Write a line-trimming helper. Our WAT lines have leading spaces. We need to strip them to compare against instruction names.

fn vm_trim(line: []const u8) []const u8 {
    var s: usize = 0;
    while (s < line.len and (line[s] == ' ' or line[s] == '\t')) {
        s += 1;
    }
    return line[s..];
}

And a helper to check if a line starts with a given prefix:

fn starts_with(hay: []const u8, needle: []const u8) bool {
    if (hay.len < needle.len) {
        return false;
    }
    for (needle, 0..) |c, i| {
        if (hay[i] != c) {
            return false;
        }
    }
    return true;
}

We'll use starts_with constantly -- starts_with(line, "i64.const "), starts_with(line, "local.get $"), etc. Each instruction has a recognizable prefix, and the operand (a number or a name) follows it.