// Package glm, "garage layout manager", implements the transition of garage
// layout states based on the currently active layout and the desired layout
// defined by the user.
package glm

import (
	"context"
	"errors"
	"fmt"
	"io/fs"
	"isle/daemon/daecommon"
	"isle/garage"
	"isle/jsonutil"
	"isle/toolkit"
	"net/netip"
	"path/filepath"
)

// GarageLayoutManager (GLM) tracks the currently active set of storage
// allocations and calculates actions required to transition from the active set
// into a target set.
//
// GLM will make sure that when allocations are removed they are properly
// drained prior to being fully removed from the cluster.
type GarageLayoutManager interface {

	// GetActiveAllocations returns the currently active set of allocations, as
	// recorded by the last call to CommitStateTransition. Returns an empty set
	// if CommitStateTransition has never been called.
	GetActiveAllocations(context.Context) (
		[]daecommon.ConfigStorageAllocation, error,
	)

	// SetActiveAllocations overwrites the stored active allocations, if any, to
	// the given one.
	SetActiveAllocations(context.Context, []daecommon.ConfigStorageAllocation) error

	// Validate checks the target allocation set for any inconsistencies with
	// the currently active one, and returns an error if one is found.
	Validate(
		_ context.Context, targetAllocs []daecommon.ConfigStorageAllocation,
	) error

	// CalculateStateTransition accepts a set of known nodes from an up-to-date
	// [garage.ClusterStatus] and the target allocation set for the host, and
	// returns a StateTransition describing the actions which should be taken.
	//
	// If the host is not running any garage instances, and therefore cannot
	// determine the known nodes, nil should be passed instead.
	CalculateStateTransition(
		_ context.Context,
		knownNodes []garage.KnownNode,
		targetAllocs []daecommon.ConfigStorageAllocation,
	) (
		StateTransition, error,
	)

	// CommitStateTransition should be called after the CalculateStateTransition
	// returns a StateTransition, and the StateTransition's prescribed actions
	// have been successfully carried out.
	CommitStateTransition(context.Context, StateTransition) error
}

type garageLayoutManager struct {
	dir    toolkit.Dir
	hostIP netip.Addr
}

// NewGarageLayoutManager initializes and returns a GarageLayoutManager which
// will use the given directory to store state, and which is managing the layout
// for the host with the given IP.
func NewGarageLayoutManager(
	dir toolkit.Dir, hostIP netip.Addr,
) GarageLayoutManager {
	return &garageLayoutManager{dir, hostIP}
}

const glmStateFile = "glm.json"

type glmState struct {
	ActiveAllocations []daecommon.ConfigStorageAllocation
}

func (glm *garageLayoutManager) get() (glmState, bool, error) {
	var (
		path  = filepath.Join(glm.dir.Path, glmStateFile)
		state glmState
	)

	err := jsonutil.LoadFile(&state, path)
	if errors.Is(err, fs.ErrNotExist) {
		return glmState{}, false, nil
	} else if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return glmState{}, false, err
	}

	return state, true, nil
}

func (glm *garageLayoutManager) set(state glmState) error {
	path := filepath.Join(glm.dir.Path, glmStateFile)
	return jsonutil.WriteFile(state, path, 0600)
}

func (glm *garageLayoutManager) GetActiveAllocations(context.Context) (
	[]daecommon.ConfigStorageAllocation, error,
) {
	state, _, err := glm.get()
	return state.ActiveAllocations, err
}

func (glm *garageLayoutManager) SetActiveAllocations(
	_ context.Context,
	allocs []daecommon.ConfigStorageAllocation,
) error {
	return glm.set(glmState{allocs})
}

func (glm *garageLayoutManager) Validate(
	_ context.Context, targetAllocs []daecommon.ConfigStorageAllocation,
) error {
	state, ok, err := glm.get()
	if err != nil {
		return fmt.Errorf("reading state: %w", err)
	} else if !ok {
		// If there is no previously state then we assume the new state can't
		// conflict with it.
		return nil
	}

	return validateTargetAllocs(
		state.ActiveAllocations,
		targetAllocs,
	)
}

func (glm *garageLayoutManager) CalculateStateTransition(
	_ context.Context,
	knownNodes []garage.KnownNode,
	targetAllocs []daecommon.ConfigStorageAllocation,
) (
	StateTransition, error,
) {
	state, ok, err := glm.get()
	if err != nil {
		return StateTransition{}, fmt.Errorf("reading state: %w", err)
	}

	if ok {
		// If there is no previously state then we assume the new state can't
		// conflict with it.
		err = validateTargetAllocs(state.ActiveAllocations, targetAllocs)
		if err != nil {
			return StateTransition{}, fmt.Errorf(
				"validating target allocations: %w", err,
			)
		}
	}

	allKnownNodes, knownNodes := knownNodes, knownNodes[:0]
	for _, node := range allKnownNodes {
		if node.Addr.Addr() == glm.hostIP {
			knownNodes = append(knownNodes, node)
		}
	}

	return calcStateTransition(
		state.ActiveAllocations, knownNodes, targetAllocs,
	), nil
}

func (glm *garageLayoutManager) CommitStateTransition(
	_ context.Context, stateTransition StateTransition,
) error {
	return glm.set(glmState{
		ActiveAllocations: stateTransition.ActiveAllocations(),
	})
}