Pad/internal/editor/wrap_index.go

package editor

// WrapIndex tracks, for each logical line, how many visual (wrapped) lines
// it occupies when the editor renders with word wrap enabled.
//
// Why it exists: every scroll-to-content mapping in the editor runs in
// visual-line space. A logical line k that wraps into W(k) visual lines has
// document height V(k)*lineHeight where V(k) = sum of W over lines [0,k).
// The scroll offset, the window start line, the sub-line draw offset, the
// max scroll and tap positioning all decompose against V, so scrolling past
// a wrapped line advances the viewport by the wrapped line's full height
// instead of jumping over its wrapped remainder.
//
// Counts start as estimates of 1 (one visual line per logical line) until
// the renderer shapes the line; the shaped window's glyph layout corrects
// the counts for the lines it covered, every frame (applyWrapCounts). An
// all-ones index is exactly the legacy no-wrap mapping, so behavior before
// the first shaping pass — and with word wrap off — is unchanged.
//
// A Fenwick tree answers prefix sums (VisualsBefore, TotalVisuals) and
// per-line corrections in O(log n). Line insertions and deletions rebuild
// the structure in O(n), the same cost class as LineIndex maintenance.
//
// int32 is safe for the values stored: the max total visual lines is bounded
// by the file size (every visual line holds at least one byte; the editable
// cap is 50 MB), and per-line counts are bounded by the same amount.
//
// Memory: two int32 arrays, i.e. 8 bytes per logical line on top of the
// existing LineIndex (4 bytes/line). Realistic note files are negligible;
// a 50 MB file of one-character lines (~50M lines) would cost ~400 MB here
// and ~200 MB in LineIndex already.
type WrapIndex struct {
	counts []int32 // visual lines per logical line (1 = estimate or single line)
	tree   []int32 // Fenwick tree over counts (1-indexed, len = n+1)
}

// NewWrapIndex returns an index for nLines logical lines, all estimated at
// one visual line.
func NewWrapIndex(nLines int) *WrapIndex {
	if nLines < 0 {
		nLines = 0
	}
	w := &WrapIndex{
		counts: make([]int32, nLines),
		tree:   make([]int32, nLines+1),
	}
	for i := range w.counts {
		w.counts[i] = 1
	}
	w.rebuild()
	return w
}

// rebuild recomputes the Fenwick tree from counts (linear build).
func (w *WrapIndex) rebuild() {
	n := len(w.counts)
	copy(w.tree[1:n+1], w.counts)
	for i := 1; i <= n; i++ {
		j := i + (i & -i)
		if j <= n {
			w.tree[j] += w.tree[i]
		}
	}
}

// Len is the number of logical lines tracked.
func (w *WrapIndex) Len() int { return len(w.counts) }

// Get returns the current visual-line count for line i.
func (w *WrapIndex) Get(i int) int32 { return w.counts[i] }

// Set updates the visual-line count for line i. No-op if unchanged, so the
// per-frame correction pass only touches lines whose wrap actually changed.
func (w *WrapIndex) Set(i int, c int32) {
	if i < 0 || i >= len(w.counts) || c <= 0 {
		return
	}
	if w.counts[i] == c {
		return
	}
	delta := c - w.counts[i]
	w.counts[i] = c
	for j := i + 1; j < len(w.tree); j += j & -j {
		w.tree[j] += delta
	}
}

// SetRange updates consecutive lines' counts (the shaped-window correction
// pass). Bounds are clamped.
func (w *WrapIndex) SetRange(start int, counts []int32) {
	for i, c := range counts {
		w.Set(start+i, c)
	}
}

// VisualsBefore returns V(k): the total number of visual lines occupied by
// logical lines [0,k). Out-of-range k is clamped.
func (w *WrapIndex) VisualsBefore(k int) int32 {
	if k < 0 {
		return 0
	}
	if k > len(w.counts) {
		k = len(w.counts)
	}
	var s int32
	for i := k; i > 0; i -= i & -i {
		s += w.tree[i]
	}
	return s
}

// TotalVisuals returns the total number of visual lines in the document.
func (w *WrapIndex) TotalVisuals() int32 {
	return w.VisualsBefore(len(w.counts))
}

// LineForVisual returns the logical line that contains visual line v
// (0-indexed): the smallest k with V(k+1) > v. Clamped to [0, n-1];
// for an empty index it returns 0.
//
// Implemented as a binary search over VisualsBefore (O(log^2 n)); at the
// file sizes this app supports (tens of thousands of lines) that is far
// below a microsecond and keeps the Fenwick code simple.
func (w *WrapIndex) LineForVisual(v int32) int {
	n := len(w.counts)
	if n == 0 {
		return 0
	}
	if v <= 0 {
		return 0
	}
	if v >= w.TotalVisuals() {
		return n - 1
	}
	// Invariant: V(k) is monotone non-decreasing in k, and V(k+1) > V(k)
	// (counts >= 1), so the search is well-defined.
	lo, hi := 0, n-1
	for lo < hi {
		mid := (lo + hi) / 2
		if w.VisualsBefore(mid+1) > v {
			hi = mid
		} else {
			lo = mid + 1
		}
	}
	return lo
}

// InsertLines adds n lines (each estimated at one visual line) at position
// pos. pos is clamped to [0, Len].
func (w *WrapIndex) InsertLines(pos, n int) {
	if n <= 0 {
		return
	}
	if pos < 0 {
		pos = 0
	}
	if pos > len(w.counts) {
		pos = len(w.counts)
	}
	newCounts := make([]int32, 0, len(w.counts)+n)
	newCounts = append(newCounts, w.counts[:pos]...)
	for i := 0; i < n; i++ {
		newCounts = append(newCounts, 1)
	}
	newCounts = append(newCounts, w.counts[pos:]...)
	w.counts = newCounts
	w.tree = make([]int32, len(w.counts)+1)
	w.rebuild()
}

// DeleteLines removes n lines at position pos. Clamped to what exists.
func (w *WrapIndex) DeleteLines(pos, n int) {
	if n <= 0 {
		return
	}
	if pos < 0 {
		pos = 0
	}
	if pos >= len(w.counts) {
		return
	}
	if pos+n > len(w.counts) {
		n = len(w.counts) - pos
	}
	w.counts = append(w.counts[:pos], w.counts[pos+n:]...)
	w.tree = make([]int32, len(w.counts)+1)
	w.rebuild()
}

// ResetAll sets every line back to the estimate of one visual line. Used
// when the wrap width changes (window resize), which invalidates every
// shaped count; the visible window is re-corrected on the next frame.
func (w *WrapIndex) ResetAll() {
	for i := range w.counts {
		w.counts[i] = 1
	}
	w.rebuild()
}