= The Blaise Interface File (`.bif`) Format
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== Purpose

A `.bif` ("Blaise Interface File") is the serialised form of a unit's
**interface section** — every type, const, var, routine, generic body,
and inlinable body that the compiler needs in order to consume the unit
*without re-parsing the source*.

It is the Blaise equivalent of FPC's `.ppu` or C/C++'s precompiled
header: a stable wire format the compiler can read at unit-resolve time
to populate a `TUnitInterface` (see `uUnitInterface.pas`) and feed
import into the symbol table.

There are two ways a `.bif` is consumed:

. **Standalone file** next to the source (or in a cache dir), read via
  `ReadUnitInterfaceFromFile`. Used by tooling and tests.
. **Embedded section** inside a companion `.o`, read via
  `LoadEmbeddedBifString`. This is the production path used by the
  unit loader's auto-discovery (`uUnitLoader.LoadTransitive`). See
  <<embedding>>.

Both paths feed the same parser (`ReadUnitInterface`) and produce the
same in-memory `TUnitInterface`.

== Design Constraints

The format is **text**, not binary. The rationale (from the unit
header):

> Why text and not binary: easier to diff and inspect during
> development, no endianness concerns, no bit-twiddling. Once the
> layout stabilises we can re-encode as a compact binary form without
> changing the surface API.

Three structural rules underpin everything else:

. **Length-prefixed strings everywhere.** Every variable-length payload
  is encoded as `<decimal-len>:` followed by exactly `<len>` raw bytes.
  No escaping is needed — the reader consumes by byte count, never by
  delimiter scan. Embedded `:`, newlines, quotes, and `END` markers
  all flow through unmolested.

. **A single primitive in the reader.** Everything — counts, booleans,
  Int64 values, qualified type refs — is encoded as an lpstr. The
  reader has exactly one byte-consumption primitive (`ReadLpstrAt`),
  plus thin wrappers (`ReadInt64At`, `ReadFlagsAt`, `ReadTag`). Adding
  a new field type is mechanical.

. **Section-tagged framing.** Each top-level section opens with a tag
  line (`META`, `TYPE 5`, `CONST 2`, …) and closes with `END`.
  Out-of-order sections are tolerated; unknown sections are not.

== Top-Level Layout

A complete `.bif` has the following structure:

----
BLAISE-IFACE <version>\n        <-- magic + format version
<unit-name lpstr>\n             <-- unit identifier
META
  <meta-line>
END
TYPE <count>
  <type-line> × count
END
CONST <count>
  <const-line> × count
END
VAR <count>
  <var-line> × count
END
ROUT <count>
  <routine-line> × count
END
GENROUT <count>
  <generic-routine-line> × count
END
INLINE <count>
  <inline-body-line> × count
END
----

Section order is fixed in the writer (`WriteUnitInterface` at
`uUnitInterfaceIO.pas:895`). The reader uses `ReadTag` to dispatch and
will accept sections in any order, but does not skip unknown tags —
they raise `EIfaceFormatError`.

=== Magic + Version

`BLAISE-IFACE 2` is the current header. The version field is the
constant `IFACE_VERSION` in `uUnitInterfaceIO.pas`. The version was
bumped from `1` → `2` when the `META` block grew `CompilerId` and
`SourceModTime` fields (see <<versioning>>).

A wrong magic or version raises `EIfaceFormatError` immediately from
`ReadHeader`.

=== Unit Name

The single line after the header is `<lpstr>` carrying the unit's
canonical name as written in the `unit ... ;` declaration. Case is
preserved.

== Encoding Primitives

[cols="1,2,3",options="header"]
|===
| Primitive | Wire form | Notes

| `lpstr`
| `<n>:<n bytes>`
| Length-prefixed byte sequence. The only variable-length primitive.

| `Int64`
| lpstr containing decimal digits
| Signed. No size prefix on the digits themselves.

| `Count` / `Bool`
| lpstr containing `'0'`, `'1'`, …
| Same shape as Int64; called out separately for documentation only.

| `QualRef`
| lpstr containing `"UnitName.TypeName"`
| For cross-unit type references. `$builtin.Integer` denotes a
  primitive; `.<empty>` denotes "untyped" (consts of an inferred
  literal type).

| `Flags`
| lpstr containing a decimal byte
| Bit-packed booleans. Bit assignments are call-site specific
  (see `EncodeFlags`, `EncodeParamFlags`).

| Enum members
| lpstr containing `Name=Ord,Name=Ord,...`
| Inner format is comma-separated `name=ordinal` pairs.
|===

A subtle but load-bearing detail: lpstrs are **concatenated without
separators**. The reader doesn't skip whitespace between fields on a
record line — it advances exactly `<len>` bytes for each field and
the next field starts at the next byte. Newlines exist only between
*lines* (added by the writer via `TStringList.Add`), not between
fields within a line.

== Section: META

----
META\n
<source-file lpstr><source-hash lpstr><compiler-id lpstr>
  <source-mod-time Int64><used-units stringlist>\n
END\n
----

Fields:

`SourceFile`::
  Absolute or repo-relative path the iface was generated from. Used at
  load time for staleness checks — if the file exists on the search
  path, the loader hashes it and compares to `SourceHash`.

`SourceHash`::
  FNV-1a 64-bit hash, lowercase hex (16 chars), of the source file's
  contents at write time. Empty string means "format predates
  hashing" — the loader treats that as a forced miss and falls back to
  recompiling from source. See `ContentHashFnv1a64`.

`CompilerId`::
  The string `COMPILER_ID` from `uCompilerId.pas` baked in at the
  moment of write. Format: `blaise-<version>-<channel>+<phase>`.
  Used as the freshness signal when source isn't available on the
  search path.

`SourceModTime`::
  `Int64` modification timestamp captured at write. Currently
  informational — the loader trusts the hash, not the mtime.

`UsedUnits`::
  List of unit names this unit `uses`. Stored so the loader can
  recurse without re-parsing the source.

A `stringlist` is encoded as `<count lpstr>` followed by `<count>`
lpstrs back-to-back.

== Section: TYPE

----
TYPE <count>\n
<kind lpstr><name lpstr><payload...>  (× count)
END\n
----

Each type record opens with two lpstrs: the discriminator (`enum`,
`set`, `alias`, `record`, `class`, `interface`, `proc`,
`generic-class`, `generic-interface`) and the type name. The payload
shape depends on the kind:

[cols="1,3",options="header"]
|===
| Kind | Payload

| `enum`
| lpstr containing `Name=Ord,Name=Ord,...`

| `set`
| `<base-type-name lpstr>`

| `alias`
| `<aliased-type-name lpstr>`

| `record`
| `<is-packed bool><field-list>`

| `class`
| `<parent qualref><instance-size lpstr><attributes stringlist>` +
  `<implements stringlist><field-list><method-list>`

| `interface`
| `<parent-name lpstr><method-decl-list>`

| `proc`
| `<is-function bool><is-method-ptr bool><return-type-name lpstr>` +
  `<param-count lpstr>(<param-name><param-type><param-flags>) × count`

| `generic-class`
| `<type-param-list>(<as for class>)`

| `generic-interface`
| `<type-param-list><parent-name lpstr><method-decl-list>`
|===

A `field-list` is a count lpstr followed by `<name><type-name><is-weak>`
triples — flattened so multi-name declarations like `X, Y: Integer`
produce two records.

A `method-list` and `method-decl-list` are structurally similar but
not identical:

* `method-list` (from `EncodeMethodSig`) carries class-method extras
  the **codegen** needs: `IsVirtual`, `IsOverride`, `ResolvedQbeName`,
  `VTableSlot`. Used by classes.
* `method-decl-list` (from `EncodeMethodDecl`) lacks the resolved
  names — interfaces don't carry codegen state. Used by interfaces.

A `type-param-list` is a count lpstr followed by
`<param-name><constraint>` pairs.

Three TTypeDef kinds are **not** serialised: anything for which
`TypeEntryKind` returns `''`. These are filtered out of the count and
skipped in the write loop. The reader will never see them.

== Section: CONST

----
CONST <count>\n
<name lpstr><type qualref><int-value Int64><str-value lpstr><flags>  (× count)
END\n
----

A single record carries both the integer and string representations;
the flags byte (low 2 bits) tells the reader which is live:

* bit 0 = `IsString` — `<str-value>` is the value
* bit 1 = `IsFloat` — `<str-value>` is a decimal float string
* otherwise — `<int-value>` is the value

`<type qualref>` is `<UnitName>.<TypeName>` or `$builtin.Integer` /
`.<empty>` for inferred-typed literals.

== Section: VAR

----
VAR <count>\n
<name lpstr><type qualref>  (× count)
END\n
----

Unit-level variable declarations. No initial value — initialisation
runs in the unit's initialisation section (which is in the
*implementation*, not the interface, and therefore not in the `.bif`).

== Section: ROUT

----
ROUT <count>\n
<name lpstr><is-function lpstr><return-type qualref>
  <param-count lpstr>
  (<param-name><param-type><param-flags>) × param-count
(× count)
END\n
----

Free routines (not methods). `<is-function>` is `'0'` or `'1'`
encoded *as a decimal-digit lpstr*, not as an `EncodeBool` lpstr — a
historical inconsistency, not a bug. The reader uses
`ReadDecimalAt`-equivalent parsing.

Param flags (`EncodeParamFlags`):

* bit 0 = `IsVarParam`
* bit 1 = `IsConstParam`
* bit 2 = `IsOpenArray`

== Section: GENROUT

----
GENROUT <count>\n
<name lpstr><type-param-list><method-decl><block>  (× count)
END\n
----

Generic *free* routines. The interface needs the full body because
instantiation happens at the call site — the consumer has to be able
to clone-substitute the body locally. `<block>` is a serialised
`TBlock` (see <<ast-encoding>>).

Generic *type* bodies (e.g. `TList<T>`) are skipped here — they ride
along inside their owning class's `generic-class` TYPE record.

== Section: INLINE

----
INLINE <count>\n
<routine-name lpstr><block>  (× count)
END\n
----

Inline-eligible routine bodies. The body is a serialised `TBlock`
chunk encoded with the same expression/statement protocol as GENROUT.
A consumer that wants to inline a call recovers the AST from this
section instead of re-parsing the source.

[#ast-encoding]
== AST Encoding (Generic & Inline Bodies)

`TASTExpr` / `TASTStmt` / `TBlock` are serialised inline within the
surrounding lpstr stream. Each node is `<kind-tag lpstr>` followed by
a kind-specific tail of further lpstrs. `nil` children are encoded as
`<kind 'nil'>`.

Kind tags (current set in `EncodeExpr` / `EncodeStmt`):

[cols="1,2,3",options="header"]
|===
| Tag | Node class | Tail

| `int`
| `TIntLiteral`
| `<value lpstr>`

| `float`
| `TFloatLiteral`
| `<value lpstr>` (decimal string)

| `str`
| `TStringLiteral`
| `<value lpstr>`

| `nillit`
| `TNilLiteral`
| _none_

| `id`
| `TIdentExpr`
| `<name lpstr>`

| `bin`
| `TBinaryExpr`
| `<op-ord lpstr><lhs expr><rhs expr>`

| `not`
| `TNotExpr`
| `<expr>`

| `call`
| `TFuncCallExpr`
| `<name lpstr><arg expr-list>`

| `mcall`
| `TMethodCallExpr`
| `<obj-name lpstr><method-name lpstr><obj-expr><arg expr-list>`

| `field`
| `TFieldAccessExpr`
| `<record-name lpstr><field-name lpstr><base expr><prop-index expr>`

| `deref`
| `TDerefExpr`
| `<expr>`

| `addr`
| `TAddrOfExpr`
| `<expr>`

| `ssub`
| `TStringSubscriptExpr`
| `<str expr><index expr>`

| `alit`
| `TArrayLiteralExpr`
| `<elements expr-list>`

| `isop` / `asop` / `supp`
| `TIsExpr` / `TAsExpr` / `TSupportsExpr`
| `<obj expr><type-name lpstr>(+<out-var-name> for supp)`
|===

(Statement kinds — `assign`, `if`, `while`, `repeat`, `for`, `case`,
`break`, `continue`, `exit`, `try`, `raise`, `with`-`stub`, `block`,
etc. — follow the same pattern. See `EncodeStmt` for the full table.)

**Versioning implication**: any new node class added to `uAST.pas`
that is reachable from a generic or inline body must get a new kind
tag here, with reader-side dispatch in `ReadExpr` / `ReadStmt`. The
reader's `else` branch raises `EIfaceFormatError`, so the failure
mode is loud and well-localised. The version field bumps at the same
time.

[#embedding]
== Embedding `.bif` Inside `.o`

The production path doesn't ship `.bif` as a sidecar file. Instead,
the `.bif` bytes are embedded inside the companion object file under
a non-loaded section, via GNU `objcopy`:

[source,shell]
----
# Embed
objcopy --add-section .blaise.iface=foo.bif \
        --set-section-flags .blaise.iface=noload,readonly \
        foo.o foo.o

# Extract
objcopy --dump-section .blaise.iface=out.bif foo.o /dev/null
----

Section name varies by object format (`uIfaceObject.pas`):

[cols="1,1",options="header"]
|===
| Format | Section name
| ELF (Linux)        | `.blaise.iface`
| PE/COFF (Windows)  | `.bliface` (8-char limit)
|===

PE/COFF support is stubbed out — only the constant is wired. The
runtime currently only emits ELF.

**Why embed at all?** From `uIfaceObject.pas`:

> Keep .bif and .o in lock-step. When the iface lives as its own
> file next to the .o, a build-system glitch or stale copy can
> produce a mismatch where the compiler trusts an interface for
> symbols that the object file no longer exports. Embedding the
> bytes inside the .o makes the artefacts inseparable — copying the
> .o brings the iface along, and a regenerated .o can't carry an
> older iface by accident.

The section is marked `noload` so it never reaches the final
executable. GNU `ld` drops it at link time. `clang -fuse-ld=lld`
preserves the same behaviour.

[#versioning]
== Versioning & Compatibility

The `IFACE_VERSION` constant is the only version surface. The format
gates compatibility on three checks, in order:

. **Magic mismatch.** Anything other than `BLAISE-IFACE` at byte zero
  is rejected outright.
. **Version mismatch.** A `.bif` with a version other than
  `IFACE_VERSION` is rejected. There is no down-conversion; producers
  and consumers must use the same compiler revision (or the
  `CompilerId`-match path, below, must apply).
. **Content drift.** Even with a matching version, a `.bif` may be
  stale relative to its source. The loader (`uUnitLoader.ValidateIface`)
  decides:

  ** **Source available on path** → recompute FNV-1a hash of the
     source bytes; if it matches `SourceHash`, accept; otherwise
     recompile from source.
  ** **Source not available** → compare the embedded `CompilerId` to
     this compiler's `COMPILER_ID`. Match → accept (the binary `.o`
     and its `.bif` were produced together by an identical compiler
     and there's no way to do better). Mismatch → fatal: we have no
     way to regenerate.

**When to bump `IFACE_VERSION`:**

* Adding a new top-level section (e.g. `MACRO`, `RESOURCE`).
* Changing the field set of an existing record kind.
* Adding a new TYPE-kind discriminator.
* Adding a new AST node kind referenced from generic/inline bodies.

**When not to bump:**

* Adding a new entry to a `stringlist` (the count drives parsing).
* Adding a new flag bit to an existing flags byte (the bits we don't
  use are presently zero; widening is forward-compatible as long as
  readers mask the bits they care about).
* Cosmetic changes to non-content (whitespace between sections is
  consumed and ignored).

== Failure Modes & Diagnostics

`EIfaceFormatError` is the single error type. It's raised by
`ReadLpstrAt` (bad length syntax, length runs off the end of input),
`ReadTag` / `ReadHeader` (wrong magic / version / section tag),
`ReadExpr` / `ReadStmt` (unknown kind tag), and the per-section
readers (unexpected `END` or unexpected non-`END`).

Higher up, `uUnitLoader.LoadIfaceFromObject` catches the exception
and falls back to compiling from source — a malformed embedded iface
is recoverable as long as the `.pas` is on the search path. The
warning surfaces to stderr:

----
warning: unreadable iface in foo.o: <message>
----

A stale-but-readable iface (source hash mismatch) produces:

----
note: foo.o iface stale vs source on path; recompiling from source
----

A binary-only dep with a wrong compiler id produces an error and
fails the build:

----
error: foo.o iface compiled by 'blaise-0.9.0-dev+6c-M' (this compiler
is 'blaise-0.9.0-dev+6c-N'); source unavailable to rebuild
----

== Round-Trip Guarantee

`ReadUnitInterface(WriteUnitInterface(I))` is structurally equal to
`I` for every field the format encodes. **Fields the format does not
encode** (mostly implementation-section state — initialisation blocks,
private types, finalization order) are not part of the iface contract
and are not carried.

The "fields encoded" list is exactly:

* Unit `Name`, `UsedUnits`
* `Consts`, `Vars`
* `Types` where `TypeEntryKind <> ''` (i.e. except untaggable kinds)
* `Routines` (free routine signatures)
* `GenericBodies` where `not G.IsType` (generic free routines with
  bodies)
* `InlineBodies` (inline-eligible routine bodies)
* META: `SourceFile`, `SourceHash`, `CompilerId`, `SourceModTime`

If a producer populates a field not in this list and expects the
consumer to see it, that's a bug in the producer's expectations, not
in the format.

== References

[cols="1,2",options="header"]
|===
| File | Role

| `compiler/src/main/pascal/uUnitInterface.pas`
| In-memory data model — `TUnitInterface` and its sub-records

| `compiler/src/main/pascal/uUnitInterfaceIO.pas`
| Serialiser + parser; the canonical source of truth for the format

| `compiler/src/main/pascal/uIfaceObject.pas`
| `.bif` ↔ `.o` embedding via `objcopy`

| `compiler/src/main/pascal/uUnitLoader.pas`
| Auto-discovery + freshness validation at unit-resolve time

| `compiler/src/main/pascal/uCompilerId.pas`
| The `COMPILER_ID` constant baked into the META block

| `docs/extending-ast.adoc`
| Companion doc — what to do when adding a new AST node that needs
  serialisation
|===