// Package conversation manages the saved history, model inference, tool
// execution, and live event streaming for a single conversation.
//
// A [Conversation] coordinates access to one conversation in SQLite:
//   - [Conversation.Reserve] reserves the conversation for one
//     [Inference], and [Inference.Run] runs the model and tool-call loop.
//     Only one inference call or message edit may run at a time;
//     concurrent calls return [ErrBusy].
//   - [Conversation.Subscribe] synchronizes a client's cached messages with
//     saved history and streams live updates from an active inference run.
//   - Message and tool-call edit methods modify saved history when no
//     inference is running, using optimistic timestamp checks
//     ([ErrRevisionConflict]) to prevent lost updates.
package conversation

import (
	"context"
	"database/sql"
	"encoding/json/jsontext"
	"encoding/json/v2"
	"errors"
	"fmt"
	"log/slog"
	"runtime"
	"slices"
	"sync"
	"time"
	"uuid"

	"code.chimeric.al/chimerical/odidere/internal/db"
	"code.chimeric.al/chimerical/odidere/internal/llm"
	"code.chimeric.al/chimerical/odidere/internal/tool"
)

const (
	// interruptedWarning indicates a tool call started but did not finish
	// normally, so its effects are unknown.
	interruptedWarning = `▸ WARNING: the tool started, but its outcome is unknown.
It may have had effects. Do not assume it is safe to repeat.`

	// resultSaveTimeout is the time limit for saving one tool result.
	resultSaveTimeout = time.Second
)

var (
	// ErrApprovalRequired indicates that saved tool calls await user
	// approval.
	ErrApprovalRequired = errors.New("tool approval required")

	// ErrBusy indicates that inference is ongoing.
	ErrBusy = errors.New("conversation busy")

	// ErrMaxIterations indicates that the model-request limit was reached
	// without a final assistant message.
	ErrMaxIterations = errors.New("max iterations exceeded")

	// ErrRevisionConflict indicates that the message was changed before
	// the caller's revision.
	ErrRevisionConflict = errors.New("message revision conflict")
)

// DecodeMessages converts saved rows into llm.Messages, validating each.
// It stops at the first row that fails to unmarshal or validate; the
// returned slice is nil on error.
func DecodeMessages(rows []db.Message) ([]llm.Message, error) {
	msgs := make([]llm.Message, len(rows))
	for i, row := range rows {
		if err := json.Unmarshal(
			row.Content, &msgs[i],
		); err != nil {
			return nil, fmt.Errorf(
				"unmarshal message %q: %w", row.Id, err,
			)
		}
		if err := msgs[i].Validate(); err != nil {
			return nil, fmt.Errorf(
				"invalid message %q: %w", row.Id, err,
			)
		}
	}
	return msgs, nil
}

// New constructs an in-memory conversation manager.
func New(
	cfg Config,
	sdb *db.SQLite,
	log *slog.Logger,
	tools *tool.Registry,
) (*Conversation, error) {
	if err := cfg.Validate(); err != nil {
		return nil, fmt.Errorf("invalid config: %w", err)
	}
	if sdb == nil {
		return nil, fmt.Errorf("missing database")
	}
	if log == nil {
		return nil, fmt.Errorf("missing logger")
	}
	if tools == nil {
		return nil, fmt.Errorf("missing tool registry")
	}

	return &Conversation{
		id:    cfg.Id,
		db:    sdb,
		dbr:   db.New(sdb.Reader),
		dbw:   db.New(sdb.Writer),
		log:   log,
		tools: tools,
	}, nil
}

// Config identifies the saved conversation to manage.
type Config struct {
	Id string
}

// Validate checks the configuration without accessing the database.
func (cfg Config) Validate() error {
	if cfg.Id == "" {
		return fmt.Errorf("missing conversation id")
	}
	return nil
}

// Conversation is an in-memory handle for one saved conversation.
type Conversation struct {
	id    string
	db    *db.SQLite
	dbr   *db.Queries
	dbw   *db.Queries
	log   *slog.Logger
	tools *tool.Registry

	// mu synchronizes access to cancel and inf.
	mu sync.Mutex
	// cancel is used to stop inference.
	// When set, it indicates inference is reserved or ongoing.
	cancel context.CancelFunc
	// inf shares the reserved Inference's events with subscribers.
	// When set, it indicates inference is reserved or ongoing.
	inf *Inference
}

// MessageManifest identifies one saved message cached by a client.
type MessageManifest struct {
	Id        string
	UpdatedAt time.Time
}

// EventType distinguishes the events delivered by Subscribe.
type EventType string

const (
	EventAssistantUpdate EventType = "assistant_update"
	EventDone            EventType = "done"
	EventMessageDeleted  EventType = "message_deleted"
	EventMessageSaved    EventType = "message_saved"
)

// Event is a conversation update delivered to subscribers.
type Event interface {
	// Type reports the kind of the event.
	Type() EventType
}

// MessageSaved reports a message was saved.
type MessageSaved struct {
	Message db.Message `json:"message"`
}

func (MessageSaved) Type() EventType { return EventMessageSaved }

// MessageDeleted reports a message ID the client's manifest holds but
// the saved history no longer contains.
type MessageDeleted struct {
	MessageId string `json:"message_id"`
}

func (MessageDeleted) Type() EventType { return EventMessageDeleted }

// AssistantUpdate reports the next streamed fragment of one assistant
// message.
type AssistantUpdate struct {
	MessageId string          `json:"message_id"`
	Chunk     llm.StreamChunk `json:"chunk"`
}

func (AssistantUpdate) Type() EventType { return EventAssistantUpdate }

// Done reports that the Inference the subscriber joined has returned.
// Error is empty on success.
type Done struct {
	Error string `json:"error,omitempty"`
}

func (Done) Type() EventType { return EventDone }

// Inference is one reserved inference call and its events.
// [Conversation.Reserve] creates it, and [Inference.Run] runs it.
// [Inference.Events] and [Conversation.Subscribe] use its events to send
// past and live updates to subscribers.
type Inference struct {
	c      *Conversation
	client *llm.Client
	req    InferenceRequest
	// ctx is canceled by cancel, which Stop and Delete also call.
	ctx    context.Context
	cancel context.CancelFunc

	// mu serializes access to the fields below.
	mu sync.Mutex
	// started reports whether Run has been called.
	started bool
	// changed notifies waiting subscribers of new events or completion.
	// Each publish closes changed and replaces it with a new channel;
	// finish closes it without replacement.
	changed chan struct{}
	// done reports whether the inference work has finished.
	done bool
	// err reports why inference failed, or nil on success.
	err error
	// events holds all published events in order.
	// Events are added, never removed.
	// Each subscriber keeps its own read position in events.
	events []Event
}

// publish appends e to the events and wakes waiting subscribers.
// The caller must hold inf.mu.
func (inf *Inference) publish(e Event) {
	inf.events = append(inf.events, e)
	close(inf.changed)
	inf.changed = make(chan struct{})
}

// finish marks the call complete and records err.
// It closes changed to wake waiting subscribers.
// The channel stays closed because the call is complete.
func (inf *Inference) finish(err error) {
	inf.mu.Lock()
	defer inf.mu.Unlock()

	inf.done = true
	inf.err = err
	close(inf.changed)
}

// Events streams the call's events from the start, then ends with
// [Done]. The channel closes after Done or when ctx is canceled.
// Canceling ctx stops delivery; it does not stop the call.
func (inf *Inference) Events(ctx context.Context) <-chan Event {
	ch := make(chan Event, 64)
	go func() {
		defer close(ch)
		inf.follow(ctx, ch, nil)
	}()
	return ch
}

// follow sends the call's events to ch from index 0, skipping
// [MessageSaved] and [AssistantUpdate] events whose message ID is in
// skip. It waits for new events until the call finishes, then sends
// [Done]. It returns false if ctx is canceled first.
func (inf *Inference) follow(
	ctx context.Context,
	ch chan Event,
	skip map[string]struct{},
) bool {
	pos := 0
	for {
		// Copy the slice header for unread events and the current
		// notification channel. Because the call only appends to
		// events, reading batch after Unlock is safe.
		inf.mu.Lock()
		batch := inf.events[pos:]
		pos = len(inf.events)
		wait, done, err := inf.changed, inf.done, inf.err
		inf.mu.Unlock()

		for _, e := range batch {
			var id string
			switch e := e.(type) {
			case MessageSaved:
				id = e.Message.Id
			case AssistantUpdate:
				id = e.MessageId
			}
			if id != "" {
				if _, ok := skip[id]; ok {
					continue
				}
			}
			if !send(ctx, ch, e) {
				return false
			}
		}
		if done {
			var errStr string
			if err != nil {
				errStr = err.Error()
			}
			return send(ctx, ch, Done{Error: errStr})
		}
		// Wait for the call to close wait (when it appends new
		// events or finishes), or for the subscriber to cancel ctx.
		select {
		case <-wait:
		case <-ctx.Done():
			return false
		}
	}
}

// Subscribe streams events that synchronize a client's message cache with
// the saved conversation and any reserved or ongoing [Inference].
//
// manifest lists the message IDs and timestamps the caller already holds.
// An empty manifest requests all saved messages.
//
// Events arrive on the returned channel in two phases:
//  1. Cache diff: [MessageSaved] for each saved message missing from or
//     newer than manifest, followed by [MessageDeleted] for each ID in
//     manifest that is no longer saved.
//  2. Active inference: if an [Inference] is reserved or running when
//     Subscribe loads the database, Subscribe streams its
//     [MessageSaved] and [AssistantUpdate] events, then ends with [Done].
//     If no inference is reserved or running, Subscribe sends no [Done]
//     event.
//
// The channel closes when delivery finishes or ctx is canceled. Canceling
// ctx stops delivery to this subscriber; it does not stop an ongoing
// [Inference].
func (c *Conversation) Subscribe(
	ctx context.Context,
	manifest []MessageManifest,
) (<-chan Event, error) {
	// Check for a reserved or ongoing Inference.
	c.mu.Lock()
	inf := c.inf
	c.mu.Unlock()

	// Inference.Run holds inf.mu while it commits a message and
	// appends the MessageSaved event to inf.events. Holding inf.mu
	// across this read ensures that any message committed after this
	// point will appear in inf.events, and any message already in rows
	// will be recorded in the diff's sent map.
	if inf != nil {
		inf.mu.Lock()
	}
	rows, err := c.dbr.SelectMessagesConversationId(ctx, c.id)
	if inf != nil {
		inf.mu.Unlock()
	}
	if err != nil {
		return nil, fmt.Errorf("load messages: %w", err)
	}
	events, sent := diffMessages(rows, manifest)

	// Buffer events so bursts (such as the initial diff or fast token
	// streaming) do not cause a context switch on every event.
	ch := make(chan Event, 64)
	go func() {
		defer close(ch)

		for _, e := range events {
			if !send(ctx, ch, e) {
				return
			}
		}
		if inf == nil {
			return
		}
		// Skip events for messages that the initial diff already
		// sent. If the client's manifest already had a message from
		// this call, it is not in sent and will be re-sent here; the
		// client deduplicates by ID and UpdatedAt.
		inf.follow(ctx, ch, sent)
	}()
	return ch, nil
}

// send writes e to ch and returns true, or returns false if ctx is
// canceled before ch can accept e. This prevents a slow or disconnected
// subscriber from blocking the producer goroutine when ch is full.
func send(ctx context.Context, ch chan Event, e Event) bool {
	select {
	case ch <- e:
		return true
	case <-ctx.Done():
		return false
	}
}

// diffMessages compares saved rows against manifest.
//
// It returns:
//   - events: [MessageSaved] for each row missing from or newer than
//     manifest (in saved order), followed by [MessageDeleted] for each
//     ID in manifest that is not in rows.
//   - sent: the set of message IDs included in the [MessageSaved] events.
func diffMessages(
	rows []db.Message,
	manifest []MessageManifest,
) (events []Event, sent map[string]struct{}) {
	var (
		// known maps each client-cached message id to its timestamp.
		known = make(map[string]time.Time, len(manifest))
		// saved holds ids of all messages currently in the database.
		saved = make(map[string]struct{}, len(rows))
	)
	for _, m := range manifest {
		known[m.Id] = m.UpdatedAt
	}

	// Emit MessageSaved for new or updated rows, in saved order.
	sent = map[string]struct{}{}
	for _, row := range rows {
		saved[row.Id] = struct{}{}
		if at, ok := known[row.Id]; ok && at.Equal(row.UpdatedAt) {
			continue
		}
		events = append(events, MessageSaved{Message: row})
		sent[row.Id] = struct{}{}
	}

	// Emit MessageDeleted for client-cached ids no longer in the database.
	for id := range known {
		if _, ok := saved[id]; !ok {
			events = append(events, MessageDeleted{MessageId: id})
		}
	}

	return events, sent
}

// InferenceRequest supplies input and settings for one inference/tool loop.
// Runtime dependencies are passed separately to Inference.
type InferenceRequest struct {
	// Message supplies new user input, including text and attachments.
	// nil continues from saved history without adding a message.
	Message *llm.Message
	// Model is the LLM model ID. If empty, the default model is used.
	Model string
	// MaxIterations caps the agent loop iterations. Zero means unlimited.
	MaxIterations int
	// ThinkingBudgetTokens is the llama.cpp reasoning budget.
	// (-1 = unlimited, 0 = off).
	ThinkingBudgetTokens *int
	// Timeout limits the inference time (e.g., "10m").
	Timeout time.Duration
	// Stream selects provider streaming, not HTTP response delivery.
	// Failed streams never save partial assistant messages.
	// False requests a completed provider response.
	Stream bool
}

// Validate checks inference limits and optional user input.
func (r InferenceRequest) Validate() error {
	if r.MaxIterations < 0 {
		return fmt.Errorf("negative max_iterations")
	}
	if r.ThinkingBudgetTokens != nil && *r.ThinkingBudgetTokens < -1 {
		return fmt.Errorf("invalid thinking_budget_tokens")
	}
	if r.Timeout < 0 {
		return fmt.Errorf("negative timeout")
	}
	if m := r.Message; m != nil {
		if m.Role != llm.RoleUser || len(m.ToolCalls) != 0 ||
			m.ToolCallID != "" || m.Reasoning != "" ||
			m.ReasoningContent != "" || m.Refusal != "" {
			return fmt.Errorf("invalid message")
		}
		if err := m.Validate(); err != nil {
			return fmt.Errorf("invalid message: %w", err)
		}
	}
	return nil
}

// Reserve validates req and reserves the conversation for one inference
// call. It returns ErrBusy if an inference call is reserved or running.
//
// ctx bounds the call's work, not the call to Reserve: the call stops
// when ctx is canceled, when req.Timeout expires, or on Stop or Delete.
// Pass a context that outlives any single HTTP request, so a client
// disconnect does not stop the call.
//
// The caller must call [Inference.Run] on the result. Until Run returns,
// the conversation stays busy: other calls to Reserve and all edits
// return ErrBusy, even if Run is never called.
func (c *Conversation) Reserve(
	ctx context.Context,
	client *llm.Client,
	req InferenceRequest,
) (*Inference, error) {
	// Reject invalid input before accepting work.
	if client == nil {
		return nil, fmt.Errorf("missing llm client")
	}
	if err := req.Validate(); err != nil {
		return nil, fmt.Errorf("invalid inference request: %w", err)
	}

	// Limit the work duration and allow Stop to cancel it.
	var cancel context.CancelFunc
	if req.Timeout > 0 {
		ctx, cancel = context.WithTimeout(ctx, req.Timeout)
	} else {
		ctx, cancel = context.WithCancel(ctx)
	}

	// Check busy and reserve in one critical section, so no other
	// Reserve or edit can start between the check and the reservation.
	inf := &Inference{
		c:       c,
		client:  client,
		req:     req,
		ctx:     ctx,
		cancel:  cancel,
		changed: make(chan struct{}),
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		cancel()
		return nil, ErrBusy
	}
	c.cancel = cancel
	c.inf = inf
	return inf, nil
}

// Run resolves outstanding tool calls in saved history, saves optional
// new user input, and runs the model/tool loop, blocking until the work
// and result-save attempts finish. Pending approvals return
// ErrApprovalRequired after sibling calls finish their save attempts;
// new user input is not saved while calls remain unresolved.
//
// When Run returns, it sends [Done] to subscribers and releases the
// reservation. Call Run once; a second call returns an error.
func (inf *Inference) Run() (err error) {
	inf.mu.Lock()
	started := inf.started
	inf.started = true
	inf.mu.Unlock()
	if started {
		return fmt.Errorf("inference already run")
	}

	c := inf.c
	defer func() {
		// Every exit path, including errors, finishes the call.
		// Every failing path below must assign err, not shadow it.
		inf.finish(err)
		inf.cancel()
		c.mu.Lock()
		c.cancel = nil
		c.inf = nil
		c.mu.Unlock()
	}()

	var (
		ctx    = inf.ctx
		client = inf.client
		req    = inf.req
	)

	// Check that the conversation exists.
	conv, err := c.dbr.SelectConversationId(ctx, c.id)
	if err != nil {
		return fmt.Errorf("load conversation: %w", err)
	}
	log := c.log.With(
		slog.String("conversation_id", conv.Id),
	)

	// Load saved rows in conversation order.
	rows, err := c.dbr.SelectMessagesConversationId(ctx, c.id)
	if err != nil {
		return fmt.Errorf("load history: %w", err)
	}
	base := len(rows)

	// Convert db.Message into llm.Message; validate.
	history, err := DecodeMessages(rows)
	if err != nil {
		return fmt.Errorf("decode history: %w", err)
	}

	// Find the last message that is not a tool result.
	// For an assistant role with tool calls, run calls without a result.
	var (
		aId       string
		remaining []llm.ToolCall
		answered  = map[string]struct{}{}
	)
	for i, m := range slices.Backward(history) {
		if m.Role == llm.RoleTool {
			answered[m.ToolCallID] = struct{}{}
			continue
		}
		if len(m.ToolCalls) != 0 {
			aId = rows[i].Id
			for _, call := range m.ToolCalls {
				if _, ok := answered[call.ID]; !ok {
					remaining = append(remaining, call)
				}
			}
		}
		// Done: we've found the last non-tool message.
		break
	}

	// Resolve tool calls before adding input or requesting inference.
	if len(remaining) != 0 {
		log.InfoContext(
			ctx,
			"executing remaining tool calls",
			slog.Int("count", len(remaining)),
		)
		var tres []llm.Message
		tres, err = c.callTools(ctx, log, aId, remaining)
		if err != nil {
			return fmt.Errorf("call tools: %w", err)
		}
		history = append(history, tres...)
	}

	// Save new user input before requesting a completion.
	if req.Message != nil {
		log.InfoContext(ctx, "persisting new message")

		var saved *db.Message
		inf.mu.Lock()
		saved, err = c.insertMessage(
			ctx, nil, *req.Message, nil, nil,
		)
		if err == nil {
			inf.publish(MessageSaved{Message: *saved})
		}
		inf.mu.Unlock()
		if err != nil {
			return fmt.Errorf("insert message: %w", err)
		}
		log.InfoContext(
			ctx,
			"persisted new user message",
			slog.String("id", saved.Id),
		)

		// Give an untitled conversation its title before the model
		// runs, so a slow completion does not delay the title. A
		// failure here costs the title, not the run: the next user
		// message tries again.
		if err := c.setTitle(ctx, *req.Message); err != nil {
			log.WarnContext(
				ctx, "failed to derive conversation title",
				slog.Any("error", err),
			)
		}

		// Continue from the updated history.
		history = append(history, *req.Message)
	}

	// Loop for inference.
	var (
		completed bool
		start     time.Time
		tools     = c.tools.ToAPI()
	)
	log.InfoContext(ctx, "starting inference")
	start = time.Now()
	for i := 0; req.MaxIterations == 0 || i < req.MaxIterations; i++ {
		var (
			cp = llm.CompletionsParams{
				Messages:             history,
				Model:                req.Model,
				ThinkingBudgetTokens: req.ThinkingBudgetTokens,
				Tools:                tools,
			}
			// aid identifies the assistant message this iteration
			// produces, created before the request so the
			// assistant_update events and the saved row share it.
			aid    = uuid.NewV4()
			res    *llm.CompletionResult
			row    *db.Message
			stream *llm.CompletionStream
		)
		if req.Stream {
			stream, err = client.CompletionsStream(ctx, cp)
			if err != nil {
				err = fmt.Errorf("completions stream: %w", err)
				break
			}
			for stream.Next() {
				inf.mu.Lock()
				inf.publish(AssistantUpdate{
					MessageId: aid.String(),
					Chunk:     stream.Current(),
				})
				inf.mu.Unlock()
			}
			err = errors.Join(
				stream.Err(), stream.Close(),
			)
			if err != nil {
				break
			}
			sr := stream.Result()
			res = &sr
		} else {
			res, err = client.Completions(ctx, cp)
			if err != nil {
				err = fmt.Errorf("completions: %w", err)
				break
			}
		}

		// Reject invalid assistant output before saving it.
		if res.Message.Role != llm.RoleAssistant {
			err = fmt.Errorf(
				"invalid message role: %q", res.Message.Role)
			break
		}
		if err = res.Message.Validate(); err != nil {
			err = fmt.Errorf("invalid assistant message: %w", err)
			break
		}
		// Persist the assistant message before acting on it, so an
		// interruption leaves a committed record.
		inf.mu.Lock()
		row, err = c.insertMessage(
			ctx, &aid, res.Message, &res.Metadata, nil,
		)
		if err == nil {
			inf.publish(MessageSaved{Message: *row})
		}
		inf.mu.Unlock()
		if err != nil {
			err = fmt.Errorf("insert message: %w", err)
			break
		}
		history = append(history, res.Message)

		// If there are no more tool calls, we're done.
		if len(res.Message.ToolCalls) == 0 {
			completed = true
			break
		}

		// Resolve the reply's tool calls before the next request.
		tres, toolErr := c.callTools(
			ctx, log, row.Id, res.Message.ToolCalls,
		)
		if toolErr != nil {
			err = errors.Join(toolErr, ctx.Err())
			break
		}
		history = append(history, tres...)
	}
	duration := time.Since(start)
	if err == nil && !completed {
		err = ErrMaxIterations
	}
	if err != nil {
		log.ErrorContext(
			ctx,
			"inference failed",
			slog.Any("error", err),
			slog.Duration("duration", duration),
		)
		return err
	}
	log.InfoContext(
		ctx,
		"inference complete",
		slog.Duration("duration", duration),
		slog.Int("messages", len(history)-base),
	)
	return nil
}

// callTools runs the tool calls of the assistant message aId, up to
// twice the CPU count concurrently, and collects each persisted result.
// Each call saves its own result, so the returned order may differ from
// the saved order; results are keyed by tool call ID. The error joins
// every failure; if non-nil, at least one call has no result.
func (c *Conversation) callTools(
	ctx context.Context,
	log *slog.Logger,
	aId string,
	calls []llm.ToolCall,
) ([]llm.Message, error) {
	// Load each call's execution status.
	executions, err := c.dbr.SelectExecutionsMessageId(
		ctx, aId,
	)
	if err != nil {
		return nil, fmt.Errorf("load executions: %w", err)
	}
	callExecutions := make(map[string]*db.Execution, len(executions))
	for i := range executions {
		callExecutions[executions[i].ToolCallId] = &executions[i]
	}

	// Run calls concurrently, up to twice the CPU count at once.
	type outcome struct {
		err error
		msg llm.Message
	}
	var (
		completed = make(chan outcome, len(calls))
		sem       = make(chan struct{}, runtime.NumCPU()*2)
	)
	for _, call := range calls {
		go func() {
			select {
			case sem <- struct{}{}:
				defer func() { <-sem }()
			case <-ctx.Done():
				completed <- outcome{err: ctx.Err()}
				return
			}
			msg, err := c.callTool(
				ctx, log, aId, call, callExecutions[call.ID],
			)
			completed <- outcome{msg: msg, err: err}
		}()
	}

	// Wait for every worker, even if another worker fails.
	var (
		errs    []error
		results []llm.Message
	)
	for range calls {
		result := <-completed
		if result.err != nil {
			log.ErrorContext(
				ctx,
				"tool call failed",
				slog.Any("error", result.err),
			)
			errs = append(errs, result.err)
			continue
		}
		results = append(results, result.msg)
	}
	return results, errors.Join(errs...)
}

// callTool runs one tool call and saves the tool message that reports
// the outcome to the model.
//
// The record is a row in the executions table, saved before the tool
// starts. The row shows whether the tool may have run:
//   - No row: the tool never started. It is safe to run again.
//   - Row with status 'pending': the tool requires approval and never
//     started; it runs once approved.
//   - Row with status 'running': the tool may have run and had effects.
//     It is never retried.
//   - Row with status 'completed': the tool finished or never started,
//     and the tool message is saved.
//
// A nil execution is a new call: recorded running, or pending if the
// tool requires approval, in which case ErrApprovalRequired is returned
// before running. A recorded execution decides the rest: pending returns
// ErrApprovalRequired, approved is marked running and run, any other
// status is refused.
//
// Results:
//   - A message and a nil error: the message is saved. The message
//     reports the tool's outcome: exit code and output, the error that
//     kept the tool from starting, or the error that interrupted it.
//   - An error: the outcome was not recorded. The tool may have run and
//     had effects.
//
// The message contains:
//   - If the tool finished: its output.
//   - If it never started: the error.
//   - If it started but was stopped, or its output was cut off: the
//     error, a warning that its effects are unknown, and any output.
//
// Limitations:
//   - callTool relies on [tool.Registry.Execute] returning a nil result
//     only when the process never started.
func (c *Conversation) callTool(
	ctx context.Context,
	log *slog.Logger,
	aId string,
	call llm.ToolCall,
	execution *db.Execution,
) (llm.Message, error) {
	log = log.With(
		slog.String("tool_call_id", call.ID),
		slog.String("name", call.Function.Name),
	)

	// Decide whether and how the call may run. A case that ends without
	// returning has cleared the call to run and continues to the tail.
	switch {
	case execution == nil:
		// Record a new call before running it or requesting approval.
		status := db.ExecutionStatusRunning
		if definition, ok := c.tools.Get(call.Function.Name); ok &&
			definition.RequiresApproval {
			status = db.ExecutionStatusPending
		}
		var err error
		execution, err = c.dbw.InsertExecution(
			ctx,
			db.InsertExecutionParams{
				MessageId:  aId,
				ToolCallId: call.ID,
				Status:     status,
			},
		)
		if err != nil {
			return llm.Message{}, fmt.Errorf(
				"insert execution for call %q: %w", call.ID, err,
			)
		}
		if status == db.ExecutionStatusPending {
			return llm.Message{}, fmt.Errorf(
				"call %q: %w", call.ID, ErrApprovalRequired,
			)
		}
	case execution.Status == db.ExecutionStatusPending:
		return llm.Message{}, fmt.Errorf(
			"call %q: %w", call.ID, ErrApprovalRequired,
		)
	case execution.Status == db.ExecutionStatusApproved:
		if err := c.dbw.UpdateExecutionStatus(
			ctx,
			db.UpdateExecutionStatusParams{
				Id:     execution.Id,
				Status: db.ExecutionStatusRunning,
			},
		); err != nil {
			return llm.Message{}, fmt.Errorf(
				"start approved call %q: %w", call.ID, err,
			)
		}
	default:
		// An existing row that may have run is never retried.
		return llm.Message{}, fmt.Errorf(
			"call %q has status %q but no result; will not retry",
			call.ID, execution.Status,
		)
	}

	log.InfoContext(
		ctx,
		"calling tool",
		slog.String("args", call.Function.Arguments),
	)
	// Run the tool.
	tres, toolErr := c.tools.Execute(
		ctx, call.Function.Name, call.Function.Arguments,
	)
	var content string
	switch {
	case toolErr == nil:
		log.InfoContext(
			ctx,
			"called tool",
			slog.Duration(
				"duration",
				tres.FinishedAt.Sub(tres.StartedAt),
			),
		)
		content = tres.FormatResult()
	case tres == nil:
		log.ErrorContext(
			ctx,
			"tool did not start",
			slog.Any("error", toolErr),
		)
		content = toolErr.Error()
	default:
		log.ErrorContext(
			ctx,
			"tool interrupted",
			slog.Any("error", toolErr),
			slog.Duration(
				"duration",
				tres.FinishedAt.Sub(tres.StartedAt),
			),
		)
		// Retain output produced before interruption.
		content = toolErr.Error() + "\n" +
			interruptedWarning + "\n" +
			tres.FormatResult()
	}

	// Build the tool result msg.
	msg := llm.Message{
		Role:       llm.RoleTool,
		Name:       call.Function.Name,
		ToolCallID: call.ID,
		ContentParts: []llm.ContentPart{{
			Type: llm.ContentTypeText,
			Text: content,
		}},
	}
	if err := msg.Validate(); err != nil {
		return llm.Message{}, fmt.Errorf(
			"invalid message %q: %w", call.ID, err,
		)
	}
	// Attempt to save the result, even in case of cancellation.
	// Tool calls are expensive and may have side-effects; avoid throwing
	// away completed work. The status update, if any, commits with the
	// result.
	saveCtx, cancel := context.WithTimeout(
		context.WithoutCancel(ctx), resultSaveTimeout,
	)
	defer cancel()

	// Mark finished tools and failed launches as completed.
	// Keep interrupted calls as ExecutionStatusRunning;
	// they may have had effects.
	var update *db.UpdateExecutionStatusParams
	if toolErr == nil || tres == nil {
		update = &db.UpdateExecutionStatusParams{
			Id:     execution.Id,
			Status: db.ExecutionStatusCompleted,
		}
	}
	// c.inf is set for the whole of Inference.Run, the only caller path
	// to callTool.
	c.mu.Lock()
	inf := c.inf
	c.mu.Unlock()

	// Hold inf.mu across the commit so a concurrent Subscribe's diff
	// sees the row and its event together.
	inf.mu.Lock()
	saved, err := c.insertMessage(saveCtx, nil, msg, nil, update)
	if err == nil {
		inf.publish(MessageSaved{Message: *saved})
	}
	inf.mu.Unlock()
	if err != nil {
		log.ErrorContext(
			ctx,
			"failed to save tool result",
			slog.Any("error", err),
		)
		return llm.Message{}, fmt.Errorf(
			"save tool result %q: %w", call.ID, err,
		)
	}
	return msg, nil
}

// Stop cancels ongoing inference. It returns without waiting.
// Inference may still be finishing after Stop is called.
// Stop copies cancel to prevent canceling a new inference request.
func (c *Conversation) Stop() {
	c.mu.Lock()
	cancel := c.cancel
	c.mu.Unlock()

	if cancel != nil {
		cancel()
	}
}

// insertMessage stores msg under id, or a new version 4 UUID if id is
// nil, with its optional inference metadata at the next sequence in
// the conversation; nil metadata stores NULL. If executionUpdate is
// non-nil, its status change commits in the same transaction. The
// caller chooses the execution and a valid status transition.
func (c *Conversation) insertMessage(
	ctx context.Context,
	id *uuid.UUID,
	msg llm.Message,
	metadata *llm.Metadata,
	executionUpdate *db.UpdateExecutionStatusParams,
) (*db.Message, error) {
	content, err := json.Marshal(msg)
	if err != nil {
		return nil, fmt.Errorf("marshal content: %w", err)
	}
	var md jsontext.Value
	if metadata != nil {
		encoded, err := json.Marshal(
			metadata, json.FormatNilSliceAsNull(true),
		)
		if err != nil {
			return nil, fmt.Errorf("marshal metadata: %w", err)
		}
		md = jsontext.Value(encoded)
	}

	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	sequence, err := qtx.NextSequence(ctx, c.id)
	if err != nil {
		return nil, fmt.Errorf("get next message sequence: %w", err)
	}
	saved, err := qtx.InsertMessage(ctx, db.InsertMessageParams{
		Content:        content,
		ConversationId: c.id,
		Id:             id,
		Metadata:       md,
		Role:           db.MessageRole(msg.Role),
		Sequence:       sequence,
	})
	if err != nil {
		return nil, fmt.Errorf("insert message: %w", err)
	}
	if executionUpdate != nil {
		if err := qtx.UpdateExecutionStatus(
			ctx, *executionUpdate,
		); err != nil {
			return nil, fmt.Errorf(
				"update execution status: %w", err,
			)
		}
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit message tx: %w", err)
	}

	return saved, nil
}

// InsertMessage saves a new user or assistant message at the next
// sequence, validating it first. It returns ErrBusy while Inference is
// ongoing.
func (c *Conversation) InsertMessage(
	ctx context.Context,
	msg llm.Message,
) (*db.Message, error) {
	if msg.Role != llm.RoleAssistant && msg.Role != llm.RoleUser {
		return nil, fmt.Errorf(
			"invalid message role: %q", msg.Role,
		)
	}
	if err := msg.Validate(); err != nil {
		return nil, fmt.Errorf("invalid message: %w", err)
	}

	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	saved, err := c.insertMessage(ctx, nil, msg, nil, nil)
	if err != nil {
		return nil, err
	}

	// A user message can give an untitled conversation its title.
	// A failure here costs the title, not the saved message.
	if msg.Role == llm.RoleUser {
		if err := c.setTitle(ctx, msg); err != nil {
			c.log.WarnContext(
				ctx, "failed to derive conversation title",
				slog.Any("error", err),
			)
		}
	}
	return saved, nil
}

// UpdateMessage replaces a message's model-visible content; its role may
// not change. A mismatch between updatedAt and the saved updated_at
// returns ErrRevisionConflict. Inference metadata is preserved, and may
// no longer reflect initially generated output. It returns ErrBusy while
// Inference is ongoing.
func (c *Conversation) UpdateMessage(
	ctx context.Context,
	mId string,
	updatedAt time.Time,
	message llm.Message,
) (*db.Message, error) {
	if err := message.Validate(); err != nil {
		return nil, fmt.Errorf("invalid message: %w", err)
	}

	content, err := json.Marshal(message)
	if err != nil {
		return nil, fmt.Errorf("marshal message: %w", err)
	}

	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return nil, fmt.Errorf("load message %q: %w", mId, err)
	}
	// Refuse to update a message that has been changed.
	if !row.UpdatedAt.Equal(updatedAt) {
		return nil, ErrRevisionConflict
	}
	// Validate the role update.
	if llm.Role(row.Role) != message.Role {
		return nil, fmt.Errorf("invalid role: %q", message.Role)
	}

	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	updated, err := qtx.UpdateMessageContent(
		ctx,
		db.UpdateMessageContentParams{
			Id:      row.Id,
			Content: content,
		},
	)
	if err != nil {
		return nil, fmt.Errorf("update message %q: %w", row.Id, err)
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit message tx: %w", err)
	}
	return updated, nil
}

// DeleteMessage removes one saved message, checking for conflicting
// changes. A mismatch between updatedAt and the saved updated_at returns
// ErrRevisionConflict. It returns ErrBusy while Inference is ongoing.
func (c *Conversation) DeleteMessage(
	ctx context.Context,
	mId string,
	updatedAt time.Time,
) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return fmt.Errorf("load message %q: %w", mId, err)
	}

	// Refuse to delete a message if it has been updated.
	if !row.UpdatedAt.Equal(updatedAt) {
		return ErrRevisionConflict
	}

	// Delete the row and close the sequence gap it leaves, in one
	// transaction.
	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	if err := qtx.DeleteMessageId(ctx, row.Id); err != nil {
		return fmt.Errorf("delete message %q: %w", row.Id, err)
	}
	if err := qtx.UpdateMessagesSequence(ctx, c.id, row.Sequence); err != nil {
		return fmt.Errorf(
			"shift sequences after %d: %w", row.Sequence, err,
		)
	}
	if err := tx.Commit(); err != nil {
		return fmt.Errorf("commit message tx: %w", err)
	}
	return nil
}

// AddToolCall appends a tool call to a saved assistant message.
// mId identifies that assistant; updatedAt is the message's
// updated_at from the caller's last read; a mismatch returns
// ErrRevisionConflict. This edit does not execute the tool: it creates
// no execution row and no result. It returns ErrBusy while Inference is
// ongoing.
func (c *Conversation) AddToolCall(
	ctx context.Context,
	mId string,
	updatedAt time.Time,
	call llm.ToolCall,
) (*db.Message, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return nil, fmt.Errorf("load message %q: %w", mId, err)
	}

	// Refuse to update a message that has been changed.
	if !row.UpdatedAt.Equal(updatedAt) {
		return nil, ErrRevisionConflict
	}
	if row.Role != db.MessageRoleAssistant {
		return nil, fmt.Errorf("invalid role: %q", row.Role)
	}

	var message llm.Message
	if err := json.Unmarshal(
		row.Content, &message,
	); err != nil {
		return nil, fmt.Errorf(
			"unmarshal message %q: %w", row.Id, err,
		)
	}

	if err := message.AddToolCall(call); err != nil {
		return nil, fmt.Errorf("message %q: %w", row.Id, err)
	}

	content, err := json.Marshal(message)
	if err != nil {
		return nil, fmt.Errorf("marshal updated message: %w", err)
	}

	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	updated, err := qtx.UpdateMessageContent(
		ctx,
		db.UpdateMessageContentParams{
			Id:      row.Id,
			Content: content,
		},
	)
	if err != nil {
		return nil, fmt.Errorf("update message %q: %w", row.Id, err)
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit message tx: %w", err)
	}
	return updated, nil
}

// UpdateToolCall replaces a call in a saved assistant message. mId
// identifies the message; callId identifies the call within it. The
// replacement must keep the existing call ID: the call's execution
// record and any saved result are keyed by it. updatedAt is the
// assistant message's updated_at from the caller's last read; a mismatch
// returns ErrRevisionConflict. It returns ErrBusy while Inference is
// ongoing.
func (c *Conversation) UpdateToolCall(
	ctx context.Context,
	mId string,
	callId string,
	updatedAt time.Time,
	call llm.ToolCall,
) (*db.Message, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return nil, fmt.Errorf("load message %q: %w", mId, err)
	}

	// Refuse to update a message that has been changed.
	if !row.UpdatedAt.Equal(updatedAt) {
		return nil, ErrRevisionConflict
	}
	// Validate the role update.
	if row.Role != db.MessageRoleAssistant {
		return nil, fmt.Errorf("invalid role: %q", row.Role)
	}

	var message llm.Message
	if err := json.Unmarshal(
		row.Content, &message,
	); err != nil {
		return nil, fmt.Errorf(
			"unmarshal message %q: %w", row.Id, err,
		)
	}

	if err := message.UpdateToolCall(call); err != nil {
		return nil, fmt.Errorf("message %q: %w", row.Id, err)
	}

	content, err := json.Marshal(message)
	if err != nil {
		return nil, fmt.Errorf("marshal updated message: %w", err)
	}

	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	updated, err := qtx.UpdateMessageContent(
		ctx,
		db.UpdateMessageContentParams{
			Id:      row.Id,
			Content: content,
		},
	)
	if err != nil {
		return nil, fmt.Errorf("update message %q: %w", row.Id, err)
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit message tx: %w", err)
	}
	return updated, nil
}

// DeleteToolCall removes a call from a saved assistant message, deleting
// the call's saved result and execution record with it. mId identifies
// the parent assistant. updatedAt is the assistant's updated_at from the
// caller's last read; a mismatch returns ErrRevisionConflict. It returns
// ErrBusy while Inference is ongoing.
func (c *Conversation) DeleteToolCall(
	ctx context.Context,
	mId string,
	callId string,
	updatedAt time.Time,
) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return fmt.Errorf("load assistant %q: %w", mId, err)
	}

	// Refuse to delete if the assistant has been changed.
	if !row.UpdatedAt.Equal(updatedAt) {
		return ErrRevisionConflict
	}
	if row.Role != db.MessageRoleAssistant {
		return fmt.Errorf("message %q is not an assistant", mId)
	}
	var message llm.Message
	if err := json.Unmarshal(
		row.Content, &message,
	); err != nil {
		return fmt.Errorf(
			"unmarshal assistant %q: %w", row.Id, err,
		)
	}
	var hasCall bool
	for _, call := range message.ToolCalls {
		if call.ID == callId {
			hasCall = true
			break
		}
	}
	if !hasCall {
		return fmt.Errorf(
			"assistant %q has no call %q", row.Id, callId,
		)
	}

	// Remove the call from the assistant's content.
	calls := make([]llm.ToolCall, 0, len(message.ToolCalls)-1)
	for _, call := range message.ToolCalls {
		if call.ID != callId {
			calls = append(calls, call)
		}
	}
	message.ToolCalls = calls
	content, err := json.Marshal(message)
	if err != nil {
		return fmt.Errorf("marshal assistant: %w", err)
	}

	// Delete the call, its result, and its execution record together.
	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	sequence, err := qtx.SelectMessageSequenceToolResult(
		ctx, c.id, row.Sequence,
	)
	if err != nil {
		return fmt.Errorf("find tool result sequence: %w", err)
	}
	result, err := qtx.SelectConversationToolResult(
		ctx, c.id, row.Sequence, sequence, callId,
	)
	if err != nil && !errors.Is(err, sql.ErrNoRows) {
		return fmt.Errorf("load tool result %q: %w", callId, err)
	}

	if _, err := qtx.UpdateMessageContent(ctx, db.UpdateMessageContentParams{
		Id:      row.Id,
		Content: content,
	}); err != nil {
		return fmt.Errorf("update assistant %q: %w", row.Id, err)
	}
	if result != nil {
		if err := qtx.DeleteMessageId(ctx, result.Id); err != nil {
			return fmt.Errorf("delete result %q: %w", result.Id, err)
		}
		if err := qtx.UpdateMessagesSequence(
			ctx, c.id, result.Sequence,
		); err != nil {
			return fmt.Errorf(
				"shift sequences after %d: %w",
				result.Sequence, err,
			)
		}
	}
	if err := qtx.DeleteExecutionMessageIdToolCallId(
		ctx, row.Id, callId,
	); err != nil {
		return fmt.Errorf(
			"delete execution for call %q: %w", callId, err,
		)
	}
	if err := tx.Commit(); err != nil {
		return fmt.Errorf("commit tool call tx: %w", err)
	}
	return nil
}

// AddToolResult saves textual output for a call in the assistant message
// mId, right after that assistant's last saved tool result; later
// messages shift down by one. It does not execute the tool, and it fails
// if the call already has a result. updatedAt is the assistant message's
// updated_at from the caller's last read; a mismatch returns
// ErrRevisionConflict. The call's execution is marked completed, or
// created as completed if missing. It returns ErrBusy while Inference is
// ongoing.
func (c *Conversation) AddToolResult(
	ctx context.Context,
	mId string,
	callId string,
	updatedAt time.Time,
	content string,
) (*db.Message, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return nil, fmt.Errorf("load assistant %q: %w", mId, err)
	}

	// Refuse to add a result if the assistant has been changed.
	if !row.UpdatedAt.Equal(updatedAt) {
		return nil, ErrRevisionConflict
	}
	if row.Role != db.MessageRoleAssistant {
		return nil, fmt.Errorf("message %q is not an assistant", mId)
	}
	var message llm.Message
	if err := json.Unmarshal(
		row.Content, &message,
	); err != nil {
		return nil, fmt.Errorf(
			"unmarshal assistant %q: %w", row.Id, err,
		)
	}
	var call *llm.ToolCall
	for i := range message.ToolCalls {
		if message.ToolCalls[i].ID == callId {
			call = &message.ToolCalls[i]
			break
		}
	}
	if call == nil {
		return nil, fmt.Errorf(
			"assistant %q has no call %q", row.Id, callId,
		)
	}

	// Build the tool result message.
	result := llm.Message{
		Role:       llm.RoleTool,
		Name:       call.Function.Name,
		ToolCallID: call.ID,
		ContentParts: []llm.ContentPart{{
			Type: llm.ContentTypeText,
			Text: content,
		}},
	}
	if err := result.Validate(); err != nil {
		return nil, fmt.Errorf("invalid tool result: %w", err)
	}
	encoded, err := json.Marshal(result)
	if err != nil {
		return nil, fmt.Errorf(
			"marshal tool result %q: %w", callId, err,
		)
	}

	// Find the position, free it, insert the result, and complete the
	// execution together, so the position and duplicate check stay valid.
	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	sequence, err := qtx.SelectMessageSequenceToolResult(
		ctx, c.id, row.Sequence,
	)
	if err != nil {
		return nil, fmt.Errorf("find tool result sequence: %w", err)
	}
	if _, err := qtx.SelectConversationToolResult(
		ctx, c.id, row.Sequence, sequence, callId,
	); err == nil {
		return nil, fmt.Errorf("call %q already has a result", callId)
	} else if !errors.Is(err, sql.ErrNoRows) {
		return nil, fmt.Errorf("check tool result %q: %w", callId, err)
	}
	if err := qtx.UpdateMessagesSequenceIncrement(
		ctx, c.id, sequence,
	); err != nil {
		return nil, fmt.Errorf(
			"shift sequences from %d: %w", sequence, err,
		)
	}
	msg, err := qtx.InsertMessage(ctx, db.InsertMessageParams{
		Content:        encoded,
		ConversationId: c.id,
		Role:           db.MessageRoleTool,
		Sequence:       sequence,
	})
	if err != nil {
		return nil, fmt.Errorf("save tool result %q: %w", callId, err)
	}
	if err := qtx.InsertExecutionCompleted(
		ctx, row.Id, callId,
	); err != nil {
		return nil, fmt.Errorf("complete execution %q: %w", callId, err)
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit tool result tx: %w", err)
	}
	return msg, nil
}

// UpdateToolResult replaces the saved textual output for a call in the
// assistant message mId. Unlike AddToolResult, updatedAt is the tool
// result message's updated_at (not the assistant message's); a mismatch
// returns ErrRevisionConflict. It does not execute the tool, and fails
// if the call has no result. The execution is marked completed, or
// created as completed if missing, settling an interrupted call's
// running row. It returns ErrBusy while Inference is ongoing.
func (c *Conversation) UpdateToolResult(
	ctx context.Context,
	mId string,
	callId string,
	updatedAt time.Time,
	content string,
) (*db.Message, error) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		return nil, ErrBusy
	}

	row, err := c.dbr.SelectConversationMessageId(ctx, c.id, mId)
	if err != nil {
		return nil, fmt.Errorf("load assistant %q: %w", mId, err)
	}
	if row.Role != db.MessageRoleAssistant {
		return nil, fmt.Errorf("message %q is not an assistant", mId)
	}
	var message llm.Message
	if err := json.Unmarshal(
		row.Content, &message,
	); err != nil {
		return nil, fmt.Errorf(
			"unmarshal assistant %q: %w", row.Id, err,
		)
	}
	var call *llm.ToolCall
	for i := range message.ToolCalls {
		if message.ToolCalls[i].ID == callId {
			call = &message.ToolCalls[i]
			break
		}
	}
	if call == nil {
		return nil, fmt.Errorf(
			"assistant %q has no call %q", row.Id, callId,
		)
	}

	// Build the replacement tool result message.
	result := llm.Message{
		Role:       llm.RoleTool,
		Name:       call.Function.Name,
		ToolCallID: call.ID,
		ContentParts: []llm.ContentPart{{
			Type: llm.ContentTypeText,
			Text: content,
		}},
	}
	if err := result.Validate(); err != nil {
		return nil, fmt.Errorf("invalid tool result: %w", err)
	}
	encoded, err := json.Marshal(result)
	if err != nil {
		return nil, fmt.Errorf(
			"marshal tool result %q: %w", callId, err,
		)
	}

	// Load and update the result, and complete the execution
	// together, so the revision check stays valid.
	tx, err := c.db.Writer.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	qtx := c.dbw.WithTx(tx)
	sequence, err := qtx.SelectMessageSequenceToolResult(
		ctx, c.id, row.Sequence,
	)
	if err != nil {
		return nil, fmt.Errorf("find tool result sequence: %w", err)
	}
	saved, err := qtx.SelectConversationToolResult(
		ctx, c.id, row.Sequence, sequence, callId,
	)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, fmt.Errorf("call %q has no result", callId)
	}
	if err != nil {
		return nil, fmt.Errorf("load tool result %q: %w", callId, err)
	}
	// Refuse to update a result that has been changed.
	if !saved.UpdatedAt.Equal(updatedAt) {
		return nil, ErrRevisionConflict
	}
	updated, err := qtx.UpdateMessageContent(
		ctx,
		db.UpdateMessageContentParams{
			Id:      saved.Id,
			Content: encoded,
		},
	)
	if err != nil {
		return nil, fmt.Errorf("update tool result %q: %w", callId, err)
	}
	if err := qtx.InsertExecutionCompleted(
		ctx, row.Id, callId,
	); err != nil {
		return nil, fmt.Errorf("complete execution %q: %w", callId, err)
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit tool result tx: %w", err)
	}
	return updated, nil
}

// Delete removes the saved conversation and all its messages and
// execution records.
// If inference is ongoing, it is canceled.
func (c *Conversation) Delete(ctx context.Context) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.cancel != nil {
		c.cancel()
	}
	err := c.dbw.DeleteConversationId(ctx, c.id)
	if err != nil {
		return fmt.Errorf("delete conversation: %w", err)
	}
	return nil
}