# Coding standards

Conventions for danos source. The overriding one, from which most of the rest follows:

> **Names are spelled out in full. An identifier is not abbreviated unless the
> abbreviation is an acronym.**

`interruptDispatch`, not `intDisp`. `message_len`, not `message_len` (`msg` expands, `len`
is a Zig idiom — see the exceptions). `devices_broker`, not `devices_broker`. `scheduler`, not
`sched`. The cost of a longer name is paid once, at the keyboard; the cost of a
cryptic one is paid every time the code is read, by everyone who reads it. In a
microkernel whose whole argument is that a human can hold each piece in their head,
that trade is not close.

## The rule, precisely

**Acronyms and initialisms stay.** They *are* the full name — expanding them would make
the code worse, not better. `IPC`, `MMIO`, `DMA`, `IRQ`, `TSS`, `GDT`, `IDT`, `APIC`,
`GSI`, `HPET`, `ACPI`, `PCI`, `EOI`, `BAR`, `ECAM`, `MSI`, `CPU`, `ELF`, `ABI`, `UEFI`,
`MMU`, `TLB`, `ISR`, `ISA`, `GAS`, `HAL`, `PMM`, `VMM`, `VFS`, `HID`, `HCD`, `SMP`,
`AML`, `MADT`, `MCFG`, `FADT`, `RSDP`, `XSDT`, `RSDT`, `GOP`, `EDID`, `TSC`, `PIT`,
`RTC`, `LAPIC`, `SIPI`. In code they carry whatever case the surrounding convention
demands: `Hal` the type, `hal` the variable, `mapMmio` the function.

**Everything else is spelled out.** If it's a word with letters removed, restore them:

| Abbreviation | Full |
|---|---|
| `proto` | `protocol` |
| `msg` | `message` |
| `desc` | `descriptor` |
| `res` | `resource` |
| `recv` | `receive` |
| `buf` | `buffer` |
| `cur` | `current` |
| `src` / `dst` | `source` / `destination` |
| `idx` | `index` |
| `addr` | `address` |
| `reg` | `register` |
| `prev` | `previous` |
| `cfg` / `config` | `configuration` |
| `arch` | `architecture` |
| `sched` | `scheduler` |
| `dev` | `device` |
| `sys` / `syscall` | `system` / `system_call` |
| `info` | `information` |
| `dt` | `device_tree` |
| `ep` | `endpoint` |
| `rt` | `runtime` |
| `func` | `function` |
| `phys` / `virt` | `physical` / `virtual` |
| `wq` | `wait_queue` |

This list is illustrative, not exhaustive. The rule is the rule; when you meet a new
abbreviation, expand it.

## Exceptions

Three, and only three.

1. **Foreign ABI names are spelled exactly as the ABI spells them — but only inside
   the layer that *is* that ABI.** A function that *is* the C or POSIX interface keeps
   its name: `fopen`, `fwrite`, `fread`, `malloc`, `calloc`, `realloc`, `free`,
   `memcpy`, `mmap`, `munmap`, `open`, `read`, `write`, `close`, `lseek`, `stat`,
   `errno`, `O_CREAT`. We don't get to rename `fwrite` to `fileWrite` — it wouldn't be
   `fwrite` any more.

   **This exception is scoped to a file that *is* a foreign ABI, and nothing else.**
   danos has no such file today: the old `library/posix/` compatibility shim was retired
   once its callers moved to the danos-native `runtime.fs`, since a hand-rolled POSIX
   layer is premature until danos actually needs it (see
   [zig-self-hosting.md](zig-self-hosting.md)). The exception will apply again to the
   `std.os.danos` seam when danos becomes a real Zig target — that module *is* the C-ABI
   `system` interface, so it keeps `open`/`read`/`errno`/`O_CREAT`. **Everywhere else,
   Zig/danos naming applies with no exception**: a concept POSIX also has gets a danos
   name — the VFS wire protocol carries a `FileStatus`, not a `Stat`, and a `create`
   flag, not `O_CREAT`; the boundary is where `stat`→`status` and `O_CREAT`→`create` get
   mapped. (The `syscall` *wrappers* elsewhere are not an exception — they wrap the
   private danos ABI, so they use danos names.)

2. **Zig idioms are spelled the way Zig spells them.** Three names are the language's,
   not ours, and are left alone:
   - **`init` / `deinit`** — the constructor convention (`std.ArrayList.init`), not a
     shortening of "initialize".
   - **`len` / `ptr`** — the slice field names (`slice.len`, `slice.ptr`). Our own
     structs use bare `len`/`ptr` fields to mirror them, so a reader carries one
     mental model. (Compounds still expand: a field is `message_len`, not
     `message_length` — `len` is kept, `msg` is not.)
   - The builtins (`@min`, `@max`, `@memcpy`) and `allocator.alloc` / `.create` are
     Zig's spelling.

   The rule governs the names *we* coin.

3. **Single-letter variables in a trivial local scope.** `for (items) |item, i|` may
   keep `i`; a coordinate may be `x`, `y`. The moment the scope is big enough that the
   letter's meaning isn't obvious on sight, give it a real name. When in doubt, name it.

That's all — no Unix-abbreviation exception. The source directories are full words
(`system`, `library`, not `src`/`lib`), and there is no daemon `d` suffix: a driver
lives in `system/drivers/` and a service in `system/services/`, so the *location*
already says what it is. Encoding the role in the name too (`busd`, `vfsd`) is
redundant — the program is just `ps2-bus`, `vfs`. Don't put in a name what its directory
already tells you.

## A note on collisions

Two identifiers can legitimately expand to the same word. When they do, keep both
meaningful by renaming one to its *specific* identity rather than the generic
expansion. Two cases resolved this way:

- The `config` module (compile-time tunables — `maximum_cpus`, `timer_hz`) would
  collide with `cfg` (a `PlatformConfiguration` value) at `configuration`. The module
  became **`parameters`**, which is what it holds.
- The kernel `device.zig` module would collide with `dev` (a device value) at
  `device`. The module alias became **`device_model`**, which is what it is — the
  device data model (`Device`, `DeviceTree`, `ResourceKind`).
- The `Namespace` module alias (`ns`/`nsp` across the AML files) collides with a
  `Namespace` **instance**. Resolved by dropping the module alias entirely — the two
  types it provided are imported directly (`const Node = @import("namespace.zig").Node;`)
  — which frees `namespace` for the instance.

A related case is one abbreviation with two meanings. In the AML code, `op` means
**opcode** (`opcodes.zig`, the `*_opcode` constants) but `Op` in `BinaryOperation` /
`LogicOperation` means **operation** — distinguished by case. The per-opcode parser
handlers, formerly `opName`/`opField`, are `parseName`/`parseField`: they *parse* the
opcode's structure, which says what they do without overloading "op".

## Case and file names

Within those spelling rules, follow Zig's own conventions:

- **Types** — `PascalCase`: `DeviceDescriptor`, `Endpoint`, `WaitQueue`.
- **Functions** — `camelCase`: `mapUserDeviceInto`, `notifyFromIsr`.
- **Variables, fields, constants** — `snake_case`: `message_length`, `devices_broker`,
  `notify_badge_bit`.

**File names are `kebab-case`.** A file named for a multi-word thing hyphenates it:
`device-tree.zig`, `ipc-synchronous.zig`, `vfs-protocol.zig`, `devices-broker.zig`. A
single word or acronym needs no hyphen: `scheduler.zig`, `paging.zig`, `apic.zig`,
`idt.zig`. (The module *alias* a file is imported under still follows the code
conventions above — `snake_case` — because it's an identifier, not a filename.)

**A sub-project's entry point repeats its directory's name** — `init/init.zig`,
`runtime/runtime.zig`, `ps2-bus/ps2-bus.zig` — and the sub-project is addressed by the
*directory* (`system/services/init`, `library/runtime`), with the repeated leaf
resolving away. See the repository-layout section of [README.md](README.md).

## Named values, not magic numbers

The naming rule has a twin: **a value with meaning gets a name, too.** The same
principle drives both — a reader should never have to leave the code to understand it.
An abbreviated *name* forces a reader to guess; a bare *number* forces them worse, out
to a spec or a header or a comment three files away, to learn what the value even *is*.
If `0x0C` is the PCI serial-bus class, the code says `BaseClass.serial_bus`, not `0x0C`;
if `0x04` is the ACPI IRQ resource descriptor, it says `SmallResourceType.irq`, not
`0x04`. The number is an implementation detail of the name — recorded once, where the
name is defined, and never spelled again at a use site.

**Prefer an `enum`** when the values form a set (device classes, AML opcodes, resource
descriptor types, states): the type then also says *which* set a value belongs to, and
the compiler rejects a value from the wrong one. A lone `pub const` with a descriptive
name suffices for a one-off (`const large_descriptor_bit = 0x80`). Reach for the enum
the moment code elsewhere compares against, packs, or produces the value — a packed PCI
class triple is written from named parts (`.serial_bus`, `.usb`, `.xhci`), never as
`0x0C_03_30` under a comment that decodes the bytes.

The exceptions are the numbers that carry no hidden meaning: `0` and `1` as plain zero
and one, an index step, a field width, a bit shift. `x + 1`, `buffer[0]`, and `<< 8`
need no christening — there is nothing to look up. The test is exactly the naming test:
*would a reader have to look this up to know what it means?* If yes, name it. This is
what `opcodes.zig`'s `*_opcode` constants, `acpi-ids`'s `HardwareId`, and `pci-class`'s
class enums already are — reference data defined once and named everywhere it is used.

## Why acronyms are the line

Because an acronym has no letters to restore. `MMIO` doesn't become "memory mapped
input output" in code — that expansion is what the acronym *is for*. But `msg` is just
`message` with three letters stolen, and stealing them buys nothing a reader wants. The
test for "is this an abbreviation I must expand" is simply: *is there a longer word this
is a clipped form of?* If yes, write the word. If it's an initialism standing in for a
phrase, leave it.

## Zen of Zig

* Communicate intent precisely.
* Edge cases matter.
* Favor reading code over writing code.
* Only one obvious way to do things.
* Runtime crashes are better than bugs.
* Compile errors are better than runtime crashes.
* Incremental improvements.
* Avoid local maximums.
* Reduce the amount one must remember.
* Focus on code rather than style.
* Resource allocation may fail; resource deallocation must succeed.
* Memory is a resource.
* Together we serve the users.
