treerack/arena.go

package treerack

type arena struct {

	// indicates fixed size pages from the pool
	f bool

	// tolerates pages of different size, but expects the page size to be min w + 1
	p *pool[[]int]

	// the max size of the stored data blocks. Expected to be larger than 0
	w int

	// pages storing the cursors for the indexes. The cursors point to the position of the first record
	// for an index in a. The total range of indexes can be larger then the sum of the page lengths,
	// depending on the offset. Initialized to -1, which means a cursor not pointing to a data item
	c [][]int

	// pages storing the records. Every record belongs to an index in c. The size of a record is w
	// + 1, where w fields represent the data, and one field hold the cursor pointing to the next record
	// belonging to the same index. The cursors point to the cursor position, the data fields are before the
	// cursor position. This means 0 is not a valid value for a cursor, and represents an empty record
	// placeholder. The pages in a are initialized to 0. Records are not split between pages
	a [][]int

	// virtual size of c. The actual size is l - o
	l int

	// offset in between the total virtual range of the indexes and the first index available in c. By
	// default it is 0, until one or more pages are removed from the beginning of c as a result of prune.
	// All the other indicators and the input indexes are interpreted for the total virtual range of the
	// arena
	o int

	// available size for records. The virtual total length of the pages in a starting from the original
	// zero.
	m int

	// virutal total size occupied by records. Effectively, the next available position in a, unless s >= m - w -
	// 1
	s int

	// total size of pages trimmed from the arena
	t int
}

func newArena(w int, p *pool[[]int], f bool) *arena {
	return &arena{
		f: f,
		p: p,
		w: w,
	}
}

func (a *arena) findPosition(pages [][]int, p int) ([]int, int) {
	if a.f {
		if len(pages) == 0 {
			return nil, -1
		}

		s := len(pages[0])
		if p >= s*len(pages) {
			return nil, -1
		}

		return pages[p/s], p % s
	}

	for i := 0; i < len(pages); i++ {
		if p < len(pages[i]) {
			return pages[i], p
		}

		p -= len(pages[i])
	}

	return nil, -1
}

func matchValues(v, q []int) bool {
	for i := 0; i < len(q); i++ {
		if v[i] != q[i] {
			return false
		}
	}

	return true
}

func (a *arena) cursor(index int) *int {
	index -= a.o
	if index < 0 {
		return nil
	}

	p, i := a.findPosition(a.c, index)
	if i < 0 {
		return nil
	}

	return &p[i]
}

func (a *arena) stepCursor(c *int) *int {
	p, i := a.findPosition(a.a, *c-a.t)
	return &p[i]
}

func (a *arena) recordAt(c int) []int {
	p, i := a.findPosition(a.a, c-a.t)
	return p[i-a.w : i+1]
}

func (a *arena) allocCursor(index int) {
	for index >= a.l {
		p := a.p.get()
		for i := range p {
			p[i] = -1
		}

		a.c = append(a.c, p)
		a.l += len(p)
	}
}

func (a *arena) allocArena() {
	if a.s+a.w+1 <= a.m {
		return
	}

	p := a.p.get()
	for i := range p {
		p[i] = 0
	}

	a.a = append(a.a, p)
	a.m += len(p)
}

func (a *arena) append(c int, v ...int) int {
	p := a.a[len(a.a)-1]
	i := a.s - a.m + len(p)
	if i < 0 {
		i = 0
		a.s = a.m - len(p)
	}

	copy(p[i:i+a.w], v)
	a.s += a.w + 1
	p[i+a.w] = c
	return a.s - 1
}

func (a *arena) clearCursor(c *int, q ...int) {
	if len(q) > a.w {
		return
	}

	for c != nil && *c > 0 {
		r := a.recordAt(*c)
		if matchValues(r[:len(r)-1], q) {
			*c, r[a.w] = r[a.w], 0
			continue
		}

		c = a.stepCursor(c)
	}
}

func (a *arena) lastActivePosition(p []int) int {
	for i := len(p) - len(p)%(a.w+1) - 1; i > 0; i -= a.w + 1 {
		if p[i] == 0 {
			continue
		}

		return i
	}

	return -1
}

func (a *arena) trim() {
	for {
		if len(a.a) == 0 {
			break
		}

		if a.lastActivePosition(a.a[0]) > 0 {
			break
		}

		a.p.put(a.a[0])
		a.t += len(a.a[0])
		a.a = a.a[1:]
	}

	for {
		if len(a.a) == 0 {
			break
		}

		last := len(a.a) - 1
		p := a.lastActivePosition(a.a[last])
		if p > 0 {
			a.s = a.m - len(a.a[last]) + p + 1
			break
		}

		a.p.put(a.a[last])
		a.m -= len(a.a[last])
		a.s = a.m
		a.a = a.a[:last]
	}
}

func (a *arena) len() int {
	return a.l
}

func (a *arena) has(index int, q ...int) bool {
	if len(q) == 0 {
		return true
	}

	if len(q) > a.w {
		return false
	}

	for c := a.cursor(index); c != nil && *c > 0; c = a.stepCursor(c) {
		r := a.recordAt(*c)
		if matchValues(r[:len(r)-1], q) {
			return true
		}
	}

	return false
}

// it does not copy the data
func (a *arena) get(index int, q ...int) [][]int {
	if len(q) > a.w {
		return nil
	}

	var values [][]int
	for c := a.cursor(index); c != nil && *c > 0; c = a.stepCursor(c) {
		r := a.recordAt(*c)
		v := r[:len(r)-1]
		if matchValues(v, q) {
			values = append(values, v)
		}
	}

	return values
}

// it does not copy the data
func (a *arena) set(index int, v ...int) {
	if a.has(index, v...) {
		return
	}

	a.allocCursor(index)
	c := a.cursor(index)

	// index can be in pruned range
	if c == nil {
		return
	}

	a.allocArena()
	*c = a.append(*c, v...)
}

func (a *arena) del(index int, q ...int) {
	c := a.cursor(index)
	a.clearCursor(c, q...)
	a.trim()
}

func (a *arena) prune(from, to int) {
	var p int
	for _, c := range a.c {
		if p+len(c) <= from-a.o {
			p += len(c)
			continue
		}

		if p >= to-a.o {
			break
		}

		for i := max(0, from-a.o-p); i < min(len(c), to-a.o-p); i++ {
			a.clearCursor(&c[i])
		}

		p += len(c)
	}

	for {
		if len(a.c) == 0 {
			break
		}

		var used bool
		c := a.c[0]
		if a.o+len(c) > to {
			break
		}

		for _, ci := range c {
			if ci > 0 {
				used = true
				break
			}
		}

		if used {
			break
		}

		a.o += len(c)
		a.p.put(c)
		a.c = a.c[1:]
	}

	for {
		if len(a.c) == 0 {
			break
		}

		var used bool
		last := len(a.c) - 1
		c := a.c[last]
		for _, ci := range c {
			if ci > 0 {
				used = true
				break
			}
		}

		if used {
			break
		}

		a.l -= len(c)
		a.p.put(c)
		a.c = a.c[:last]
	}

	a.trim()
}