package classes

import (
	"slices"
	"time"
)

// Slot is what file numbering needs of one row of the slot table: its
// group, its file number (nil while unnumbered), when it was first seen and
// whether it is listed. A row left out keeps its number: its file keeps the
// hole instead of the later files shifting (D-30, RESEARCH Pitfall 5).
type Slot struct {
	ID        uint64
	Grp       string
	FileNo    *int
	FirstSeen time.Time
	Listed    bool
}

// AssignSlots numbers the rows for files of size addresses and reports
// whether any number changed. Without renumber, a numbered row keeps its
// number and every listed row without one is appended to the last file of
// its group while that file has room (counting the holes of left-out rows),
// then to the next file, oldest first (first seen, then id). With renumber
// (the file size changed), every group's listed rows are numbered again
// from file 1, oldest first, and the left-out rows lose their numbers.
func AssignSlots(rows []*Slot, size int, renumber bool) bool {
	if size < 1 {
		size = 1
	}
	groups := map[string][]*Slot{}
	var order []string
	for _, r := range rows {
		if _, ok := groups[r.Grp]; !ok {
			order = append(order, r.Grp)
		}
		groups[r.Grp] = append(groups[r.Grp], r)
	}
	changed := false
	set := func(r *Slot, n *int) {
		if (r.FileNo == nil) != (n == nil) || (r.FileNo != nil && *r.FileNo != *n) {
			changed = true
		}
		r.FileNo = n
	}
	for _, grp := range order {
		members := groups[grp]
		slices.SortStableFunc(members, func(a, b *Slot) int {
			if c := a.FirstSeen.Compare(b.FirstSeen); c != 0 {
				return c
			}
			switch {
			case a.ID < b.ID:
				return -1
			case a.ID > b.ID:
				return 1
			}
			return 0
		})
		if renumber {
			i := 0
			for _, r := range members {
				if !r.Listed {
					set(r, nil)
					continue
				}
				n := i/size + 1
				set(r, &n)
				i++
			}
			continue
		}
		counts := map[int]int{}
		last := 0
		for _, r := range members {
			if r.FileNo == nil {
				continue
			}
			counts[*r.FileNo]++
			last = max(last, *r.FileNo)
		}
		for _, r := range members {
			if r.FileNo != nil || !r.Listed {
				continue
			}
			if last == 0 || counts[last] >= size {
				last++
			}
			n := last
			set(r, &n)
			counts[last]++
		}
	}
	return changed
}