package service

import (
	"context"
	"errors"
	"fmt"

	"sourcecraft.dev/bigbes/sr-ht-spec/core"
	"sourcecraft.dev/bigbes/sr-ht-spec/db"
	"sourcecraft.dev/bigbes/sr-ht-spec/gitx"
)

// Merge lands an open proposal onto the space's approved head and records how it
// was authorized.
//
// This is the state machine the design calls "approve merges immediately":
// there is no separate approved-then-merged step, because with one reviewer a
// merge is the approval. Phase 4's browser button calls it with ApprovalHuman;
// auto-merge policy calls the same path with ApprovalPolicy (see Propose), so
// the merge model runs identically whether a human clicked or a pattern
// matched — the only difference is the approval kind recorded, which is what a
// reader needs to tell reviewed content from firehose.
//
// The failure modes are the design's: a base that moved under the proposal is
// ErrStale (type-assert the chain to *gitx.StaleError for which document), and a
// proposal already in the approved history is ErrAlreadyMerged rather than a
// confusing staleness 409.
func (s *Service) Merge(ctx context.Context, ref core.SpaceRef, proposalID int, approval core.Approval) (Proposal, error) {
	sp, row, err := s.openProposalRow(ctx, ref, proposalID)
	if err != nil {
		return Proposal{}, err
	}
	return s.mergeProposal(ctx, sp, row, approval)
}

// Reject resolves an open proposal to rejected. There is no request-changes
// cycle — with one reviewer, a proposal you dislike is rejected and the agent
// proposes again — so this is the whole of the "do not merge" path. The branch
// and row are kept: the proposal URL still resolves and shows the outcome.
func (s *Service) Reject(ctx context.Context, ref core.SpaceRef, proposalID int) (Proposal, error) {
	_, row, err := s.openProposalRow(ctx, ref, proposalID)
	if err != nil {
		return Proposal{}, err
	}
	if err := s.store.RejectProposal(ctx, row.ID); err != nil {
		return Proposal{}, resolveProposalErr(err, row.ID)
	}
	rejected, err := s.GetProposal(ctx, row.ID)
	if err != nil {
		return Proposal{}, err
	}
	s.emit(EventProposalRejected, rejected)
	return rejected, nil
}

// mergeProposal is the merge itself, shared by the public Merge and by
// auto-merge. It assumes row is the proposal's current row and sp its open
// space.
func (s *Service) mergeProposal(ctx context.Context, sp *Space, row *db.Proposal, approval core.Approval) (Proposal, error) {
	if _, err := core.ParseApproval(string(approval)); err != nil {
		return Proposal{}, err
	}
	if row.State != core.StateOpen {
		return Proposal{}, fmt.Errorf("%w: proposal %d is %s", ErrProposalNotOpen, row.ID, row.State)
	}

	head, err := sp.Repo.ApprovedHead(ctx)
	if err != nil {
		return Proposal{}, readErr(err, "read approved head of %s", sp.Ref)
	}
	proposalHead, err := sp.Repo.BranchHead(ctx, row.Branch)
	if err != nil {
		return Proposal{}, readErr(err, "read head of %s in %s", row.Branch, sp.Ref)
	}
	base, err := sp.Repo.ResolveRev(ctx, row.BaseRev)
	if err != nil {
		return Proposal{}, readErr(err, "resolve base %s of %s in %s", row.BaseRev, row.Branch, sp.Ref)
	}

	// A branch still sitting on its base carries no commits. It is neither
	// mergeable nor "already merged": its tip is trivially an ancestor of the
	// approved head (the branch was cut there), which the ancestry check below
	// would misread as a completed merge. Say "nothing to merge" first — the
	// same qualification PlanRepairs makes for exactly this reason.
	if proposalHead == base {
		return Proposal{}, fmt.Errorf("service: proposal %d has no changes to merge: %w",
			row.ID, gitx.ErrUnsupportedChange)
	}

	// Already-merged proposals need an ancestry check, not a staleness check
	// (design): once the approved head carries the proposal's own blobs, the
	// blob comparison is trivially "changed" and gitx.Merge would return a
	// confusing 409. Testing IsAncestor(proposalHead, head) first is the only
	// thing that tells "landed" from "conflicts".
	already, err := sp.Repo.IsAncestor(ctx, proposalHead, head)
	if err != nil {
		return Proposal{}, readErr(err, "ancestry of %s in %s", row.Branch, sp.Ref)
	}
	if already {
		return Proposal{}, fmt.Errorf("%w: proposal %d (%s)", ErrAlreadyMerged, row.ID, row.Branch)
	}

	// The owner approves and the owner commits, so both identities are the
	// instance owner. A merge carries no agent trailers: it is a human (or
	// policy) act, and the agent provenance rides on the proposal's own commits,
	// which stay visible as the merge commit's second parent.
	sig := s.ownerSignature()
	res, err := sp.Repo.Merge(ctx, gitx.MergeRequest{
		Branch: row.Branch,
		Base:   row.BaseRev,
		Meta: gitx.CommitMeta{
			Message:   fmt.Sprintf("Merge proposal %d: %s", row.ID, row.Title),
			Author:    sig,
			Committer: sig,
		},
	})
	if err != nil {
		return Proposal{}, mergeErr(err, sp.Ref, row.ID)
	}

	// Refs are already truth for the merge; this makes Postgres agree, and does
	// the row flip and the document-registry move in one transaction so a reader
	// never sees a merged proposal whose documents are still registered at their
	// pre-merge paths.
	docs := make([]db.DocRef, 0, len(res.Docs))
	for _, d := range res.Docs {
		id, err := core.ParseDocID(d.DocID)
		if err != nil {
			return Proposal{}, fmt.Errorf("service: merged document %q at %q in %s carries an unusable id: %w",
				d.DocID, d.Path, sp.Ref, err)
		}
		docs = append(docs, db.DocRef{ID: id, Path: d.Path})
	}
	mergedRev := res.Commit.String()
	if err := s.store.MergeProposal(ctx, db.Merge{
		ProposalID: row.ID,
		SpaceID:    sp.ID,
		Approval:   approval,
		MergedRev:  mergedRev,
		Docs:       docs,
	}); err != nil {
		// The ref moved but the row did not: exactly the crash state the
		// reconciler repairs (RepairMarkMerged). Surface it rather than
		// reporting a failed merge — the merge commit is on the approved branch
		// and reads already see it.
		return Proposal{}, fmt.Errorf("service: proposal %d merged to %s in %s but its row could not be updated "+
			"(the reconciler will repair it): %w", row.ID, short(mergedRev), sp.Ref, err)
	}

	// mergeProposal is the single merge point — both the public Merge and
	// auto-merge reach it — so PROPOSAL_MERGED fires here, once per merge.
	merged, err := s.GetProposal(ctx, row.ID)
	if err != nil {
		return Proposal{}, err
	}
	s.emit(EventProposalMerged, merged)
	return merged, nil
}

// openProposalRow resolves a proposal by id within a named space, opening the
// space's repository. A proposal id that belongs to another space is reported
// as not found rather than acted on across the space boundary.
func (s *Service) openProposalRow(ctx context.Context, ref core.SpaceRef, proposalID int) (*Space, *db.Proposal, error) {
	sp, err := s.OpenSpace(ctx, ref)
	if err != nil {
		return nil, nil, err
	}
	row, err := s.store.GetProposal(ctx, proposalID)
	if err != nil {
		if errors.Is(err, db.ErrNotFound) {
			return nil, nil, fmt.Errorf("%w: proposal %d", ErrNotFound, proposalID)
		}
		return nil, nil, fmt.Errorf("service: look up proposal %d: %w", proposalID, err)
	}
	if row.SpaceID != sp.ID {
		return nil, nil, fmt.Errorf("%w: proposal %d is not in %s", ErrNotFound, proposalID, ref)
	}
	return sp, row, nil
}

// ownerSignature is the git identity every merge commit carries: the instance
// owner, stamped with the reconciler-injectable clock so provenance stays
// testable without sleeping.
func (s *Service) ownerSignature() gitx.Signature {
	o := s.cfg.Instance.OwnerSignature()
	return gitx.Signature{Name: o.Name, Email: o.Email, When: s.now().UTC()}
}

// mergeErr maps a gitx merge failure onto this package's sentinels. A staleness
// error becomes ErrStale while keeping the *gitx.StaleError in the chain, so a
// surface that wants to tell the agent which document went stale type-asserts to
// it and one that only needs the 409 matches ErrStale.
func mergeErr(err error, ref core.SpaceRef, proposalID int) error {
	var stale *gitx.StaleError
	if errors.As(err, &stale) {
		return fmt.Errorf("%w: proposal %d against %s: %w", ErrStale, proposalID, ref, err)
	}
	return fmt.Errorf("service: merge proposal %d in %s: %w", proposalID, ref, err)
}

// resolveProposalErr maps a db resolution failure (reject) onto this package's
// sentinels: a missing row is ErrNotFound, and a row that has already merged or
// been rejected is ErrProposalNotOpen.
func resolveProposalErr(err error, proposalID int) error {
	switch {
	case errors.Is(err, db.ErrNotFound):
		return fmt.Errorf("%w: proposal %d", ErrNotFound, proposalID)
	case errors.Is(err, db.ErrProposalNotOpen):
		return fmt.Errorf("%w: proposal %d", ErrProposalNotOpen, proposalID)
	default:
		return fmt.Errorf("service: reject proposal %d: %w", proposalID, err)
	}
}