package agent

import (
	"context"
	"fmt"
	"sync"
)

// SubagentSlotStatus is the queue lifecycle shown for background task/fleet
// items that share the session scheduler.
type SubagentSlotStatus string

const (
	SubagentSlotQueued  SubagentSlotStatus = "queued"
	SubagentSlotRunning SubagentSlotStatus = "running"
	SubagentSlotDone    SubagentSlotStatus = "done"
	SubagentSlotFailed  SubagentSlotStatus = "failed"
)

// AcquireRequest describes a sub-agent slot request against the session pool.
type AcquireRequest struct {
	// Writer is true for writer-capable runs (task without read_only, profile
	// that is not read-only, fleet items that can write).
	Writer bool
	// WritePaths is the claim held while the slot is active. Empty for
	// read-only work. Whole-workspace claims count as writers and serialize
	// against every other writer.
	WritePaths WritePathSet
	// Nested fails immediately when no capacity is free instead of queueing.
	// Nested sub-agents must not block waiting for a parent-held slot.
	Nested bool
	// Label is optional diagnostics text.
	Label string
}

// SubagentScheduler is a session-scoped concurrency controller shared by task,
// fleet, parallel_tasks, profile skills, and nested sub-agents.
type SubagentScheduler struct {
	mu sync.Mutex

	maxTotal   int
	maxWriters int

	activeTotal   int
	activeWriters int
	activeLive    []liveClaim
	nextClaimID   int64
	// parentClaims are write paths held by the parent agent during a write-tool
	// Execute. They block overlapping subagent claims without consuming a
	// subagent concurrency slot (parent is not a subagent).
	parentClaims []WritePathSet

	// waiters are FIFO waiters for non-nested acquires.
	waiters []*schedulerWaiter
}

type schedulerWaiter struct {
	req    AcquireRequest
	ready  chan struct{}
	failed error
	id     int64
}

// NewSubagentScheduler builds a scheduler with the given limits (normalized).
func NewSubagentScheduler(maxTotal, maxWriters int) *SubagentScheduler {
	maxTotal, maxWriters = NormalizeConcurrencyLimits(maxTotal, maxWriters)
	return &SubagentScheduler{maxTotal: maxTotal, maxWriters: maxWriters}
}

// Limits returns the effective total/writer caps.
func (s *SubagentScheduler) Limits() (total, writers int) {
	if s == nil {
		return DefaultMaxSubagentConcurrency, DefaultMaxParallelWriters
	}
	return s.maxTotal, s.maxWriters
}

// Acquire reserves a concurrency slot (and optional write claim). Nested
// requests fail immediately when capacity is exhausted. Non-nested requests
// queue until capacity is free or ctx is cancelled.
//
// The returned release function must be called exactly once when the sub-agent
// finishes. release is safe to call even if Acquire returns an error (no-op).
func (s *SubagentScheduler) Acquire(ctx context.Context, req AcquireRequest) (release func(), err error) {
	release, _, err = s.AcquireWithID(ctx, req)
	return release, err
}

// AcquireWithID is Acquire plus the live claim id used by Realize/MarkOpaque.
func (s *SubagentScheduler) AcquireWithID(ctx context.Context, req AcquireRequest) (release func(), claimID int64, err error) {
	noop := func() {}
	if s == nil {
		return noop, 0, nil
	}
	if ctx == nil {
		ctx = context.Background()
	}

	s.mu.Lock()
	if ok, reason := s.canStartIncomingLocked(req); ok {
		id := s.activateLocked(req)
		s.mu.Unlock()
		return s.makeReleaseID(id), id, nil
	} else if req.Nested {
		s.mu.Unlock()
		return noop, 0, fmt.Errorf("subagent concurrency limit reached (%s); nested subagents fail fast to avoid parent/child slot deadlock", reason)
	}

	w := &schedulerWaiter{req: req, ready: make(chan struct{})}
	s.waiters = append(s.waiters, w)
	s.mu.Unlock()

	select {
	case <-w.ready:
		if w.failed != nil {
			return noop, 0, w.failed
		}
		return s.makeReleaseID(w.id), w.id, nil
	case <-ctx.Done():
		s.mu.Lock()
		s.removeWaiterLocked(w)
		s.pumpWaitersLocked()
		s.mu.Unlock()
		select {
		case <-w.ready:
			if w.failed == nil {
				s.makeReleaseID(w.id)()
			}
		default:
		}
		return noop, 0, ctx.Err()
	}
}

// TryClaimWritePaths checks whether paths conflict with active claims without
// taking a concurrency slot. Used for diagnostics; prefer ReserveParentWrite
// for parent agent writes so the check is not TOCTOU with subagent Acquire.
func (s *SubagentScheduler) TryClaimWritePaths(paths WritePathSet) error {
	if s == nil || paths.Empty() {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.conflictLocked(paths)
}

// Realize records path-bound writes against an active claim. Directory and
// whole-workspace declarations shrink to the realized files when no opaque
// mutation has occurred. Same-file realizes from two live writers fail.
func (s *SubagentScheduler) Realize(id int64, paths WritePathSet) error {
	if s == nil || id == 0 || paths.Empty() {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	idx := s.liveIndexLocked(id)
	if idx < 0 {
		return fmt.Errorf("write path is claimed by a running background subagent; wait for it to finish before writing the same path")
	}
	claim := s.activeLive[idx]
	if claim.opaque {
		return nil
	}
	nextPaths := mergeRealized(claim.realized, paths)
	next := fileReservation(claim.declared.WorkspaceRoot, nextPaths)
	if err := s.conflictAgainstOthersLocked(id, next); err != nil {
		return err
	}
	claim.realized = nextPaths
	s.activeLive[idx] = claim
	s.pumpWaitersLocked()
	return nil
}

// MarkOpaque upgrades a live claim to a whole-workspace reservation (bash/MCP).
func (s *SubagentScheduler) MarkOpaque(id int64) error {
	if s == nil || id == 0 {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	idx := s.liveIndexLocked(id)
	if idx < 0 {
		return fmt.Errorf("write path is claimed by a running background subagent; wait for it to finish before writing the same path")
	}
	claim := s.activeLive[idx]
	if claim.opaque {
		return nil
	}
	next := wholeReservation(claim.declared.WorkspaceRoot)
	if err := s.conflictAgainstOthersLocked(id, next); err != nil {
		return err
	}
	claim.opaque = true
	s.activeLive[idx] = claim
	return nil
}

// ReserveParentWrite holds paths against overlapping subagent claims for the
// duration of a parent write-tool Execute. It does not consume subagent
// concurrency slots. On conflict it fails immediately (parent cannot queue
// behind background jobs mid-tool-call). release must be called once when the
// write finishes so queued subagents can proceed.
func (s *SubagentScheduler) ReserveParentWrite(paths WritePathSet) (release func(), err error) {
	noop := func() {}
	if s == nil || paths.Empty() {
		return noop, nil
	}
	s.mu.Lock()
	if err := s.conflictLocked(paths); err != nil {
		s.mu.Unlock()
		return noop, err
	}
	s.parentClaims = append(s.parentClaims, paths)
	s.mu.Unlock()

	var once sync.Once
	return func() {
		once.Do(func() {
			s.mu.Lock()
			s.parentClaims = removeClaim(s.parentClaims, paths)
			s.pumpWaitersLocked()
			s.mu.Unlock()
		})
	}, nil
}

// ActiveWriterClaims returns a snapshot of subagent + parent write claims.
func (s *SubagentScheduler) ActiveWriterClaims() []WritePathSet {
	if s == nil {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make([]WritePathSet, 0, len(s.activeLive)+len(s.parentClaims))
	for _, live := range s.activeLive {
		if !live.writer {
			continue
		}
		res := live.reservation()
		if res.Empty() {
			if live.declared.Empty() {
				continue
			}
			res = live.declared
		}
		out = append(out, res)
	}
	out = append(out, s.parentClaims...)
	return out
}

func (s *SubagentScheduler) conflictLocked(paths WritePathSet) error {
	if paths.WholeWorkspace {
		for _, live := range s.activeLive {
			if live.writer {
				return fmt.Errorf("write path is claimed by a running background subagent; wait for it to finish before writing the same path")
			}
		}
	}
	return s.conflictAgainstOthersLocked(0, paths)
}

func (s *SubagentScheduler) conflictAgainstOthersLocked(skipID int64, paths WritePathSet) error {
	if paths.Empty() {
		return nil
	}
	for _, live := range s.activeLive {
		if live.id == skipID {
			continue
		}
		if ScheduleOverlaps(live.reservation(), paths) {
			return fmt.Errorf("write path is claimed by a running background subagent; wait for it to finish before writing the same path")
		}
	}
	for _, active := range s.parentClaims {
		if ScheduleOverlaps(active, paths) {
			return fmt.Errorf("write path is claimed by another parent write in progress")
		}
	}
	return nil
}

func (s *SubagentScheduler) makeReleaseID(id int64) func() {
	var once sync.Once
	return func() {
		once.Do(func() {
			s.mu.Lock()
			s.deactivateIDLocked(id)
			s.pumpWaitersLocked()
			s.mu.Unlock()
		})
	}
}

func (s *SubagentScheduler) liveIndexLocked(id int64) int {
	for i, live := range s.activeLive {
		if live.id == id {
			return i
		}
	}
	return -1
}

func (s *SubagentScheduler) canStartLocked(req AcquireRequest) (bool, string) {
	if s.activeTotal >= s.maxTotal {
		return false, fmt.Sprintf("total concurrency %d/%d", s.activeTotal, s.maxTotal)
	}
	if !req.Writer {
		return true, ""
	}
	if s.activeWriters >= s.maxWriters {
		return false, fmt.Sprintf("writer concurrency %d/%d", s.activeWriters, s.maxWriters)
	}
	if req.WritePaths.WholeWorkspace {
		for _, live := range s.activeLive {
			if live.writer {
				return false, "whole-workspace claim conflicts with a running writer"
			}
		}
	}
	for _, live := range s.activeLive {
		if ScheduleOverlaps(req.WritePaths, live.reservation()) {
			return false, "write path conflict with a running subagent"
		}
	}
	for _, active := range s.parentClaims {
		if ScheduleOverlaps(req.WritePaths, active) {
			return false, "write path conflict with a parent write in progress"
		}
	}
	return true, ""
}

func (s *SubagentScheduler) activateLocked(req AcquireRequest) int64 {
	s.activeTotal++
	s.nextClaimID++
	id := s.nextClaimID
	if req.Writer {
		s.activeWriters++
	}
	s.activeLive = append(s.activeLive, liveClaim{id: id, writer: req.Writer, declared: req.WritePaths})
	return id
}

func (s *SubagentScheduler) deactivateIDLocked(id int64) {
	idx := s.liveIndexLocked(id)
	if idx < 0 {
		return
	}
	if s.activeTotal > 0 {
		s.activeTotal--
	}
	if s.activeLive[idx].writer && s.activeWriters > 0 {
		s.activeWriters--
	}
	s.activeLive = append(s.activeLive[:idx], s.activeLive[idx+1:]...)
}

func (s *SubagentScheduler) pumpWaitersLocked() {
	if len(s.waiters) == 0 {
		return
	}
	remaining := s.waiters[:0]
	// A blocked whole-workspace writer is a FIFO barrier for later writers,
	// while read-only work may still use otherwise available capacity.
	wholeWriterPending := false
	for _, w := range s.waiters {
		if wholeWriterPending && w.req.Writer {
			remaining = append(remaining, w)
			continue
		}
		if ok, _ := s.canStartLocked(w.req); ok {
			w.id = s.activateLocked(w.req)
			close(w.ready)
			continue
		}
		remaining = append(remaining, w)
		if w.req.Writer && w.req.WritePaths.WholeWorkspace {
			wholeWriterPending = true
		}
	}
	s.waiters = remaining
}

func (s *SubagentScheduler) removeWaiterLocked(target *schedulerWaiter) {
	if len(s.waiters) == 0 {
		return
	}
	out := s.waiters[:0]
	for _, w := range s.waiters {
		if w == target {
			continue
		}
		out = append(out, w)
	}
	s.waiters = out
}

func removeClaim(claims []WritePathSet, target WritePathSet) []WritePathSet {
	for i, c := range claims {
		if writeClaimEqual(c, target) {
			return append(claims[:i], claims[i+1:]...)
		}
	}
	return claims
}

func writeClaimEqual(a, b WritePathSet) bool {
	if a.WholeWorkspace != b.WholeWorkspace || a.WorkspaceRoot != b.WorkspaceRoot {
		return false
	}
	if len(a.Paths) != len(b.Paths) {
		return false
	}
	for i := range a.Paths {
		if a.Paths[i] != b.Paths[i] {
			return false
		}
	}
	return true
}