fdu-core 0.1.0

//! Stored-state identity: which requests a stored tier may answer.
//!
//! A **tier** is a unit of identity: entries and their roll-ups, `.gitignore` control
//! state, and content records. A **store** holds one or more tiers: a metadata snapshot
//! holds the entry and control tiers, a content sidecar holds the content tier, and a
//! retained [`Index`](crate::Index) holds all three. A store records the identity of every
//! tier it holds, and a stored tier answers a request only when the request's identity
//! for that tier is one it serves.
//!
//! Each identity is the engine fingerprint plus exactly the request parts that change the
//! tier's values. Operational settings such as worker counts, batch sizes, and scan order
//! never appear here, so they can never invalidate a store, and a request part that
//! changes a tier's values always does.

use crate::content::{
    AnalysisSet, AnalyzerId, AnalyzerVersion, ContentProvenance, OptionsFingerprint,
};
use crate::control::ControlLimits;
use crate::engine_contract::ScanScope;

/// Version of the fixed `.gitignore` control semantics, the first thing
/// [`ControlTierIdentity::ignore_rules_fingerprint`] hashes.
const IGNORE_RULES_VERSION: u64 = 2;

/// Which entries a scan retains: its depth, symlink, filesystem-boundary, hidden-entry,
/// and special-object settings.
///
/// This is [`ScanScope`] without its type-rules, reducer-set, and ignore-rules
/// fingerprints: the filesystem-admission identity of a validated scan configuration. A
/// store's entry tier records it, and an opened root binds it in
/// [`EngineVersion::scope`](crate::EngineVersion::scope). Root binding and execution policy
/// are deliberately absent, so two roots may share it without claiming to be the same live
/// session.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
pub struct EntryScope {
    /// Maximum retained relative depth, or unlimited when absent.
    pub max_depth: Option<usize>,
    /// Whether directory symlinks are followed.
    pub follow_symlinks: bool,
    /// Whether traversal stays on the root filesystem.
    pub one_filesystem: bool,
    /// Identity of leading-dot component admission and its exact-name allowlist.
    pub hidden_fingerprint: u64,
    /// Whether filesystem objects outside files, directories, and symlinks are excluded.
    pub exclude_special: bool,
}

/// Identity of an entry tier: the entries a store holds and the roll-ups derived from
/// them.
///
/// `.gitignore` observation is deliberately not part of it. Reading control files changes
/// which entries are classified as ignored, never which entries exist or what they
/// measure, so a store taken with observation on and one taken with it off hold equal
/// entry tiers. That is what lets a later projection serve one from the other.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
pub struct EntryTierIdentity {
    /// The engine fingerprint of the build that produced the tier
    /// ([`crate::snapshot::engine_fingerprint`]).
    pub engine: u64,
    /// Which entries a scan retains.
    pub scope: EntryScope,
    /// Identity of the type-classification rules roll-ups were tallied under.
    pub type_rules_fingerprint: u64,
    /// Identity of the enabled reducer set.
    pub reducers_fingerprint: u64,
}

impl EntryTierIdentity {
    /// The entry tier identity this build gives an index of `scope`.
    pub fn of_scope(scope: ScanScope) -> Self {
        Self {
            engine: crate::snapshot::engine_fingerprint(),
            scope: scope.entry_scope(),
            type_rules_fingerprint: scope.type_rules_fingerprint,
            reducers_fingerprint: scope.reducers_fingerprint,
        }
    }
}

/// Identity of a `.gitignore` control tier.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
pub enum ControlTierIdentity {
    /// No control file was read and no entry was classified, so the tier cannot say
    /// whether any entry is ignored.
    NotObserved,
    /// Control files were read and admitted under `limits`.
    Observed {
        /// The budget and line limit that decided which control files apply.
        limits: ControlLimits,
    },
}

impl ControlTierIdentity {
    /// Whether the tier observed control state.
    pub const fn is_observed(self) -> bool {
        matches!(self, Self::Observed { .. })
    }

    /// The ignore-rules fingerprint a [`ScanScope`] with this control tier carries.
    ///
    /// Zero is reserved for [`Self::NotObserved`], which is what
    /// [`ScanScope::observes_controls`] tests. An observed tier hashes the control
    /// semantics version and each limit in turn with FNV-1a and is never zero, so a scope
    /// taken under one budget or line limit never matches one taken under another.
    pub fn ignore_rules_fingerprint(self) -> u64 {
        const FNV_OFFSET_BASIS: u64 = 0xcbf2_9ce4_8422_2325;
        const FNV_PRIME: u64 = 0x100_0000_01b3;
        const UNBOUNDED: u8 = 0;
        const BOUNDED: u8 = 1;

        let Self::Observed { limits } = self else {
            return 0;
        };
        let mut fingerprint = FNV_OFFSET_BASIS;
        let mut mix = |bytes: &[u8]| {
            for byte in bytes {
                fingerprint ^= u64::from(*byte);
                fingerprint = fingerprint.wrapping_mul(FNV_PRIME);
            }
        };
        mix(&IGNORE_RULES_VERSION.to_le_bytes());
        for limit in [limits.budget, limits.line_limit] {
            match limit {
                None => mix(&[UNBOUNDED]),
                Some(limit) => {
                    mix(&[BOUNDED]);
                    mix(&u64::try_from(limit).unwrap_or(u64::MAX).to_le_bytes());
                }
            }
        }
        fingerprint.max(1)
    }
}

/// The identity of every tier a metadata snapshot holds.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
pub struct SnapshotIdentity {
    /// The entry tier.
    pub entries: EntryTierIdentity,
    /// The `.gitignore` control tier.
    pub controls: ControlTierIdentity,
}

impl SnapshotIdentity {
    /// The scope an index holding these tiers records.
    pub fn scan_scope(self) -> ScanScope {
        let EntryScope {
            max_depth,
            follow_symlinks,
            one_filesystem,
            hidden_fingerprint,
            exclude_special,
        } = self.entries.scope;
        ScanScope {
            max_depth,
            follow_symlinks,
            one_filesystem,
            hidden_fingerprint,
            exclude_special,
            ignore_rules_fingerprint: self.controls.ignore_rules_fingerprint(),
            type_rules_fingerprint: self.entries.type_rules_fingerprint,
            reducers_fingerprint: self.entries.reducers_fingerprint,
        }
    }
}

/// Identity of a content tier: the per-file analysis records a sidecar holds.
///
/// The entry tier's identity rather than the whole snapshot's, because no metric depends on
/// `.gitignore` observation: every regular file in scope is an analysis candidate whether
/// or not it is ignored. Then the analyzer set the records were produced for, and the
/// analyzers' identities, versions, and options.
///
/// The type rules the records were classified under are the entry tier's, and stated only
/// there: a record's [`ContentProvenance`] is this identity's entry-tier type rules and its
/// [`AnalyzerProvenance`].
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct ContentTierIdentity {
    /// The entry tier the records were analyzed over, including their type rules.
    pub entries: EntryTierIdentity,
    /// The analyzer set the tier holds records for.
    pub analysis: AnalysisSet,
    /// The options and analyzer versions the records were produced under.
    pub provenance: AnalyzerProvenance,
}

/// The analyzers a content tier's records were produced by, and the options they ran with.
///
/// A record's [`ContentProvenance`] without its type-rules fingerprint, which the content
/// tier's [`EntryTierIdentity`] holds.
#[derive(Clone, PartialEq, Eq, Debug, Hash)]
pub struct AnalyzerProvenance {
    /// Identity of the semantic analyzer options.
    pub options_fingerprint: OptionsFingerprint,
    /// Each analyzer the records ran, with its version, in the order the set enables them.
    pub analyzers: Vec<(AnalyzerId, AnalyzerVersion)>,
}

impl ContentTierIdentity {
    /// The provenance each record of this tier carries.
    pub(crate) fn record_provenance(&self) -> ContentProvenance {
        ContentProvenance {
            type_rules_fingerprint: self.entries.type_rules_fingerprint,
            options_fingerprint: self.provenance.options_fingerprint,
            analyzers: self.provenance.analyzers.clone(),
        }
    }

    /// Identity produced by this build for the requested entry tier and analyzers.
    pub fn for_request(entries: EntryTierIdentity, analysis: AnalysisSet) -> Self {
        let provenance = ContentProvenance::for_request(
            crate::content::AnalysisRequest { profile: analysis, ..Default::default() },
            entries.type_rules_fingerprint,
        );
        Self {
            entries,
            analysis,
            provenance: AnalyzerProvenance {
                options_fingerprint: provenance.options_fingerprint,
                analyzers: provenance.analyzers,
            },
        }
    }

    /// Admit a stored content identity and return the projection that may consume it.
    ///
    /// Equality is the only lawful content projection today. Adding another relation
    /// requires implementing its record and tier projection here, not widening a reader.
    #[must_use]
    pub fn admit(&self, stored: &Self) -> Option<ContentAdmission<'_>> {
        self.admit_parts(
            stored.entries,
            stored.analysis,
            stored.provenance.options_fingerprint,
            &stored.provenance.analyzers,
        )
    }

    fn admit_parts(
        &self,
        entries: EntryTierIdentity,
        analysis: AnalysisSet,
        options: OptionsFingerprint,
        analyzers: &[(AnalyzerId, AnalyzerVersion)],
    ) -> Option<ContentAdmission<'_>> {
        (entries == self.entries
            && analysis == self.analysis
            && options == self.provenance.options_fingerprint
            && analyzers == self.provenance.analyzers)
            .then_some(ContentAdmission { identity: self })
    }

    /// Admission for an observation already tied to this entry tier by its candidate.
    /// This compares borrowed components, without allocating provenance per file.
    pub(crate) fn admit_record(
        &self,
        analysis: AnalysisSet,
        provenance: &ContentProvenance,
    ) -> Option<ContentAdmission<'_>> {
        self.admit_parts(
            EntryTierIdentity {
                type_rules_fingerprint: provenance.type_rules_fingerprint,
                ..self.entries
            },
            analysis,
            provenance.options_fingerprint,
            &provenance.analyzers,
        )
    }
}

/// Proof that stored content can be projected to one requested identity.
///
/// The private constructor is the shared admission relation. Consumers apply this proof
/// to records or a borrowed record set; the proof never widens the requested analyzer set.
#[must_use = "admission must be applied before consuming stored content"]
#[derive(Clone, Copy, Debug)]
pub struct ContentAdmission<'a> {
    identity: &'a ContentTierIdentity,
}

impl<'a> ContentAdmission<'a> {
    /// The identity the projected content answers.
    pub const fn identity(self) -> &'a ContentTierIdentity {
        self.identity
    }

    pub(crate) fn record(self, record: crate::content::FileAnalysis) -> Option<AdmittedRecord<'a>> {
        record
            .matches_profile(self.identity.analysis)
            .then_some(AdmittedRecord { identity: self.identity, record })
    }

    pub(crate) fn project(
        self,
        content: &crate::content::ContentIndex,
    ) -> Option<ContentProjection<'_>> {
        let _admission = self.identity.admit(content.identity()?)?;
        Some(ContentProjection { content })
    }
}

/// A record set projected through the content serving relation for one request.
#[derive(Clone, Copy)]
pub(crate) struct ContentProjection<'a> {
    content: &'a crate::content::ContentIndex,
}

impl<'a> ContentProjection<'a> {
    pub(crate) fn identity(self) -> &'a ContentTierIdentity {
        self.content.identity().expect("admitted tier has an identity")
    }
    pub(crate) fn len(self) -> usize {
        self.content.len()
    }
    pub(crate) fn state(self) -> Option<crate::content::ContentTierState> {
        self.content.state()
    }
    pub(crate) fn file(self, path: &std::path::Path) -> Option<&'a crate::content::FileAnalysis> {
        self.content.file(path)
    }
    pub(crate) fn records(
        self,
    ) -> impl Iterator<Item = (&'a std::path::Path, &'a crate::content::FileAnalysis)> {
        self.content.records()
    }
}

/// A decoded record whose identity and analyzer slots have passed admission.
#[must_use]
pub(crate) struct AdmittedRecord<'a> {
    identity: &'a ContentTierIdentity,
    record: crate::content::FileAnalysis,
}

impl AdmittedRecord<'_> {
    pub(crate) fn value(&self) -> &crate::content::FileAnalysis {
        &self.record
    }

    pub(crate) fn into_record(self) -> crate::content::FileAnalysis {
        self.record
    }

    pub(crate) fn for_tier(
        self,
        wanted: &ContentTierIdentity,
    ) -> Option<crate::content::FileAnalysis> {
        wanted.admit(self.identity)?.record(self.record).map(AdmittedRecord::into_record)
    }
}

/// How a stored tier answers a request.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
pub enum Serves {
    /// The stored identity equals the requested one, so the stored tier holds what a cold
    /// run of the request would build.
    Exact,
    /// The stored entry tier equals the requested one, and an observed control tier can
    /// be discarded to produce the controls-off index a cold run would build.
    ProjectControlsOff,
    /// The stored tier cannot answer the request, which is a miss.
    Refuse,
}

/// Whether a snapshot of the `stored` identity answers a request for `wanted`.
///
/// Equality: a snapshot serves exactly the request whose identity for every tier equals
/// its own. Any relation beyond equality arrives with a projection that yields what a cold
/// run of `wanted` would, and is proven by its own test.
pub fn serves_snapshot(stored: SnapshotIdentity, wanted: SnapshotIdentity) -> Serves {
    if stored == wanted {
        Serves::Exact
    } else if stored.entries == wanted.entries
        && stored.controls.is_observed()
        && wanted.controls == ControlTierIdentity::NotObserved
    {
        Serves::ProjectControlsOff
    } else {
        Serves::Refuse
    }
}

// ---- write rules ----
//
// Each tier is written by what an absent item in it means. An absent entry changes every
// roll-up above it, so the entry tier is written only when the pass verified all of it.
// An absent content record is a miss that reads the file again, so records are written one
// at a time, each when the pass verified it.

/// Whether `index`'s entry tier, and the control tier stored with it, may be written.
///
/// Only a complete, fresh index: a snapshot missing an entry would be served as the tree's
/// totals on the next run, and an older complete snapshot is better than that.
pub(crate) fn entries_writable(index: &crate::Index) -> bool {
    index.freshness() == crate::Freshness::Fresh
        && index.state().coverage == crate::engine_contract::Coverage::Complete
}

/// Whether the content record `record` for the file at `path` may be written.
///
/// A record is written when it describes a file this pass verified: the index holds a
/// regular file there whose fingerprint is the record's, the entry was scanned or
/// revalidated by the pass rather than retained from a snapshot, and reading it did not
/// fail. A file the pass verified was listed by its parent, so its subtree was verified
/// down to it. A record under a subtree the pass could not verify describes a retained
/// file nobody checked, so it is left out, and the next run that verifies the file reads
/// it again.
pub(crate) fn content_record_writable(
    index: &crate::Index,
    path: &std::path::Path,
    record: &crate::content::FileAnalysis,
) -> bool {
    if !record.is_reusable() {
        return false;
    }
    // A complete, fresh pass verified every entry, and the content tier holds only records
    // that match their live entry, because a metadata change invalidates a file's record
    // and a commit checks the entry it lands on. So only a partial pass asks per file, and
    // the common write pays no lookup per record.
    if entries_writable(index) {
        return true;
    }
    let crate::PathState::Present { kind: crate::EntryKind::File, attrs } = index.path_state(path)
    else {
        return false;
    };
    attrs.fingerprint() == record.fingerprint
        && index.provenance(path).is_some_and(crate::Provenance::is_verified)
}

/// Whether `index`'s content tier may be written beside the store that holds
/// `stored_entries`, the entry tier of the snapshot already stored for its root, if any.
///
/// After a complete pass, always: the snapshot is written with it. After a partial pass,
/// only beside a snapshot of the same entry tier, which the sidecar pairs with, because a partial
/// pass under another identity writes no snapshot, so replacing the sidecar would evict the
/// records that pair with the snapshot that stays.
pub(crate) fn content_tier_writable(
    index: &crate::Index,
    stored_entries: impl FnOnce() -> Option<EntryTierIdentity>,
) -> bool {
    entries_writable(index)
        || stored_entries().is_some_and(|stored| stored == index.snapshot_identity().entries)
}

// ---- fixed-width codecs ----
//
// Every store writes its tier identities in these encodings. Each is fixed-width and
// canonical: one identity has exactly one encoding, and a decoder refuses any byte that no
// encoder writes, so two stores hold equal identities exactly when their encoded bytes are
// equal. The engine fingerprint is not encoded here: a store writes it once, in its
// prologue beside the magic and format version, and every tier identity it holds shares
// it.

/// Encoded width of an optional bound: a tag, then eight value bytes.
pub(crate) const BOUND_BYTES: usize = 1 + 8;

/// Encoded width of an [`EntryTierIdentity`] after its engine fingerprint: the maximum
/// depth as a bound, the scope flags, and the hidden-entry, type-rules, and reducer-set
/// fingerprints.
pub(crate) const ENTRY_TIER_BYTES: usize = BOUND_BYTES + 1 + 8 + 8 + 8;

/// Encoded width of a [`ControlTierIdentity`]: the observation tag, then the budget and the
/// line limit, each a bound.
pub(crate) const CONTROL_TIER_BYTES: usize = 1 + 2 * BOUND_BYTES;

/// Encoded width of a [`SnapshotIdentity`] after its engine fingerprint.
pub(crate) const SNAPSHOT_IDENTITY_BYTES: usize = ENTRY_TIER_BYTES + CONTROL_TIER_BYTES;

/// Scope flag for symlink-following traversal.
const SCOPE_FOLLOW_SYMLINKS: u8 = 1 << 0;
/// Scope flag for staying on the root filesystem.
const SCOPE_ONE_FILESYSTEM: u8 = 1 << 1;
/// Scope flag for excluding native special objects.
const SCOPE_EXCLUDE_SPECIAL: u8 = 1 << 2;
/// Every scope flag this encoding defines.
const SCOPE_KNOWN_FLAGS: u8 = SCOPE_FOLLOW_SYMLINKS | SCOPE_ONE_FILESYSTEM | SCOPE_EXCLUDE_SPECIAL;

/// Control tier tag for a tier that observed nothing, whose limit fields are all zero.
const CONTROLS_NOT_OBSERVED: u8 = 0;
/// Control tier tag for an observed tier, whose limit fields follow.
const CONTROLS_OBSERVED: u8 = 1;

/// Bound tag for no bound, whose eight value bytes are zero.
const UNBOUNDED: u8 = 0;
/// Bound tag for a bound, whose value is the eight bytes that follow.
const BOUNDED: u8 = 1;

// Every bound is a `usize`, which fits the eight bytes that encode it on every target, so a
// bound is never refused or aliased at encode.
const _: () = assert!(usize::BITS <= u64::BITS, "a bound fits its eight encoded bytes");

/// Fills a fixed-width encoding field by field.
struct FixedWriter<const N: usize> {
    bytes: [u8; N],
    at: usize,
}

impl<const N: usize> FixedWriter<N> {
    const fn new() -> Self {
        Self { bytes: [0; N], at: 0 }
    }

    fn put(&mut self, field: &[u8]) {
        let end = self.at + field.len();
        self.bytes[self.at..end].copy_from_slice(field);
        self.at = end;
    }

    /// Write an optional bound as its tag and eight value bytes, zero when unbounded.
    ///
    /// A tag rather than a reserved value, so no bound can be mistaken for a sentinel.
    fn put_bound(&mut self, bound: Option<usize>) {
        let (tag, value) = match bound {
            None => (UNBOUNDED, 0),
            // Lossless: the width assertion above holds on every target this compiles for.
            Some(bound) => (BOUNDED, u64::try_from(bound).unwrap_or(u64::MAX)),
        };
        self.put(&[tag]);
        self.put(&value.to_le_bytes());
    }

    fn finish(self) -> [u8; N] {
        debug_assert_eq!(self.at, N, "every field of a fixed-width encoding is written");
        self.bytes
    }
}

/// A field holds bytes no encoder writes.
struct NotEncoded;

/// Reads a fixed-width encoding field by field.
struct FixedReader<'a> {
    rest: &'a [u8],
}

impl FixedReader<'_> {
    fn take<const W: usize>(&mut self) -> [u8; W] {
        let (field, rest) =
            self.rest.split_first_chunk::<W>().expect("a fixed-width encoding holds every field");
        self.rest = rest;
        *field
    }

    fn u8(&mut self) -> u8 {
        self.take::<1>()[0]
    }

    fn u64(&mut self) -> u64 {
        u64::from_le_bytes(self.take())
    }

    /// Read [`FixedWriter::put_bound`], refusing a tag or value no encoder writes.
    fn bound(&mut self) -> Result<Option<usize>, NotEncoded> {
        match (self.u8(), self.u64()) {
            (UNBOUNDED, 0) => Ok(None),
            (BOUNDED, value) => usize::try_from(value).map(Some).map_err(|_| NotEncoded),
            _ => Err(NotEncoded),
        }
    }
}

impl EntryTierIdentity {
    /// Encode every field but the engine fingerprint, which the store's prologue carries.
    pub(crate) fn encode(self) -> [u8; ENTRY_TIER_BYTES] {
        let scope = self.scope;
        let mut flags = 0u8;
        if scope.follow_symlinks {
            flags |= SCOPE_FOLLOW_SYMLINKS;
        }
        if scope.one_filesystem {
            flags |= SCOPE_ONE_FILESYSTEM;
        }
        if scope.exclude_special {
            flags |= SCOPE_EXCLUDE_SPECIAL;
        }
        let mut out = FixedWriter::new();
        out.put_bound(scope.max_depth);
        out.put(&[flags]);
        out.put(&scope.hidden_fingerprint.to_le_bytes());
        out.put(&self.type_rules_fingerprint.to_le_bytes());
        out.put(&self.reducers_fingerprint.to_le_bytes());
        out.finish()
    }

    /// Decode [`Self::encode`] under the engine fingerprint of the store that holds it, or
    /// `None` for a field no encoder writes.
    pub(crate) fn decode(engine: u64, bytes: &[u8; ENTRY_TIER_BYTES]) -> Option<Self> {
        let mut fields = FixedReader { rest: bytes };
        let max_depth = fields.bound().ok()?;
        let flags = fields.u8();
        if flags & !SCOPE_KNOWN_FLAGS != 0 {
            return None;
        }
        let scope = EntryScope {
            max_depth,
            follow_symlinks: flags & SCOPE_FOLLOW_SYMLINKS != 0,
            one_filesystem: flags & SCOPE_ONE_FILESYSTEM != 0,
            hidden_fingerprint: fields.u64(),
            exclude_special: flags & SCOPE_EXCLUDE_SPECIAL != 0,
        };
        Some(Self {
            engine,
            scope,
            type_rules_fingerprint: fields.u64(),
            reducers_fingerprint: fields.u64(),
        })
    }
}

impl ControlTierIdentity {
    /// Encode the observation and, when observed, both limits.
    pub(crate) fn encode(self) -> [u8; CONTROL_TIER_BYTES] {
        let mut out = FixedWriter::new();
        match self {
            Self::NotObserved => out.put(&[CONTROLS_NOT_OBSERVED; CONTROL_TIER_BYTES]),
            Self::Observed { limits } => {
                out.put(&[CONTROLS_OBSERVED]);
                out.put_bound(limits.budget);
                out.put_bound(limits.line_limit);
            }
        }
        out.finish()
    }

    /// Decode [`Self::encode`], or `None` for a tag or value no encoder writes.
    pub(crate) fn decode(bytes: &[u8; CONTROL_TIER_BYTES]) -> Option<Self> {
        let mut fields = FixedReader { rest: bytes };
        match fields.u8() {
            CONTROLS_NOT_OBSERVED => {
                bytes[1..].iter().all(|byte| *byte == 0).then_some(Self::NotObserved)
            }
            CONTROLS_OBSERVED => {
                let budget = fields.bound().ok()?;
                let line_limit = fields.bound().ok()?;
                Some(Self::Observed { limits: ControlLimits { budget, line_limit } })
            }
            _ => None,
        }
    }
}

impl SnapshotIdentity {
    /// Encode the entry tier's fields, then the control tier.
    pub(crate) fn encode(self) -> [u8; SNAPSHOT_IDENTITY_BYTES] {
        let mut out = FixedWriter::new();
        out.put(&self.entries.encode());
        out.put(&self.controls.encode());
        out.finish()
    }

    /// Decode [`Self::encode`] under the engine fingerprint of the snapshot that holds it.
    pub(crate) fn decode(engine: u64, bytes: &[u8; SNAPSHOT_IDENTITY_BYTES]) -> Option<Self> {
        let mut fields = FixedReader { rest: bytes };
        Some(Self {
            entries: EntryTierIdentity::decode(engine, &fields.take())?,
            controls: ControlTierIdentity::decode(&fields.take())?,
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ScanConfig;

    fn limits(budget: Option<usize>, line_limit: Option<usize>) -> ControlLimits {
        ControlLimits { budget, line_limit }
    }

    #[test]
    fn snapshot_serving_is_equality_plus_observation_on_to_off() {
        let base = ScanConfig::default().snapshot_identity();
        assert_eq!(serves_snapshot(base, base), Serves::Exact);

        let entries = base.entries;
        let mut refused = vec![
            SnapshotIdentity {
                entries: EntryTierIdentity { engine: entries.engine ^ 1, ..entries },
                ..base
            },
            SnapshotIdentity {
                entries: EntryTierIdentity {
                    type_rules_fingerprint: entries.type_rules_fingerprint ^ 1,
                    ..entries
                },
                ..base
            },
            SnapshotIdentity {
                entries: EntryTierIdentity {
                    reducers_fingerprint: entries.reducers_fingerprint ^ 1,
                    ..entries
                },
                ..base
            },
            SnapshotIdentity {
                controls: ControlTierIdentity::Observed { limits: limits(None, None) },
                ..base
            },
        ];
        for config in [
            ScanConfig { max_depth: Some(1), ..ScanConfig::default() },
            ScanConfig { one_filesystem: true, ..ScanConfig::default() },
            ScanConfig { exclude_special: true, ..ScanConfig::default() },
            ScanConfig {
                hidden: Some(std::sync::Arc::new(crate::HiddenPolicy::prune_hidden(
                    std::iter::empty::<std::ffi::OsString>(),
                ))),
                ..ScanConfig::default()
            },
        ] {
            refused.push(config.snapshot_identity());
        }
        for wanted in refused {
            assert_eq!(serves_snapshot(base, wanted), Serves::Refuse, "{wanted:?}");
            assert_eq!(serves_snapshot(wanted, base), Serves::Refuse, "{wanted:?}");
        }
        let blind = SnapshotIdentity { controls: ControlTierIdentity::NotObserved, ..base };
        assert_eq!(serves_snapshot(base, blind), Serves::ProjectControlsOff);
        assert_eq!(serves_snapshot(blind, base), Serves::Refuse);
    }

    #[test]
    fn control_settings_change_only_the_control_tier() {
        let base = ScanConfig::default();
        for config in [
            ScanConfig { read_controls: false, ..base.clone() },
            ScanConfig { control_limits: limits(None, Some(1)), ..base.clone() },
            ScanConfig {
                read_controls: false,
                control_limits: limits(Some(1), None),
                ..base.clone()
            },
        ] {
            assert_eq!(config.snapshot_identity().entries, base.snapshot_identity().entries);
            assert_ne!(config.snapshot_identity().controls, base.snapshot_identity().controls);
        }
        // Limits decide nothing when nothing is observed, so they leave no trace.
        let blind = ScanConfig { read_controls: false, ..base.clone() };
        let blind_other_limits = ScanConfig { control_limits: limits(None, None), ..blind.clone() };
        assert_eq!(blind.snapshot_identity(), blind_other_limits.snapshot_identity());
        assert_eq!(blind.control_identity(), ControlTierIdentity::NotObserved);
    }

    #[test]
    fn the_ignore_rules_fingerprint_reserves_zero_for_an_unobserved_tier() {
        assert_eq!(ControlTierIdentity::NotObserved.ignore_rules_fingerprint(), 0);
        let defaults = ControlLimits::default();
        let observed = [
            defaults,
            limits(None, defaults.line_limit),
            limits(defaults.budget, None),
            limits(None, None),
            // The same values in each other's places are a different tier.
            limits(defaults.line_limit, defaults.budget),
        ]
        .map(|limits| ControlTierIdentity::Observed { limits }.ignore_rules_fingerprint());
        for (index, fingerprint) in observed.iter().enumerate() {
            assert_ne!(*fingerprint, 0);
            assert!(!observed[index + 1..].contains(fingerprint), "{observed:?}");
        }
    }

    #[test]
    fn the_scope_an_identity_composes_is_the_one_a_scan_records() {
        for config in [
            ScanConfig::default(),
            ScanConfig { read_controls: false, ..ScanConfig::default() },
            ScanConfig { control_limits: limits(None, None), ..ScanConfig::default() },
            ScanConfig { max_depth: Some(3), exclude_special: true, ..ScanConfig::default() },
        ] {
            let identity = config.snapshot_identity();
            let scope = identity.scan_scope();
            assert_eq!(scope, config.scope());
            assert_eq!(EntryTierIdentity::of_scope(scope), identity.entries);
            assert_eq!(scope.observes_controls(), identity.controls.is_observed());

            let index = crate::Index::new_with_config("/root", &config);
            assert_eq!(index.snapshot_identity(), identity);
            assert_eq!(index.control_identity(), config.control_identity());
        }
    }

    #[test]
    fn each_tier_is_writable_by_its_own_rule() {
        let config = ScanConfig::default();
        let entries = config.snapshot_identity().entries;
        let other =
            ScanConfig { max_depth: Some(2), ..ScanConfig::default() }.snapshot_identity().entries;

        let mut complete = crate::Index::new_with_config("/root", &config);
        complete.set_initial_freshness(true);
        assert!(entries_writable(&complete));
        for stored in [None, Some(entries), Some(other)] {
            assert!(
                content_tier_writable(&complete, || stored),
                "a complete pass writes: {stored:?}"
            );
        }

        let mut partial = crate::Index::new_with_config("/root", &config);
        partial.set_initial_freshness(false);
        assert!(!entries_writable(&partial), "an absent entry would change totals");
        assert!(content_tier_writable(&partial, || Some(entries)));
        for stored in [None, Some(other)] {
            assert!(!content_tier_writable(&partial, || stored), "mismatched pair: {stored:?}");
        }

        let mut unverified = complete.clone();
        unverified.mark_unverified();
        assert!(!entries_writable(&unverified), "a cache-only index verified nothing");
    }

    /// Identities that each differ from the default in one encoded field, including the
    /// edges of each range, so a codec that dropped any one field would alias two of them.
    fn identities() -> Vec<SnapshotIdentity> {
        let base = ScanConfig::default().snapshot_identity();
        let entries = base.entries;
        let defaults = ControlLimits::default();
        assert_eq!(entries.scope.max_depth, None);
        assert!(defaults.budget.is_some() && defaults.line_limit.is_some());
        let mut all = vec![base];
        for scope in [
            EntryScope { max_depth: Some(0), ..entries.scope },
            // The largest bound is a bound, never the unbounded depth.
            EntryScope { max_depth: Some(usize::MAX), ..entries.scope },
            EntryScope { follow_symlinks: true, ..entries.scope },
            EntryScope { one_filesystem: true, ..entries.scope },
            EntryScope { exclude_special: true, ..entries.scope },
            EntryScope { hidden_fingerprint: u64::MAX, ..entries.scope },
        ] {
            all.push(SnapshotIdentity { entries: EntryTierIdentity { scope, ..entries }, ..base });
        }
        for entries in [
            EntryTierIdentity { type_rules_fingerprint: 0, ..entries },
            EntryTierIdentity { reducers_fingerprint: u64::MAX, ..entries },
        ] {
            all.push(SnapshotIdentity { entries, ..base });
        }
        for controls in [
            ControlTierIdentity::NotObserved,
            ControlTierIdentity::Observed { limits: limits(None, defaults.line_limit) },
            ControlTierIdentity::Observed { limits: limits(Some(0), defaults.line_limit) },
            ControlTierIdentity::Observed { limits: limits(defaults.budget, None) },
            ControlTierIdentity::Observed { limits: limits(defaults.budget, Some(usize::MAX)) },
        ] {
            all.push(SnapshotIdentity { controls, ..base });
        }
        all
    }

    #[test]
    fn every_identity_round_trips_through_its_fixed_width_encoding() {
        let identities = identities();
        let encoded = identities.iter().map(|identity| identity.encode()).collect::<Vec<_>>();
        for (identity, bytes) in identities.iter().zip(&encoded) {
            let engine = identity.entries.engine;
            assert_eq!(SnapshotIdentity::decode(engine, bytes), Some(*identity));
            let (entry_bytes, control_bytes) = bytes.split_at(ENTRY_TIER_BYTES);
            assert_eq!(
                EntryTierIdentity::decode(engine, entry_bytes.try_into().expect("width")),
                Some(identity.entries)
            );
            assert_eq!(
                ControlTierIdentity::decode(control_bytes.try_into().expect("width")),
                Some(identity.controls)
            );
        }
        // Canonical: distinct identities never share an encoding.
        for (index, bytes) in encoded.iter().enumerate() {
            assert!(!encoded[index + 1..].contains(bytes), "{:?}", identities[index]);
        }
    }

    #[test]
    fn bytes_no_encoder_writes_are_refused() {
        let base = ScanConfig::default().snapshot_identity();
        let engine = base.entries.engine;
        let bounded = EntryTierIdentity {
            scope: EntryScope { max_depth: Some(3), ..base.entries.scope },
            ..base.entries
        };
        let (depth_tag_at, depth_at, flags_at) = (0, 1, BOUND_BYTES);
        let mut forged_entries = Vec::new();
        let mut unknown_flag = base.entries.encode();
        unknown_flag[flags_at] |= 1 << 7;
        forged_entries.push(unknown_flag);
        let mut unknown_depth_tag = bounded.encode();
        assert_eq!(unknown_depth_tag[depth_tag_at], BOUNDED);
        unknown_depth_tag[depth_tag_at] = 2;
        forged_entries.push(unknown_depth_tag);
        let mut unbounded_depth_with_a_value = base.entries.encode();
        assert_eq!(unbounded_depth_with_a_value[depth_tag_at], UNBOUNDED);
        unbounded_depth_with_a_value[depth_at] = 1;
        forged_entries.push(unbounded_depth_with_a_value);
        for bytes in forged_entries {
            assert_eq!(EntryTierIdentity::decode(engine, &bytes), None, "{bytes:?}");
        }

        let observed = ControlTierIdentity::Observed { limits: limits(None, Some(1)) };
        let controls = observed.encode();
        let (budget_tag_at, budget_at, line_tag_at) = (1, 2, 1 + BOUND_BYTES);
        assert_eq!(controls[budget_tag_at], UNBOUNDED);
        assert_eq!(controls[line_tag_at], BOUNDED);
        let mut forged = Vec::new();
        let mut unknown_tag = controls;
        unknown_tag[0] = 2;
        forged.push(unknown_tag);
        let mut unknown_limit_tag = controls;
        unknown_limit_tag[line_tag_at] = 2;
        forged.push(unknown_limit_tag);
        let mut unbounded_with_a_value = controls;
        unbounded_with_a_value[budget_at] = 1;
        forged.push(unbounded_with_a_value);
        let mut unobserved_with_limits = controls;
        unobserved_with_limits[0] = CONTROLS_NOT_OBSERVED;
        forged.push(unobserved_with_limits);
        for bytes in forged {
            assert_eq!(ControlTierIdentity::decode(&bytes), None, "{bytes:?}");
        }
    }

    /// A content tier states its type rules once, in its entry tier, and every record it
    /// holds carries exactly those rules.
    #[test]
    fn a_content_tier_holds_its_records_type_rules_in_its_entry_tier() {
        use crate::content::AnalysisRequest;

        let entries = ScanConfig::default().snapshot_identity().entries;
        let request = AnalysisRequest { profile: AnalysisSet::ALL, ..AnalysisRequest::default() };
        let records = ContentProvenance::for_request(request, entries.type_rules_fingerprint);
        let identity = ContentTierIdentity::for_request(entries, request.profile);
        assert_eq!(identity.record_provenance(), records);
        assert!(identity.admit_record(request.profile, &records).is_some());

        let other_rules = ContentProvenance::for_request(request, !entries.type_rules_fingerprint);
        assert!(identity.admit_record(request.profile, &other_rules).is_none(), "other type rules");
        let lines = AnalysisSet::NONE.with_lines();
        assert!(identity.admit_record(lines, &records).is_none(), "another analyzer set's label");
    }

    #[test]
    fn content_admission_refuses_every_identity_difference_and_applies_exact_identity() {
        let entries = ScanConfig::default().snapshot_identity().entries;
        let wanted = ContentTierIdentity::for_request(entries, AnalysisSet::ALL);
        assert_eq!(wanted.admit(&wanted).expect("exact admission").identity(), &wanted);
        let changes: &[fn(&mut ContentTierIdentity)] = &[
            |identity| identity.entries.engine ^= 1,
            |identity| identity.entries.scope.max_depth = Some(1),
            |identity| identity.entries.type_rules_fingerprint ^= 1,
            |identity| identity.entries.reducers_fingerprint ^= 1,
            |identity| identity.analysis = AnalysisSet::LINES_ONLY,
            |identity| identity.provenance.options_fingerprint.0 ^= 1,
            |identity| identity.provenance.analyzers[0].1.0 += 1,
            |identity| {
                identity.provenance.analyzers.pop();
            },
        ];
        for change in changes {
            let mut other = wanted.clone();
            change(&mut other);
            assert!(wanted.admit(&other).is_none(), "stored mismatch: {other:?}");
            assert!(other.admit(&wanted).is_none(), "requested mismatch: {other:?}");
        }
    }

    #[test]
    fn the_engine_fingerprint_comes_from_the_store_not_the_encoding() {
        let identity = ScanConfig::default().snapshot_identity();
        let bytes = identity.encode();
        let other = SnapshotIdentity::decode(!identity.entries.engine, &bytes).expect("decode");
        assert_eq!(other.entries.engine, !identity.entries.engine);
        assert_eq!(serves_snapshot(other, identity), Serves::Refuse);
    }
}