// SPDX-License-Identifier: AGPL-3.0-or-later

package web

import (
	"context"
	"net/http"
	"slices"

	"github.com/a-h/templ"

	"bnema.dev/ermite/internal/domain"
	"bnema.dev/ermite/internal/ports"
)

// compareDepth bounds the commits read on each side to find where a branch
// left its target.
const compareDepth = maxComparePages * pageSize

// compare shows a branch against the default branch of the repository: the
// commits it is ahead by and, for the owner, the form that opens a merge
// request.
//
// The API has no "diff between two branches" call outside a merge request, so
// the page shows the diff only when it is exact without one: the branch is
// ahead by a single commit that sits on the tip of the target (that commit's
// own diff is then the whole change). Otherwise it lists the commits, which
// link to their own diffs, and says the whole diff comes with the merge
// request.
//
//hexcheck:ignore suspicious-business-logic-in-adapter builds the compare page from the use case results; the merge rules are the use case's
func (s *Server) compare(ctx context.Context, r *http.Request) (templ.Component, int, error) {
	d, err := s.loadRepo(ctx, r.PathValue("repo"))
	if err != nil {
		return nil, 0, err
	}
	src, rest, ok := resolveRef(d.branches, r.PathValue("rest"))
	if !ok || rest != "" || d.head == nil {
		return nil, 0, errNotFound
	}
	tgt := *d.head
	name := d.name()

	p := ComparePage{
		Source: visibleText(src.ShortName), Target: visibleText(tgt.ShortName),
		SourceHref: repoHref(name, "tree", src.ShortName), TargetHref: repoHref(name, "tree", tgt.ShortName),
		CLI: Clone{ID: "cli-create", Lines: []string{
			"ermite mr create " + shellWord(name) + " --source " + shellWord(src.ShortName) + " --title <title>",
		}},
	}

	same := src.ShortName == tgt.ShortName
	var ahead []domain.Commit
	exact, behind := false, false
	if same {
		p.Note = "this is the default branch: there is nothing to compare it with."
		p.AheadText = "0 commits"
	} else {
		ahead, exact, behind, err = s.commitsAhead(ctx, d.repo.Name, src.CommitID.String(), tgt.CommitID.String())
		if err != nil {
			return nil, 0, err
		}
		now := s.cfg.Now()
		for _, c := range ahead {
			p.Ahead = append(p.Ahead, commitRow(name, c, now))
		}
		p.AheadText = plural(len(ahead), "commit", "commits")
		if !exact {
			p.AheadText += " or more"
		}
		switch {
		case !exact:
			p.Note = "the branch left " + p.Target + " too long ago to tell where: only the newest commits are listed."
		case len(ahead) == 0:
			p.Note = "nothing to merge: " + p.Target + " already has every commit of " + p.Source + "."
		case behind:
			p.Note = p.Target + " has moved on since the branch started: a merge request can only be merged after " +
				p.Source + " is rebased on " + p.Target + " and pushed again."
		}
	}

	// The exact diff: one commit on top of the tip of the target.
	if exact && !behind && len(ahead) == 1 {
		res, err := s.getCommit(ctx, d.repo.Name, ahead[0].ID.String())
		if err != nil {
			return nil, 0, err
		}
		p.HasDiff = true
		p.Files = diffFiles(res.Diff.Files)
		p.NFiles, p.Adds, p.Dels = res.Stats.FilesChanged, res.Stats.Additions, res.Stats.Deletions
		if n := diffNote(res.Diff, len(p.Files), p.NFiles); n != "" {
			p.Note = n
		}
		lines, invisible := diffLines(p.Files)
		p.Invisible = invisible
		self := repoHref(name, "compare", src.ShortName)
		p.DiffView = newDiffView(r, lines, self+"?view=unified", self+"?view=split")
	} else if len(ahead) > 1 && exact {
		if p.Note == "" {
			p.Note = "the whole diff of the branch is on its merge request; each commit links to its own."
		}
	}

	// An open request for the same pair, or the form to open one.
	var existing *domain.MergeRequest
	if !same {
		existing, err = s.openRequestFor(ctx, d.repo.Name, src.ShortName, tgt.ShortName)
		if err != nil {
			return nil, 0, err
		}
	}
	switch {
	case existing != nil:
		p.Existing = &Link{Label: "#" + itoa(int(existing.Number)) + " " + visibleText(existing.Title.String()), Href: mrHref(name, existing.Number)}
	case SignedIn(ctx) && !same && len(ahead) > 0:
		title := ""
		if len(ahead) == 1 {
			title = ahead[0].Summary
		} else {
			title = ahead[len(ahead)-1].Summary
		}
		p.Create = &CreateForm{
			Action: mrBase(name), Source: src.ShortName, Target: tgt.ShortName,
			Title: cutRunes(title, 120),
		}
	}

	l := s.repoLayout(ctx, d, "merge-requests")
	l.Title = "compare " + p.Source + " · " + name + " · " + s.instance
	pinRef(&l, target{rev: src.CommitID.String(), name: src.ShortName})
	l.Crumbs[0].Current = false
	l.Crumbs = append(l.Crumbs,
		Crumb{Label: "merge requests", Href: mrBase(name)},
		Crumb{Label: "compare", Current: false},
		Crumb{Label: p.Source, Current: true},
	)
	l.HasRail = true
	return Compare(l, p), http.StatusOK, nil
}

// commitsAhead lists the commits of the source that the target lacks, newest
// first. exact is false when the walk gave up before finding where the source
// meets the target. behind is true when the target has commits the source
// lacks (the tip of the target is not an ancestor of the source).
//
//hexcheck:ignore suspicious-business-logic-in-adapter walks the history pages the use case returns to find where the branch left its target; the page needs it, the forge decides nothing
func (s *Server) commitsAhead(ctx context.Context, repo domain.RepoName, srcTip, tgtTip string) (ahead []domain.Commit, exact, behind bool, err error) {
	if srcTip == tgtTip {
		return nil, true, false, nil
	}
	onTarget := map[string]bool{}
	tokenT := ""
	for len(onTarget) < compareDepth {
		res, err := s.commitList(ctx, repo, tgtTip, "", pageSize, tokenT)
		if err != nil {
			return nil, false, false, err
		}
		for _, c := range res.Commits {
			onTarget[c.ID.String()] = true
		}
		if tokenT = res.NextPageToken; tokenT == "" {
			break
		}
	}
	token := ""
	for n := 0; n < compareDepth; {
		res, err := s.commitList(ctx, repo, srcTip, "", pageSize, token)
		if err != nil {
			return nil, false, false, err
		}
		for _, c := range res.Commits {
			n++
			if onTarget[c.ID.String()] {
				// The first commit of the source that the target has: where the
				// branch left it. The target moved on when this is not its tip.
				return ahead, true, c.ID.String() != tgtTip, nil
			}
			ahead = append(ahead, c)
		}
		if token = res.NextPageToken; token == "" {
			// The whole history of the source is ahead: no common history.
			return ahead, true, true, nil
		}
	}
	return ahead, false, !slices.ContainsFunc(ahead, func(c domain.Commit) bool { return c.ID.String() == tgtTip }), nil
}

// openRequestFor finds the open merge request with this source and target.
func (s *Server) openRequestFor(ctx context.Context, repo domain.RepoName, source, target string) (*domain.MergeRequest, error) {
	token := ""
	for range maxOpenPages {
		res, err := s.cfg.MergeRequests.List(ctx, principal(ctx), repo, ports.ListMergeRequestsParams{
			State: domain.MRStateOpen, PageSize: pageSize, PageToken: token,
		})
		if err != nil {
			return nil, err
		}
		for i := range res.MergeRequests {
			if m := &res.MergeRequests[i]; m.Source == source && m.Target == target {
				return m, nil
			}
		}
		if token = res.NextPageToken; token == "" {
			break
		}
	}
	return nil, nil
}

// cutRunes shortens s to at most n characters.
func cutRunes(s string, n int) string {
	r := []rune(s)
	if len(r) <= n {
		return s
	}
	return string(r[:n])
}