Fix 076, add 077,078 sentinels and many-item pointers

exercises/077_sentinels2.zig

@@ -0,0 +1,66 @@
+//
+// ------------------------------------------------------------
+//  TOP SECRET  TOP SECRET  TOP SECRET  TOP SECRET  TOP SECRET
+// ------------------------------------------------------------
+//
+// Are you ready for the THE TRUTH about Zig string literals?
+//
+// Here it is:
+//
+//     @TypeOf("foo") == *const [3:0]u8
+//
+// Which means a string literal is a "constant pointer to a
+// zero-terminated (null-terminated) fixed-size array of u8".
+//
+// Now you know. You've earned it. Welcome to the secret club!
+//
+// ------------------------------------------------------------
+//
+// Why do we bother using a zero/null sentinel to terminate
+// strings in Zig when we already have a known length?
+//
+// Versatility! Zig strings are compatible with C strings (which
+// are null-terminated) AND can be coerced to a variety of other
+// Zig types:
+// 
+//     const a: [5]u8 = "array".*;
+//     const b: *const [16]u8 = "pointer to array";
+//     const c: []const u8 = "slice";
+//     const d: [:0]const u8 = "slice with sentinel";
+//     const e: [*:0]const u8 = "many-item pointer with sentinel";
+//     const f: [*]const u8 = "many-item pointer";
+//
+// All but 'f' may be printed. (A many-item pointer without a
+// sentinel is not safe to print because we don't know where it
+// ends!)
+//
+const print = @import("std").debug.print;
+
+const WeirdContainer = struct {
+    data: [*]const u8,
+    length: usize,
+};
+
+pub fn main() void {
+    // WeirdContainer is an awkward way to house a string.
+    //
+    // Being a many-item pointer (with no sentinel termination),
+    // the 'data' field "loses" the length information AND the
+    // sentinel termination of the string literal "Weird Data!".
+    //
+    // Luckily, the 'length' field makes it possible to still
+    // work with this value.
+    const foo = WeirdContainer {
+        .data = "Weird Data!",
+        .length = 11,
+    };
+
+    // How do we get a printable value from 'foo'? One way is to
+    // turn it into something with a known length. We do have a
+    // length... You've actually solved this problem before!
+    //
+    // Here's a big hint: do you remember how to take a slice?
+    const printable = ???;
+
+    print("{s}\n", .{printable});
+}