diff --git a/Rakefile b/Rakefile
index 4e8cbbc..1f175f8 100644
--- a/Rakefile
+++ b/Rakefile
@@ -17,10 +17,6 @@ CLEAN.include 'build/boot', 'build/valid'
 CLEAN.include 'doc/*.pdf'
 CLOBBER.include 'build'
 
-def compile(*arguments)
-  sh(ENV.fetch('CC', 'gcc'), '-fpie', '-g', *arguments)
-end
-
 def run(exe)
   ENV.fetch('QEMU', '').split << exe
 end
@@ -50,75 +46,74 @@ task :convert do
   end
 end
 
-rule /^build\/[[:alpha:]]+\/stage[[:digit:]]+\/cl$/ => ->(match) {
-  "#{match}.o"
-} do |t|
-  arguments_path = Pathname.new('boot') + Pathname.new(t.name).dirname.basename + 'linker.arg'
-  if arguments_path.exist?
-    arguments1 = ['--dynamic-linker', '/lib32/ld-linux-riscv32-ilp32d.so.1', '/usr/lib/crt1.o', '/usr/lib/crti.o', '-lc']
-    arguments2 = ['/usr/lib/crtn.o']
-  else
-    arguments1 = arguments2 = []
+file 'build/build.ninja' => ['build'] do |t|
+  File.open t.name, 'w' do |f|
+    f << <<~NINJA
+      builddir = build
+      cflags = -fpie -g
+
+      rule cc
+        command = gcc $cflags -nostdlib -o $out $in
+
+      rule as
+        command = gcc $cflags -c -o $out $in
+
+      rule link1
+        command = ld -o $out $in
+
+      rule link2
+        command = ld -o $out --dynamic-linker /lib32/ld-linux-riscv32-ilp32d.so.1 /usr/lib/crt1.o /usr/lib/crti.o -lc $in /usr/lib/crtn.o
+
+      rule boot1
+        command = build/boot/stage1/cl < \$in > \$out
+
+      rule valid1
+        command = build/valid/stage1/cl < \$in > \$out
+    NINJA
+    STAGES.each do |stage|
+      stage_number = stage.delete_prefix('stage').to_i
+
+      f << <<~NINJA
+
+        rule valid#{stage_number}
+          command = build/valid/stage#{stage_number}/cl < \$in > \$out
+
+        rule boot#{stage_number}
+          command = build/boot/stage#{stage_number}/cl < \$in > \$out
+      NINJA
+    end
+    f << <<~NINJA
+
+      build build/boot/stage1/cl: cc boot/stage1.s
+
+      build build/valid/stage1/cl.s: boot1 boot/stage1.s | build/boot/stage1/cl
+      build build/valid/stage1/cl.o: as build/valid/stage1/cl.s
+      build build/valid/stage1/cl: link1 build/valid/stage1/cl.o
+    NINJA
+    STAGES.each do |stage|
+      stage_number = stage.delete_prefix('stage').to_i
+
+      arguments_path = Pathname.new('boot') + stage + 'linker.arg'
+      if arguments_path.exist?
+        link = 'link2'
+      else
+        link = 'link1'
+      end
+      boot_stage = "build/boot/stage#{stage_number}"
+      valid_stage = "build/valid/stage#{stage_number}"
+      f << <<~NINJA
+
+        build #{boot_stage}/cl.s: valid#{stage_number.pred} boot/stage#{stage_number}/cl.elna | build/valid/stage#{stage_number.pred}/cl
+        build #{boot_stage}/cl.o: as #{boot_stage}/cl.s
+        build #{boot_stage}/cl: #{link} #{boot_stage}/cl.o
+
+        build #{valid_stage}/cl.s: boot#{stage_number} boot/stage#{stage_number}/cl.elna | #{boot_stage}/cl
+        build #{valid_stage}/cl.o: as #{valid_stage}/cl.s
+        build #{valid_stage}/cl: #{link} #{valid_stage}/cl.o
+      NINJA
+    end
+    f << "\ndefault build/valid/stage18/cl\n"
   end
-
-  sh(ENV.fetch('LD', 'ld'), '-o', t.name, *arguments1, *t.prerequisites, *arguments2)
-end
-
-rule /^build\/[[:alpha:]]+\/stage[[:digit:]]+\/cl.o$/ => ->(match) {
-  match.ext('.s')
-} do |t|
-  compile('-c', '-o', t.name, *t.prerequisites)
-end
-
-STAGES.each do |stage|
-  previous = stage.delete_prefix('stage').to_i.pred
-
-  directory "build/valid/#{stage}"
-  directory "build/boot/#{stage}"
-
-  file "build/valid/#{stage}/cl.s" => ["build/boot/#{stage}/cl", "boot/#{stage}/cl.elna", "build/valid/#{stage}"] do |t|
-    exe, source = t.prerequisites
-
-    cat_arguments = ['cat', source]
-    last_stdout, wait_threads = Open3.pipeline_r(cat_arguments, run(exe))
-
-    IO.copy_stream last_stdout, t.name
-  end
-
-  file "build/boot/#{stage}/cl.s" => ["build/valid/stage#{previous}/cl", "boot/#{stage}/cl.elna", "build/boot/#{stage}"] do |t|
-    exe, source = t.prerequisites
-
-    cat_arguments = ['cat', source]
-    last_stdout, wait_threads = Open3.pipeline_r(cat_arguments, run(exe))
-
-    IO.copy_stream last_stdout, t.name
-  end
-end
-
-#
-# Stage 1.
-#
-
-directory 'build/valid/stage1'
-directory 'build/boot/stage1'
-
-file 'build/valid/stage1/cl' => ['build/valid/stage1.s'] do |t|
-  compile('-nostdlib', '-o', t.name, *t.prerequisites)
-end
-
-file 'build/valid/stage1.s' => ['build/boot/stage1/cl', 'boot/stage1.s', 'build/valid/stage1'] do |t|
-  source, exe, = t.prerequisites.partition { |prerequisite| prerequisite.end_with? '.s' }
-
-  cat_arguments = ['cat', *source]
-  last_stdout, wait_threads = Open3.pipeline_r(cat_arguments, run(exe.first))
-
-  IO.copy_stream last_stdout, t.name
-end
-
-file 'build/boot/stage1/cl' => ['build/boot/stage1', 'boot/stage1.s'] do |t|
-  source = t.prerequisites.select { |prerequisite| prerequisite.end_with? '.s' }
-
-  compile('-nostdlib', '-o', t.name, *source)
 end
 
 rule '.pdf' => '.adoc' do |t|