Zig: Alloctaion-less experiment

person-noalloc.zig

// Small allocation-less experiment.
// For sure, there are ways to not allocate at all: passing stack memory locations down.
// This does mean everything needs to be defined higher up where it is ultimately consumed fully

const std = @import("std");

const Person = struct {
    name: []const u8 = "",
    age: u8 = 0,
};

pub fn main() !void {

    // because the struct has default values, we don't need to specify things here
    var person = Person{};

    var name_buf = [_]u8{0}**64;
    var age_buf = [_]u8{0}**4;

    const name = try readLine("Name", &name_buf);
    // Interesting effect: if e.StreamTooLong error occurs (more chars than the buffer can handle)
    //  and we return/exit , the rest of stdin is left alone. The console then receives it - and thus executes the rest of the string
    const age = std.fmt.parseInt(u8, readLine("Age", &age_buf) catch |e| {std.debug.print("{}\n", .{e}); return;}, 10) catch |err| {
        std.debug.print("{}\n", .{err});
        return;
    };

    // This initializes a stack buffer.
    // We can pass it down to other stacks, no need for allocation.
    var return_message = [_]u8{0}**128;

    // Populate the struct with constants, and produce a message into the buffer
    populate(&person, name, age, &return_message) catch |err| {
        std.debug.print("Something really bad happened! {}\n", .{err});
    };

    std.debug.print("Person: {{name={s}, age={d}}}\n", person);
    std.debug.print("[{s}]\n", .{return_message});
}

fn readLine(prompt:[] const u8, buf:[]u8) ![]const u8 {
    // Returns a slice of the buffer containing the line data we want
    var reader = std.io.getStdIn().reader();
    std.debug.print("{s}: ", .{prompt});
    _ = try reader.readUntilDelimiterOrEof(buf, '\n');

    // Buffer now contains like "Alice\n" or "20\n", with subsequent zero-bytes
    // We need to remove _both_ (ideally we want to iterate, detect "\n", and return slice there)
    const trimmed = std.mem.trim(u8, buf, &[_]u8{'\n', 0});

    // std.debug.print("[{s}]\n", .{trimmed});
    return trimmed;
}


fn populate(p:*Person, name:[]const u8, age:u8, return_message:[]u8) !void {
    p.name = name;
    p.age = age;
    _ = try std.fmt.bufPrint(return_message, "{s} is {d} years young!", .{name, age});
}