package lane

import (
	"context"
	"sync"
)

// ConvarReader is the narrow surface the registry uses to read
// per-lane concurrency caps from convars at startup and on Reload.
//
// Why an interface (not pkg/convar directly): registry is a generic
// primitive and shouldn't import the application convar package.
// Production wires a thin adapter; tests pass a fake.
type ConvarReader interface {
	Int(ctx context.Context, name string, def int) int
}

// ConvarReaderFunc adapts a closure into a ConvarReader.
type ConvarReaderFunc func(ctx context.Context, name string, def int) int

// Int satisfies ConvarReader.
func (f ConvarReaderFunc) Int(ctx context.Context, name string, def int) int {
	if f == nil {
		return def
	}
	return f(ctx, name, def)
}

// Registry is a manager of named lanes. The default policy is
// fair-share; lanes are created lazily on first GetOrCreate, with
// concurrency read from convar `lanes.<name>.max_concurrent` (default
// 1). Reload re-reads convars and updates each lane's MaxConcurrent
// in place — useful for runtime tuning without losing in-flight work.
//
// Why a singleton-ish manager vs constructing lanes ad-hoc: the
// registry is the integration point where mort.go wires lanes once
// and every subsystem (LLM transport, skill runner) looks them up by
// name. Lazy creation lets the registry stay schema-free — adding a
// new lane is just "ask for it by name".
//
// Test: registry_test.go covers GetOrCreate identity, convar read,
// and Reload.
type Registry struct {
	mu      sync.RWMutex
	lanes   map[string]Lane
	convars ConvarReader
	// policyFactory is the queue policy constructor used for new
	// lanes. Defaults to NewFairSharePolicy. Tests substitute FIFO
	// when they want deterministic ordering.
	policyFactory func() queuePolicy
}

// NewRegistry constructs a registry. convars may be nil — lanes
// fall back to the registry's default concurrency (1).
func NewRegistry(convars ConvarReader) *Registry {
	return &Registry{
		lanes:         make(map[string]Lane),
		convars:       convars,
		policyFactory: NewFairSharePolicy,
	}
}

// SetPolicyFactory overrides the default policy used for new lanes.
// Existing lanes are unchanged. Used by tests; production keeps the
// fair-share default.
func (r *Registry) SetPolicyFactory(f func() queuePolicy) {
	if f == nil {
		f = NewFairSharePolicy
	}
	r.mu.Lock()
	r.policyFactory = f
	r.mu.Unlock()
}

// Get returns the named lane or nil if it has not been created.
// Useful in admin/UI code that wants to show only existing lanes
// without creating new ones as a side effect.
func (r *Registry) Get(name string) Lane {
	r.mu.RLock()
	defer r.mu.RUnlock()
	return r.lanes[name]
}

// StatsReader is the read-only stats surface exposed to admin / user
// dashboards (Discord queue commands, /skills/admin/queues web view).
// *Registry satisfies it; tests substitute a fake.
//
// Why a narrow interface (vs passing *Registry around): the consumers
// only need stats and lane lookup — no creation or mutation surface.
// Keeping the dep narrow makes mocks trivial in webui + skills tests.
type StatsReader interface {
	// List returns a snapshot of every registered lane.
	List() []Lane

	// Lookup returns the lane by name, or nil. Mirrors Registry.Get
	// (named differently to avoid the "Get" verb confusion in
	// dashboards that primarily call Stats).
	Lookup(name string) Lane
}

// Lookup satisfies the StatsReader surface alongside Registry.Get. We
// expose both verbs so the dashboard code reads naturally without
// forcing existing call sites that use Get() to migrate.
func (r *Registry) Lookup(name string) Lane { return r.Get(name) }

// GetOrCreate returns the named lane, creating it lazily on first
// call. Concurrency is read from convar `lanes.<name>.max_concurrent`
// (default 1). The policy is the registry's policy factory (default
// fair-share).
//
// Why convar name `lanes.<name>.max_concurrent` (not
// `skills.lane.<name>.max_concurrent`): pkg/lane is generic — the
// skills system happens to be the first caller, but the LLM transport
// wrapper (Phase 3) and other future runners will use the same
// registry. The convar namespace `lanes.*` keeps lane configuration
// in one place. The skills system can adopt different convar names
// if it prefers; in that case, mort.go reads them and calls
// SetMaxConcurrent on the resulting lanes after creation.
func (r *Registry) GetOrCreate(ctx context.Context, name string) Lane {
	r.mu.RLock()
	if l, ok := r.lanes[name]; ok {
		r.mu.RUnlock()
		return l
	}
	r.mu.RUnlock()

	r.mu.Lock()
	defer r.mu.Unlock()
	// Double-check after upgrading the lock.
	if l, ok := r.lanes[name]; ok {
		return l
	}
	maxConcurrent := r.readConcurrency(ctx, name)
	policy := r.policyFactory()
	if policy == nil {
		policy = NewFairSharePolicy()
	}
	l := NewWithPolicy(name, maxConcurrent, policy)
	r.lanes[name] = l
	return l
}

// List returns a snapshot of all registered lanes. Iteration order is
// not guaranteed (Go map randomization).
func (r *Registry) List() []Lane {
	r.mu.RLock()
	defer r.mu.RUnlock()
	out := make([]Lane, 0, len(r.lanes))
	for _, l := range r.lanes {
		out = append(out, l)
	}
	return out
}

// Names returns the registered lane names. Used for the admin
// "list all lanes" surface.
func (r *Registry) Names() []string {
	r.mu.RLock()
	defer r.mu.RUnlock()
	out := make([]string, 0, len(r.lanes))
	for n := range r.lanes {
		out = append(out, n)
	}
	return out
}

// Reload re-reads convars for every registered lane and calls
// SetMaxConcurrent on each. Existing running jobs continue to run;
// new dispatches respect the updated cap.
//
// Why a manual Reload instead of reading convars at every dispatch:
// dispatch is on the hot path; reading a convar there for every
// queued job is wasteful. A periodic Reload (every minute, say) is
// cheap and good enough for human-driven config changes.
func (r *Registry) Reload(ctx context.Context) {
	r.mu.RLock()
	defer r.mu.RUnlock()
	for name, l := range r.lanes {
		n := r.readConcurrency(ctx, name)
		l.SetMaxConcurrent(n)
	}
}

// readConcurrency reads `lanes.<name>.max_concurrent` with a default
// of 1. Defensive against a nil ConvarReader and against negative
// values (clamped to 1).
func (r *Registry) readConcurrency(ctx context.Context, name string) int {
	if r.convars == nil {
		return 1
	}
	n := r.convars.Int(ctx, "lanes."+name+".max_concurrent", 1)
	if n <= 0 {
		return 1
	}
	return n
}