Add integer function to EPSG and ESRI types (#333)

Author: juliohm

src/codes.jl

@@ -11,6 +11,13 @@ See also [`EPSG`](@ref), [`ESRI`](@ref).
 """
 abstract type CRSCode end
 
+"""
+    CoordRefSystems.integer(::Type{<:CRSCode})
+
+Get the integer code associated with the `CRSCode` type.
+"""
+function integer end
+
 """
     EPSG{code}
 
@@ -21,12 +28,7 @@ See [EPSG Geodetic Parameter Dataset](https://en.wikipedia.org/wiki/EPSG_Geodeti
 """
 abstract type EPSG{Code} <: CRSCode end
 
-"""
-    ESRI{code}
-
-ESRI dataset `code`. Codes can be searched at [epsg.io](https://epsg.io).
-"""
-abstract type ESRI{Code} <: CRSCode end
+integer(::Type{EPSG{Code}}) where {Code} = Code
 
 function wkt2(::Type{EPSG{Code}}) where {Code}
   epsg = joinpath(pkgdir(@__MODULE__), "artifacts", "epsg-wkt2")
@@ -50,6 +52,15 @@ function wkt2(::Type{EPSG{Code}}) where {Code}
   str
 end
 
+"""
+    ESRI{code}
+
+ESRI dataset `code`. Codes can be searched at [epsg.io](https://epsg.io).
+"""
+abstract type ESRI{Code} <: CRSCode end
+
+integer(::Type{ESRI{Code}}) where {Code} = Code
+
 function wkt2(::Type{ESRI{Code}}) where {Code}
   throw(ArgumentError("""
                       OGC WKT-CRS version 2 does not support ESRI:$Code.

test/codes.jl

@@ -0,0 +1,4 @@
+@testset "Codes" begin
+  @test CoordRefSystems.integer(EPSG{4326}) == 4326
+  @test CoordRefSystems.integer(ESRI{102100}) == 102100
+end

test/runtests.jl

@@ -71,6 +71,7 @@ testfiles = [
   "crs/traits.jl",
 
   # EPSG/ESRI codes
+  "codes.jl",
   "get.jl",
 
   # WKT strings