Time to actually run the WAT. The stack machine has one rule: i64.const N pushes N on the stack. Let's get that one working first; i64.add comes next.

To dispatch, we'll compare each line to a known instruction name. Two helpers, both at file scope:

fn stringsEqual(
    a: []const u8,
    b: []const u8,
) bool {
    if (a.len != b.len) {
        return false;
    }
    var i: usize = 0;
    while (i < a.len) {
        if (a[i] != b[i]) {
            return false;
        }
        i += 1;
    }
    return true;
}

fn stringStartsWith(
    s: []const u8,
    prefix: []const u8,
) bool {
    if (s.len < prefix.len) {
        return false;
    }
    return stringsEqual(s[0..prefix.len], prefix);
}

stringsEqual for full matches like "i64.add". stringStartsWith for "i64.const " -- the prefix is fixed but the digit after isn't.

In main, walk the WAT and dispatch on each line. For now the WAT only has consts. If a line starts with "i64.const ", parse the last char as a digit and push. Anything else is an error.

pub fn main() void {
    // (interpreter section unchanged)

    const wat: [:0]const u8 =
        \\i64.const 3
        \\i64.const 4
    ;

    var stack: [16]i64 = undefined;
    var stack_pointer: usize = 0;

    var wat_pos: usize = 0;
    while (wat_pos < wat.len) {
        var wat_line_end: usize = wat_pos;
        while (wat_line_end < wat.len and wat[wat_line_end] != '\n') {
            wat_line_end += 1;
        }
        const line: []const u8 = wat[wat_pos..wat_line_end];

        // YOU: dispatch on line.
        //   If it starts with "i64.const ", parse line[line.len - 1] as a digit, push.
        //   Otherwise, printString("error: unknown instruction\n").

        wat_pos = wat_line_end + 1;
    }

    printString("top of stack: ");
    printNumber(stack[stack_pointer - 1]);
    printChar('\n');
}