.
		l.restoreScrollLine = s.ScrollLine
		l.restoreScrollSub = sub // fraction of the line height
		l.restorePinDeadline = time.Now().Add(restorePinTimeout)
	} else {
		l.restoreScrollLine = -1
	}
	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
}

// restorePinTimeout bounds the line-pin: after this long the index around
// the restored window has either settled or the user has moved on.
const restorePinTimeout = 2 * time.Second

// releaseRestorePin clears the restore line-pin (user or search took over
// the viewport). Must be called on the logic goroutine.
func (l *Logic) releaseRestorePin() {
	l.restoreScrollLine = -1
}

// refreshRestorePin re-derives the scroll offset from the pinned line under
// the CURRENT WrapIndex (V(L)*lh + sub, the same mapping the layout uses),
// so wrap-count corrections landing below the pinned line cannot drag the
// viewport off it. Releases the pin when the deadline passes. Must be
// called on the logic goroutine, after a wrap correction has been applied.
func (l *Logic) refreshRestorePin() {
	if l.restoreScrollLine < 0 {
		return
	}
	if time.Now().After(l.restorePinDeadline) {
		l.restoreScrollLine = -1
		return
	}
	cb := l.state.Editor.ChunkedBuffer
	if cb == nil || cb.WrapIndex == nil {
		return
	}
	w := cb.WrapIndex
	if l.restoreScrollLine >= w.Len() {
		return
	}
	lh := EffectiveLineHeight()
	if gl := l.state.Editor.GlyphLayout; gl.LineHeight > 0 {
		lh = gl.LineHeight
	}
	// No MaxScroll clamp here: the correction that triggered this refresh
	// just grew the index, so the layout's MaxScroll (computed before it) is
	// stale and would under-clamp the re-derived offset; the layout pass of
	// the emitted frame clamps to the fresh value. An edit shrinking the
	// file mid-pin is covered the same way.
	off := ui.Dp(float64(w.VisualsBefore(l.restoreScrollLine))*float64(lh) + l.restoreScrollSub*float64(lh))
	if off != l.state.ScrollOffset {
		l.state.ScrollOffset = off
		l.emitFrame()
	}
}

// fontPinTimeout bounds the pinch font-pin: rewrap corrections keep landing
// for a while after the last pinch frame (shaping lags the font change);
// after this long the user has moved on and the pin stands down.
const fontPinTimeout = 2 * time.Second

// setFontPin arms/updates the pinch font-pin from a pre-change capture
// (see captureContentPin: the glyph under the pinch center + offset from its
// baseline) and applies the IMMEDIATE anchor: the continuous content
// coordinate under the center scaled by the font ratio. Exact until the
// rewrap lands; the shaped-layout feedback (refreshFontPin) then snaps the
// pinned character exactly onto the center. Must be called on the logic
// goroutine.
func (l *Logic) setFontPin(pin contentPin, m float64, ratio float64) {
	l.fontPin = pin
	l.fontPinM = m
	l.fontPinArmed = true
	l.fontPinDeadline = time.Now().Add(fontPinTimeout)
	s := l.state
	s.rescaleScrollAnchored(ratio, float64(s.EditorRegion.Y)+m)
	// No emitFrame here: the caller (input path or debug command) emits the
	// frame carrying the new scale and the rescaled offset.
}

// refreshFontPin re-derives the scroll offset from a freshly shaped layout so
// the pinned content point stays under the pinch center (see
// refineContentPin). Runs on every layout feedback while armed: the font
// change's own re-shaping is the first, and rewrap corrections follow it. If
// the pinned byte is not in the shaped window (no layout yet, or the point
// is in a blank margin), the (line, fragment, sub-line) fallback anchor
// stands in. An edit landing since the capture (EditSeq mismatch) or leaving
// the editor page disarms the pin: its byte no longer names the same content.
// No MaxScroll clamp: the layout pass of the emitted frame clamps to the
// fresh value. Must be called on the logic goroutine.
func (l *Logic) refreshFontPin(fb ui.LayoutFeedback) {
	if !l.fontPinArmed {
		return
	}
	if time.Now().After(l.fontPinDeadline) {
		l.fontPinArmed = false
		return
	}
	s := l.state
	if s.page != EditorPage || fb.EditSeq != l.fontPin.EditSeq {
		l.fontPinArmed = false
		return
	}
	l.fontPinDeadline = time.Now().Add(fontPinTimeout)
	// Skip feedback shaped at a DIFFERENT scale: during a pinch every frame
	// changes the scale, so all but the latest feedback carry layouts whose
	// LineHeight no longer matches the geometry the pin computes in. A
	// stale-scale layout would place the point by the old geometry for one
	// frame (a visible jump) before the next corrects it.
	if fb.GlyphLayout.LineHeight > 0 &&
		math.Abs(float64(fb.GlyphLayout.LineHeight)-float64(EffectiveLineHeight())) > 0.5 {
		return
	}
	old := s.ScrollOffset
	vk := -1
	if cb := s.Editor.ChunkedBuffer; cb != nil && cb.WrapIndex != nil && fb.WindowStartLine >= 0 {
		vk = int(cb.WrapIndex.VisualsBefore(fb.WindowStartLine))
	}
	if l.fontPin.HaveGlyph && vk >= 0 {
		if off, ok := refineContentPin(fb.GlyphLayout, vk, fb.WindowStartByte, l.fontPin.Dy, l.fontPinM, l.fontPin.Byte); ok {
			s.ScrollOffset = off
		} else {
			s.applyFontPin(l.fontPin.Line, l.fontPin.Frag, l.fontPin.Sub, l.fontPinM)
		}
	} else {
		s.applyFontPin(l.fontPin.Line, l.fontPin.Frag, l.fontPin.Sub, l.fontPinM)
	}
	if s.ScrollOffset != old {
		l.emitFrame()
	}
}

// releaseFontPin disarms the pinch font-pin: the user (a scroll, a debug
// command, a file switch) has taken over the viewport. Must be called on
// the logic goroutine.
func (l *Logic) releaseFontPin() {
	l.fontPinArmed = false
}

// 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 = ""
}