package world

import (
	"testing"

	"github.com/stretchr/testify/require"
)

func TestWorldDeltaFixedToCanvasPx_RemainderAccumulatesPositive(t *testing.T) {
	t.Parallel()

	// zoom=1: px = (deltaWorldFp * 1000) / 1e6
	// For deltaWorldFp=1, each step contributes 0 px with remainder,
	// and after 1000 steps it must become 1 px total.
	zoomFp := SCALE
	var rem int64
	sum := 0

	for i := 0; i < 1000; i++ {
		sum += worldDeltaFixedToCanvasPx(1, zoomFp, &rem)
	}

	require.Equal(t, 1, sum)
}

func TestWorldDeltaFixedToCanvasPx_RemainderAccumulatesNegative(t *testing.T) {
	t.Parallel()

	zoomFp := SCALE
	var rem int64
	sum := 0

	for i := 0; i < 1000; i++ {
		sum += worldDeltaFixedToCanvasPx(-1, zoomFp, &rem)
	}

	require.Equal(t, -1, sum)
}

func TestComputePanShiftPx_FirstCallRequiresFullRedraw(t *testing.T) {
	t.Parallel()

	w := NewWorld(10, 10)

	params := RenderParams{
		ViewportWidthPx:  100,
		ViewportHeightPx: 80,
		MarginXPx:        25,
		MarginYPx:        20,
		CameraXWorldFp:   5 * SCALE,
		CameraYWorldFp:   5 * SCALE,
		CameraZoom:       1.0,
	}

	_, _, err := w.ComputePanShiftPx(params)
	require.ErrorIs(t, err, errIncrementalStateNotReady)
}

func TestComputePanShiftPx_ZoomOrViewportChangeForcesFullRedraw(t *testing.T) {
	t.Parallel()

	w := NewWorld(10, 10)

	base := RenderParams{
		ViewportWidthPx:  100,
		ViewportHeightPx: 80,
		MarginXPx:        25,
		MarginYPx:        20,
		CameraXWorldFp:   5 * SCALE,
		CameraYWorldFp:   5 * SCALE,
		CameraZoom:       1.0,
	}
	require.NoError(t, w.CommitFullRedrawState(base))

	changed := base
	changed.CameraZoom = 2.0

	_, _, err := w.ComputePanShiftPx(changed)
	require.ErrorIs(t, err, errIncrementalZoomMismatch)

	changed2 := base
	changed2.ViewportWidthPx = 101

	_, _, err = w.ComputePanShiftPx(changed2)
	require.ErrorIs(t, err, errIncrementalZoomMismatch)
}

func TestComputePanShiftPx_PanRightShiftsImageLeft(t *testing.T) {
	t.Parallel()

	w := NewWorld(10, 10)

	params := RenderParams{
		ViewportWidthPx:  100,
		ViewportHeightPx: 80,
		MarginXPx:        25,
		MarginYPx:        20,
		CameraXWorldFp:   5 * SCALE,
		CameraYWorldFp:   5 * SCALE,
		CameraZoom:       1.0,
	}
	require.NoError(t, w.CommitFullRedrawState(params))

	// Move camera right by 1 world unit => world rect minX increases by 1 unit,
	// so content moves left by 1px at zoom=1 => image shift should be -1.
	params2 := params
	params2.CameraXWorldFp += 1 * SCALE

	dx, dy, err := w.ComputePanShiftPx(params2)
	require.NoError(t, err)
	require.Equal(t, -1, dx)
	require.Equal(t, 0, dy)
}

func TestComputePanShiftPx_PanUpShiftsImageDown(t *testing.T) {
	t.Parallel()

	w := NewWorld(10, 10)

	params := RenderParams{
		ViewportWidthPx:  100,
		ViewportHeightPx: 80,
		MarginXPx:        25,
		MarginYPx:        20,
		CameraXWorldFp:   5 * SCALE,
		CameraYWorldFp:   5 * SCALE,
		CameraZoom:       1.0,
	}
	require.NoError(t, w.CommitFullRedrawState(params))

	// Move camera up by 1 world unit => world rect minY decreases by 1 unit,
	// so content moves down by 1px => image shift should be +1 in dy.
	params2 := params
	params2.CameraYWorldFp -= 1 * SCALE

	dx, dy, err := w.ComputePanShiftPx(params2)
	require.NoError(t, err)
	require.Equal(t, 0, dx)
	require.Equal(t, 1, dy)
}

func TestComputePanShiftPx_SubPixelPanAccumulatesToOnePixel(t *testing.T) {
	t.Parallel()

	w := NewWorld(10, 10)

	params := RenderParams{
		ViewportWidthPx:  100,
		ViewportHeightPx: 80,
		MarginXPx:        25,
		MarginYPx:        20,
		CameraXWorldFp:   5 * SCALE,
		CameraYWorldFp:   5 * SCALE,
		CameraZoom:       1.0,
	}
	require.NoError(t, w.CommitFullRedrawState(params))

	// Pan camera right by 0.001 world units (1 fixed-point) 1000 times.
	// At zoom=1 this should accumulate to a 1px content shift left, hence image shift -1.
	totalDx := 0
	p := params
	for i := 0; i < 1000; i++ {
		p.CameraXWorldFp += 1
		dx, dy, err := w.ComputePanShiftPx(p)
		require.NoError(t, err)
		require.Equal(t, 0, dy)
		totalDx += dx
	}

	require.Equal(t, -1, totalDx)
}