// Package exporter builds an Excalidraw scene JSON from a project
// snapshot per docs/design.md §4 ("Export — DB → Excalidraw").
//
// The exporter is a pure function on a *db.Snapshot — no DB access, no
// IO — so it's trivial to unit-test against fixtures and gives the
// caller (the HTTP handler) a clean handoff: build scene → upload.
package exporter

import (
	"crypto/rand"
	"encoding/json"
	"fmt"
	"math/big"
	"sort"

	"mgit.msbls.de/m/cablegui/internal/db"
)

// Scene is the top-level Excalidraw file format. Keys mirror what the
// official Excalidraw JSON contains (we only emit the keys mxdrw cares
// about for rendering — `appState`, `files`, `libraryItems` etc. can be
// added later if m needs them).
type Scene struct {
	Type     string    `json:"type"`
	Version  int       `json:"version"`
	Source   string    `json:"source"`
	Elements []Element `json:"elements"`
	AppState AppState  `json:"appState"`
	Files    Files     `json:"files"`
}

type AppState struct {
	GridSize       *int   `json:"gridSize"`
	ViewBackground string `json:"viewBackgroundColor"`
}

type Files struct{}

// Element is one node in the scene. Excalidraw's wire format has a lot
// of optional fields; we only emit the ones that matter for the shapes
// we draw. Extra null/zero fields are fine in Excalidraw (it merges
// defaults). Pointer fields stay nil-omitted via omitempty so the
// payload stays clean.
type Element struct {
	ID              string       `json:"id"`
	Type            string       `json:"type"`
	X               float64      `json:"x"`
	Y               float64      `json:"y"`
	Width           float64      `json:"width"`
	Height          float64      `json:"height"`
	Angle           float64      `json:"angle"`
	StrokeColor     string       `json:"strokeColor"`
	BackgroundColor string       `json:"backgroundColor"`
	FillStyle       string       `json:"fillStyle"`
	StrokeWidth     int          `json:"strokeWidth"`
	StrokeStyle     string       `json:"strokeStyle"`
	Roughness       int          `json:"roughness"`
	Opacity         int          `json:"opacity"`
	GroupIDs        []string     `json:"groupIds"`
	FrameID         *string      `json:"frameId"`
	Roundness       *Roundness   `json:"roundness"`
	Seed            int64        `json:"seed"`
	Version         int          `json:"version"`
	VersionNonce    int64        `json:"versionNonce"`
	IsDeleted       bool         `json:"isDeleted"`
	BoundElements   []BoundRef   `json:"boundElements,omitempty"`
	Updated         int64        `json:"updated"`
	Link            *string      `json:"link"`
	Locked          bool         `json:"locked"`

	// Element-type-specific extras
	Name string `json:"name,omitempty"`

	// Text-element fields
	Text         string `json:"text,omitempty"`
	FontSize     int    `json:"fontSize,omitempty"`
	FontFamily   int    `json:"fontFamily,omitempty"`
	TextAlign    string `json:"textAlign,omitempty"`
	VerticalAlign string `json:"verticalAlign,omitempty"`
	ContainerID  *string `json:"containerId,omitempty"`
	OriginalText string `json:"originalText,omitempty"`
	LineHeight   float64 `json:"lineHeight,omitempty"`

	// Arrow-element fields
	Points       [][2]float64  `json:"points,omitempty"`
	StartBinding *Binding      `json:"startBinding,omitempty"`
	EndBinding   *Binding      `json:"endBinding,omitempty"`
	StartArrowhead *string     `json:"startArrowhead,omitempty"`
	EndArrowhead   *string     `json:"endArrowhead,omitempty"`
	LastCommittedPoint *[2]float64 `json:"lastCommittedPoint,omitempty"`
}

type Roundness struct {
	Type int `json:"type"`
}

type BoundRef struct {
	ID   string `json:"id"`
	Type string `json:"type"`
}

type Binding struct {
	ElementID string   `json:"elementId"`
	Focus     float64  `json:"focus"`
	Gap       float64  `json:"gap"`
}

// IDAssignment is the result of running BuildScene: the scene to upload
// + the per-row excalidraw_id assignments that the caller should
// persist so the next export reuses the same ids (Excalidraw collab
// cursors / comments / undo history survive that way; design §4.2).
type IDAssignment struct {
	Frames    map[int64]string `json:"frames"`
	Devices   map[int64]string `json:"devices"`
	Ports     map[int64]string `json:"ports"`
	IOMarkers map[int64]string `json:"io_markers"`
	Cables    map[int64]string `json:"cables"`
	Clamps    map[int64]string `json:"clamps"`
}

// BuildScene transforms a project snapshot into an Excalidraw Scene +
// the id-assignment side-table.
//
// nowMilli is the Updated timestamp (one millisecond stamp for every
// element keeps re-exports consistent — mxdrw treats wildly-different
// updateds as edit-noise).
//
// genID is a 21-char ID factory. Tests pass a deterministic generator
// to lock element ids down across asserts. Production uses Generate21.
func BuildScene(snap *db.Snapshot, nowMilli int64, genID func() string) (*Scene, *IDAssignment) {
	a := &IDAssignment{
		Frames:    map[int64]string{},
		Devices:   map[int64]string{},
		Ports:     map[int64]string{},
		IOMarkers: map[int64]string{},
		Cables:    map[int64]string{},
		Clamps:    map[int64]string{},
	}
	// idFor: reuse the existing excalidraw_id if present, else mint one.
	idFor := func(existing *string) string {
		if existing != nil && *existing != "" {
			return *existing
		}
		return genID()
	}

	cableTypeColor := map[int64]string{}
	for _, t := range snap.CableTypes {
		cableTypeColor[t.ID] = t.Color
	}

	// We'll need: device-id → element-id, port-id → element-id, io-id → element-id
	// for binding arrows.
	deviceElID := map[int64]string{}
	portElID := map[int64]string{}
	ioElID := map[int64]string{}
	frameElID := map[int64]string{}

	var els []Element

	// Frames first (Excalidraw renders later elements on top; frames are
	// containers that go on the bottom).
	for _, f := range snap.Frames {
		elID := idFor(f.ExcalidrawID)
		a.Frames[f.ID] = elID
		frameElID[f.ID] = elID
		els = append(els, Element{
			ID:              elID,
			Type:            "frame",
			X:               f.X,
			Y:               f.Y,
			Width:           f.Width,
			Height:          f.Height,
			StrokeColor:     "#bbbbbb",
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			Name:            f.Name,
		})
	}

	// Devices: rectangle + bound text with the device's name. Excalidraw
	// uses a `containerId` pointer on the text to bind it to the rect,
	// and `boundElements` on the rect to point back at the text.
	for _, d := range snap.Devices {
		rectID := idFor(d.ExcalidrawID)
		a.Devices[d.ID] = rectID
		deviceElID[d.ID] = rectID
		textID := genID()
		var frameRef *string
		if d.FrameID != nil {
			if v, ok := frameElID[*d.FrameID]; ok {
				frameRef = &v
			}
		}
		// Rect
		els = append(els, Element{
			ID:              rectID,
			Type:            "rectangle",
			X:               d.X,
			Y:               d.Y,
			Width:           d.Width,
			Height:          d.Height,
			StrokeColor:     d.Color,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Roundness:       &Roundness{Type: 3},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			BoundElements:   []BoundRef{{ID: textID, Type: "text"}},
		})
		// Bound text — name centered on the rect.
		els = append(els, Element{
			ID:              textID,
			Type:            "text",
			X:               d.X,
			Y:               d.Y + d.Height/2 - 8,
			Width:           d.Width,
			Height:          16,
			StrokeColor:     d.Color,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			Text:            d.Name,
			OriginalText:    d.Name,
			FontSize:        16,
			FontFamily:      1,
			TextAlign:       "center",
			VerticalAlign:   "middle",
			ContainerID:     &rectID,
			LineHeight:      1.25,
		})
	}

	// Ports — small ellipses at device.x + port.x_offset (positional,
	// not containerId-bound per the seed drawing's grammar; design §4.1).
	for _, p := range snap.Ports {
		elID := idFor(p.ExcalidrawID)
		a.Ports[p.ID] = elID
		portElID[p.ID] = elID
		// Locate the parent device for absolute pos + frame ref.
		var dev *db.Device
		for i := range snap.Devices {
			if snap.Devices[i].ID == p.DeviceID {
				dev = &snap.Devices[i]
				break
			}
		}
		if dev == nil {
			continue
		}
		var frameRef *string
		if dev.FrameID != nil {
			if v, ok := frameElID[*dev.FrameID]; ok {
				frameRef = &v
			}
		}
		color := cableTypeColor[p.TypeID]
		if color == "" {
			color = "#1e1e1e"
		}
		els = append(els, Element{
			ID:              elID,
			Type:            "ellipse",
			X:               dev.X + p.XOffset - 6,
			Y:               dev.Y + p.YOffset - 4,
			Width:           12,
			Height:          9,
			StrokeColor:     color,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Roundness:       &Roundness{Type: 2},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
		})
	}

	// IO markers — diamonds with bound "IO" (or m's label) text.
	powerColor := ""
	for _, t := range snap.CableTypes {
		if t.Name == "Power" {
			powerColor = t.Color
			break
		}
	}
	if powerColor == "" {
		powerColor = "#e03131"
	}
	for _, m := range snap.IOMarkers {
		elID := idFor(m.ExcalidrawID)
		a.IOMarkers[m.ID] = elID
		ioElID[m.ID] = elID
		textID := genID()
		var frameRef *string
		if m.FrameID != nil {
			if v, ok := frameElID[*m.FrameID]; ok {
				frameRef = &v
			}
		}
		els = append(els, Element{
			ID:              elID,
			Type:            "diamond",
			X:               m.X,
			Y:               m.Y,
			Width:           30,
			Height:          30,
			StrokeColor:     powerColor,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Roundness:       &Roundness{Type: 2},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			BoundElements:   []BoundRef{{ID: textID, Type: "text"}},
		})
		els = append(els, Element{
			ID:              textID,
			Type:            "text",
			X:               m.X,
			Y:               m.Y + 7,
			Width:           30,
			Height:          16,
			StrokeColor:     powerColor,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			Text:            m.Label,
			OriginalText:    m.Label,
			FontSize:        11,
			FontFamily:      1,
			TextAlign:       "center",
			VerticalAlign:   "middle",
			ContainerID:     &elID,
			LineHeight:      1.25,
		})
	}

	// Clamps — small grey rounded squares (v5 §11.7). Distinct from the
	// red IO marker diamonds so m can tell routing anchors from wall
	// outlets at a glance.
	const clampSize = 12.0
	for _, cl := range snap.Clamps {
		elID := idFor(cl.ExcalidrawID)
		a.Clamps[cl.ID] = elID
		var frameRef *string
		if cl.FrameID != nil {
			if v, ok := frameElID[*cl.FrameID]; ok {
				frameRef = &v
			}
		}
		els = append(els, Element{
			ID:              elID,
			Type:            "rectangle",
			X:               cl.X - clampSize/2,
			Y:               cl.Y - clampSize/2,
			Width:           clampSize,
			Height:          clampSize,
			StrokeColor:     "#555555",
			BackgroundColor: "#888888",
			FillStyle:       "solid",
			StrokeWidth:     1,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			FrameID:         frameRef,
			Roundness:       &Roundness{Type: 3},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
		})
	}

	// Pre-group cable_clamps by cable for the arrow mid-points pass.
	clampsByCable := map[int64][]db.CableClamp{}
	for _, cc := range snap.CableClamps {
		clampsByCable[cc.CableID] = append(clampsByCable[cc.CableID], cc)
	}
	for _, arr := range clampsByCable {
		// Already sorted by ListCableClamps (ORDER BY cable_id, ord),
		// but defend against unsorted inputs.
		sort.Slice(arr, func(i, j int) bool { return arr[i].Ord < arr[j].Ord })
	}
	clampPos := map[int64][2]float64{}
	for _, cl := range snap.Clamps {
		clampPos[cl.ID] = [2]float64{cl.X, cl.Y}
	}

	// Cables — arrows with startBinding/endBinding to the port / device /
	// IO marker excalidraw_ids. Endpoint anchors (the visible "from" /
	// "to" points) come from the same anchor logic the canvas uses.
	for _, c := range snap.Cables {
		elID := idFor(c.ExcalidrawID)
		a.Cables[c.ID] = elID
		fromAnchor, fromRef := exportAnchor(c.FromPortID, c.FromDeviceID, c.FromIOID,
			snap, deviceElID, portElID, ioElID)
		toAnchor, toRef := exportAnchor(c.ToPortID, c.ToDeviceID, c.ToIOID,
			snap, deviceElID, portElID, ioElID)
		// fromRef/toRef are nil when the endpoint row vanished (manual
		// cable referencing a deleted port, say). Skip rather than emit
		// a half-bound arrow.
		if fromRef == nil || toRef == nil {
			continue
		}
		color := cableTypeColor[c.TypeID]
		if color == "" {
			color = "#1e1e1e"
		}
		startArr := ""
		endArr := "arrow"
		// Excalidraw arrow `points` is relative to (X, Y). We anchor at
		// the from-point, so vertex 0 is always (0, 0). Mid-vertices
		// (clamps) and the final to-vertex are offsets from there.
		pts := [][2]float64{{0, 0}}
		for _, cc := range clampsByCable[c.ID] {
			pos, ok := clampPos[cc.ClampID]
			if !ok {
				continue
			}
			pts = append(pts, [2]float64{pos[0] - fromAnchor[0], pos[1] - fromAnchor[1]})
		}
		pts = append(pts, [2]float64{toAnchor[0] - fromAnchor[0], toAnchor[1] - fromAnchor[1]})
		els = append(els, Element{
			ID:              elID,
			Type:            "arrow",
			X:               fromAnchor[0],
			Y:               fromAnchor[1],
			Width:           toAnchor[0] - fromAnchor[0],
			Height:          toAnchor[1] - fromAnchor[1],
			StrokeColor:     color,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     2,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			Points:          pts,
			StartArrowhead:  &startArr,
			EndArrowhead:    &endArr,
			StartBinding:    bindingPtr(fromRef),
			EndBinding:      bindingPtr(toRef),
		})
	}

	// Legend in the top-left of the first frame (or at 20,20 if there
	// are no frames). One text row per cable_type, stacked vertically.
	legendX, legendY := 20.0, 20.0
	if len(snap.Frames) > 0 {
		legendX = snap.Frames[0].X + 10
		legendY = snap.Frames[0].Y + 10
	}
	for i, t := range snap.CableTypes {
		els = append(els, Element{
			ID:              genID(),
			Type:            "text",
			X:               legendX,
			Y:               legendY + float64(i*18),
			Width:           80,
			Height:          16,
			StrokeColor:     t.Color,
			BackgroundColor: "transparent",
			FillStyle:       "solid",
			StrokeWidth:     1,
			StrokeStyle:     "solid",
			Roughness:       0,
			Opacity:         100,
			GroupIDs:        []string{},
			Seed:            randInt(),
			Version:         1,
			VersionNonce:    randInt(),
			Updated:         nowMilli,
			Text:            t.Name,
			OriginalText:    t.Name,
			FontSize:        16,
			FontFamily:      1,
			TextAlign:       "left",
			VerticalAlign:   "top",
			LineHeight:      1.25,
		})
	}

	scene := &Scene{
		Type:     "excalidraw",
		Version:  2,
		Source:   "cablegui",
		Elements: els,
		AppState: AppState{
			GridSize:       nil,
			ViewBackground: "#ffffff",
		},
		Files: Files{},
	}
	return scene, a
}

func bindingPtr(b *Binding) *Binding {
	if b == nil {
		return nil
	}
	return b
}

// exportAnchor returns (x,y) + a Binding for the endpoint kind passed in.
func exportAnchor(portID, deviceID, ioID *int64, snap *db.Snapshot,
	devElID, portElID, ioElID map[int64]string,
) ([2]float64, *Binding) {
	if portID != nil {
		// Find the port + its parent device.
		for _, p := range snap.Ports {
			if p.ID != *portID {
				continue
			}
			for _, d := range snap.Devices {
				if d.ID == p.DeviceID {
					id := portElID[p.ID]
					return [2]float64{d.X + p.XOffset, d.Y + p.YOffset}, &Binding{ElementID: id, Focus: 0, Gap: 1}
				}
			}
		}
	}
	if deviceID != nil {
		for _, d := range snap.Devices {
			if d.ID != *deviceID {
				continue
			}
			id := devElID[d.ID]
			return [2]float64{d.X + d.Width/2, d.Y + d.Height/2}, &Binding{ElementID: id, Focus: 0, Gap: 1}
		}
	}
	if ioID != nil {
		for _, m := range snap.IOMarkers {
			if m.ID != *ioID {
				continue
			}
			id := ioElID[m.ID]
			return [2]float64{m.X + 15, m.Y + 15}, &Binding{ElementID: id, Focus: 0, Gap: 1}
		}
	}
	return [2]float64{}, nil
}

// Generate21 mints a 21-char base62 identifier, the shape Excalidraw
// uses for element ids (nanoid-style). crypto/rand source.
func Generate21() string {
	const alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
	buf := make([]byte, 21)
	max := big.NewInt(int64(len(alphabet)))
	for i := range buf {
		n, err := rand.Int(rand.Reader, max)
		if err != nil {
			// crypto/rand failure is unrecoverable in practice; fall back
			// to a deterministic alphabet position so callers see a panic-
			// adjacent symptom rather than a half-initialised id.
			return fmt.Sprintf("crypto-rand-failed-%d", i)
		}
		buf[i] = alphabet[n.Int64()]
	}
	return string(buf)
}

// randInt returns a non-negative int64 derived from crypto/rand for
// Excalidraw's `seed` / `versionNonce`. Excalidraw treats these as
// noise — only the IDs and the structural fields matter.
func randInt() int64 {
	n, err := rand.Int(rand.Reader, big.NewInt(1<<62))
	if err != nil {
		return 0
	}
	return n.Int64()
}

// MarshalScene returns the scene as Excalidraw-flavoured JSON.
func MarshalScene(s *Scene) ([]byte, error) {
	return json.Marshal(s)
}