package sprite

import (
	"fmt"
	"image/color"
)

// Sheet lays out several equally sized sprites in a row-major grid:
// index 0 is top-left, then left-to-right, then next row. A 1D strip is
// simply a sheet with one row (or one column).
type Sheet struct {
	Sprites      []*Sprite
	Cols, Rows   int
	CellW, CellH int
}

// NewSheet composes sprites into a sheet with the given number of columns.
// cols <= 0 puts everything in a single row. Every sprite must have the
// same dimensions so the sheet can be indexed cell by cell.
func NewSheet(sprites []*Sprite, cols int) (*Sheet, error) {
	if len(sprites) == 0 {
		return nil, fmt.Errorf("a sheet needs at least one sprite")
	}
	w, h := sprites[0].Bounds()
	for _, sp := range sprites[1:] {
		sw, sh := sp.Bounds()
		if sw != w || sh != h {
			return nil, fmt.Errorf("sprite %q is %dx%d but %q is %dx%d — all sprites in a sheet must be the same size",
				sp.Name, sw, sh, sprites[0].Name, w, h)
		}
	}
	if cols <= 0 || cols > len(sprites) {
		cols = len(sprites)
	}
	rows := (len(sprites) + cols - 1) / cols
	return &Sheet{Sprites: sprites, Cols: cols, Rows: rows, CellW: w, CellH: h}, nil
}

// Bounds returns the total sheet size in pixels.
func (sh *Sheet) Bounds() (w, h int) { return sh.Cols * sh.CellW, sh.Rows * sh.CellH }

// At returns the pixel at (x, y). Cells past the last sprite (when the
// sprite count doesn't fill the grid) are transparent.
func (sh *Sheet) At(x, y int) color.NRGBA {
	col, row := x/sh.CellW, y/sh.CellH
	idx := row*sh.Cols + col
	if idx >= len(sh.Sprites) {
		return Transparent
	}
	return sh.Sprites[idx].At(x%sh.CellW, y%sh.CellH)
}

// FileBase appends the layout to an output name so the file itself
// documents cell size and orientation: <base>_<cellW>x<cellH>_<cols>x<rows>
// e.g. walk_16x16_4x2 = 16x16 cells, 4 columns, 2 rows, read row by row.
func (sh *Sheet) FileBase(base string) string {
	return fmt.Sprintf("%s_%dx%d_%dx%d", base, sh.CellW, sh.CellH, sh.Cols, sh.Rows)
}