package prebuilds

import (
	"slices"
	"time"

	"github.com/google/uuid"

	"github.com/coder/quartz"

	"github.com/coder/coder/v2/coderd/database"
)

// ActionType represents the type of action needed to reconcile prebuilds.
type ActionType int

const (
	// ActionTypeUndefined represents an uninitialized or invalid action type.
	ActionTypeUndefined ActionType = iota

	// ActionTypeCreate indicates that new prebuilds should be created.
	ActionTypeCreate

	// ActionTypeDelete indicates that existing prebuilds should be deleted.
	ActionTypeDelete

	// ActionTypeBackoff indicates that prebuild creation should be delayed.
	ActionTypeBackoff
)

// PresetSnapshot is a filtered view of GlobalSnapshot focused on a single preset.
// It contains the raw data needed to calculate the current state of a preset's prebuilds,
// including running prebuilds, in-progress builds, and backoff information.
type PresetSnapshot struct {
	Preset     database.GetTemplatePresetsWithPrebuildsRow
	Running    []database.GetRunningPrebuiltWorkspacesRow
	InProgress []database.CountInProgressPrebuildsRow
	Backoff    *database.GetPresetsBackoffRow
}

// ReconciliationState represents the processed state of a preset's prebuilds,
// calculated from a PresetSnapshot. While PresetSnapshot contains raw data,
// ReconciliationState contains derived metrics that are directly used to
// determine what actions are needed (create, delete, or backoff).
// For example, it calculates how many prebuilds are eligible, how many are
// extraneous, and how many are in various transition states.
type ReconciliationState struct {
	Actual     int32 // Number of currently running prebuilds
	Desired    int32 // Number of prebuilds desired as defined in the preset
	Eligible   int32 // Number of prebuilds that are ready to be claimed
	Extraneous int32 // Number of extra running prebuilds beyond the desired count

	// Counts of prebuilds in various transition states
	Starting int32
	Stopping int32
	Deleting int32
}

// ReconciliationActions represents actions needed to reconcile the current state with the desired state.
// Based on ActionType, exactly one of Create, DeleteIDs, or BackoffUntil will be set.
type ReconciliationActions struct {
	// ActionType determines which field is set and what action should be taken
	ActionType ActionType

	// Create is set when ActionType is ActionTypeCreate and indicates the number of prebuilds to create
	Create int32

	// DeleteIDs is set when ActionType is ActionTypeDelete and contains the IDs of prebuilds to delete
	DeleteIDs []uuid.UUID

	// BackoffUntil is set when ActionType is ActionTypeBackoff and indicates when to retry creating prebuilds
	BackoffUntil time.Time
}

func (ra *ReconciliationActions) IsNoop() bool {
	return ra.Create == 0 && len(ra.DeleteIDs) == 0 && ra.BackoffUntil.IsZero()
}

// CalculateState computes the current state of prebuilds for a preset, including:
// - Actual: Number of currently running prebuilds
// - Desired: Number of prebuilds desired as defined in the preset
// - Eligible: Number of prebuilds that are ready to be claimed
// - Extraneous: Number of extra running prebuilds beyond the desired count
// - Starting/Stopping/Deleting: Counts of prebuilds in various transition states
//
// The function takes into account whether the preset is active (using the active template version)
// and calculates appropriate counts based on the current state of running prebuilds and
// in-progress transitions. This state information is used to determine what reconciliation
// actions are needed to reach the desired state.
func (p PresetSnapshot) CalculateState() *ReconciliationState {
	var (
		actual     int32
		desired    int32
		eligible   int32
		extraneous int32
	)

	// #nosec G115 - Safe conversion as p.Running slice length is expected to be within int32 range
	actual = int32(len(p.Running))

	if p.isActive() {
		desired = p.Preset.DesiredInstances.Int32
		eligible = p.countEligible()
		extraneous = max(actual-desired, 0)
	}

	starting, stopping, deleting := p.countInProgress()

	return &ReconciliationState{
		Actual:     actual,
		Desired:    desired,
		Eligible:   eligible,
		Extraneous: extraneous,

		Starting: starting,
		Stopping: stopping,
		Deleting: deleting,
	}
}

// CalculateActions determines what actions are needed to reconcile the current state with the desired state.
// The function:
// 1. First checks if a backoff period is needed (if previous builds failed)
// 2. If the preset is inactive (template version is not active), it will delete all running prebuilds
// 3. For active presets, it calculates the number of prebuilds to create or delete based on:
//   - The desired number of instances
//   - Currently running prebuilds
//   - Prebuilds in transition states (starting/stopping/deleting)
//   - Any extraneous prebuilds that need to be removed
//
// The function returns a ReconciliationActions struct that will have exactly one action type set:
// - ActionTypeBackoff: Only BackoffUntil is set, indicating when to retry
// - ActionTypeCreate: Only Create is set, indicating how many prebuilds to create
// - ActionTypeDelete: Only DeleteIDs is set, containing IDs of prebuilds to delete
func (p PresetSnapshot) CalculateActions(clock quartz.Clock, backoffInterval time.Duration) (*ReconciliationActions, error) {
	// TODO: align workspace states with how we represent them on the FE and the CLI
	//	     right now there's some slight differences which can lead to additional prebuilds being created

	// TODO: add mechanism to prevent prebuilds being reconciled from being claimable by users; i.e. if a prebuild is
	// 		 about to be deleted, it should not be deleted if it has been claimed - beware of TOCTOU races!

	actions, needsBackoff := p.needsBackoffPeriod(clock, backoffInterval)
	if needsBackoff {
		return actions, nil
	}

	if !p.isActive() {
		return p.handleInactiveTemplateVersion()
	}

	return p.handleActiveTemplateVersion()
}

// isActive returns true if the preset's template version is the active version, and it is neither deleted nor deprecated.
// This determines whether we should maintain prebuilds for this preset or delete them.
func (p PresetSnapshot) isActive() bool {
	return p.Preset.UsingActiveVersion && !p.Preset.Deleted && !p.Preset.Deprecated
}

// handleActiveTemplateVersion deletes excess prebuilds if there are too many,
// otherwise creates new ones to reach the desired count.
func (p PresetSnapshot) handleActiveTemplateVersion() (*ReconciliationActions, error) {
	state := p.CalculateState()

	// If we have more prebuilds than desired, delete the oldest ones
	if state.Extraneous > 0 {
		return &ReconciliationActions{
			ActionType: ActionTypeDelete,
			DeleteIDs:  p.getOldestPrebuildIDs(int(state.Extraneous)),
		}, nil
	}

	// Calculate how many new prebuilds we need to create
	// We subtract starting prebuilds since they're already being created
	prebuildsToCreate := max(state.Desired-state.Actual-state.Starting, 0)

	return &ReconciliationActions{
		ActionType: ActionTypeCreate,
		Create:     prebuildsToCreate,
	}, nil
}

// handleInactiveTemplateVersion deletes all running prebuilds except those already being deleted
// to avoid duplicate deletion attempts.
func (p PresetSnapshot) handleInactiveTemplateVersion() (*ReconciliationActions, error) {
	prebuildsToDelete := len(p.Running)
	deleteIDs := p.getOldestPrebuildIDs(prebuildsToDelete)

	return &ReconciliationActions{
		ActionType: ActionTypeDelete,
		DeleteIDs:  deleteIDs,
	}, nil
}

// needsBackoffPeriod checks if we should delay prebuild creation due to recent failures.
// If there were failures, it calculates a backoff period based on the number of failures
// and returns true if we're still within that period.
func (p PresetSnapshot) needsBackoffPeriod(clock quartz.Clock, backoffInterval time.Duration) (*ReconciliationActions, bool) {
	if p.Backoff == nil || p.Backoff.NumFailed == 0 {
		return nil, false
	}
	backoffUntil := p.Backoff.LastBuildAt.Add(time.Duration(p.Backoff.NumFailed) * backoffInterval)
	if clock.Now().After(backoffUntil) {
		return nil, false
	}

	return &ReconciliationActions{
		ActionType:   ActionTypeBackoff,
		BackoffUntil: backoffUntil,
	}, true
}

// countEligible returns the number of prebuilds that are ready to be claimed.
// A prebuild is eligible if it's running and its agents are in ready state.
func (p PresetSnapshot) countEligible() int32 {
	var count int32
	for _, prebuild := range p.Running {
		if prebuild.Ready {
			count++
		}
	}
	return count
}

// countInProgress returns counts of prebuilds in transition states (starting, stopping, deleting).
// These counts are tracked at the template level, so all presets sharing the same template see the same values.
func (p PresetSnapshot) countInProgress() (starting int32, stopping int32, deleting int32) {
	for _, progress := range p.InProgress {
		num := progress.Count
		switch progress.Transition {
		case database.WorkspaceTransitionStart:
			starting += num
		case database.WorkspaceTransitionStop:
			stopping += num
		case database.WorkspaceTransitionDelete:
			deleting += num
		}
	}

	return starting, stopping, deleting
}

// getOldestPrebuildIDs returns the IDs of the N oldest prebuilds, sorted by creation time.
// This is used when we need to delete prebuilds, ensuring we remove the oldest ones first.
func (p PresetSnapshot) getOldestPrebuildIDs(n int) []uuid.UUID {
	// Sort by creation time, oldest first
	slices.SortFunc(p.Running, func(a, b database.GetRunningPrebuiltWorkspacesRow) int {
		return a.CreatedAt.Compare(b.CreatedAt)
	})

	// Take the first N IDs
	n = min(n, len(p.Running))
	ids := make([]uuid.UUID, n)
	for i := 0; i < n; i++ {
		ids[i] = p.Running[i].ID
	}

	return ids
}