package beads

import (
	"context"
	"sort"
	"strings"
)

// milestonePrefix marks labels that name a milestone; the rollup groups by them.
const milestonePrefix = "milestone:"

// MilestoneView is the opaque .Data handed to milestones.html.
type MilestoneView struct {
	Milestones []MilestoneDetail
	Unlabeled  int // issues carrying no milestone label
	Total      int // all issues read
}

// MilestoneDetail is one milestone's rollup and the issues under it, arranged
// as a shallow hierarchy: the milestone-typed issue(s) first, then epics with
// their subtasks nested one level below, then everything else.
type MilestoneDetail struct {
	Name       string // label with the "milestone:" prefix stripped
	Label      string // full label, for filter links back to the board
	Total      int
	Done       int // closed
	InProgress int
	Open       int             // open (or unknown) — the remaining work
	Heads      []Card          // issue_type == "milestone" — the milestone's own issue(s)
	Epics      []MilestoneEpic // epics in the milestone, each with its nested subtasks
	Loose      []Card          // members that are neither heads, epics, nor nested subtasks
}

// MilestoneEpic is an epic inside a milestone together with the milestone
// members nested under it (parent-child edges pointing at the epic).
type MilestoneEpic struct {
	Card     Card
	Done     int // closed children, for the "d/t" rollup on the epic row
	Total    int
	Children []Card
}

// Pct is the milestone's completion percentage (0..100) for the progress bar.
func (m MilestoneDetail) Pct() int {
	if m.Total == 0 {
		return 0
	}
	return m.Done * 100 / m.Total
}

// BuildMilestones reads the issues and their labels, then groups by milestone
// label. An issue with several milestone labels counts under each. Missing
// labels/statuses tables degrade to empty (no milestones), never an error.
func BuildMilestones(ctx context.Context, sess BrowseSession, ref string) (*MilestoneView, error) {
	issues, _, err := readRows(ctx, sess, ref, "issues")
	if err != nil {
		return nil, err
	}
	labels, _, _ := readRowsOptional(ctx, sess, ref, "labels")
	statuses, _, _ := readRowsOptional(ctx, sess, ref, "custom_statuses")
	deps, _, _ := readRowsOptional(ctx, sess, ref, "dependencies")

	// child issue → its parent-child parents; used to nest tasks under epics.
	parentsByChild := map[string][]string{}
	if deps != nil {
		cols := indexCols(deps.Columns)
		for _, r := range deps.Rows {
			if !strings.EqualFold(cell(cols, r, "type"), "parent-child") {
				continue
			}
			child := cell(cols, r, "issue_id")
			parent := cell(cols, r, "depends_on_issue_id")
			if child != "" && parent != "" {
				parentsByChild[child] = append(parentsByChild[child], parent)
			}
		}
	}

	catByStatus := indexStatusCategories(statuses)
	labelsByIssue := indexLabels(labels)

	issueCols := indexCols(issues.Columns)
	byLabel := map[string]*MilestoneDetail{}
	cardsByLabel := map[string][]Card{}
	unlabeled := 0
	for _, r := range issues.Rows {
		id := cell(issueCols, r, "id")
		cat := statusCategory(cell(issueCols, r, "status"), catByStatus)
		card := Card{
			ID:       id,
			Title:    cell(issueCols, r, "title"),
			Type:     cell(issueCols, r, "issue_type"),
			Priority: cell(issueCols, r, "priority"),
			Assignee: cell(issueCols, r, "assignee"),
			Category: cat,
		}
		seen := false
		for _, l := range labelsByIssue[id] {
			if !strings.HasPrefix(l, milestonePrefix) {
				continue
			}
			seen = true
			md := byLabel[l]
			if md == nil {
				md = &MilestoneDetail{Name: strings.TrimPrefix(l, milestonePrefix), Label: l}
				byLabel[l] = md
			}
			md.Total++
			switch cat {
			case "closed":
				md.Done++
			case "in_progress":
				md.InProgress++
			default:
				md.Open++
			}
			cardsByLabel[l] = append(cardsByLabel[l], card)
		}
		if !seen {
			unlabeled++
		}
	}

	names := make([]string, 0, len(byLabel))
	for l := range byLabel {
		names = append(names, l)
	}
	sort.Strings(names)
	out := make([]MilestoneDetail, 0, len(names))
	for _, l := range names {
		md := byLabel[l]
		md.arrange(cardsByLabel[l], parentsByChild)
		out = append(out, *md)
	}

	return &MilestoneView{Milestones: out, Unlabeled: unlabeled, Total: len(issues.Rows)}, nil
}

// arrange splits a milestone's member cards into the display hierarchy: heads
// (issue_type "milestone") on top, then epics with the members parent-child'ed
// under them, then the leftovers. Only members of this milestone participate —
// membership stays purely label-based; a child nests only when its epic carries
// the same milestone label.
func (md *MilestoneDetail) arrange(cards []Card, parentsByChild map[string][]string) {
	epicByID := map[string]*MilestoneEpic{}
	var epics []*MilestoneEpic
	for _, c := range cards {
		switch {
		case strings.EqualFold(c.Type, "milestone"):
			md.Heads = append(md.Heads, c)
		case strings.EqualFold(c.Type, "epic"):
			e := &MilestoneEpic{Card: c}
			epicByID[c.ID] = e
			epics = append(epics, e)
		}
	}
	for _, c := range cards {
		if strings.EqualFold(c.Type, "milestone") || strings.EqualFold(c.Type, "epic") {
			continue
		}
		var home *MilestoneEpic
		for _, p := range parentsByChild[c.ID] {
			if e := epicByID[p]; e != nil {
				home = e
				break
			}
		}
		if home == nil {
			md.Loose = append(md.Loose, c)
			continue
		}
		home.Total++
		if c.Category == "closed" {
			home.Done++
		}
		home.Children = append(home.Children, c)
	}

	sortMilestoneCards(md.Heads)
	sortMilestoneCards(md.Loose)
	sort.SliceStable(epics, func(i, j int) bool {
		return milestoneCardLess(epics[i].Card, epics[j].Card)
	})
	for _, e := range epics {
		sortMilestoneCards(e.Children)
		md.Epics = append(md.Epics, *e)
	}
}

// sortMilestoneCards orders a milestone's issues open-work-first (closed sinks to
// the bottom), then by priority, then id — a stable, deterministic order.
func sortMilestoneCards(cards []Card) {
	sort.SliceStable(cards, func(i, j int) bool {
		return milestoneCardLess(cards[i], cards[j])
	})
}

func milestoneCardLess(a, b Card) bool {
	ca, cb := a.Category == "closed", b.Category == "closed"
	if ca != cb {
		return !ca
	}
	pa, pb := priorityRank(a.Priority), priorityRank(b.Priority)
	if pa != pb {
		return pa < pb
	}
	return a.ID < b.ID
}