speed up x/y for #19

DESCRIPTION

@@ -1,6 +1,6 @@
 Package: vaster
 Title: Tools for Raster Grid Logic 
-Version: 0.0.1.9002
+Version: 0.0.1.9003
 Authors@R: c(person("Michael", "Sumner", email = "[email protected]", role = c("aut", "cre"), comment = c(ORCID = "0000-0002-2471-7511")))
 Description: Provides raster grid logic, the grid operations that don't require access to materialized data, i.e. most of them. 
  Grids are arrays with dimension and extent, and many operations are functions of just the dimension 'nrows', 'ncols' or 
@@ -11,7 +11,7 @@ Description: Provides raster grid logic, the grid operations that don't require
 License: MIT + file LICENSE
 Encoding: UTF-8
 Roxygen: list(markdown = TRUE)
-RoxygenNote: 7.3.0
+RoxygenNote: 7.3.1
 URL: https://github.com/hypertidy/vaster, https://hypertidy.github.io/vaster/
 BugReports: https://github.com/hypertidy/vaster/issues
 Suggests: 

NEWS.md

@@ -1,5 +1,7 @@
 # vaster dev
 
+* Speed up `x_from_col` and `y_from_row`, fixes #19. 
+
 * New functions `rasterio_idx()` (was affinity::raster_to_rasterio), `rasterio0` (was affinity::raster_io0) and
 internal for now `rasterio_crop()`. 
 

R/coordinates.R

@@ -142,7 +142,9 @@ x_from_col <- function(dimension, extent = NULL, col) {
 
   col[col < 1] <- NA
   col[col > dimension[1L]] <- NA
-  x_centre(dimension, extent = extent)[col]
+  xres <- x_res(dimension, extent)
+  extent[1] - xres/2 + col * xres
+
 }
 
 #' @name coordinates
@@ -156,7 +158,9 @@ y_from_row <- function(dimension, extent = NULL, row) {
 
   row[row < 1] <- NA
   row[row > dimension[2]] <- NA
-  rev(y_centre(dimension, extent = extent))[row]
+  yres <- y_res(dimension, extent)
+  extent[4] + yres/2 - row * yres
+
 }
 
 #' @name coordinates