package world

import (
	"errors"
	"fmt"
	"math"
)

var (
	errInvalidCameraZoom = errors.New("invalid camera zoom")
)

const (
	// SCALE is the fixed-point multiplier used across the package.
	// A real value of 1.0 is represented as SCALE.
	SCALE = 1000

	// MIN_ZOOM and MAX_ZOOM define the supported zoom range in fixed-point form.
	// They are reserved for future validation/clamping logic.
	MIN_ZOOM = int(SCALE / 4)  // 0.25x
	MAX_ZOOM = int(SCALE * 32) // 32x

	// cellSizeMin and cellSizeMax bound the automatically selected grid cell size.
	cellSizeMin = 16 * SCALE
	cellSizeMax = 512 * SCALE
)

// Rect is a half-open rectangle in fixed-point world coordinates:
// [minX, maxX) x [minY, maxY).
type Rect struct {
	minX, maxX int
	minY, maxY int
}

// wrap maps value into the half-open interval [0, size).
// It supports negative input values and is used for torus coordinates.
func wrap(value, size int) int {
	r := value % size
	if r < 0 {
		r += size
	}
	return r
}

// clamp limits value to the closed interval [minValue, maxValue].
func clamp(value, minValue, maxValue int) int {
	if value < minValue {
		return minValue
	}
	if value > maxValue {
		return maxValue
	}
	return value
}

// ceilDiv returns ceil(a / b) for positive integers.
func ceilDiv(a, b int) int {
	return (a + b - 1) / b
}

// floorDiv returns floor(a / b) for b > 0 and supports negative a.
func floorDiv(a, b int) int {
	if b <= 0 {
		panic("floorDiv: non-positive divisor")
	}

	q := a / b
	r := a % b
	if r != 0 && a < 0 {
		q--
	}
	return q
}

// fixedPoint converts a real value into the package fixed-point representation
// using nearest-integer rounding.
func fixedPoint(v float64) int {
	return int(math.Round(v * SCALE))
}

// abs returns the absolute value of v.
func abs(v int) int {
	if v < 0 {
		return -v
	}
	return v
}

// viewportPxToWorldFixed converts a viewport size in pixels into the visible
// world size in fixed-point coordinates for the given fixed-point zoom.
func viewportPxToWorldFixed(viewportWidthPx, viewportHeightPx, cameraZoomFp int) (int, int) {
	return PixelSpanToWorldFixed(viewportWidthPx, cameraZoomFp),
		PixelSpanToWorldFixed(viewportHeightPx, cameraZoomFp)
}

// worldToCell maps a world coordinate to a grid cell index.
// The input coordinate is wrapped on the torus before the cell is computed.
// The function panics when the grid configuration is invalid.
func worldToCell(value, worldSize, cells, cellSize int) int {
	if cells <= 0 || cellSize <= 0 {
		panic(fmt.Sprintf("worldToCell: cells=%d cellSize=%d", cells, cellSize))
	}

	wrappedValue := wrap(value, worldSize)
	cell := wrappedValue / cellSize
	if cell >= cells {
		cell = cells - 1
	}
	return cell
}

// splitByWrap splits a half-open rectangle by torus wrap into 1..4 rectangles
// fully contained inside [0, W) x [0, H).
func splitByWrap(W, H, minX, maxX, minY, maxY int) []Rect {
	width := maxX - minX
	height := maxY - minY

	if width <= 0 || height <= 0 {
		return nil
	}

	if width >= W {
		minX = 0
		maxX = W
	}
	if height >= H {
		minY = 0
		maxY = H
	}

	type xPart struct {
		minX, maxX int
	}

	xParts := make([]xPart, 0, 2)

	if minX >= 0 && maxX <= W {
		xParts = append(xParts, xPart{minX: minX, maxX: maxX})
	} else {
		wrappedMinX := wrap(minX, W)
		wrappedMaxX := wrap(maxX, W)

		if wrappedMinX < wrappedMaxX {
			xParts = append(xParts, xPart{minX: wrappedMinX, maxX: wrappedMaxX})
		} else {
			xParts = append(xParts, xPart{minX: wrappedMinX, maxX: W})
			if wrappedMaxX > 0 {
				xParts = append(xParts, xPart{minX: 0, maxX: wrappedMaxX})
			}
		}
	}

	result := make([]Rect, 0, 4)

	for _, xp := range xParts {
		if minY >= 0 && maxY <= H {
			result = append(result, Rect{
				minX: xp.minX, maxX: xp.maxX,
				minY: minY, maxY: maxY,
			})
			continue
		}

		wrappedMinY := wrap(minY, H)
		wrappedMaxY := wrap(maxY, H)

		if wrappedMinY < wrappedMaxY {
			result = append(result, Rect{
				minX: xp.minX, maxX: xp.maxX,
				minY: wrappedMinY, maxY: wrappedMaxY,
			})
		} else {
			result = append(result, Rect{
				minX: xp.minX, maxX: xp.maxX,
				minY: wrappedMinY, maxY: H,
			})
			if wrappedMaxY > 0 {
				result = append(result, Rect{
					minX: xp.minX, maxX: xp.maxX,
					minY: 0, maxY: wrappedMaxY,
				})
			}
		}
	}

	return result
}

// PixelSpanToWorldFixed converts a span in pixels into a span in fixed-point
// world coordinates for the given fixed-point zoom.
func PixelSpanToWorldFixed(spanPx int, zoomFp int) int {
	return (spanPx * SCALE * SCALE) / zoomFp
}

// shortestWrappedDelta returns a canonical torus representation of the pair
// (from, to) along a single axis of length size.
//
// The resulting delta (b - a) is normalized into the half-open interval
// [-size/2, size/2). This makes the tie-case deterministic:
// when the points are exactly half a world apart, wrap is always applied in
// the direction that produces the negative delta.
func shortestWrappedDelta(from, to, size int) (a int, b int) {
	a, b = from, to

	delta := to - from
	half := size / 2

	if delta >= half {
		a += size
		return
	}
	if delta < -half {
		b += size
		return
	}

	return
}

// CameraZoomToWorldFixed converts a UI-facing zoom multiplier into the package
// fixed-point representation used by world-space calculations.
//
// The input zoom is expected to be a finite positive real value where 1.0 means
// the neutral zoom level. The result is rounded to the nearest fixed-point value.
//
// An error is returned when the input is invalid or when rounding would produce
// a non-positive fixed-point zoom.
func CameraZoomToWorldFixed(cameraZoom float64) (int, error) {
	if cameraZoom <= 0 || math.IsNaN(cameraZoom) || math.IsInf(cameraZoom, 0) {
		return 0, errInvalidCameraZoom
	}

	zoomFp := int(math.Round(cameraZoom * SCALE))
	if zoomFp <= 0 {
		return 0, errInvalidCameraZoom
	}

	return zoomFp, nil
}

// mustCameraZoomToWorldFixed is the panic-on-error variant of
// cameraZoomToWorldFixed. It is intended for internal code paths where invalid
// zoom is considered a programmer or integration error and must fail fast.
func mustCameraZoomToWorldFixed(cameraZoom float64) int {
	zoomFp, err := CameraZoomToWorldFixed(cameraZoom)
	if err != nil {
		panic(err)
	}
	return zoomFp
}