package lane

import (
	"sort"
	"time"
)

// fairSharePolicy implements queuePolicy with per-user sub-queues.
// Dequeue rotates through users round-robin so one user can't starve
// others. Within a user's queue, higher priority comes first; ties
// broken FIFO.
//
// Why round-robin not weighted-fair: simple, no tuning. If user A has
// 5 queued and user B has 1, user B's job runs after at most one of
// user A's jobs. That matches the v6 spec's "Steve queues 10, Dave
// queues 1, Dave gets in after at most 1 of Steve's" guarantee.
//
// Why a separate file: keeps pool.go focused on the in-memory pool;
// the queue policy is a swap-out. v7 may add weighted fair share or
// strict priority.
type fairSharePolicy struct {
	// perUser maps caller_id → ordered sub-queue (priority desc,
	// FIFO ties).
	perUser map[string][]*queuedJob
	// users is the round-robin rotation order. A user is appended
	// when they first enqueue; removed when their sub-queue empties.
	users []string
	// nextIdx is the index into users for the next Dequeue. Wraps
	// modulo len(users).
	nextIdx int
}

// NewFairSharePolicy returns a queuePolicy with per-user round-robin
// dequeue and priority-ordered FIFO within each user's sub-queue.
//
// Why exported: lets tests (and future callers in pkg/logic/skills)
// construct lanes with explicit fair-share policy via NewWithPolicy.
func NewFairSharePolicy() queuePolicy {
	return &fairSharePolicy{
		perUser: make(map[string][]*queuedJob),
	}
}

// NewWithFairShare constructs a Lane backed by a pool with fair-share
// queueing. Convenience wrapper used by the registry default.
func NewWithFairShare(name string, maxConcurrent int) Lane {
	return NewWithPolicy(name, maxConcurrent, NewFairSharePolicy())
}

// Enqueue adds the job to the caller's sub-queue, sorted by priority
// (higher first) with FIFO tie-breaking.
func (f *fairSharePolicy) Enqueue(j *queuedJob) {
	user := j.job.CallerID()
	if _, ok := f.perUser[user]; !ok {
		f.perUser[user] = []*queuedJob{}
		f.users = append(f.users, user)
	}
	sub := f.perUser[user]
	// Insert sorted by priority desc; FIFO ties via stable insert
	// after the last entry of equal-or-higher priority.
	//
	// Why sort.Search: O(log n) within a single user's queue. Since
	// per-user backlog is typically small, even a linear scan would
	// be fine, but sort.Search keeps the worst case bounded.
	i := sort.Search(len(sub), func(i int) bool {
		return sub[i].job.Priority() < j.job.Priority()
	})
	sub = append(sub, nil)
	copy(sub[i+1:], sub[i:])
	sub[i] = j
	f.perUser[user] = sub
}

// Dequeue rotates users round-robin until it finds a non-empty
// sub-queue. Returns nil when all sub-queues are empty.
//
// Why a single-pass loop bounded by len(users): a user whose sub-queue
// is empty stays in `users` only briefly (we delete on the empty
// transition); a single rotation through `users` always finds a non-
// empty sub-queue if one exists, and an empty rotation means truly
// empty.
func (f *fairSharePolicy) Dequeue() *queuedJob {
	if len(f.users) == 0 {
		return nil
	}
	for tries := 0; tries < len(f.users); tries++ {
		// Bounds-safe selection — len(users) might shrink during
		// iteration, so re-bound on every iteration.
		if f.nextIdx >= len(f.users) {
			f.nextIdx = 0
		}
		user := f.users[f.nextIdx]
		sub := f.perUser[user]
		// Advance the cursor for next time, regardless of whether
		// we picked from this user. A round-robin pass that finds
		// every user empty exits the loop.
		f.nextIdx++
		if len(sub) == 0 {
			continue
		}
		j := sub[0]
		sub[0] = nil
		sub = sub[1:]
		if len(sub) == 0 {
			// User's sub-queue is now empty — remove from rotation.
			delete(f.perUser, user)
			f.users = removeStringAt(f.users, f.nextIdx-1)
			// f.nextIdx-1 is the index we just dequeued from. After
			// removing, nextIdx now points at the next user (if any),
			// so we don't decrement.
			if f.nextIdx > len(f.users) {
				f.nextIdx = 0
			}
		} else {
			f.perUser[user] = sub
		}
		return j
	}
	return nil
}

// Cancel walks every sub-queue looking for a matching job ID. Returns
// true if found and removed.
//
// Why O(n) scan: callers cancel by job ID without knowing the user.
// Could maintain a jobID → user index for O(1) cancel; deferred to
// later if profiling shows it matters. n is bounded by total queued
// jobs across all users.
func (f *fairSharePolicy) Cancel(jobID string) bool {
	for user, sub := range f.perUser {
		for i, j := range sub {
			if j.job.ID() == jobID {
				// Remove from sub-queue.
				j.done <- jobResult{err: ErrCancelled}
				f.perUser[user] = append(sub[:i], sub[i+1:]...)
				if len(f.perUser[user]) == 0 {
					delete(f.perUser, user)
					f.users = removeString(f.users, user)
					if f.nextIdx > len(f.users) {
						f.nextIdx = 0
					}
				}
				return true
			}
		}
	}
	return false
}

// Len returns the total queued count across every sub-queue.
func (f *fairSharePolicy) Len() int {
	total := 0
	for _, sub := range f.perUser {
		total += len(sub)
	}
	return total
}

// OldestEnqueueTime returns the earliest enqueue time across every
// sub-queue. Returns nil if every queue is empty.
func (f *fairSharePolicy) OldestEnqueueTime() *time.Time {
	var oldest *time.Time
	for _, sub := range f.perUser {
		for _, j := range sub {
			if oldest == nil || j.enqueuedAt.Before(*oldest) {
				t := j.enqueuedAt
				oldest = &t
			}
		}
	}
	return oldest
}

// removeString returns a new slice with the first occurrence of target
// removed. Order is preserved (round-robin order matters).
func removeString(s []string, target string) []string {
	for i, v := range s {
		if v == target {
			return append(s[:i], s[i+1:]...)
		}
	}
	return s
}

// removeStringAt returns a new slice with the element at idx removed.
// Order is preserved. idx is bounds-checked defensively.
func removeStringAt(s []string, idx int) []string {
	if idx < 0 || idx >= len(s) {
		return s
	}
	return append(s[:idx], s[idx+1:]...)
}