package db

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"strconv"
	"time"

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

// BranchPrefix is the ref namespace agents may write. The refs rule — an agent
// token can only update refs under this prefix, and only the owner can move the
// approved branch — is the boundary that actually bounds the damage a confused
// agent can do.
const BranchPrefix = "proposals/"

// ProposalBranch is the branch name for a proposal id: "proposals/42". The row
// stores it verbatim (proposal.branch) because the row, not this function, is
// what the reconciler compares against the refs it finds.
func ProposalBranch(id int) string { return BranchPrefix + strconv.Itoa(id) }

// Proposal is a bundle of document edits awaiting review: a branch under
// BranchPrefix plus this row.
//
// BaseRev is the If-Match value the agent sent when the proposal was opened —
// the space's approved-head sha at the time it read the document — and it does
// not move as the proposal accumulates edits. Agent and AgentSession are
// mandatory provenance: one shared token still yields a full audit trail,
// because the identity strings, not the credential, are what say who did what.
//
// Approval and MergedRev are empty until the proposal merges; Resolved is nil
// until it leaves the open state.
type Proposal struct {
	ID           int
	SpaceID      int
	Title        string
	Rationale    string
	BaseRev      string
	Branch       string
	State        core.ProposalState
	Approval     core.Approval
	MergedRev    string
	Agent        string
	AgentSession string
	Created      time.Time
	Resolved     *time.Time
}

// Merge is everything one merge writes to Postgres: the proposal's transition
// to merged, and the new location of every document the merge touched. The two
// are one invariant — a merged proposal whose documents are still registered at
// their old paths would break link resolution and the next staleness check —
// so MergeProposal writes them in a single transaction.
type Merge struct {
	ProposalID int
	SpaceID    int
	Approval   core.Approval
	MergedRev  string
	// Docs is the (id, path) set of the documents the merge landed, at their
	// paths in the merge commit. Empty is legal but unusual: it means the
	// proposal touched nothing the registry tracks.
	Docs []DocRef
}

const proposalSelect = `
SELECT id, space_id, title, COALESCE(rationale, ''), base_rev, branch, state,
       COALESCE(approval, ''), COALESCE(merged_rev, ''), agent, agent_session,
       created, resolved
FROM proposal`

func scanProposal(sc rowScanner) (*Proposal, error) {
	var (
		p        Proposal
		state    string
		approval string
		resolved sql.NullTime
	)
	if err := sc.Scan(&p.ID, &p.SpaceID, &p.Title, &p.Rationale, &p.BaseRev,
		&p.Branch, &state, &approval, &p.MergedRev, &p.Agent, &p.AgentSession,
		&p.Created, &resolved); err != nil {
		return nil, err
	}
	parsedState, err := core.ParseProposalState(state)
	if err != nil {
		return nil, fmt.Errorf("proposal %d: %w", p.ID, err)
	}
	p.State = parsedState
	if approval != "" {
		parsedApproval, err := core.ParseApproval(approval)
		if err != nil {
			return nil, fmt.Errorf("proposal %d: %w", p.ID, err)
		}
		p.Approval = parsedApproval
	}
	if resolved.Valid {
		t := resolved.Time
		p.Resolved = &t
	}
	return &p, nil
}

// OpenProposal inserts a new proposal in the open state and returns it with its
// id, branch and creation time filled in. p.SpaceID, p.Title, p.BaseRev,
// p.Agent and p.AgentSession must be set; State, Approval, MergedRev and
// Resolved are ignored on input — a proposal is always born open.
//
// The branch name derives from the generated id ("proposals/42"), so id and
// branch are allocated in one statement: taking the id in a first round trip
// and writing the branch in a second would leave a window where a crash yields
// a row whose branch names nothing.
func (s *Store) OpenProposal(ctx context.Context, p *Proposal) (*Proposal, error) {
	if p.Agent == "" || p.AgentSession == "" {
		return nil, fmt.Errorf("open proposal: agent identity and session are required provenance")
	}
	if p.BaseRev == "" {
		return nil, fmt.Errorf("open proposal: base rev is required")
	}
	const q = `
WITH next AS (SELECT nextval(pg_get_serial_sequence('proposal', 'id')) AS id)
INSERT INTO proposal (id, space_id, title, rationale, base_rev, branch, state,
                      agent, agent_session, created)
SELECT next.id, $1, $2, $3, $4, $5::text || next.id::text, $6, $7, $8, $9
FROM next
RETURNING id, branch, created`
	out := *p
	out.State = core.StateOpen
	out.Approval = ""
	out.MergedRev = ""
	out.Resolved = nil
	err := s.q.QueryRowContext(ctx, q,
		p.SpaceID, p.Title, nullable(p.Rationale), p.BaseRev, BranchPrefix,
		string(core.StateOpen), p.Agent, p.AgentSession, time.Now().UTC(),
	).Scan(&out.ID, &out.Branch, &out.Created)
	if err != nil {
		return nil, fmt.Errorf("open proposal: %w", err)
	}
	return &out, nil
}

// GetProposal resolves a proposal by id. Returns ErrNotFound if it does not
// exist. Proposal URLs are stable and shareable — a link still resolves after
// merge or rejection, showing the outcome — so this is the same lookup whatever
// the state.
func (s *Store) GetProposal(ctx context.Context, id int) (*Proposal, error) {
	q := proposalSelect + ` WHERE id = $1`
	p, err := scanProposal(s.q.QueryRowContext(ctx, q, id))
	if errors.Is(err, sql.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, fmt.Errorf("get proposal %d: %w", id, err)
	}
	return p, nil
}

// ListProposalsByState lists proposals in one state, newest first — the inbox
// query ("N proposals waiting on you"), served by ix_proposal_state_created.
// limit <= 0 means no limit.
func (s *Store) ListProposalsByState(ctx context.Context, state core.ProposalState, limit int) ([]*Proposal, error) {
	if _, err := core.ParseProposalState(string(state)); err != nil {
		return nil, err
	}
	q := proposalSelect + ` WHERE state = $1 ORDER BY created DESC, id DESC`
	args := []any{string(state)}
	if limit > 0 {
		q += ` LIMIT $2`
		args = append(args, limit)
	}
	rows, err := s.q.QueryContext(ctx, q, args...)
	if err != nil {
		return nil, fmt.Errorf("list proposals state=%s: %w", state, err)
	}
	defer rows.Close()
	var out []*Proposal
	for rows.Next() {
		p, err := scanProposal(rows)
		if err != nil {
			return nil, fmt.Errorf("scan proposal: %w", err)
		}
		out = append(out, p)
	}
	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("iterate proposals: %w", err)
	}
	return out, nil
}

// MarkProposalMerged transitions a proposal to merged, recording how it was
// authorized (human or policy) and the merge commit. It writes only the row;
// use MergeProposal to update the document registry in the same transaction.
//
// The legality of the transition is enforced in SQL by `WHERE state = 'open'`,
// not by reading the row first: a check-then-write would let two concurrent
// resolutions both pass the check. When the guard bites, the current state is
// read back only to name it in the error.
func (s *Store) MarkProposalMerged(ctx context.Context, id int, approval core.Approval, mergedRev string) error {
	if _, err := core.ParseApproval(string(approval)); err != nil {
		return err
	}
	if mergedRev == "" {
		return fmt.Errorf("merge proposal %d: merged rev is required", id)
	}
	return s.resolveProposal(ctx, id, core.StateMerged, string(approval), mergedRev)
}

// RejectProposal transitions a proposal to rejected. There is no
// request-changes cycle: with one reviewer, a proposal you dislike is rejected
// and the agent proposes again.
func (s *Store) RejectProposal(ctx context.Context, id int) error {
	return s.resolveProposal(ctx, id, core.StateRejected, "", "")
}

// resolveProposal is the shared open->terminal update. next must be a legal
// destination from open; approval and mergedRev are stored as SQL NULL when
// empty, which the ck_proposal_merged constraints require for a rejection.
func (s *Store) resolveProposal(ctx context.Context, id int, next core.ProposalState, approval, mergedRev string) error {
	if err := core.StateOpen.CanTransitionTo(next); err != nil {
		return err
	}
	const q = `
UPDATE proposal
SET state = $2, approval = $3, merged_rev = $4, resolved = $5
WHERE id = $1 AND state = $6`
	res, err := s.q.ExecContext(ctx, q, id, string(next), nullable(approval),
		nullable(mergedRev), time.Now().UTC(), string(core.StateOpen))
	if err != nil {
		return fmt.Errorf("resolve proposal %d as %s: %w", id, next, err)
	}
	n, err := res.RowsAffected()
	if err != nil {
		return fmt.Errorf("resolve proposal %d: rows affected: %w", id, err)
	}
	if n == 1 {
		return nil
	}

	// Nothing moved: either the proposal is gone, or it is no longer open.
	var current string
	err = s.q.QueryRowContext(ctx, `SELECT state FROM proposal WHERE id = $1`, id).Scan(&current)
	if errors.Is(err, sql.ErrNoRows) {
		return ErrNotFound
	}
	if err != nil {
		return fmt.Errorf("resolve proposal %d: read current state: %w", id, err)
	}
	from, err := core.ParseProposalState(current)
	if err != nil {
		return fmt.Errorf("proposal %d: %w", id, err)
	}
	if err := from.CanTransitionTo(next); err != nil {
		return fmt.Errorf("proposal %d: %w", id, err)
	}
	// The row is open and the transition is legal, yet the guarded UPDATE
	// matched nothing. That cannot happen; refuse rather than report success.
	return fmt.Errorf("proposal %d: guarded update matched no row while state is %s", id, from)
}

// MergeProposal records a merge: the proposal's transition to merged and the
// new registry location of every document it landed, atomically.
//
// Git refs remain the source of truth for whether the merge happened — this row
// is what the reconciler repairs from the ref when a crash lands between the
// two. What the transaction buys is that Postgres never holds the half-state
// where the proposal reads merged but its documents are still registered at
// their pre-merge paths.
func (s *Store) MergeProposal(ctx context.Context, m Merge) error {
	if _, err := core.ParseApproval(string(m.Approval)); err != nil {
		return err
	}
	if m.MergedRev == "" {
		return fmt.Errorf("merge proposal %d: merged rev is required", m.ProposalID)
	}
	return s.InTx(ctx, func(tx *Store) error {
		if err := tx.MarkProposalMerged(ctx, m.ProposalID, m.Approval, m.MergedRev); err != nil {
			return err
		}
		return tx.UpsertDocIDs(ctx, m.SpaceID, m.Docs, m.MergedRev)
	})
}