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105 lines
3.5 KiB
Zig
105 lines
3.5 KiB
Zig
//
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// A sentinel value indicates the end of data. Let's imagine a
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// sequence of lowercase letters where uppercase 'S' is the
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// sentinel, indicating the end of the sequence:
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//
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// abcdefS
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//
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// If our sequence also allows for uppercase letters, 'S' would
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// make a terrible sentinel since it could no longer be a regular
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// value in the sequence:
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//
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// abcdQRST
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// ^-- Oops! The last letter in the sequence is R!
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//
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// A popular choice for indicating the end of a string is the
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// value 0. ASCII and Unicode call this the "Null Character".
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//
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// Zig supports sentinel-terminated arrays, slices, and pointers:
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//
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// const a: [4:0]u32 = [4:0]u32{1, 2, 3, 4};
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// const b: [:0]const u32 = &[4:0]u32{1, 2, 3, 4};
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// const c: [*:0]const u32 = &[4:0]u32{1, 2, 3, 4};
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//
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// Array 'a' stores 5 u32 values, the last of which is 0.
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// However the compiler takes care of this housekeeping detail
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// for you. You can treat 'a' as a normal array with just 4
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// items.
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//
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// Slice 'b' is only allowed to point to zero-terminated arrays
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// but otherwise works just like a normal slice.
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//
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// Pointer 'c' is exactly like the many-item pointers we learned
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// about in exercise 054, but it is guaranteed to end in 0.
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// Because of this guarantee, we can safely find the end of this
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// many-item pointer without knowing its length. (We CAN'T do
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// that with regular many-item pointers!).
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//
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// Important: the sentinel value must be of the same type as the
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// data being terminated!
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//
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const print = @import("std").debug.print;
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const sentinel = @import("std").meta.sentinel;
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pub fn main() void {
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// Here's a zero-terminated array of u32 values:
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var nums = [_:0]u32{ 1, 2, 3, 4, 5, 6 };
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// And here's a zero-terminated many-item pointer:
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const ptr: [*:0]u32 = &nums;
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// For fun, let's replace the value at position 3 with the
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// sentinel value 0. This seems kind of naughty.
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nums[3] = 0;
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// So now we have a zero-terminated array and a many-item
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// pointer that reference the same data: a sequence of
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// numbers that both ends in and CONTAINS the sentinel value.
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//
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// Attempting to loop through and print both of these should
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// demonstrate how they are similar and different.
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//
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// (It turns out that the array prints completely, including
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// the sentinel 0 in the middle. The many-item pointer stops
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// at the first sentinel value.)
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printSequence(nums);
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printSequence(ptr);
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print("\n", .{});
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}
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// Here's our generic sequence printing function. It's nearly
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// complete, but there are a couple of missing bits. Please fix
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// them!
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fn printSequence(my_seq: anytype) void {
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const my_typeinfo = @typeInfo(@TypeOf(my_seq));
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// The TypeInfo contained in my_type is a union. We use a
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// switch to handle printing the Array or Pointer fields,
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// depending on which type of my_seq was passed in:
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switch (my_typeinfo) {
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.Array => {
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print("Array:", .{});
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// Loop through the items in my_seq.
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for (???) |s| {
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print("{}", .{s});
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}
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},
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.Pointer => {
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// Check this out - it's pretty cool:
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const my_sentinel = sentinel(@TypeOf(my_seq));
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print("Many-item pointer:", .{});
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// Loop through the items in my_seq until we hit the
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// sentinel value.
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var i: usize = 0;
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while (??? != my_sentinel) {
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print("{}", .{my_seq[i]});
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i += 1;
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}
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},
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else => unreachable,
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}
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print(". ", .{});
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}
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