if(NOT DEFINED PREFIX)
  message(FATAL_ERROR "PREFIX must be defined.")
endif()

if(NOT DEFINED URL)
  message(FATAL_ERROR "URL must be defined.")
endif()

if(NOT DEFINED DOWNLOAD_DIR)
  message(FATAL_ERROR "DOWNLOAD_DIR must be defined.")
endif()

if(NOT DEFINED EXPECTED_SHA256)
  message(FATAL_ERROR "EXPECTED_SHA256 must be defined.")
endif()

if(NOT DEFINED TARGET)
  message(FATAL_ERROR "TARGET must be defined.")
endif()

set(SRC_DIR ${PREFIX}/src/${TARGET})

# Check whether the source has been downloaded. If true, skip it.
# Useful for external downloads like homebrew.
if(EXISTS "${SRC_DIR}" AND IS_DIRECTORY "${SRC_DIR}")
  file(GLOB EXISTED_FILES "${SRC_DIR}/*")
  if(EXISTED_FILES)
    message(STATUS "${SRC_DIR} is found and not empty, skipping download and extraction. ")
    return()
  endif()
endif()

# Taken from ExternalProject_Add.  Let's hope we can drop this one day when
# ExternalProject_Add allows you to disable SHOW_PROGRESS on the file download.
if(TIMEOUT)
  set(timeout_args TIMEOUT ${timeout})
  set(timeout_msg "${timeout} seconds")
else()
  set(timeout_args "# no TIMEOUT")
  set(timeout_msg "none")
endif()

string(REGEX MATCH "[^/\\?]*$" fname "${URL}")
if(NOT "${fname}" MATCHES "(\\.|=)(bz2|tar|tgz|tar\\.gz|zip)$")
  string(REGEX MATCH "([^/\\?]+(\\.|=)(bz2|tar|tgz|tar\\.gz|zip))/.*$" match_result "${URL}")
  set(fname "${CMAKE_MATCH_1}")
endif()
if(NOT "${fname}" MATCHES "(\\.|=)(bz2|tar|tgz|tar\\.gz|zip)$")
  message(FATAL_ERROR "Could not extract tarball filename from url:\n  ${url}")
endif()
string(REPLACE ";" "-" fname "${fname}")

set(file ${DOWNLOAD_DIR}/${fname})
message(STATUS "file: ${file}")

message(STATUS "downloading...
     src='${URL}'
     dst='${file}'
     timeout='${timeout_msg}'")

file(DOWNLOAD ${URL} ${file}
  ${timeout_args}
  ${hash_args}
  STATUS status
  LOG log)

list(GET status 0 status_code)
list(GET status 1 status_string)

if(NOT status_code EQUAL 0)
  message(FATAL_ERROR "error: downloading '${URL}' failed
  status_code: ${status_code}
  status_string: ${status_string}
  log: ${log}
")
endif()

set(NULL_SHA256 "0000000000000000000000000000000000000000000000000000000000000000")

# Allow users to use "SKIP" or "skip" as the sha256 to skip checking the hash.
# You can still use the all zeros hash too.
if((EXPECTED_SHA256 STREQUAL "SKIP") OR (EXPECTED_SHA256 STREQUAL "skip"))
  set(EXPECTED_SHA256 ${NULL_SHA256})
endif()

# We could avoid computing the SHA256 entirely if a NULL_SHA256 was given,
# but we want to warn users of an empty file.
file(SHA256 ${file} ACTUAL_SHA256)
if(ACTUAL_SHA256 STREQUAL "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855")
  # File was empty.  It's likely due to lack of SSL support.
  message(FATAL_ERROR
    "Failed to download ${URL}.  The file is empty and likely means CMake "
    "was built without SSL support.  Please use a version of CMake with "
    "proper SSL support.  See "
    "https://github.com/neovim/neovim/wiki/Building-Neovim#build-prerequisites "
    "for more information.")
elseif((NOT EXPECTED_SHA256 STREQUAL NULL_SHA256) AND
       (NOT EXPECTED_SHA256 STREQUAL ACTUAL_SHA256))
  # Wasn't a NULL SHA256 and we didn't match, so we fail.
  message(FATAL_ERROR
    "Failed to download ${URL}.  Expected a SHA256 of "
    "${EXPECTED_SHA256} but got ${ACTUAL_SHA256} instead.")
endif()

message(STATUS "downloading... done")

# Slurped from a generated extract-TARGET.cmake file.
message(STATUS "extracting...
     src='${file}'
     dst='${SRC_DIR}'")

if(NOT EXISTS "${file}")
  message(FATAL_ERROR "error: file to extract does not exist: '${file}'")
endif()

# Prepare a space for extracting:
#
set(i 1234)
while(EXISTS "${SRC_DIR}/../ex-${TARGET}${i}")
  math(EXPR i "${i} + 1")
endwhile()
set(ut_dir "${SRC_DIR}/../ex-${TARGET}${i}")
file(MAKE_DIRECTORY "${ut_dir}")

# Extract it:
#
message(STATUS "extracting... [tar xfz]")
execute_process(COMMAND ${CMAKE_COMMAND} -E tar xfz ${file}
  WORKING_DIRECTORY ${ut_dir}
  RESULT_VARIABLE rv)

if(NOT rv EQUAL 0)
  message(STATUS "extracting... [error clean up]")
  file(REMOVE_RECURSE "${ut_dir}")
  message(FATAL_ERROR "error: extract of '${file}' failed")
endif()

# Analyze what came out of the tar file:
#
message(STATUS "extracting... [analysis]")
file(GLOB contents "${ut_dir}/*")
list(LENGTH contents n)
if(NOT n EQUAL 1 OR NOT IS_DIRECTORY "${contents}")
  set(contents "${ut_dir}")
endif()

# Move "the one" directory to the final directory:
#
message(STATUS "extracting... [rename]")
file(REMOVE_RECURSE ${SRC_DIR})
get_filename_component(contents ${contents} ABSOLUTE)
file(RENAME ${contents} ${SRC_DIR})

# Clean up:
#
message(STATUS "extracting... [clean up]")
file(REMOVE_RECURSE "${ut_dir}")

message(STATUS "extracting... done")