ExternalData: Generalize hash algo/ext handling

Use private global variables _ExternalData_REGEX_(ALGO|EXT) to match the
possible hash algorithm names and extensions in regular expressions.
Use "file(<algo>)" instead of "cmake -E md5sum" to compute hashes
without a child process and to support more hash algorithms.

Modules/ExternalData.cmake

@@ -261,18 +261,16 @@ endfunction()
 #-----------------------------------------------------------------------------
 # Private helper interface
 
+set(_ExternalData_REGEX_ALGO "MD5")
+set(_ExternalData_REGEX_EXT "md5")
 set(_ExternalData_SELF "${CMAKE_CURRENT_LIST_FILE}")
 get_filename_component(_ExternalData_SELF_DIR "${_ExternalData_SELF}" PATH)
 
 function(_ExternalData_compute_hash var_hash algo file)
-  if("${algo}" STREQUAL "MD5")
-    # TODO: Errors
-    execute_process(COMMAND "${CMAKE_COMMAND}" -E md5sum "${file}"
-      OUTPUT_VARIABLE output)
-    string(SUBSTRING "${output}" 0 32 hash)
+  if("${algo}" MATCHES "^${_ExternalData_REGEX_ALGO}$")
+    file("${algo}" "${file}" hash)
     set("${var_hash}" "${hash}" PARENT_SCOPE)
   else()
-    # TODO: Other hashes.
     message(FATAL_ERROR "Hash algorithm ${algo} unimplemented.")
   endif()
 endfunction()
@@ -295,7 +293,7 @@ function(_ExternalData_atomic_write file content)
 endfunction()
 
 function(_ExternalData_link_content name var_ext)
-  if("${ExternalData_LINK_CONTENT}" MATCHES "^(MD5)$")
+  if("${ExternalData_LINK_CONTENT}" MATCHES "^(${_ExternalData_REGEX_ALGO})$")
     set(algo "${ExternalData_LINK_CONTENT}")
   else()
     message(FATAL_ERROR
@@ -305,7 +303,7 @@ function(_ExternalData_link_content name var_ext)
   _ExternalData_compute_hash(hash "${algo}" "${name}")
   get_filename_component(dir "${name}" PATH)
   set(staged "${dir}/.ExternalData_${algo}_${hash}")
-  set(ext ".md5")
+  string(TOLOWER ".${algo}" ext)
   _ExternalData_atomic_write("${name}${ext}" "${hash}\n")
   file(RENAME "${name}" "${staged}")
   set("${var_ext}" "${ext}" PARENT_SCOPE)
@@ -533,7 +531,7 @@ endmacro()
 function(_ExternalData_arg_find_files pattern regex)
   file(GLOB globbed RELATIVE "${top_src}" "${top_src}/${pattern}*")
   foreach(entry IN LISTS globbed)
-    if("x${entry}" MATCHES "^x(.*)(\\.md5)$")
+    if("x${entry}" MATCHES "^x(.*)(\\.(${_ExternalData_REGEX_EXT}))$")
       set(relname "${CMAKE_MATCH_1}")
       set(alg "${CMAKE_MATCH_2}")
     else()
@@ -716,8 +714,8 @@ if("${ExternalData_ACTION}" STREQUAL "fetch")
   file(READ "${name}${ext}" hash)
   string(STRIP "${hash}" hash)
 
-  if("${ext}" STREQUAL ".md5")
-    set(algo "MD5")
+  if("${ext}" MATCHES "^\\.(${_ExternalData_REGEX_EXT})$")
+    string(TOUPPER "${CMAKE_MATCH_1}" algo)
   else()
     message(FATAL_ERROR "Unknown hash algorithm extension \"${ext}\"")
   endif()