package main

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"os"
	"reflect"
	"sort"
	"strings"

	"github.com/novox/mesh-controller/internal/inventory"
)

// No change to a machine takes effect unseen (novox/hq ADR 0217, to-be 44).
//
// Two incidents in two days had one shape: a change took effect that nobody saw before it did. A
// provisioner read an unreadable file as "nobody asks" and dropped seven databases (issue 241); one
// placement set for one module on one machine replaced its whole settings layer, and the plan then
// ran the module on an empty directory (novox/hq issue 304). Here are the three guards: what a settings layer
// loses is said and refused unless meant; what a push will change can be read before it is sent; and
// a running container's data moving holds that machine's push until the operator says so. Each is
// silent when nothing is at stake, so an ordinary change goes exactly as before.

// ---- 1. what a settings layer loses ------------------------------------------------------------

// settingsChange is what replacing one layer with another adds, changes and removes, by dotted path
// to each leaf — `places.config`, not only `places` — because a layer that keeps a key and loses one
// of its entries has lost something just the same: on 2026-10-05 a node's `places` kept its name and
// lost three of its four directories.
func settingsChange(before, after map[string]any) (added, changed, removed []string) {
	was, now := leaves(before, ""), leaves(after, "")
	for path, v := range now {
		old, had := was[path]
		switch {
		case !had:
			added = append(added, path)
		case !reflect.DeepEqual(old, v):
			changed = append(changed, path)
		}
	}
	for path := range was {
		if _, kept := now[path]; !kept {
			removed = append(removed, path)
		}
	}
	sort.Strings(added)
	sort.Strings(changed)
	sort.Strings(removed)
	return added, changed, removed
}

// leaves flattens a settings layer to its leaves by dotted path; a list is a leaf, compared whole.
func leaves(m map[string]any, prefix string) map[string]any {
	out := map[string]any{}
	for k, v := range m {
		path := k
		if prefix != "" {
			path = prefix + "." + k
		}
		if inner, ok := v.(map[string]any); ok && len(inner) > 0 {
			for p, leaf := range leaves(inner, path) {
				out[p] = leaf
			}
			continue
		}
		out[path] = v
	}
	return out
}

// ---- 2. what a push will change -----------------------------------------------------------------

// sentResource is what is kept of one resource a machine was sent: enough to say what changed and
// whether a container's data moved, and nothing that could carry a secret — a file's content is
// kept as a digest, never as text (the record lands in the store's own backups).
type sentResource struct {
	ID   string `json:"id"`
	Type string `json:"type"`
	// Digest is of the whole resource as it was sent.
	Digest string `json:"digest"`
	// Fields is each field's digest, so a change can be named by field.
	Fields map[string]string `json:"fields"`
	// Volumes is a container's mounts as sent — paths, which are not secrets, and what a moved
	// data directory is read from.
	Volumes []string `json:"volumes,omitempty"`
	// Image is a container's image as sent — a reference, not a secret — so the machine can be told
	// which images its last declaration named, and keep them (novox/hq ADR 0251, `images`). Absent in
	// a summary kept before it was.
	Image string `json:"image,omitempty"`
}

// summarize is what is kept of a declaration body: its resources, in the order sent.
func summarize(body []byte) ([]sentResource, error) {
	var envelope struct {
		Resources []map[string]any `json:"resources"`
	}
	if err := json.Unmarshal(body, &envelope); err != nil {
		return nil, fmt.Errorf("a declaration that is not one: %w", err)
	}
	out := make([]sentResource, 0, len(envelope.Resources))
	for _, r := range envelope.Resources {
		s := sentResource{ID: fmt.Sprint(r["id"]), Type: fmt.Sprint(r["type"]), Digest: digestValue(r),
			Fields: map[string]string{}}
		for k, v := range r {
			s.Fields[k] = digestValue(v)
		}
		if s.Type == "container" {
			if image, ok := r["image"].(string); ok {
				s.Image = image
			}
			if vols, ok := r["volumes"].([]any); ok {
				for _, v := range vols {
					s.Volumes = append(s.Volumes, fmt.Sprint(v))
				}
			}
		}
		out = append(out, s)
	}
	return out, nil
}

func digestValue(v any) string {
	raw, _ := json.Marshal(v)
	sum := sha256.Sum256(raw)
	return hex.EncodeToString(sum[:8])
}

// changedResource is one resource sent differently, with the fields that differ.
type changedResource struct {
	ID, Type string
	Fields   []string
}

// sentDiff is what would be sent now against what was sent last, by resource id.
type sentDiff struct {
	Added, Removed []string
	Changed        []changedResource
}

func (d sentDiff) empty() bool {
	return len(d.Added) == 0 && len(d.Removed) == 0 && len(d.Changed) == 0
}

func diffSent(before, after []sentResource) sentDiff {
	was := map[string]sentResource{}
	for _, r := range before {
		was[r.ID] = r
	}
	var d sentDiff
	seen := map[string]bool{}
	for _, r := range after {
		seen[r.ID] = true
		old, had := was[r.ID]
		if !had {
			d.Added = append(d.Added, r.ID)
			continue
		}
		if old.Digest == r.Digest {
			continue
		}
		c := changedResource{ID: r.ID, Type: r.Type}
		for k, v := range r.Fields {
			if old.Fields[k] != v {
				c.Fields = append(c.Fields, k)
			}
		}
		for k := range old.Fields {
			if _, kept := r.Fields[k]; !kept {
				c.Fields = append(c.Fields, k)
			}
		}
		sort.Strings(c.Fields)
		d.Changed = append(d.Changed, c)
	}
	for _, r := range before {
		if !seen[r.ID] {
			d.Removed = append(d.Removed, r.ID)
		}
	}
	sort.Strings(d.Added)
	sort.Strings(d.Removed)
	sort.Slice(d.Changed, func(a, b int) bool { return d.Changed[a].ID < d.Changed[b].ID })
	return d
}

// writeDiff says a diff the way a person reads one: what is added, removed and changed, and a
// changed container's fields — a container sent differently is a container recreated.
func writeDiff(w io.Writer, node string, d sentDiff, moves []dataMove) {
	if d.empty() {
		fmt.Fprintf(w, "%s: nothing would change — it was last sent exactly this\n", node)
		return
	}
	fmt.Fprintf(w, "%s would change:\n", node)
	for _, id := range d.Added {
		fmt.Fprintf(w, "  + %s\n", id)
	}
	for _, id := range d.Removed {
		fmt.Fprintf(w, "  - %s\n", id)
	}
	for _, c := range d.Changed {
		what := "changed"
		if c.Type == "container" {
			what = "recreated"
		}
		fmt.Fprintf(w, "  ~ %s %s: %s\n", c.ID, what, strings.Join(c.Fields, ", "))
	}
	writeMoves(w, node, moves)
}

// ---- 3. a running module's data moving ------------------------------------------------------------

// dataMove is a container keeping a mount at the same place inside it with a different directory
// on the machine behind it: its data moving — or, as on 2026-10-05, a module about to start on an
// empty directory because the placement that pointed at its data was lost.
type dataMove struct {
	Container, Inside, From, To string
}

// Module is the module the container belongs to: resource ids are `<module>.<id>`.
func (m dataMove) Module() string {
	module, _, _ := strings.Cut(m.Container, ".")
	return module
}

// movesIn is every data move between what a machine was sent and what it would be sent. A new
// container, a mount added or dropped and a changed image are not moves.
func movesIn(before, after []sentResource) []dataMove {
	was := map[string]sentResource{}
	for _, r := range before {
		if r.Type == "container" {
			was[r.ID] = r
		}
	}
	var out []dataMove
	for _, r := range after {
		old, had := was[r.ID]
		if r.Type != "container" || !had {
			continue
		}
		from := mountSources(old.Volumes)
		for inside, to := range mountSources(r.Volumes) {
			if prev, mounted := from[inside]; mounted && prev != to {
				out = append(out, dataMove{Container: r.ID, Inside: inside, From: prev, To: to})
			}
		}
	}
	sort.Slice(out, func(a, b int) bool {
		if out[a].Container != out[b].Container {
			return out[a].Container < out[b].Container
		}
		return out[a].Inside < out[b].Inside
	})
	return out
}

// mountSources is a container's mounts by the place inside it: `source:inside[:options]`.
func mountSources(volumes []string) map[string]string {
	out := map[string]string{}
	for _, v := range volumes {
		parts := strings.SplitN(v, ":", 3)
		if len(parts) < 2 {
			continue
		}
		out[parts[1]] = parts[0]
	}
	return out
}

func writeMoves(w io.Writer, node string, moves []dataMove) {
	for _, m := range moves {
		fmt.Fprintf(w, "  ! %s: %s moves from %s to %s\n", m.Container, m.Inside, m.From, m.To)
	}
}

// heldMoves is the moves in a push to one machine that the operator has not said to send (`--move
// <module>`); empty when there is nothing to hold, including a machine never sent anything before.
func heldMoves(ctx context.Context, inv *inventory.Inventory, node string, body []byte, allowed map[string]bool) ([]dataMove, error) {
	prev, err := inv.SentSummary(ctx, node)
	if err != nil || len(prev) == 0 {
		return nil, err
	}
	var before []sentResource
	if err := json.Unmarshal(prev, &before); err != nil {
		// A record this version cannot read compares with nothing: holding every push for it would
		// stop the mesh on a format, which is not what is at stake. Said, so it is not passed over.
		fmt.Fprintf(os.Stderr, "%s: what it was last sent is kept in a form this version does not read, "+
			"so whether its data would move is not known: %v\n", node, err)
		// empty-on-error: said above; the send replaces the unreadable record with one this version reads
		return nil, nil
	}
	after, err := summarize(body)
	if err != nil {
		return nil, err
	}
	var held []dataMove
	for _, m := range movesIn(before, after) {
		if !allowed[m.Module()] {
			held = append(held, m)
		}
	}
	return held, nil
}

// sayDataHeld tells the operator why a machine was not sent, and how to send it.
func sayDataHeld(w io.Writer, node string, moves []dataMove) {
	modules := map[string]bool{}
	for _, m := range moves {
		modules[m.Module()] = true
	}
	var names []string
	for m := range modules {
		names = append(names, m)
	}
	sort.Strings(names)
	fmt.Fprintf(w, "held %s: a running module's data would move (novox/hq ADR 0217)\n", node)
	writeMoves(w, node, moves)
	fmt.Fprintf(w, "  if that is meant: push %s --move %s\n", node, strings.Join(names, ","))
}

// writePlanDiff says what sending body to a machine would change against what it was last sent.
func writePlanDiff(ctx context.Context, inv *inventory.Inventory, node string, body []byte) error {
	after, err := summarize(body)
	if err != nil {
		return err
	}
	prev, err := inv.SentSummary(ctx, node)
	if err != nil {
		return err
	}
	if len(prev) == 0 {
		fmt.Printf("%s: what it was last sent is not kept yet — the first push from this version keeps "+
			"it; %d resource(s) would be sent\n", node, len(after))
		return nil
	}
	var before []sentResource
	if err := json.Unmarshal(prev, &before); err != nil {
		return fmt.Errorf("%s: what it was last sent is kept in a form this version does not read: %w", node, err)
	}
	writeDiff(os.Stdout, node, diffSent(before, after), movesIn(before, after))
	return nil
}