Implement checksum verification (#4)

This implements the -c / --check switch to verify checksums previously generated by ksum, similar in behaviour to other checksum programs such as sha256sum.

A test suite is also added to test the behaviour of ksum, including tests against various test vectors.

The --warn, --quiet, --strict switches are also implemented (only relevant when verifying checksums)

.gitignore

@@ -1,2 +1,3 @@
 obj
 bin
+__pycache__

.travis.yml

@@ -1,5 +1,7 @@
 language: c
 
+dist: bionic
+
 services:
   - docker
 

README.md

@@ -38,6 +38,37 @@ $ cat file1 | ksum --sha3-256 -
 be5215abf72333a73b992dafdf4ab59884b948452e0015cfaddaa0b87a0e4515  -
 ```
 
+### Checking checksums
+
+The `-c` or `--check` switch can be used to read checksums from the `FILE`s
+and check them.
+
+For example, to generate a file called `checksums.txt` that contains the
+checksums of `file1`, `file2`, and `file3`:
+
+```
+$ ksum --sha3-256 file1 file2 file3 > checksums.txt
+```
+
+Then, to verify the checksums use the `--check` switch (or its `-c` short
+version):
+```
+$ ksum --sha3-256 --check checksums.txt
+```
+
+`ksum` exits with a status of 0 if all checksums are valid. Otherwise,
+`ksum` prints diagnostic messages and exits with a non-zero status.
+Here's an example error output:
+```
+$ ksum --sha3-256 --check checksums.txt
+file2: FAILED
+ksum: WARNING: checksums.txt: 1 computed checksum did NOT match
+```
+
+Note that `ksum` must be invoked with the same settings that was used to
+generate the checksums. This is particularly important for algorithms
+that are customizable, e.g. KMAC, ParallelHash, cSHAKE.
+
 ### Variable output length
 
 Several of the algorithms supported by `ksum` can output a variable length
@@ -187,7 +218,6 @@ The table also includes the output of other checksum programs for reference
 ## TODO
 
 Things not yet implemented:
-  * Check mode to verify files against pre-generated checksums.
   * Testing against test vectors.
     (although `libkeccak` itself is tested against test vectors)
 

src/argument_parser.ads

@@ -30,10 +30,17 @@ is
    type Argument_Handler is
      access not null procedure (Short_Name, Long_Name, Arg : in String);
 
+   type Group_Type is
+     (Main_Switches,
+      Check_Switches,
+      Customization,
+      Algorithms);
+
    type Switch_Descriptor is record
       Short_Name   : Unbounded_String;
       Long_Name    : Unbounded_String;
       Description  : Unbounded_String;
+      Group        : Group_Type;
       Has_Argument : Boolean;
       Handler      : Argument_Handler;
    end record;