// SPDX-License-Identifier: Unlicense OR MIT

/*

Package op implements operations for updating a user interface.

Gio programs use operations, or ops, for describing their user
interfaces. There are operations for drawing, defining input
handlers, changing window properties as well as operations for
controlling the execution of other operations.

Ops represents a list of operations. The most important use
for an Ops list is to describe a complete user interface update
to a ui/app.Window's Update method.

Drawing a colored square:

	import "gioui.org/unit"
	import "gioui.org/app"
	import "gioui.org/op/paint"

	var w app.Window
	var e system.FrameEvent
	ops := new(op.Ops)
	...
	ops.Reset()
	paint.ColorOp{Color: ...}.Add(ops)
	paint.PaintOp{Rect: ...}.Add(ops)
	e.Frame(ops)

State

An Ops list can be viewed as a very simple virtual machine: it has an implicit
mutable state stack and execution flow can be controlled with macros.

The StackOp saves the current state to the state stack and restores it later:

	ops := new(op.Ops)
	var stack op.StackOp
	// Save the current state, in particular the transform.
	stack.Push(ops)
	// Apply a transform to subsequent operations.
	op.TransformOp{}.Offset(...).Add(ops)
	...
	// Restore the previous transform.
	stack.Pop()

The MacroOp records a list of operations to be executed later:

	ops := new(op.Ops)
	var macro op.MacroOp
	macro.Record(ops)
	// Record operations by adding them.
	op.InvalidateOp{}.Add(ops)
	...
	// End recording.
	macro.Stop()

	// replay the recorded operations by calling Add:
	macro.Add(ops)

*/
package op

import (
	"encoding/binary"
	"math"
	"time"

	"gioui.org/f32"
	"gioui.org/internal/opconst"
)

// Ops holds a list of operations. Operations are stored in
// serialized form to avoid garbage during construction of
// the ops list.
type Ops struct {
	// version is incremented at each Reset.
	version int
	// data contains the serialized operations.
	data []byte
	// External references for operations.
	refs []interface{}

	stackDepth int
	macroDepth int
}

// StackOp can save and restore the operation state
// in a stack-like manner.
type StackOp struct {
	stackDepth int
	macroDepth int
	active     bool
	ops        *Ops
}

// MacroOp can record a list of operations for later
// use.
type MacroOp struct {
	recording bool
	ops       *Ops
	version   int
	pc        pc
}

// InvalidateOp requests a redraw at the given time. Use
// the zero value to request an immediate redraw.
type InvalidateOp struct {
	At time.Time
}

// TransformOp applies a transform to the current transform.
type TransformOp struct {
	// TODO: general transformations.
	offset f32.Point
}

type pc struct {
	data int
	refs int
}

// Push (save) the current operations state.
func (s *StackOp) Push(o *Ops) {
	if s.active {
		panic("unbalanced push")
	}
	s.active = true
	s.ops = o
	o.stackDepth++
	s.stackDepth = o.stackDepth
	s.macroDepth = o.macroDepth
	data := o.Write(opconst.TypePushLen)
	data[0] = byte(opconst.TypePush)
}

// Pop (restore) a previously Pushed operations state.
func (s *StackOp) Pop() {
	if !s.active {
		panic("unbalanced pop")
	}
	if s.ops.stackDepth != s.stackDepth {
		panic("unbalanced pop")
	}
	if s.ops.macroDepth != s.macroDepth {
		panic("pop in a different macro than push")
	}
	s.active = false
	s.ops.stackDepth--
	data := s.ops.Write(opconst.TypePopLen)
	data[0] = byte(opconst.TypePop)
}

// Reset the Ops, preparing it for re-use.
func (o *Ops) Reset() {
	o.stackDepth = 0
	// Leave references to the GC.
	for i := range o.refs {
		o.refs[i] = nil
	}
	o.data = o.data[:0]
	o.refs = o.refs[:0]
	o.version++
}

// Data is for internal use only.
func (o *Ops) Data() []byte {
	return o.data
}

// Refs is for internal use only.
func (o *Ops) Refs() []interface{} {
	return o.refs
}

// Version is for internal use only.
func (o *Ops) Version() int {
	return o.version
}

// Write is for internal use only.
func (o *Ops) Write(n int, refs ...interface{}) []byte {
	o.data = append(o.data, make([]byte, n)...)
	o.refs = append(o.refs, refs...)
	return o.data[len(o.data)-n:]
}

func (o *Ops) pc() pc {
	return pc{data: len(o.data), refs: len(o.refs)}
}

// Record a macro of operations.
func (m *MacroOp) Record(o *Ops) {
	if m.recording {
		panic("already recording")
	}
	m.recording = true
	m.ops = o
	m.ops.macroDepth++
	m.pc = o.pc()
	// Reserve room for a macro definition. Updated in Stop.
	m.ops.Write(opconst.TypeMacroDefLen)
	m.fill()
}

// Stop ends a previously started recording.
func (m *MacroOp) Stop() {
	if !m.recording {
		panic("not recording")
	}
	m.ops.macroDepth--
	m.recording = false
	m.fill()
}

func (m *MacroOp) fill() {
	pc := m.ops.pc()
	// Fill out the macro definition reserved in Record.
	data := m.ops.data[m.pc.data:]
	data = data[:opconst.TypeMacroDefLen]
	data[0] = byte(opconst.TypeMacroDef)
	bo := binary.LittleEndian
	bo.PutUint32(data[1:], uint32(pc.data))
	bo.PutUint32(data[5:], uint32(pc.refs))
	m.version = m.ops.version
}

// Add the recorded list of operations. The Ops
// argument may be different than the Ops argument
// passed to Record.
func (m MacroOp) Add(o *Ops) {
	if m.recording {
		panic("a recording is in progress")
	}
	if m.ops == nil {
		return
	}
	data := o.Write(opconst.TypeMacroLen, m.ops)
	data[0] = byte(opconst.TypeMacro)
	bo := binary.LittleEndian
	bo.PutUint32(data[1:], uint32(m.pc.data))
	bo.PutUint32(data[5:], uint32(m.pc.refs))
	bo.PutUint32(data[9:], uint32(m.version))
}

func (r InvalidateOp) Add(o *Ops) {
	data := o.Write(opconst.TypeRedrawLen)
	data[0] = byte(opconst.TypeInvalidate)
	bo := binary.LittleEndian
	// UnixNano cannot represent the zero time.
	if t := r.At; !t.IsZero() {
		nanos := t.UnixNano()
		if nanos > 0 {
			bo.PutUint64(data[1:], uint64(nanos))
		}
	}
}

// Offset the transformation.
func (t TransformOp) Offset(o f32.Point) TransformOp {
	return t.Multiply(TransformOp{o})
}

// Invert the transformation.
func (t TransformOp) Invert() TransformOp {
	return TransformOp{offset: t.offset.Mul(-1)}
}

// Transform a point.
func (t TransformOp) Transform(p f32.Point) f32.Point {
	return p.Add(t.offset)
}

// Multiply by a transformation.
func (t TransformOp) Multiply(t2 TransformOp) TransformOp {
	return TransformOp{
		offset: t.offset.Add(t2.offset),
	}
}

func (t TransformOp) Add(o *Ops) {
	data := o.Write(opconst.TypeTransformLen)
	data[0] = byte(opconst.TypeTransform)
	bo := binary.LittleEndian
	bo.PutUint32(data[1:], math.Float32bits(t.offset.X))
	bo.PutUint32(data[5:], math.Float32bits(t.offset.Y))
}