parse

        pub fn parse(text: []const u8) Parsed {
            if (text.len < 2) return .incomplete;
            const ip4_prefix = "::ffff:";
            if (std.ascii.startsWithIgnoreCase(text, ip4_prefix)) {
                const parsed = Ip4Address.parse(text[ip4_prefix.len..], 0) catch
                    return .{ .invalid_ip4_mapping = ip4_prefix.len };
                const b = parsed.bytes;
                return .{ .success = .{
                    .bytes = .{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, b[0], b[1], b[2], b[3] },
                    .interface_name = null,
                } };
            }
            // Has to be u16 elements to handle 3-digit hex numbers from compression.
            var parts: [8]u16 = @splat(0);
            var parts_i: u8 = 0;
            var text_i: u8 = 0;
            var digit_i: u8 = 0;
            var compress_start: ?u8 = null;
            var interface_name_text: ?[]const u8 = null;
            const State = union(enum) { digit, end };
            state: switch (State.digit) {
                .digit => c: switch (text[text_i]) {
                    'a'...'f' => |c| {
                        const digit = c - 'a' + 10;
                        parts[parts_i] = (std.math.mul(u16, parts[parts_i], 16) catch return .{
                            .overflow = text_i,
                        }) + digit;
                        if (digit_i == 4) return .{ .invalid_byte = text_i };
                        digit_i += 1;
                        text_i += 1;
                        if (text.len - text_i == 0) {
                            parts_i += 1;
                            continue :state .end;
                        }
                        continue :c text[text_i];
                    },
                    'A'...'F' => |c| continue :c c - 'A' + 'a',
                    '0'...'9' => |c| {
                        const digit = c - '0';
                        parts[parts_i] = (std.math.mul(u16, parts[parts_i], 16) catch return .{
                            .overflow = text_i,
                        }) + digit;
                        if (digit_i == 4) return .{ .invalid_byte = text_i };
                        digit_i += 1;
                        text_i += 1;
                        if (text.len - text_i == 0) {
                            parts_i += 1;
                            continue :state .end;
                        }
                        continue :c text[text_i];
                    },
                    ':' => {
                        if (digit_i == 0) {
                            if (compress_start != null) return .{ .invalid_byte = text_i };
                            if (text_i == 0) {
                                text_i += 1;
                                if (text[text_i] != ':') return .{ .invalid_byte = text_i };
                                assert(parts_i == 0);
                            }
                            compress_start = parts_i;
                            text_i += 1;
                            if (text.len - text_i == 0) continue :state .end;
                            continue :c text[text_i];
                        } else {
                            parts_i += 1;
                            if (parts.len - parts_i == 0) continue :state .end;
                            digit_i = 0;
                            text_i += 1;
                            if (text.len - text_i == 0) return .incomplete;
                            continue :c text[text_i];
                        }
                    },
                    '%' => {
                        if (digit_i == 0) return .{ .invalid_byte = text_i };
                        parts_i += 1;
                        text_i += 1;
                        const name = text[text_i..];
                        if (name.len == 0) return .incomplete;
                        interface_name_text = name;
                        text_i = std.math.cast(u8, text.len) orelse return .{ .overflow = text.len };
                        continue :state .end;
                    },
                    else => return .{ .invalid_byte = text_i },
                },
                .end => {
                    if (text.len - text_i != 0) return .{ .junk_after_end = text_i };
                    const remaining = parts.len - parts_i;
                    if (compress_start) |s| {
                        const src = parts[s..parts_i];
                        @memmove(parts[parts.len - src.len ..], src);
                        @memset(parts[s..][0..remaining], 0);
                    } else {
                        if (remaining != 0) return .incomplete;
                    }

                    for (&parts) |*part| part.* = @byteSwap(part.*);

                    return .{ .success = .{
                        .bytes = @bitCast(parts),
                        .interface_name = interface_name_text,
                    } };
                },
            }
        }