build: make exercise output more reliable

Currently, ZiglingStep prints the raw exercise output.  This is not a
problem when executing `zig build` from the shell, but in a unit test it
is necessary to know when the exercise output ends.

Document that Exercise.output should not have trailing whitespace.
Ensure this is true by adding a check in the validate_exercises
function.

Remove trailing whitespace in exercises 68 and 99.

Simplify the output validation in ZiglingStep.makeInternal.
Checking that the length of the actual and expected output is the same
is not necessary, since trailing whitespace has been removed.  Simply do
an exact comparison.

Print the trimmed exercise output, instead of the raw output.
This will ensure that the exercise output always ends with only one LF
character.

Fix some small coding style issues.

build.zig

@@ -18,7 +18,8 @@ pub const Exercise = struct {
     main_file: []const u8,
 
     /// This is the desired output of the program.
-    /// A program passes if its output ends with this string.
+    /// A program passes if its output, excluding trailing whitespace, is equal
+    /// to this string.
     output: []const u8,
 
     /// This is an optional hint to give if the program does not succeed.
@@ -365,7 +366,7 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "068_comptime3.zig",
-        .output = "Minnow (1:32, 4 x 2)\nShark (1:16, 8 x 5)\nWhale (1:1, 143 x 95)\n",
+        .output = "Minnow (1:32, 4 x 2)\nShark (1:16, 8 x 5)\nWhale (1:1, 143 x 95)",
     },
     .{
         .main_file = "069_comptime4.zig",
@@ -516,7 +517,7 @@ const exercises = [_]Exercise{
     },
     .{
         .main_file = "099_formatting.zig",
-        .output = "\n X |  1   2   3   4   5   6   7   8   9  10  11  12  13  14  15 \n---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+\n 1 |  1   2   3   4   5   6   7   8   9  10  11  12  13  14  15 \n\n 2 |  2   4   6   8  10  12  14  16  18  20  22  24  26  28  30 \n\n 3 |  3   6   9  12  15  18  21  24  27  30  33  36  39  42  45 \n\n 4 |  4   8  12  16  20  24  28  32  36  40  44  48  52  56  60 \n\n 5 |  5  10  15  20  25  30  35  40  45  50  55  60  65  70  75 \n\n 6 |  6  12  18  24  30  36  42  48  54  60  66  72  78  84  90 \n\n 7 |  7  14  21  28  35  42  49  56  63  70  77  84  91  98 105 \n\n 8 |  8  16  24  32  40  48  56  64  72  80  88  96 104 112 120 \n\n 9 |  9  18  27  36  45  54  63  72  81  90  99 108 117 126 135 \n\n10 | 10  20  30  40  50  60  70  80  90 100 110 120 130 140 150 \n\n11 | 11  22  33  44  55  66  77  88  99 110 121 132 143 154 165 \n\n12 | 12  24  36  48  60  72  84  96 108 120 132 144 156 168 180 \n\n13 | 13  26  39  52  65  78  91 104 117 130 143 156 169 182 195 \n\n14 | 14  28  42  56  70  84  98 112 126 140 154 168 182 196 210 \n\n15 | 15  30  45  60  75  90 105 120 135 150 165 180 195 210 225 \n\n",
+        .output = "\n X |  1   2   3   4   5   6   7   8   9  10  11  12  13  14  15 \n---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+\n 1 |  1   2   3   4   5   6   7   8   9  10  11  12  13  14  15 \n\n 2 |  2   4   6   8  10  12  14  16  18  20  22  24  26  28  30 \n\n 3 |  3   6   9  12  15  18  21  24  27  30  33  36  39  42  45 \n\n 4 |  4   8  12  16  20  24  28  32  36  40  44  48  52  56  60 \n\n 5 |  5  10  15  20  25  30  35  40  45  50  55  60  65  70  75 \n\n 6 |  6  12  18  24  30  36  42  48  54  60  66  72  78  84  90 \n\n 7 |  7  14  21  28  35  42  49  56  63  70  77  84  91  98 105 \n\n 8 |  8  16  24  32  40  48  56  64  72  80  88  96 104 112 120 \n\n 9 |  9  18  27  36  45  54  63  72  81  90  99 108 117 126 135 \n\n10 | 10  20  30  40  50  60  70  80  90 100 110 120 130 140 150 \n\n11 | 11  22  33  44  55  66  77  88  99 110 121 132 143 154 165 \n\n12 | 12  24  36  48  60  72  84  96 108 120 132 144 156 168 180 \n\n13 | 13  26  39  52  65  78  91 104 117 130 143 156 169 182 195 \n\n14 | 14  28  42  56  70  84  98 112 126 140 154 168 182 196 210 \n\n15 | 15  30  45  60  75  90 105 120 135 150 165 180 195 210 225",
     },
     .{
         .main_file = "999_the_end.zig",
@@ -758,20 +759,20 @@ const ZiglingStep = struct {
             return err;
         };
 
-        // Allow up to 1 MB of stdout capture
+        // Allow up to 1 MB of stdout capture.
         const max_output_len = 1 * 1024 * 1024;
         const output = if (self.exercise.check_stdout)
             try child.stdout.?.reader().readAllAlloc(self.builder.allocator, max_output_len)
         else
             try child.stderr.?.reader().readAllAlloc(self.builder.allocator, max_output_len);
 
-        // at this point stdout is closed, wait for the process to terminate
+        // At this point stdout is closed, wait for the process to terminate.
         const term = child.wait() catch |err| {
             print("{s}Unable to spawn {s}: {s}{s}\n", .{ red_text, argv[0], @errorName(err), reset_text });
             return err;
         };
 
-        // make sure it exited cleanly.
+        // Make sure it exited cleanly.
         switch (term) {
             .Exited => |code| {
                 if (code != 0) {
@@ -785,10 +786,10 @@ const ZiglingStep = struct {
             },
         }
 
-        // validate the output
+        // Validate the output.
         const trimOutput = std.mem.trimRight(u8, output, " \r\n");
         const trimExerciseOutput = std.mem.trimRight(u8, self.exercise.output, " \r\n");
-        if (std.mem.indexOf(u8, trimOutput, trimExerciseOutput) == null or trimOutput.len != trimExerciseOutput.len) {
+        if (!std.mem.eql(u8, trimOutput, trimExerciseOutput)) {
             print(
                 \\
                 \\{s}----------- Expected this output -----------{s}
@@ -801,7 +802,7 @@ const ZiglingStep = struct {
             return error.InvalidOutput;
         }
 
-        print("{s}PASSED:\n{s}{s}\n", .{ green_text, output, reset_text });
+        print("{s}PASSED:\n{s}{s}\n\n", .{ green_text, trimOutput, reset_text });
     }
 
     // The normal compile step calls os.exit, so we can't use it as a library :(
@@ -1057,18 +1058,31 @@ const SkipStep = struct {
     }
 };
 
-// Check that each exercise number, excluding the last, forms the sequence `[1, exercise.len)`.
+// Check that each exercise number, excluding the last, forms the sequence
+// `[1, exercise.len)`.
+//
+// Additionally check that the output field does not contain trailing whitespace.
 fn validate_exercises() bool {
-    // Don't use the "multi-object for loop" syntax, in order to avoid a syntax error with old Zig
-    // compilers.
+    // Don't use the "multi-object for loop" syntax, in order to avoid a syntax
+    // error with old Zig compilers.
     var i: usize = 0;
     for (exercises[0 .. exercises.len - 1]) |ex| {
         i += 1;
         if (ex.number() != i) {
-            print(
-                "exercise {s} has an incorrect number: expected {}, got {s}\n",
-                .{ ex.main_file, i, ex.key() },
-            );
+            print("exercise {s} has an incorrect number: expected {}, got {s}\n", .{
+                ex.main_file,
+                i,
+                ex.key(),
+            });
+
+            return false;
+        }
+
+        const output = std.mem.trimRight(u8, ex.output, " \r\n");
+        if (output.len != ex.output.len) {
+            print("exercise {s} output field has extra trailing whitespace\n", .{
+                ex.main_file,
+            });
 
             return false;
         }