67 lines
2.4 KiB
Zig
67 lines
2.4 KiB
Zig
//
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// There is also an 'inline while'. Just like 'inline for', it
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// loops at compile time, allowing you do all sorts of
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// interesting things not possible at runtime. See if you can
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// figure out what this rather bonkers example prints:
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//
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// const foo = [3]*const [5]u8{ "~{s}~", "<{s}>", "d{s}b" };
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// comptime var i = 0;
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//
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// inline while ( i < foo.len ) : (i += 1) {
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// print(foo[i] ++ "\n", .{foo[i]});
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// }
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//
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// You haven't taken off that wizard hat yet, have you?
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//
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const print = @import("std").debug.print;
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pub fn main() void {
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// Here is a string containing a series of arithmetic
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// operations and single-digit decimal values. Let's call
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// each operation and digit pair an "instruction".
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const instructions = "+3 *5 -2 *2";
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// Here is a u32 variable that will keep track of our current
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// value in the program at runtime. It starts at 0, and we
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// will get the final value by performing the sequence of
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// instructions above.
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var value: u32 = 0;
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// This "index" variable will only be used at compile time in
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// our loop.
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comptime var i = 0;
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// Here we wish to loop over each "instruction" in the string
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// at compile time.
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//
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// Please fix this to loop once per "instruction":
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inline while (i < instructions.len) : (i += 3) {
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// This gets the digit from the "instruction". Can you
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// figure out why we subtract '0' from it?
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comptime var digit = instructions[i + 1] - '0';
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// This 'switch' statement contains the actual work done
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// at runtime. At first, this doesn't seem exciting...
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switch (instructions[i]) {
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'+' => value += digit,
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'-' => value -= digit,
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'*' => value *= digit,
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else => unreachable,
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}
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// ...But it's quite a bit more exciting than it first appears.
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// The 'inline while' no longer exists at runtime and neither
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// does anything else not touched directly by runtime
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// code. The 'instructions' string, for example, does not
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// appear anywhere in the compiled program because it's
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// not used by it!
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//
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// So in a very real sense, this loop actually converts
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// the instructions contained in a string into runtime
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// code at compile time. Guess we're compiler writers
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// now. See? The wizard hat was justified after all.
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}
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print("{}\n", .{value});
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}
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