Gim/internal/core/gap_buffer.go

package core

// GapBuffer represents a single line of text using the gap buffer data structure.
// It maintains a gap (empty space) in the buffer where the cursor is positioned,
// making insertions and deletions at the cursor position very efficient (O(1)).
type GapBuffer struct {
	buffer   []rune // The underlying buffer containing text and gap
	gapStart int    // Index where the gap starts
	gapEnd   int    // Index where the gap ends (exclusive)
}

// NewGapBuffer: creates a new gap buffer with the given initial content.
// The gap is positioned at the end of the content.
func NewGapBuffer(content string) *GapBuffer {
	runes := []rune(content)
	initialCapacity := len(runes) + 16 // Extra space for the gap
	buffer := make([]rune, initialCapacity)
	copy(buffer, runes)

	return &GapBuffer{
		buffer:   buffer,
		gapStart: len(runes),
		gapEnd:   initialCapacity,
	}
}

// NewEmptyGapBuffer: Creates a new empty gap buffer with default capacity.
func NewEmptyGapBuffer() *GapBuffer {
	initialCapacity := 16
	return &GapBuffer{
		buffer:   make([]rune, initialCapacity),
		gapStart: 0,
		gapEnd:   initialCapacity,
	}
}

// GapBuffer.String: Converts the gap buffer to a string, excluding the gap.
func (gb *GapBuffer) String() string {
	result := make([]rune, 0, gb.Len())
	result = append(result, gb.buffer[:gb.gapStart]...)
	result = append(result, gb.buffer[gb.gapEnd:]...)
	return string(result)
}

// GapBuffer.Len: Returns the length of the text (excluding the gap).
func (gb *GapBuffer) Len() int {
	return len(gb.buffer) - (gb.gapEnd - gb.gapStart)
}

// GapBuffer.GapSize: Returns the current size of the gap.
func (gb *GapBuffer) GapSize() int {
	return gb.gapEnd - gb.gapStart
}

// GapBuffer.Insert: Inserts a string at the specified position.
// This moves the gap to the position first, then inserts the text.
func (gb *GapBuffer) Insert(pos int, text string) {
	if pos < 0 || pos > gb.Len() {
		return // Invalid position
	}

	runes := []rune(text)
	if len(runes) == 0 {
		return
	}

	// Move gap to insertion position
	gb.moveGap(pos)

	// Ensure gap is large enough
	if gb.GapSize() < len(runes) {
		gb.grow(len(runes))
	}

	// Insert runes at gap start
	copy(gb.buffer[gb.gapStart:], runes)
	gb.gapStart += len(runes)
}

// GapBuffer.Delete: Deletes count runes starting at position pos.
func (gb *GapBuffer) Delete(pos, count int) {
	if pos < 0 || pos >= gb.Len() || count <= 0 {
		return
	}

	// Clamp count to not exceed buffer length
	if pos+count > gb.Len() {
		count = gb.Len() - pos
	}

	// Move gap to deletion position
	gb.moveGap(pos)

	// Expand gap to absorb deleted characters
	gb.gapEnd += count
}

// GapBuffer.RuneAt: Returns the rune at the specified position.
func (gb *GapBuffer) RuneAt(pos int) rune {
	if pos < 0 || pos >= gb.Len() {
		return 0
	}

	if pos < gb.gapStart {
		return gb.buffer[pos]
	}
	return gb.buffer[pos+gb.GapSize()]
}

// GapBuffer.Substring: Returns a substring from start to end (exclusive).
func (gb *GapBuffer) Substring(start, end int) string {
	if start < 0 {
		start = 0
	}
	if end > gb.Len() {
		end = gb.Len()
	}
	if start >= end {
		return ""
	}

	result := make([]rune, 0, end-start)
	for i := start; i < end; i++ {
		result = append(result, gb.RuneAt(i))
	}
	return string(result)
}

// GapBuffer.moveGap: Moves the gap to the specified position.
func (gb *GapBuffer) moveGap(pos int) {
	if pos < gb.gapStart {
		// Move gap left: shift text from [pos, gapStart) to [gapEnd-delta, gapEnd)
		delta := gb.gapStart - pos
		copy(gb.buffer[gb.gapEnd-delta:gb.gapEnd], gb.buffer[pos:gb.gapStart])
		gb.gapStart = pos
		gb.gapEnd -= delta
	} else if pos > gb.gapStart {
		// Move gap right: shift text from [gapEnd, gapEnd+delta) to [gapStart, gapStart+delta)
		delta := pos - gb.gapStart
		copy(gb.buffer[gb.gapStart:gb.gapStart+delta], gb.buffer[gb.gapEnd:gb.gapEnd+delta])
		gb.gapStart += delta
		gb.gapEnd += delta
	}
}

// GapBuffer.grow: Expands the buffer to accommodate at least minGapSize additional characters.
func (gb *GapBuffer) grow(minGapSize int) {
	oldLen := len(gb.buffer)
	newGapSize := minGapSize * 2 // Double the required size for future insertions
	newLen := oldLen + newGapSize - gb.GapSize()

	newBuffer := make([]rune, newLen)

	// Copy text before gap
	copy(newBuffer, gb.buffer[:gb.gapStart])

	// Copy text after gap to new position
	newGapEnd := newLen - (oldLen - gb.gapEnd)
	copy(newBuffer[newGapEnd:], gb.buffer[gb.gapEnd:])

	gb.buffer = newBuffer
	gb.gapEnd = newGapEnd
}

// GapBuffer.Set: Replaces the entire content of the gap buffer.
func (gb *GapBuffer) Set(content string) {
	runes := []rune(content)
	capacity := len(runes) + 16

	gb.buffer = make([]rune, capacity)
	copy(gb.buffer, runes)
	gb.gapStart = len(runes)
	gb.gapEnd = capacity
}

// GapBuffer.Clear: Removes all content from the gap buffer.
func (gb *GapBuffer) Clear() {
	gb.gapStart = 0
	gb.gapEnd = len(gb.buffer)
}