Pad/internal/editor/chunked_buffer.go

package editor

import (
	"bytes"
	"sort"

	"pad/internal/io/pool"
	"pad/internal/io/pool/types"
	"pad/internal/ui"
)

const (
	DefaultChunkSize = 64 * 1024 // 64 KB
	// MaxEditableFileSize is the largest file the editor will open for editing.
	// In-range files are loaded fully into memory (see Phase 3): the chunked
	// buffer keeps the raw bytes (~file size) resident and the renderer
	// virtualizes the glyph layout to the visible window. Files above this are
	// rejected with a "too large to edit" state (the browser can still list
	// them). Set from on-device measurement (see doc/development_plan.md §5).
	MaxEditableFileSize = 50 * 1024 * 1024 // 50 MB
)

// ChunkedBuffer provides chunked access to a file's content.
//
// Model (Phase 3): for in-range files the ENTIRE file is loaded into memory on
// open and split into ordered chunks (see SetContent). Each chunk keeps its
// own length; the byte offset of a chunk is the sum of the lengths of the
// chunks before it (a prefix sum), NOT a fixed i*chunkSize slot. This is what
// makes edits correct: an insert/delete changes a chunk's length and the
// prefix sums automatically shift every later chunk, so byte->chunk mapping
// stays exact. Because every chunk is resident, there is no lazy loading and
// therefore no stale-disk re-read (the old fixed-slot model could re-read a
// shifted tail chunk from disk and clobber in-memory edits).
//
// Chunks may grow/shrink with edits and are not rebalanced; that is fine for
// correctness (prefix sums) and keeps edits local to the affected chunk(s)
// rather than copying the whole file.
type ChunkedBuffer struct {
	filename   string
	chunkSize  int // target chunk size (e.g. 64 KB); actual chunks may vary
	fileLen    int64     // total content length
	chunks     [][]byte  // ordered; chunks[i] is the i-th chunk
	dirty      bool      // true if buffer has been modified
	FS         pool.FileSystem // filesystem for reads
	basePath   string          // base path for file resolution

	// line index is built asynchronously
	LineIndex *types.LineIndex

	// workerPool is retained for API compatibility; in-range files load fully
	// up front, so chunk loading no longer dispatches worker tasks.
	workerPool *pool.WorkerPool

	// lastPrefetchedChunk tracks the last chunk that was prefetched, so callers
	// can avoid redundant work. Kept for compatibility.
	lastPrefetchedChunk int

	// dirtyChunks tracks which individual chunks have been modified since they
	// were last persisted to disk.
	dirtyChunks map[int]bool

	// loadingChunks is retained for API compatibility; it is always empty for
	// in-range files (no lazy loading).
	loadingChunks map[int]bool

	// fullyLoaded is true once SetContent has populated all chunks; the buffer
	// never falls back to disk afterwards.
	fullyLoaded bool
}

// NewChunkedBuffer creates a new ChunkedBuffer.
func NewChunkedBuffer(filename string, chunkSize int, fs pool.FileSystem, basePath string) *ChunkedBuffer {
	if chunkSize <= 0 {
		chunkSize = DefaultChunkSize
	}
	return &ChunkedBuffer{
		filename:      filename,
		chunkSize:     chunkSize,
		fileLen:       0,
		chunks:        nil,
		dirtyChunks:   make(map[int]bool),
		loadingChunks: make(map[int]bool),
		FS:            fs,
		basePath:      basePath,
	}
}

// SetFileSize sets the total file length (used before SetContent, e.g. from a
// stat). It only applies when the buffer is not dirty.
func (cb *ChunkedBuffer) SetFileSize(length int64) {
	if !cb.dirty {
		cb.fileLen = length
	}
}

// FileLen returns the total content length.
func (cb *ChunkedBuffer) FileLen() int64 {
	return cb.fileLen
}

// Filename returns the filename.
func (cb *ChunkedBuffer) Filename() string {
	return cb.filename
}

// ChunkSize returns the target chunk size.
func (cb *ChunkedBuffer) ChunkSize() int {
	return cb.chunkSize
}

// SetContent replaces the buffer content with data, split into ordered chunks
// of at most chunkSize. This is the on-open load path for in-range files: the
// whole file becomes resident, so there is no lazy loading and no risk of
// re-reading stale disk data after an edit.
func (cb *ChunkedBuffer) SetContent(data []byte) {
	cb.chunks = nil
	for start := 0; start < len(data); start += cb.chunkSize {
		end := start + cb.chunkSize
		if end > len(data) {
			end = len(data)
		}
		chunk := make([]byte, end-start)
		copy(chunk, data[start:end])
		cb.chunks = append(cb.chunks, chunk)
	}
	if len(data) == 0 {
		cb.chunks = [][]byte{}
	}
	cb.fileLen = int64(len(data))
	cb.dirty = false
	cb.dirtyChunks = make(map[int]bool)
	cb.fullyLoaded = true
}

// chunkForPos returns the index of the chunk that contains byte offset pos and
// the offset of pos within that chunk. If pos is at or past the end, it returns
// the last chunk and an offset at its end (so callers can append). If the
// buffer is empty it returns (-1, 0).
func (cb *ChunkedBuffer) chunkForPos(pos int) (int, int) {
	offset := 0
	for i, chunk := range cb.chunks {
		if pos < offset+len(chunk) {
			return i, pos - offset
		}
		offset += len(chunk)
	}
	if n := len(cb.chunks); n > 0 {
		return n - 1, len(cb.chunks[n-1])
	}
	return -1, 0
}

// Content returns the bytes in [start, end) from the chunked buffer. Chunks are
// walked by their actual (prefix-sum) offsets, so the mapping stays correct
// after length-changing edits.
func (cb *ChunkedBuffer) Content(start, end int) string {
	if cb.fileLen == 0 {
		return ""
	}
	if start < 0 {
		start = 0
	}
	if end > int(cb.fileLen) {
		end = int(cb.fileLen)
	}
	if start >= end {
		return ""
	}
	var buf bytes.Buffer
	offset := 0
	for _, chunk := range cb.chunks {
		chunkStart := offset
		chunkEnd := offset + len(chunk)
		offset = chunkEnd
		if chunkEnd <= start {
			continue
		}
		if chunkStart >= end {
			break
		}
		segStart := max(start, chunkStart) - chunkStart
		segEnd := min(end, chunkEnd) - chunkStart
		if segStart < segEnd {
			buf.Write(chunk[segStart:segEnd])
		}
	}
	return buf.String()
}

// RuneIndexToByte returns the byte offset of the n-th rune (0-indexed) in the
// buffer. The IME addresses text in rune indices while the buffer is
// byte-based, so this bridges the two. It walks chunks by actual length and
// stops as soon as the n-th rune is found. If n is at or past the end it
// returns the content length.
func (cb *ChunkedBuffer) RuneIndexToByte(n int) int {
	if n <= 0 {
		return 0
	}
	if cb.fileLen == 0 {
		return 0
	}
	runes := 0
	offset := 0
	for _, chunk := range cb.chunks {
		for i := 0; i < len(chunk); i++ {
			b := chunk[i]
			// A UTF-8 rune starts at an ASCII byte (<0x80) or a multi-byte
			// lead byte (>=0xC0); 0x80-0xBF are continuation bytes.
			if b < 0x80 || b >= 0xC0 {
				if runes == n {
					return offset + i
				}
				runes++
			}
		}
		offset += len(chunk)
	}
	return int(cb.fileLen)
}

// FullContent reconstructs the entire content by concatenating the ordered
// chunks. For in-range files every chunk is resident, so this is exact and
// never returns an error.
func (cb *ChunkedBuffer) FullContent() (string, error) {
	var buf bytes.Buffer
	for _, chunk := range cb.chunks {
		buf.Write(chunk)
	}
	return buf.String(), nil
}

// loadChunk reads a chunk from disk. Retained for compatibility; in-range
// files load fully via SetContent and do not use this.
func (cb *ChunkedBuffer) loadChunk(idx int) ([]byte, error) {
	start := idx * cb.chunkSize
	chunk, err := cb.FS.ReadFileAt(cb.filename, start, cb.chunkSize)
	if err != nil {
		return nil, err
	}
	for len(cb.chunks) <= idx {
		cb.chunks = append(cb.chunks, nil)
	}
	cb.chunks[idx] = chunk
	return chunk, nil
}

// LoadChunk loads a chunk (no-op for fully-loaded in-range files).
func (cb *ChunkedBuffer) LoadChunk(idx int) {
	if cb.fullyLoaded {
		return
	}
	if idx >= 0 && idx < len(cb.chunks) && cb.chunks[idx] != nil {
		return
	}
	if _, err := cb.loadChunk(idx); err != nil {
		// Best effort; in-range files should never reach here.
	}
	cb.lastPrefetchedChunk = idx
}

// IsChunkLoaded reports whether the chunk is resident.
func (cb *ChunkedBuffer) IsChunkLoaded(idx int) bool {
	return idx >= 0 && idx < len(cb.chunks) && cb.chunks[idx] != nil
}

// IsChunkLoading reports whether a chunk load is in flight (never, for
// in-range files).
func (cb *ChunkedBuffer) IsChunkLoading(idx int) bool {
	return cb.loadingChunks[idx]
}

// LoadChunkAsync dispatches an async chunk load (no-op for in-range files,
// which are fully resident).
func (cb *ChunkedBuffer) LoadChunkAsync(idx int) {
	if cb.fullyLoaded {
		return
	}
	if cb.workerPool != nil && !cb.loadingChunks[idx] && !cb.IsChunkLoaded(idx) {
		cb.loadingChunks[idx] = true
		cb.workerPool.DispatchNonBlocking(
			pool.NewReadChunkTask(cb.filename, idx, cb.FS),
		)
	}
}

// SetWorkerPool sets the worker pool (retained for compatibility).
func (cb *ChunkedBuffer) SetWorkerPool(wp *pool.WorkerPool) {
	cb.workerPool = wp
}

// LastPrefetchedChunk returns the last prefetched chunk index.
func (cb *ChunkedBuffer) LastPrefetchedChunk() int {
	return cb.lastPrefetchedChunk
}

// Prefetch is a no-op for in-range files (all chunks resident).
func (cb *ChunkedBuffer) Prefetch(centerChunk int, radius int) {
	cb.lastPrefetchedChunk = centerChunk
}

// EvictFarChunks is intentionally a no-op for in-range files: all chunks stay
// resident so byte->chunk mapping and FullContent remain exact. (Evicting
// would reintroduce the stale-disk re-read hazard the fixed-slot model had.)
func (cb *ChunkedBuffer) EvictFarChunks(cursorPos int, radius int) {
	// no-op
}

// markDirtyChunk flags the buffer and a chunk as modified.
func (cb *ChunkedBuffer) markDirtyChunk(idx int) {
	cb.dirty = true
	if idx >= 0 {
		cb.dirtyChunks[idx] = true
	}
}

// Insert inserts text at byte position pos, splicing only the affected chunk.
func (cb *ChunkedBuffer) Insert(pos int, text string) {
	if len(text) == 0 {
		return
	}
	fileLen := int(cb.fileLen)
	if pos > fileLen {
		pos = fileLen // clamp to end
	}
	if pos < 0 {
		pos = 0
	}
	idx, local := cb.chunkForPos(pos)
	if idx < 0 {
		// Empty buffer: create the first chunk.
		cb.chunks = append(cb.chunks, []byte(text))
		cb.fileLen = int64(len(text))
		cb.markDirtyChunk(0)
		return
	}
	chunk := cb.chunks[idx]
	newChunk := make([]byte, 0, len(chunk)+len(text))
	newChunk = append(newChunk, chunk[:local]...)
	newChunk = append(newChunk, text...)
	newChunk = append(newChunk, chunk[local:]...)
	cb.chunks[idx] = newChunk
	cb.fileLen += int64(len(text))
	cb.markDirtyChunk(idx)
}

// Delete deletes n bytes starting at pos, splicing only the affected chunk(s).
func (cb *ChunkedBuffer) Delete(pos, n int) {
	if n <= 0 {
		return
	}
	fileLen := int(cb.fileLen)
	if pos < 0 {
		pos = 0
	}
	if pos >= fileLen {
		return
	}
	if pos+n > fileLen {
		n = fileLen - pos
	}
	absEnd := pos + n
	offset := 0
	for i := range cb.chunks {
		chunk := cb.chunks[i]
		chunkStart := offset
		chunkEnd := offset + len(chunk)
		offset = chunkEnd
		if chunkEnd <= pos || chunkStart >= absEnd {
			continue // no overlap with [pos, absEnd)
		}
		delStart := max(pos, chunkStart) - chunkStart
		delEnd := min(absEnd, chunkEnd) - chunkStart
		if delStart >= delEnd {
			continue
		}
		cb.chunks[i] = append(chunk[:delStart], chunk[delEnd:]...)
		cb.markDirtyChunk(i)
	}
	cb.fileLen -= int64(n)
	if cb.fileLen < 0 {
		cb.fileLen = 0
	}
	cb.dirty = true
}

// VisibleByteRange returns the byte range [start, end) that is visible on the
// current editor viewport, plus the visual line at the top of the viewport.
// This drives the renderer's virtual scrolling: only this range of text is
// laid out into GlyphLayout, keeping memory and layout cost bounded by the
// viewport, not the file. It uses actual (prefix-sum) chunk offsets so the
// range stays correct after length-changing edits.
func (cb *ChunkedBuffer) VisibleByteRange(scrollOffset ui.Dp, byteOffset int, viewportHeight ui.Dp, lineHeight ui.Dp, wordWrap bool, layout ui.GlyphLayout, visualIndex *types.VisualLineIndex) (start, end, startLine int) {
	// If wrapping and we have visual line data, use it for precise calculation
	if wordWrap && len(layout.VisualLineStarts) > 0 {
		// Calculate which visual line should be at the top based on scroll offset
		visualLine := int(scrollOffset / lineHeight)
		if visualLine >= len(layout.VisualLineStarts) {
			visualLine = len(layout.VisualLineStarts) - 1
		}

		start = layout.VisualLineStarts[visualLine] + byteOffset

		// Calculate end: enough content to fill viewport + buffer
		linesInViewport := int(viewportHeight/lineHeight) + 2
		endLine := visualLine + linesInViewport
		if endLine >= len(layout.VisualLineStarts) {
			end = int(cb.fileLen)
		} else {
			end = layout.VisualLineStarts[endLine]
		}

		// Clamp to file bounds
		if end > int(cb.fileLen) {
			end = int(cb.fileLen)
		}
		if start >= end {
			end = start + 1000 // minimum buffer
			if end > int(cb.fileLen) {
				end = int(cb.fileLen)
			}
		}
		return start, end, visualLine
	}

	// Fallback to LineIndex (logical lines) or estimate if:
	// 1. Not word wrapping
	// 2. No visual index available
	if cb.LineIndex == nil {
		start, end = cb.visibleByteRangeEstimate(scrollOffset, viewportHeight)
		return start, end, 0
	}
	start, end = cb.visibleByteRangePrecise(scrollOffset, viewportHeight)
	return start, end, 0
}

// visibleByteRangeEstimate approximates the visible byte range using
// heuristic estimates. Used when the line index is not yet available.
func (cb *ChunkedBuffer) visibleByteRangeEstimate(scrollOffset ui.Dp, viewportHeight ui.Dp) (start, end int) {
	// Use the editor's line height constant for consistency.
	lineHeight := EditorLineHeight()

	startLine := int(scrollOffset / lineHeight)
	endLine := int((scrollOffset + viewportHeight) / lineHeight)

	// Clamp line numbers to reasonable bounds
	totalLinesEstimate := 0
	if cb.fileLen > 0 {
		totalLinesEstimate = int(cb.fileLen/50) + 1 // Rough estimate: 50 bytes per line
	}
	if startLine < 0 {
		startLine = 0
	}
	if endLine > totalLinesEstimate {
		endLine = totalLinesEstimate
	}
	if startLine >= endLine {
		endLine = startLine + 1 // Ensure at least one line is visible
	}

	// Convert line numbers to byte offsets using the line index if available.
	if cb.LineIndex != nil {
		if startLine < len(cb.LineIndex.Offsets) {
			start = int(cb.LineIndex.Offsets[startLine])
		} else {
			lastKnownOffset := int64(0)
			if len(cb.LineIndex.Offsets) > 0 {
				lastKnownOffset = int64(cb.LineIndex.Offsets[len(cb.LineIndex.Offsets)-1])
			}
			linesBeyondIndex := startLine - (len(cb.LineIndex.Offsets) - 1)
			start = int(lastKnownOffset + int64(linesBeyondIndex)*50) // Estimate
		}

		if endLine < len(cb.LineIndex.Offsets) {
			end = int(cb.LineIndex.Offsets[endLine])
		} else {
			lastKnownOffset := int64(0)
			if len(cb.LineIndex.Offsets) > 0 {
				lastKnownOffset = int64(cb.LineIndex.Offsets[len(cb.LineIndex.Offsets)-1])
			}
			linesBeyondIndex := endLine - (len(cb.LineIndex.Offsets) - 1)
			end = int(lastKnownOffset + int64(linesBeyondIndex)*50) // Estimate
		}
	} else {
		// Rough byte estimation if no LineIndex
		start = startLine * 50 // rough estimate: 50 bytes per line
		end = endLine * 50
	}

	// Clamp to file bounds
	if cb.fileLen > 0 {
		if start < 0 {
			start = 0
		}
		if end > int(cb.fileLen) {
			end = int(cb.fileLen)
		}
		if end <= start {
			end = start + cb.chunkSize // Ensure at least one chunk's worth if range is invalid
		}
	} else {
		start = 0
		end = 0 // Empty file
	}

	return start, end
}

// visibleByteRangePrecise uses the LineIndex to find the exact byte range.
func (cb *ChunkedBuffer) visibleByteRangePrecise(scrollOffset ui.Dp, viewportHeight ui.Dp) (start, end int) {
	if cb.LineIndex == nil || len(cb.LineIndex.Offsets) == 0 {
		return cb.visibleByteRangeEstimate(scrollOffset, viewportHeight)
	}

	lineHeight := EditorLineHeight()

	startLine := int(scrollOffset / lineHeight)
	endLine := int((scrollOffset + viewportHeight) / lineHeight)

	if startLine < 0 {
		startLine = 0
	}
	if startLine >= len(cb.LineIndex.Offsets) {
		startLine = len(cb.LineIndex.Offsets) - 1
	}
	if endLine < 0 {
		endLine = 0
	}
	if endLine >= len(cb.LineIndex.Offsets) {
		endLine = len(cb.LineIndex.Offsets) - 1
	}
	if startLine < len(cb.LineIndex.Offsets)-1 && endLine <= startLine {
		endLine = startLine + 1
	}

	start = int(cb.LineIndex.Offsets[startLine])

	if endLine+1 < len(cb.LineIndex.Offsets) {
		end = int(cb.LineIndex.Offsets[endLine+1])
	} else {
		end = int(cb.fileLen)
	}

	if start < 0 {
		start = 0
	}
	if end > int(cb.fileLen) {
		end = int(cb.fileLen)
	}
	if end <= start {
		if start < int(cb.fileLen) {
			end = min(start+cb.chunkSize, int(cb.fileLen))
		} else {
			end = start
		}
	}

	return start, end
}

// UpdateLineIndexAfterEdit updates the LineIndex offsets after an insert or
// delete. offsetShift is the number of bytes inserted (positive) or deleted
// (negative); editPos is the byte position where the edit occurred.
func (cb *ChunkedBuffer) UpdateLineIndexAfterEdit(editPos int, offsetShift int) {
	if cb.LineIndex == nil {
		return
	}
	// Find the first offset that needs updating using binary search. All
	// offsets >= editPos are shifted by offsetShift.
	idx := sort.Search(len(cb.LineIndex.Offsets), func(i int) bool {
		return int(cb.LineIndex.Offsets[i]) >= editPos
	})
	if idx == 0 {
		idx = 1
	}
	for i := idx; i < len(cb.LineIndex.Offsets); i++ {
		cb.LineIndex.Offsets[i] += int32(offsetShift)
	}
	cb.LineIndex.Size += int64(offsetShift)
	if cb.LineIndex.Size < 0 {
		cb.LineIndex.Size = 0
	}
}

// max returns the larger of two ints.
func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

// min returns the smaller of two ints.
func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}