# Configuration Input Contract

This document owns ConfigKit's currently implemented source-input, YAML syntax,
schema, typed-decoding, loading, metadata, and ownership contracts. The parser,
owned document representation, and decoder remain private behind the public loading
lifecycle.

## Sources

ConfigKit exposes opaque sources through these constructors:

| Constructor | Current contract |
| --- | --- |
| `File(path)` | Requires a nonempty path, opens a regular file for each consumption, and closes only the library-owned file handle. Missing, inaccessible, unreadable, directory, and other nonregular inputs fail. |
| `Bytes(label, data)` | Requires a nonempty, non-secret diagnostic label and snapshots bytes during construction. Later caller mutation cannot affect the source. |
| `Reader(label, input)` | Requires a nonempty, non-secret label and non-nil reader. The reader is claimed once across all copies of the source, consumed but never closed, and cannot be reused. |
| `NoSource()` | Selects defaults-only loading, creates no source record, and marks no paths explicit. |

The zero `Source` is invalid. Constructor argument failures are retained inside the
opaque source and become safe structured diagnostics when the source is consumed.
File sources can observe file changes between consumptions; byte sources cannot.

Each source is limited to 8 MiB. A load will be limited to 64 MiB across all source
and override input when those layers are implemented. Reads request at most the
remaining allowance plus one byte so excess data is rejected rather than silently
truncated. Data returned together with a read error is not accepted as partial input.

## Single-source loading

`Load[T](source, options...)` accepts zero or one `Options[T]`. The configuration
root `T` must satisfy the application schema contract below. With no options it
starts from the Go zero value. A `Definition[T].Defaults` factory, when supplied,
runs exactly once after successful source parsing, schema construction, and optional
schema-version checking. Its result is detached before input is applied.

The current lifecycle is:

```text
validate options → parse one source → reject reserved directives → build schema
  → check the optional application version → create and clone defaults
  → strictly decode → snapshot configuration and metadata
```

`File`, `Bytes`, and `Reader` each contribute one base source with the deterministic
ID `source-1`. `NoSource` skips parsing, supplies an absent version to a configured
version checker, contributes no source, and continues through defaults, decode, and
projection. A missing file remains an error; it is never interpreted as no input.

The current `Options` boundary accepts a defaults factory and `VersionRule`.
Imports, active profiles, filesystem context, overrides, resolver and semantic
validator callbacks, and dynamic secret classification return an explicit
`options.unavailable` error because their lifecycle stages are not implemented.
The reserved profile spelling `default` selects no active profile. `NoSource`
additionally rejects imports, active profiles, and filesystem context as invalid
combinations. Input containing a reserved `_configkit` mapping key is rejected while
imports are disabled.

Every source, options, schema, version, defaults, or decode failure returns a nil
result and a `*LoadError`. Successful results contain caller-owned `Config`, detached
`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Mutating
the returned configuration or inspection records cannot alter ConfigKit's private
snapshot. ConfigKit performs no logging and exposes no effective writer yet.

### Application schema versions

`VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
decoding, the checker receives a `Version` preserving absent, explicit null, or
string input. Any other input kind is a `version.type` error and the checker does not
run. ConfigKit does not interpret the text or impose a universal version path.

The checker reports safe, value-free errors and warnings through `Validation.Error`
and `Validation.Warning`; ConfigKit resolves primary and related paths to exact or
nearest input locations. Reported errors fail loading. If the checker returns an
error without reporting one, ConfigKit emits a generic `version.hook_failed`
diagnostic and suppresses the returned error text. Warnings alone are retained on a
successful result. Callback panics remain programming failures.

### Base/default metadata

`Result.Metadata` describes every final canonical path. Exact surviving input paths
are explicit and have a base origin; omitted paths have a default origin. An explicit
parent mapping does not make omitted children explicit. Null state and available
source coordinates are retained. `History` records the Go-zero or application
default baseline followed by exact base definitions, including equal-valued input.
All metadata queries and returned nested records are detached.

## YAML documents

The private YAML boundary currently accepts exactly one nonempty document whose root
is a mapping. Comments and block or flow collections are supported. It rejects:

- empty input or an empty document;
- every additional document, including an empty one;
- duplicate decoded mapping keys;
- non-string mapping keys;
- explicit tags, anchors, aliases, and merge keys;
- a quoted or plain mapping key equal to `<<`; and
- input exceeding the fixed depth or node limits.

Mapping key comparison is exact: it performs no case folding or Unicode
normalization. The private document tree retains declaration order, exact validated
number spelling, source identifiers, and separate key/value line and column
locations. It contains no upstream parser nodes or types.

Root depth is zero. Input depth is limited to 256 and each parsed input tree to
1,000,000 nodes, counting mapping keys as input nodes. The upstream parser also has
a private calibrated depth guard; passing that guard never bypasses ConfigKit's tree
limits.

## Scalars

Scalar meaning is determined by ConfigKit rather than upstream coercion:

| Input form | Meaning |
| --- | --- |
| Quoted, literal, or folded scalar | String |
| Plain `true` or `false` | Boolean |
| Plain `null`, `~`, or an empty mapping value | Null |
| JSON decimal number | Number, retaining its exact spelling |
| Date/time-looking or other ordinary plain text | String |

JSON numbers permit an optional minus, a zero or nonzero decimal integer without a
leading zero, an optional fraction with at least one digit, and an optional decimal
exponent. Hexadecimal, octal, digit separators, leading plus, leading/trailing-dot
forms, incomplete exponents, and non-finite values are rejected. Legacy boolean
words (`yes`, `no`, `on`, and `off`) and non-lowercase variants of the reserved
boolean/null words are rejected unless quoted.

Mapping keys use the same scalar classification and must classify as strings. Quote
keys that otherwise classify as a boolean, number, or null.

## Application schema binding

ConfigKit builds a private reflection descriptor graph for each schema operation;
it keeps no global mutable descriptor cache. A configuration root must be a struct
or string-keyed map. Nested supported shapes are:

- booleans, strings, signed and unsigned integers, and finite floating-point types;
- `time.Duration` and paired `encoding.TextMarshaler`/`TextUnmarshaler` scalars;
- pointers, structs, string-keyed maps, slices, fixed arrays, and `any`; and
- recursive type definitions whose eventual application values are acyclic.

Named scalar types retain their Go identity. `uintptr`, complex numbers, functions,
channels, unsafe pointers, non-string map keys, and nonempty interfaces are not
configuration types. A custom text scalar must provide both directions on its value
or pointer method set. Building a descriptor only inspects method sets; it never
executes application methods or parser-specific hooks.

Exported struct fields use an explicit `yaml` name or the lowercase Go field name.
Unexported fields and `yaml:"-"` are ignored. `omitempty` is accepted but does not
change binding or future projection. An explicit `yaml:",inline"` flattens a struct
or pointer-to-struct, or designates at most one string-keyed inline map for otherwise
unmatched names. Ambiguous names, named inline fields, multiple inline maps, and
unsupported or malformed tag options are schema errors. Embedded fields are ordinary
named fields unless explicitly inlined. `configkit:"secret"` is the only current
ConfigKit field tag and records sensitivity for later lifecycle work.

Schema path traversal uses these same bindings. Explicit key segments address
mappings, explicit index segments address sequences, and parsed numeric pointer
tokens become indices only in a sequence. Sequence indices cannot have leading
zeroes; fixed-array indices are bounds checked. Dynamic `any` and inline-map entries
remain schema-valid for application-defined descendants.

Descriptors are reused by typed decoding, projection, and cloning so those operations
cannot acquire different field bindings.

## Defaults and typed decoding

The private decoder starts from an independently owned clone of the application's
typed defaults and applies only explicitly present input nodes. Defaults therefore
never become explicit, while equal-valued input remains explicit. Successful decode
produces detached typed state, baseline and final canonical trees, an input-presence
index, and default/input origins for the later public loading lifecycle.

| Configured input | Effect on the typed baseline |
| --- | --- |
| Absent field | Retain the default |
| Explicit false, zero, or empty string | Replace the default |
| Partial mapping or string-keyed map | Recurse and retain omitted default children or entries |
| Empty mapping | Retain existing mapping children |
| Sequence, including an empty sequence | Replace the entire sequence |
| Pointer object | Recurse into a non-nil default; otherwise allocate an object whose omitted children have ordinary Go zero values |
| Allowed null | Replace the value with nil and suppress its default |

Null is allowed only for pointers, maps, slices, and dynamic `any` values. It is an
error for scalars, structs, and arrays. New map entries and sequence elements begin
from their Go zero values; ConfigKit does not invent entry-level defaults.

Decoding is strict at every typed depth. Unknown struct fields, type mismatches,
invalid nulls, numeric overflow, and arrays with the wrong length are errors at the
original semantic path and source coordinates. String-keyed maps accept arbitrary
keys but preserve the strict schema of their values; inline maps accept unmatched
keys according to their element descriptor. Multiple independent field failures are
collected without publishing the partially decoded object.

Booleans and strings are not coerced. Integer destinations require integer syntax
and a value in the destination type's range. Float destinations accept finite JSON
numbers representable by their destination precision. Dynamic numbers become
`int64` for integer syntax and `float64` for fractional or exponent syntax, with
range and finiteness checks. Named scalar types retain their identity. Durations
decode only from strings accepted by `time.ParseDuration`; integers are rejected.
Paired text scalars decode only from strings through `encoding.TextUnmarshaler`, and
codec failures are classified without exposing application error text or input
values.

This decoder is available only through `Load`; ConfigKit exposes no generic document
conversion API.

## Typed projection and ownership

ConfigKit can internally project a schema-valid application value directly into its
owned canonical tree without YAML serialization. Projection includes every bound
field, including ordinary zero values; `omitempty` has no effect. Struct fields keep
binding order, dynamic map keys sort by decoded string, and sequences keep their
order. Integers retain exact base-10 values, finite floats use their destination
precision, durations use `time.Duration.String`, and paired text scalars use their
canonical marshaled text.

Nil pointers, maps, slices, and dynamic values project as null. Non-nil empty maps
and slices remain empty containers, preserving the distinction. Recursive acyclic
objects are supported. Cyclic values, non-finite floats, unsupported dynamic concrete
types, invalid codec output, and depth/node-limit violations fail with safe schema
classifications that omit codec error text.

The same descriptor graph creates detached typed copies. Pointers, maps, slices,
arrays, nested dynamic values, and shared references are recursively copied; shared
pointer identity is not preserved as configuration meaning. Paired text scalars are
rebuilt by unmarshaling their canonical text into fresh values, detaching private
backing storage. Ignored and unexported struct fields are zeroed rather than copied.

Private structural comparison operates on canonical trees. Mapping additions,
removals, and descendant replacements retain semantic paths. A sequence is atomic:
any sequence change reports the sequence path and never invents correspondence
between old and new indices. Cross-kind and null/container changes replace the
affected path.

These mechanisms currently support later decoding, resolution auditing, metadata,
and rendering. They are not a public generic conversion or cloning API.

## Safe failures

Source failures use stable `source.*` codes and YAML failures use stable `yaml.*`
codes declared by the root package. Diagnostics contain safe classifications,
semantic paths when applicable, and available source coordinates. Raw parser,
conversion, reader-error, and input text is not forwarded. Source labels and
diagnostic messages are application-visible and therefore must never contain
secrets; terminal control characters are escaped by `WriteDiagnostics`.