memstead-base 0.16.0

//! Anchors — engine-owned durable provenance records tying an entity to
//! the source artifacts it describes.
//!
//! An anchor is the projection pipeline's single new load-bearing
//! primitive: which artifact (in the medium's own namespace), at which
//! *grain*, under which *provenance class*, at which medium-typed
//! *version*, hashed over the **prepared** artifact form (never raw
//! bytes) where the class carries hash semantics, and the medium's
//! declared *hash stability* — so an unstable-source hash break resolves
//! as [`AnchorState::Recheck`], not [`AnchorState::Drifted`].
//!
//! ## Naming
//!
//! The `Anchor*` family is deliberately distinct from the three
//! provenance-adjacent type families already in the tree — it never
//! reuses `Provenance` / `ProvenanceKind` (the mutation-log record in
//! [`crate::provenance`]), nor `ArchiveProvenance` / `EntityProvenance` /
//! `History` (the authoring-provenance payload in
//! [`memstead_schema::archive_provenance`]). Those stay; anchors are a
//! separate concern (source→entity provenance, not mutation history nor
//! authoring lineage).
//!
//! ## Wire vocabulary (fixed contract)
//!
//! - provenance classes: `anchored` / `derived` / `authored` /
//!   `informed-by`
//! - grains: `span` / `file` / `tree` / `url` / `entity`
//! - hash stability: `stable` / `unstable`
//! - resolution states: `resolves` / `drifted` / `recheck` / `orphaned`
//!
//! The Rust identifiers around this vocabulary are the implementer's
//! choice; the wire strings are the contract and are locked by the
//! `*_wire_strings_are_stable` tests below.
//!
//! ## Storage
//!
//! Anchors persist in an engine-owned sidecar on the mem branch under
//! [`ANCHOR_SIDECAR_PATH`] (`.memstead/anchors.json`) — see
//! [`AnchorSidecar`]. The sidecar is written only through engine commits
//! (the [`crate::backend::MemBackend`] sidecar seam); every external
//! reader already filters the `.memstead/` namespace, so an anchor-only
//! commit yields no entity deltas and does not participate in `_hash`.
//!
//! ## Scope of this module
//!
//! Pure value types, wire (de)serialisation, validation (typed
//! `INVALID_ANCHOR` refusals with recovery detail), and the resolution
//! model. No storage or IO lives here — the backend seam and the
//! mutation/CLI wiring consume these types.

use std::collections::BTreeMap;

use serde::{Deserialize, Serialize};

/// Mem-relative path of the engine-owned anchors sidecar on the mem
/// branch. Lives under the `.memstead/` umbrella every external reader
/// already treats as non-entity, so an anchor-only commit produces zero
/// entity deltas.
pub const ANCHOR_SIDECAR_PATH: &str = ".memstead/anchors.json";

/// Current sidecar document schema version. Version 2 added the per-row
/// `last_observed` record; version 1 files load unchanged (the field is
/// absent) and are rewritten as version 2 on the next sidecar write.
pub const ANCHOR_SIDECAR_VERSION: u32 = 2;

/// Every sidecar version this engine reads. Anything else refuses typed:
/// a document written by a later engine is not parsed optimistically.
pub const ANCHOR_SIDECAR_VERSIONS_READ: &[u32] = &[1, 2];

/// Stable typed error code returned when an `anchors[]` element is
/// malformed. Mirrors the engine's other typed-envelope codes; the whole
/// mutation refuses and the entity is not written.
pub const INVALID_ANCHOR_CODE: &str = "INVALID_ANCHOR";

/// The pinned sentinel a publish-time redaction writes into every
/// artifact reference (`artifact`, `derived_from` entries). A fixed,
/// visibly-artificial form rather than an empty string: the anchor entry
/// stays readable (class, counts, `at_version`, hash — the trust
/// metadata), while the reference discloses nothing — and an empty
/// reference stays what it always was, malformed
/// ([`AnchorSidecar::validate_artifact_references`]).
pub const REDACTED_ARTIFACT_SENTINEL: &str = "[redacted]";

// ---------------------------------------------------------------------------
// Provenance class
// ---------------------------------------------------------------------------

/// The epistemic standing of an anchor — how the entity relates to the
/// artifact it references.
///
/// - [`Anchored`](Self::Anchored) — the entity directly reflects specific
///   artifact content (carries hash semantics).
/// - [`Derived`](Self::Derived) — the entity was computed/synthesised from
///   one or more input artifacts (carries hash semantics; lists inputs).
/// - [`Authored`](Self::Authored) — a human/agent authored the entity with
///   the artifact in view (no hash semantics; excluded from drift
///   adjudication).
/// - [`InformedBy`](Self::InformedBy) — the artifact informed the entity
///   without a content-fidelity claim (no hash semantics; excluded from
///   drift adjudication).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum AnchorProvenanceClass {
    Anchored,
    Derived,
    Authored,
    InformedBy,
}

impl AnchorProvenanceClass {
    /// Every wire string, in declaration order — the allowed set a
    /// refusal echoes for recovery.
    pub const WIRE_VALUES: &'static [&'static str] =
        &["anchored", "derived", "authored", "informed-by"];

    /// Stable wire form.
    pub fn as_wire(&self) -> &'static str {
        match self {
            AnchorProvenanceClass::Anchored => "anchored",
            AnchorProvenanceClass::Derived => "derived",
            AnchorProvenanceClass::Authored => "authored",
            AnchorProvenanceClass::InformedBy => "informed-by",
        }
    }

    /// Inverse of [`Self::as_wire`]; `None` for an unknown string so the
    /// validator can refuse it typed rather than misclassify.
    pub fn from_wire(s: &str) -> Option<Self> {
        match s {
            "anchored" => Some(AnchorProvenanceClass::Anchored),
            "derived" => Some(AnchorProvenanceClass::Derived),
            "authored" => Some(AnchorProvenanceClass::Authored),
            "informed-by" => Some(AnchorProvenanceClass::InformedBy),
            _ => None,
        }
    }

    /// Whether this class carries hash semantics. `anchored` and
    /// `derived` assert content fidelity and participate in hash-drift
    /// adjudication; `authored` and `informed-by` do not — a content
    /// change under them produces no drift state, and supplying a hash on
    /// them is a validation refusal.
    pub fn is_hash_bearing(&self) -> bool {
        matches!(
            self,
            AnchorProvenanceClass::Anchored | AnchorProvenanceClass::Derived
        )
    }
}

// ---------------------------------------------------------------------------
// Grain
// ---------------------------------------------------------------------------

/// The granularity of the artifact reference an anchor carries.
///
/// `span` / `file` / `tree` select within a path-shaped namespace; `url`
/// selects a web resource; `entity` selects another mem's entity. The
/// medium-capability matrix ([`crate::binding::medium_capabilities`])
/// decides which grains a given medium's namespace can support — a
/// mismatch (e.g. `span` on a `url`-namespace medium) refuses typed at
/// validation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum AnchorGrain {
    Span,
    File,
    Tree,
    Url,
    Entity,
}

impl AnchorGrain {
    /// Every wire string, in declaration order.
    pub const WIRE_VALUES: &'static [&'static str] = &["span", "file", "tree", "url", "entity"];

    /// Stable wire form.
    pub fn as_wire(&self) -> &'static str {
        match self {
            AnchorGrain::Span => "span",
            AnchorGrain::File => "file",
            AnchorGrain::Tree => "tree",
            AnchorGrain::Url => "url",
            AnchorGrain::Entity => "entity",
        }
    }

    /// Inverse of [`Self::as_wire`]; `None` for an unknown string.
    pub fn from_wire(s: &str) -> Option<Self> {
        match s {
            "span" => Some(AnchorGrain::Span),
            "file" => Some(AnchorGrain::File),
            "tree" => Some(AnchorGrain::Tree),
            "url" => Some(AnchorGrain::Url),
            "entity" => Some(AnchorGrain::Entity),
            _ => None,
        }
    }

    /// Whether this grain can be expressed in the medium's declared anchor
    /// namespace (the `anchor_namespace` string from the E2 capability
    /// matrix: `path` / `path+commit` / `entity` / `url`).
    ///
    /// - `span` / `file` / `tree` require a path-shaped namespace
    ///   (`path` or `path+commit`);
    /// - `url` is admitted beside every namespace: a URL is an absolute
    ///   reference that never enters a path or entity namespace, so it
    ///   collides with nothing there — and the engine never observes it
    ///   itself, so no medium capability is claimed by admitting it;
    /// - `entity` requires the `entity` namespace.
    pub fn supported_by_namespace(&self, anchor_namespace: &str) -> bool {
        let path_shaped = matches!(anchor_namespace, "path" | "path+commit");
        match self {
            AnchorGrain::Span | AnchorGrain::File | AnchorGrain::Tree => path_shaped,
            AnchorGrain::Url => true,
            AnchorGrain::Entity => anchor_namespace == "entity",
        }
    }

    /// Whether this grain selects within a path-shaped namespace.
    pub fn is_path_shaped(&self) -> bool {
        matches!(
            self,
            AnchorGrain::Span | AnchorGrain::File | AnchorGrain::Tree
        )
    }
}

// ---------------------------------------------------------------------------
// Hash stability
// ---------------------------------------------------------------------------

/// The medium's declared hash stability — whether a change in the
/// prepared-content hash is a reliable drift signal.
///
/// A `stable` medium's hash break resolves [`AnchorState::Drifted`]; an
/// `unstable` medium's hash break resolves [`AnchorState::Recheck`]
/// (the hash may have moved for reasons unrelated to the entity's claim,
/// so the engine flags it for re-examination rather than asserting drift).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum AnchorHashStability {
    Stable,
    Unstable,
}

impl AnchorHashStability {
    /// Every wire string.
    pub const WIRE_VALUES: &'static [&'static str] = &["stable", "unstable"];

    /// Stable wire form.
    pub fn as_wire(&self) -> &'static str {
        match self {
            AnchorHashStability::Stable => "stable",
            AnchorHashStability::Unstable => "unstable",
        }
    }

    /// Inverse of [`Self::as_wire`]; `None` for an unknown string.
    pub fn from_wire(s: &str) -> Option<Self> {
        match s {
            "stable" => Some(AnchorHashStability::Stable),
            "unstable" => Some(AnchorHashStability::Unstable),
            _ => None,
        }
    }
}

// ---------------------------------------------------------------------------
// Medium-typed version
// ---------------------------------------------------------------------------

/// A medium-typed pinned version the anchor was recorded against.
///
/// Which variant applies follows from the medium's namespace: a git /
/// `path+commit` medium pins a [`Commit`](Self::Commit); a graph / `entity`
/// medium pins a [`Snapshot`](Self::Snapshot) token; a web / `url` medium
/// pins an [`Etag`](Self::Etag). A plain `path` medium (mtime change
/// signal, no retrievable version) records **absent** — represented as
/// `None` on [`Anchor::at_version`], never a variant here.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(tag = "kind", content = "value", rename_all = "lowercase")]
pub enum AnchorVersion {
    /// A git commit id (`path+commit` / git namespace).
    Commit(String),
    /// A graph snapshot token (`entity` namespace).
    Snapshot(String),
    /// A web ETag (`url` namespace).
    Etag(String),
}

// ---------------------------------------------------------------------------
// Anchor
// ---------------------------------------------------------------------------

/// One durable anchor record: an entity's provenance tie to a single
/// source artifact.
///
/// This is the persisted + read shape. Malformed wire input is refused
/// upstream via [`AnchorInput::validate`], which produces this strict type
/// only when every rule holds.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Anchor {
    /// Artifact reference in the medium's own namespace — a repo-relative
    /// path, a `path@commit`, a URL, or an entity id, interpreted per
    /// [`Self::grain`] and the medium.
    pub artifact: String,
    /// The granularity of [`Self::artifact`].
    pub grain: AnchorGrain,
    /// The anchor's epistemic standing.
    pub class: AnchorProvenanceClass,
    /// The medium-typed pinned version, or `None` when the medium has no
    /// retrievable version (plain `path` / mtime).
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub at_version: Option<AnchorVersion>,
    /// Content hash over the **prepared** artifact form (never raw bytes),
    /// present only when [`Self::class`] carries hash semantics. `None`
    /// for `authored` / `informed-by`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub hash: Option<String>,
    /// The medium's declared hash stability — governs whether a hash break
    /// resolves `drifted` or `recheck`.
    pub hash_stability: AnchorHashStability,
    /// For a `derived` class: the input artifact refs the entity was
    /// derived from. Empty for every other class.
    #[serde(default, skip_serializing_if = "Vec::is_empty")]
    pub derived_from: Vec<String>,
    /// `hash(D)` of the binding that produced this anchor (E2), when a
    /// binding produced it. `None` for a manually-authored anchor with no
    /// producing binding.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub binding: Option<String>,
    /// The NAME of the source (as declared in the producing binding's
    /// `sources[]`) that produced this anchor — so a discovery run can
    /// be measured per entry point. Optional and additive: pre-existing
    /// sidecars load unchanged and are never backfilled (a guessed
    /// provenance is worse than an absent one). Validated against the
    /// producing binding's declared names only when [`Self::binding`]
    /// still resolves in the workspace.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub source: Option<String>,
    /// A `span`-grain row whose locator could NOT be checked against the
    /// artifact at write time (consistency-sweep 03/03). The write path
    /// deliberately reads no source content, so the check is possible only
    /// where the caller supplied `content`; elsewhere the anchor is accepted
    /// and this records that its span is unverified, rather than letting a
    /// later surface report it as adjudicated. Never set on a non-`span`
    /// grain. Serialized only when true, so an existing sidecar is unchanged.
    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
    pub span_unvalidated: bool,
    /// Who established this row's [`Self::hash`] baseline. `None` on every
    /// row written before the field existed, which is honest: the baseline's
    /// origin was not recorded then and guessing it would be worse than
    /// admitting it. Set at write and at backfill from then on, so a reader
    /// can tell an author-pinned baseline from an engine-inferred one.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub hash_source: Option<AnchorHashSource>,
    /// The most recent observation recorded for this row (sidecar version
    /// 2): when it was made, the prepared-content hash it saw, and the state
    /// it resolved. Written for grains the engine cannot observe itself —
    /// a `url` row adjudicated from an observer-supplied observation — so
    /// the row can age visibly (`unobserved for N days`) instead of resting
    /// in `unobserved` forever. Path and entity rows are observed live on
    /// every pass and carry none. Absent on every version-1 row.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub last_observed: Option<AnchorObservation>,
}

/// One recorded observation of an anchor's artifact (the `last_observed`
/// record of a sidecar row).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct AnchorObservation {
    /// When the observation was made — second-granularity ISO-8601 UTC
    /// (`YYYY-MM-DDTHH:MM:SSZ`), as supplied by the observer or stamped by
    /// the engine at recording time.
    pub at: String,
    /// The prepared-content hash the observation saw; `None` when the
    /// observer reported the artifact absent.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub hash: Option<String>,
    /// The state the row resolved to against that observation.
    pub state: AnchorState,
}

/// Who established an anchor's hash baseline (consistency-sweep 03/03,
/// criterion 8). A baseline that resets with no trace makes drift
/// unfalsifiable, so the origin is recorded rather than inferred.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum AnchorHashSource {
    /// The writer supplied the hash, or the content the engine hashed.
    Author,
    /// A completed verify filled a hash-less row from what it observed.
    Backfill,
}

impl AnchorHashSource {
    pub fn as_wire(self) -> &'static str {
        match self {
            AnchorHashSource::Author => "author",
            AnchorHashSource::Backfill => "backfill",
        }
    }
}

// ---------------------------------------------------------------------------
// Span locators
// ---------------------------------------------------------------------------

/// What a `span` anchor's locator (everything after the first `#`) selects.
///
/// Two forms are legal and the distinction is not cosmetic. A LINE RANGE is
/// checkable against content the write path already holds; a UNIT KEY is a
/// delivery preparation's own key (`dated-entries` writes
/// `<path>#<iso-stamp>`), whose validity only that preparation can judge, and
/// which the existing unit-absent refusal already covers where content is
/// supplied. Anything the engine can check, it checks; anything it cannot, it
/// records as unchecked.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SpanLocator<'a> {
    /// `L<start>` or `L<start>-L<end>`, 1-based and inclusive.
    Lines { start: usize, end: usize },
    /// A preparation's delivery-unit key, opaque here.
    Unit(&'a str),
}

/// Parse a `span` artifact reference's locator, or say why it cannot be one.
/// `Ok(None)` means the reference carries no locator at all.
///
/// **No locator is not a refusal**, and that is a deliberate line. A
/// `span`-grain reference naming a bare path addresses its whole file, which
/// is what a span's hash covers anyway with no preparation declared, and such
/// anchors are written today. Refusing them would be a new wall across a
/// working flow, which the plan's own criterion 4 forbids.
///
/// The refusals are the shapes that can never address anything: an EMPTY
/// locator (`path#`, which announces a span and then names none), and a
/// locator that announces itself as a line range by its `L` prefix and then
/// contradicts itself (no digits, a zero line, an end before its start). A
/// locator that does not look like a line range is a unit key and is accepted
/// here, because this function cannot know a preparation's key grammar and
/// refusing what it cannot judge would break every `dated-entries` anchor.
pub fn parse_span_locator(artifact: &str) -> Result<Option<SpanLocator<'_>>, &'static str> {
    let locator = match artifact.split_once('#') {
        None => return Ok(None),
        Some((_, loc)) if loc.trim().is_empty() => {
            return Err("the span locator after `#` is empty");
        }
        Some((_, loc)) => loc,
    };
    // Only an `L`-prefixed locator claims to be a line range. Everything else
    // is a unit key and is not this function's to judge.
    let looks_like_lines = locator.starts_with('L')
        && locator[1..]
            .chars()
            .next()
            .is_some_and(|c| c.is_ascii_digit());
    if !looks_like_lines {
        return Ok(Some(SpanLocator::Unit(locator)));
    }
    let (start_raw, end_raw) = match locator.split_once('-') {
        None => (locator, locator),
        Some((a, b)) => (a, b),
    };
    let num = |part: &str| -> Option<usize> {
        part.strip_prefix('L')
            .filter(|d| !d.is_empty() && d.chars().all(|c| c.is_ascii_digit()))
            .and_then(|d| d.parse::<usize>().ok())
    };
    let (Some(start), Some(end)) = (num(start_raw), num(end_raw)) else {
        return Err("a line-range span locator must read `L<start>` or `L<start>-L<end>`");
    };
    if start == 0 {
        return Err("line numbers are 1-based, so `L0` addresses nothing");
    }
    if end < start {
        return Err("a line-range span locator ends before it starts");
    }
    Ok(Some(SpanLocator::Lines { start, end }))
}

// ---------------------------------------------------------------------------
// Validation
// ---------------------------------------------------------------------------

/// A permissive wire-shaped anchor element as it arrives on a mutation's
/// `anchors[]` parameter. All fields are optional / string-typed so an
/// unknown class or grain surfaces as a typed [`AnchorValidationError`]
/// with recovery detail rather than an opaque serde failure. Call
/// [`Self::validate`] to obtain a strict [`Anchor`].
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct AnchorInput {
    #[serde(default)]
    pub artifact: Option<String>,
    #[serde(default)]
    pub grain: Option<String>,
    #[serde(default)]
    pub class: Option<String>,
    #[serde(default)]
    pub at_version: Option<AnchorVersion>,
    #[serde(default)]
    pub hash: Option<String>,
    /// The observed artifact CONTENT (UTF-8 text), for the engine to compute
    /// `hash` from through its preparation registry
    /// ([`crate::preparation::supplied_content_hash`]) — the write-time
    /// observation for a grain the engine cannot observe itself: a `url`
    /// anchor, because the engine never fetches. Accepted for the `span` /
    /// `file` / `url` grains; mutually exclusive with `hash`; refused on a
    /// non-hash class and on the `entity` / `tree` grains, whose prepared
    /// form is never computed from supplied bytes.
    #[serde(default)]
    pub content: Option<String>,
    #[serde(default)]
    pub hash_stability: Option<String>,
    #[serde(default)]
    pub derived_from: Option<Vec<String>>,
    #[serde(default)]
    pub binding: Option<String>,
    #[serde(default)]
    pub source: Option<String>,
}

/// A permissive wire-shaped `anchors_unset[]` element — an explicit
/// removal selector on the update surface. Each entry names an `artifact`
/// and may narrow by `grain` and/or `class`; a bare artifact selects every
/// anchor on it. String-typed like [`AnchorInput`] so an unknown grain or
/// class refuses typed (`INVALID_ANCHOR`) rather than silently selecting
/// nothing forever. Call [`Self::validate`] to obtain a strict
/// [`AnchorUnset`].
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct AnchorUnsetInput {
    #[serde(default)]
    pub artifact: Option<String>,
    #[serde(default)]
    pub grain: Option<String>,
    #[serde(default)]
    pub class: Option<String>,
}

impl AnchorUnsetInput {
    /// Validate this wire element into a strict [`AnchorUnset`], or refuse
    /// typed. Rules: artifact present and non-empty; grain / class, when
    /// supplied, must be known wire strings (absent means "any").
    pub fn validate(&self) -> Result<AnchorUnset, AnchorValidationError> {
        let artifact = self
            .artifact
            .as_deref()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .map(str::to_string)
            .ok_or(AnchorValidationError::MissingArtifact)?;
        let grain = match self.grain.as_deref() {
            None => None,
            Some(s) => Some(AnchorGrain::from_wire(s).ok_or_else(|| {
                AnchorValidationError::UnknownGrain {
                    got: Some(s.to_string()),
                    allowed: AnchorGrain::WIRE_VALUES,
                }
            })?),
        };
        let class = match self.class.as_deref() {
            None => None,
            Some(s) => Some(AnchorProvenanceClass::from_wire(s).ok_or_else(|| {
                AnchorValidationError::UnknownClass {
                    got: Some(s.to_string()),
                    allowed: AnchorProvenanceClass::WIRE_VALUES,
                }
            })?),
        };
        Ok(AnchorUnset {
            artifact,
            grain,
            class,
        })
    }
}

/// A validated explicit-removal selector: which of an entity's anchors an
/// update's `anchors_unset[]` entry removes. Selection is by artifact,
/// optionally narrowed by grain and/or class; a selector matching nothing
/// is a no-op (removal is idempotent — its job in recovery flows is "make
/// sure this is gone").
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AnchorUnset {
    /// Artifact reference to remove anchors from, exactly as stored.
    pub artifact: String,
    /// When present, only anchors of this grain are removed.
    pub grain: Option<AnchorGrain>,
    /// When present, only anchors of this class are removed.
    pub class: Option<AnchorProvenanceClass>,
}

impl AnchorUnset {
    /// Whether this selector removes `anchor`.
    pub fn matches(&self, anchor: &Anchor) -> bool {
        anchor.artifact == self.artifact
            && self.grain.is_none_or(|g| anchor.grain == g)
            && self.class.is_none_or(|c| anchor.class == c)
    }
}

/// A typed `INVALID_ANCHOR` refusal. The whole mutation refuses and the
/// entity is not written; [`Self::detail`] carries the recovery payload
/// (offending value + allowed set) the agent fixes from.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum AnchorValidationError {
    /// Provenance class is absent or not one of the allowed wire strings.
    #[error("unknown anchor provenance class {got:?}; allowed: {}", allowed.join(", "))]
    UnknownClass {
        got: Option<String>,
        allowed: &'static [&'static str],
    },
    /// Grain is absent or not one of the allowed wire strings.
    #[error("unknown anchor grain {got:?}; allowed: {}", allowed.join(", "))]
    UnknownGrain {
        got: Option<String>,
        allowed: &'static [&'static str],
    },
    /// Hash stability, when supplied, is not an allowed wire string.
    #[error("unknown anchor hash stability {got:?}; allowed: {}", allowed.join(", "))]
    UnknownHashStability {
        got: String,
        allowed: &'static [&'static str],
    },
    /// The artifact reference is missing or empty.
    #[error("anchor is missing its artifact reference")]
    MissingArtifact,
    /// A content hash (or content to hash) was supplied on a class that
    /// carries no hash semantics (`authored` / `informed-by`).
    #[error("anchor class '{class}' carries no hash semantics — a content hash is not permitted")]
    HashOnNonHashClass { class: &'static str },
    /// Both `hash` and `content` were supplied — the engine computes the
    /// hash from content, so a supplied hash beside it is ambiguous.
    #[error(
        "anchor supplies both `hash` and `content`; supply one — the engine computes the hash from `content`"
    )]
    ContentAndHash,
    /// `content` was supplied for a grain whose prepared form is never
    /// computed from supplied bytes: `entity` (computed from the live graph)
    /// or `tree` (whose prepared form, under a code map, is enumerated by
    /// the engine).
    #[error(
        "anchor grain '{grain}' does not accept `content`: its prepared form is not computed \
         from supplied bytes (accepted for span / file / url)"
    )]
    ContentNotAcceptedForGrain { grain: &'static str },
    /// `content` was supplied for a `<path>#<key>` unit under a delivery
    /// preparation, but the content yields no unit with that key.
    #[error(
        "anchor artifact {artifact:?} names a delivery unit the supplied `content` does not \
         yield; supply the whole file's content, or address a unit it contains"
    )]
    UnitAbsentFromContent { artifact: String },
    /// A `span`-grain anchor whose locator is missing, empty, or announces a
    /// line range and then contradicts itself. Refused at write: such a row
    /// can never address anything, and accepting it produces an anchor that
    /// is unadjudicable from birth.
    #[error("anchor artifact {artifact:?} is not a usable span reference: {reason}")]
    SpanLocatorUnusable {
        artifact: String,
        reason: &'static str,
    },
    /// A `span`-grain anchor whose line range lies outside the content the
    /// caller supplied. Only fires where content is in hand — the write path
    /// reads no source, and where it cannot check, the row records that
    /// instead (`span_unvalidated`).
    #[error(
        "anchor artifact {artifact:?} names lines the supplied `content` does not have \
         (it has {lines} line(s)); address a range the artifact contains"
    )]
    SpanOutsideContent { artifact: String, lines: usize },
    /// One payload named the same `(artifact, grain, class)` triple twice.
    /// That triple is the sidecar's merge identity, so the later occurrence
    /// silently replaced the earlier one and the caller was never told an
    /// anchor it wrote had gone missing. A LATER call replacing the stored
    /// row is unaffected: the unit of this refusal is one payload.
    #[error(
        "the anchors payload names {artifact:?} at grain `{grain}` and class `{class}` more \
         than once; that triple is one row, so the repeats would silently collapse to the \
         last one: send it once, or vary the grain or class"
    )]
    DuplicateAnchorTriple {
        artifact: String,
        grain: &'static str,
        class: &'static str,
    },
    /// A `source` was supplied but is empty after trimming — a source
    /// name, when present, must be one of the producing binding's
    /// declared names, and an empty string can never be one.
    #[error("anchor `source`, when present, must be a non-empty source name")]
    EmptySource,
    /// The anchor's `source` is not among the sources declared by its
    /// own (resolvable) producing binding. Carries the declared names
    /// as the recovery payload. Only fires when the `binding` hash
    /// still resolves in this workspace — an orphaned or since-edited
    /// binding accepts any non-empty name, deliberately: a legacy
    /// anchor whose binding was renamed keeps writing as long as its
    /// artifact reference is alive under the workspace-relative
    /// fallback (`mem_commands::source_dialect_anchors_join_fallback_collide_and_refuse`).
    #[error(
        "anchor `source` {got:?} is not declared by the anchor's producing binding; \
         declared sources: {}",
        declared.join(", ")
    )]
    SourceNotDeclared { got: String, declared: Vec<String> },
    /// A path-grain artifact reference that resolves under NO candidate
    /// join — neither source-relative (joined onto the declaring source's
    /// pointer, decision 26) nor workspace-relative. Refused at write time
    /// so the mutation never stores a silently dead (orphaned-at-birth)
    /// reference; the payload names every candidate tried so the agent can
    /// fix the dialect.
    #[error(
        "anchor artifact {artifact:?} resolves under no candidate path (tried: {}); artifact \
         paths are source-relative (joined onto the source's pointer) or workspace-relative — \
         write the path exactly as the brief lists it",
        candidates.join(", ")
    )]
    ArtifactUnresolvable {
        artifact: String,
        candidates: Vec<String>,
    },
    /// The grain cannot be expressed in the medium's anchor namespace
    /// (per the E2 capability matrix), e.g. `span` on a non-path medium.
    #[error(
        "anchor grain '{grain}' is unsupported by a '{medium_type}' medium: its \
         '{anchor_namespace}' namespace does not admit that grain"
    )]
    GrainNamespaceUnsupported {
        grain: &'static str,
        medium_type: String,
        anchor_namespace: &'static str,
    },
    /// A path-shaped grain (`span` / `file` / `tree`) names a URL. A URL
    /// never enters a path namespace: the web resource is addressed by the
    /// `url` grain, and a page coordinate inside it is not a path span.
    #[error(
        "anchor grain '{grain}' selects within a path namespace, but its artifact '{artifact}'          is a URL — a URL never enters a path namespace; use `grain: url` for the resource"
    )]
    PathGrainOnUrlArtifact {
        grain: &'static str,
        artifact: String,
    },
}

/// Whether an artifact string is URL-shaped (`<scheme>://…`).
pub fn looks_like_url(artifact: &str) -> bool {
    let Some((scheme, rest)) = artifact.split_once("://") else {
        return false;
    };
    !rest.is_empty()
        && scheme
            .chars()
            .next()
            .is_some_and(|c| c.is_ascii_alphabetic())
        && scheme
            .chars()
            .all(|c| c.is_ascii_alphanumeric() || matches!(c, '+' | '-' | '.'))
}

impl AnchorValidationError {
    /// The stable typed code — always [`INVALID_ANCHOR_CODE`].
    pub fn code(&self) -> &'static str {
        INVALID_ANCHOR_CODE
    }

    /// Structured recovery detail for the typed envelope: the offending
    /// field, its bad value, and the allowed set where one applies.
    pub fn detail(&self) -> BTreeMap<String, serde_json::Value> {
        let mut d = BTreeMap::new();
        match self {
            AnchorValidationError::UnknownClass { got, allowed } => {
                d.insert("field".into(), "class".into());
                d.insert("got".into(), serde_json::json!(got));
                d.insert("allowed".into(), serde_json::json!(allowed));
            }
            AnchorValidationError::UnknownGrain { got, allowed } => {
                d.insert("field".into(), "grain".into());
                d.insert("got".into(), serde_json::json!(got));
                d.insert("allowed".into(), serde_json::json!(allowed));
            }
            AnchorValidationError::UnknownHashStability { got, allowed } => {
                d.insert("field".into(), "hash_stability".into());
                d.insert("got".into(), serde_json::json!(got));
                d.insert("allowed".into(), serde_json::json!(allowed));
            }
            AnchorValidationError::MissingArtifact => {
                d.insert("field".into(), "artifact".into());
            }
            AnchorValidationError::EmptySource => {
                d.insert("field".into(), "source".into());
            }
            AnchorValidationError::SourceNotDeclared { got, declared } => {
                d.insert("field".into(), "source".into());
                d.insert("got".into(), serde_json::json!(got));
                d.insert("declared".into(), serde_json::json!(declared));
            }
            AnchorValidationError::HashOnNonHashClass { class } => {
                d.insert("field".into(), "hash".into());
                d.insert("class".into(), serde_json::json!(class));
            }
            AnchorValidationError::ContentAndHash => {
                d.insert("field".into(), "content".into());
                d.insert(
                    "expected".into(),
                    serde_json::json!("either `hash` or `content`, never both"),
                );
            }
            AnchorValidationError::ContentNotAcceptedForGrain { grain } => {
                d.insert("field".into(), "content".into());
                d.insert("grain".into(), serde_json::json!(grain));
                d.insert(
                    "accepted_grains".into(),
                    serde_json::json!(["span", "file", "url"]),
                );
            }
            AnchorValidationError::UnitAbsentFromContent { artifact } => {
                d.insert("field".into(), "content".into());
                d.insert("got".into(), serde_json::json!(artifact));
            }
            AnchorValidationError::SpanLocatorUnusable { artifact, reason } => {
                d.insert("field".into(), "artifact".into());
                d.insert("got".into(), serde_json::json!(artifact));
                d.insert("expected".into(), serde_json::json!(reason));
            }
            AnchorValidationError::SpanOutsideContent { artifact, lines } => {
                d.insert("field".into(), "artifact".into());
                d.insert("got".into(), serde_json::json!(artifact));
                d.insert("content_lines".into(), serde_json::json!(lines));
            }
            AnchorValidationError::DuplicateAnchorTriple {
                artifact,
                grain,
                class,
            } => {
                d.insert("field".into(), "anchors".into());
                d.insert(
                    "got".into(),
                    serde_json::json!({ "artifact": artifact, "grain": grain, "class": class }),
                );
                d.insert(
                    "expected".into(),
                    serde_json::json!(
                        "each (artifact, grain, class) triple at most once per payload"
                    ),
                );
            }
            AnchorValidationError::ArtifactUnresolvable {
                artifact,
                candidates,
            } => {
                d.insert("field".into(), "artifact".into());
                d.insert("got".into(), serde_json::json!(artifact));
                d.insert("candidates_tried".into(), serde_json::json!(candidates));
                d.insert(
                    "expected".into(),
                    serde_json::json!(
                        "a source-relative path (joined onto the source's pointer) or a \
                         workspace-relative path that resolves to an existing artifact"
                    ),
                );
            }
            AnchorValidationError::GrainNamespaceUnsupported {
                grain,
                medium_type,
                anchor_namespace,
            } => {
                d.insert("field".into(), "grain".into());
                d.insert("grain".into(), serde_json::json!(grain));
                d.insert("medium_type".into(), serde_json::json!(medium_type));
                d.insert(
                    "anchor_namespace".into(),
                    serde_json::json!(anchor_namespace),
                );
            }
            AnchorValidationError::PathGrainOnUrlArtifact { grain, artifact } => {
                d.insert("field".into(), "grain".into());
                d.insert("grain".into(), serde_json::json!(grain));
                d.insert("got".into(), serde_json::json!(artifact));
                d.insert(
                    "expected".into(),
                    serde_json::json!(
                        "`grain: url` for a web resource — a URL never enters a path namespace"
                    ),
                );
            }
        }
        d
    }
}

impl AnchorInput {
    /// Validate this wire element into a strict [`Anchor`], or refuse
    /// typed.
    ///
    /// `medium` — the resolving medium's `(type_name, anchor_namespace)`
    /// pair, when the mutation resolved one. When `Some`, the grain is
    /// checked against the namespace (the capability-matrix refusal);
    /// when `None` (no medium context — a manually-authored anchor), the
    /// namespace check is skipped and only the vocabulary + hash-semantics
    /// rules apply.
    ///
    /// Rules enforced (each a typed [`AnchorValidationError`]):
    /// - class present and known;
    /// - grain present and known;
    /// - artifact reference present and non-empty;
    /// - a hash (or content to hash) is supplied only on a hash-bearing
    ///   class; `content` and `hash` are mutually exclusive; `content` is
    ///   accepted only for the grains whose prepared form the registry
    ///   computes from supplied bytes (`span` / `file` / `url`), and then
    ///   `hash` is the registry's prepared hash of it;
    /// - hash stability, when supplied, is a known wire string (defaults
    ///   per grain when absent — `url` unstable, every other grain stable);
    /// - grain supported by the medium's namespace (when `medium` given).
    pub fn validate(&self, medium: Option<(&str, &str)>) -> Result<Anchor, AnchorValidationError> {
        let class = match self
            .class
            .as_deref()
            .and_then(AnchorProvenanceClass::from_wire)
        {
            Some(c) => c,
            None => {
                return Err(AnchorValidationError::UnknownClass {
                    got: self.class.clone(),
                    allowed: AnchorProvenanceClass::WIRE_VALUES,
                });
            }
        };
        let grain = match self.grain.as_deref().and_then(AnchorGrain::from_wire) {
            Some(g) => g,
            None => {
                return Err(AnchorValidationError::UnknownGrain {
                    got: self.grain.clone(),
                    allowed: AnchorGrain::WIRE_VALUES,
                });
            }
        };

        let artifact = self
            .artifact
            .as_deref()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .map(str::to_string)
            .ok_or(AnchorValidationError::MissingArtifact)?;

        // Hash stability: default per grain when absent (`url` unstable,
        // every other grain stable); refuse an unknown supplied value.
        let hash_stability = match self.hash_stability.as_deref() {
            None => crate::preparation::default_hash_stability(grain),
            Some(s) => AnchorHashStability::from_wire(s).ok_or_else(|| {
                AnchorValidationError::UnknownHashStability {
                    got: s.to_string(),
                    allowed: AnchorHashStability::WIRE_VALUES,
                }
            })?,
        };

        // A hash is only meaningful on a hash-bearing class.
        let hash = self
            .hash
            .as_deref()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .map(str::to_string);
        if (hash.is_some() || self.content.is_some()) && !class.is_hash_bearing() {
            return Err(AnchorValidationError::HashOnNonHashClass {
                class: class.as_wire(),
            });
        }
        // Supplied content: the engine computes the prepared hash through the
        // preparation registry (touchpoint A at write time) — the one way a
        // `url` anchor's recorded hash is ever the engine's prepared form.
        let hash = match self.content.as_deref() {
            None => hash,
            Some(_) if hash.is_some() => return Err(AnchorValidationError::ContentAndHash),
            Some(content) => {
                match crate::preparation::supplied_content_hash(grain, content.as_bytes()) {
                    Some(h) => Some(h),
                    None => {
                        return Err(AnchorValidationError::ContentNotAcceptedForGrain {
                            grain: grain.as_wire(),
                        });
                    }
                }
            }
        };

        // The span itself (consistency-sweep 03/03). A locator that can never
        // address anything is refused here, context-free, because no medium
        // context can rescue it. Where the caller supplied content, a line
        // range is checked against it; where they did not, the row carries
        // `span_unvalidated` so no later surface reports it as adjudicated.
        let mut span_unvalidated = false;
        if grain == AnchorGrain::Span {
            let locator = parse_span_locator(&artifact).map_err(|reason| {
                AnchorValidationError::SpanLocatorUnusable {
                    artifact: artifact.clone(),
                    reason,
                }
            })?;
            match (locator, self.content.as_deref()) {
                (Some(SpanLocator::Lines { end, .. }), Some(content)) => {
                    let lines = content.lines().count();
                    if end > lines {
                        return Err(AnchorValidationError::SpanOutsideContent {
                            artifact: artifact.clone(),
                            lines,
                        });
                    }
                }
                // A unit key with content in hand is the existing
                // unit-absent refusal's business, at the seam that knows the
                // source's preparation; a check here would have to guess it.
                (Some(SpanLocator::Unit(_)), Some(_)) => {}
                // No locator addresses the whole artifact, and the existence
                // gate already checks that the artifact is there. Nothing is
                // left unchecked, so nothing is recorded as unchecked.
                (None, _) => {}
                (Some(_), None) => span_unvalidated = true,
            }
        }

        // A path-shaped grain never names a URL: the resource is the `url`
        // grain's business, whatever medium the mem sits beside.
        if grain.is_path_shaped() && looks_like_url(&artifact) {
            return Err(AnchorValidationError::PathGrainOnUrlArtifact {
                grain: grain.as_wire(),
                artifact,
            });
        }

        // Grain must be expressible in the medium's namespace (a `url`
        // grain is admitted beside every medium — see
        // [`AnchorGrain::supported_by_namespace`]).
        if let Some((medium_type, namespace)) = medium
            && !grain.supported_by_namespace(namespace)
        {
            // Resolve the namespace to its `&'static str` so the error
            // carries a stable value even though the input came borrowed.
            let anchor_namespace = match namespace {
                "path" => "path",
                "path+commit" => "path+commit",
                "entity" => "entity",
                "url" => "url",
                _ => "path",
            };
            return Err(AnchorValidationError::GrainNamespaceUnsupported {
                grain: grain.as_wire(),
                medium_type: medium_type.to_string(),
                anchor_namespace,
            });
        }

        // `source`, when present, must be non-empty. (Whether it names a
        // source the producing binding actually declares is checked at
        // the engine seam, which can resolve the binding hash — this
        // context-free validator cannot.)
        let source = match self.source.as_deref() {
            None => None,
            Some(raw) => {
                let trimmed = raw.trim();
                if trimmed.is_empty() {
                    return Err(AnchorValidationError::EmptySource);
                }
                Some(trimmed.to_string())
            }
        };

        Ok(Anchor {
            artifact,
            grain,
            class,
            at_version: self.at_version.clone(),
            // A hash present at this point came from the writer, directly or
            // as content the engine hashed. The backfill stamps its own.
            hash_source: hash.is_some().then_some(AnchorHashSource::Author),
            hash,
            hash_stability,
            derived_from: self.derived_from.clone().unwrap_or_default(),
            binding: self
                .binding
                .as_deref()
                .map(str::trim)
                .filter(|s| !s.is_empty())
                .map(str::to_string),
            source,
            span_unvalidated,
            last_observed: None,
        })
    }
}

// ---------------------------------------------------------------------------
// Prepared-content hash
// ---------------------------------------------------------------------------

/// Compute the **prepared-content hash** of a path-grain artifact's bytes —
/// the value [`Anchor::hash`] records and hash-drift adjudication compares.
///
/// The prepared form is a deliberate, minimal canonicalization that keeps the
/// hash stable across meaningless byte noise while preserving every
/// content-bearing byte. For UTF-8 text:
///
/// - a leading BOM (U+FEFF) is stripped;
/// - CRLF / lone-CR line endings normalize to LF;
/// - trailing newlines are trimmed (final-newline presence is noise).
///
/// Interior whitespace is untouched — trailing spaces inside a line can be
/// content (markdown hard breaks), so only the two classic cross-tool noise
/// sources (encoding marks, line-ending convention) and the final-newline
/// question are canonicalized. Non-UTF-8 (binary) bytes hash as-is — no text
/// canonicalization applies to them.
///
/// The hash form reuses the house convention — SHA-256, lowercase hex,
/// truncated to 16 characters — shared by entity content hashes
/// ([`crate::entity::parser::compute_hash`]) and the change-detection digest
/// aggregate, so the engine keeps one hash shape rather than growing a
/// second normalization.
pub fn prepared_content_hash(bytes: &[u8]) -> String {
    use sha2::{Digest as _, Sha256};
    let digest = match std::str::from_utf8(bytes) {
        Ok(text) => {
            let text = text.strip_prefix('\u{feff}').unwrap_or(text);
            let normalized = text.replace("\r\n", "\n").replace('\r', "\n");
            Sha256::digest(normalized.trim_end_matches('\n').as_bytes())
        }
        Err(_) => Sha256::digest(bytes),
    };
    crate::hex_lower(&digest)[..16].to_string()
}

/// One verify-observed prepared-content hash, addressed to the anchor(s) it
/// backfills: the `(entity, artifact)` pair a hash-less hash-bearing anchor
/// is keyed by in the sidecar, plus the hash the observation computed. The
/// verify pass collects these; the engine's sidecar writer records them.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct ObservedArtifactHash {
    /// The entity id (`mem--slug`) whose anchor the hash belongs to.
    pub entity: String,
    /// The anchor's artifact reference, exactly as stored.
    pub artifact: String,
    /// The prepared-content hash observed for the artifact.
    pub hash: String,
}

// ---------------------------------------------------------------------------
// Supplied observations
// ---------------------------------------------------------------------------

/// Typed code for a malformed supplied observation row.
pub const INVALID_OBSERVATION_CODE: &str = "INVALID_OBSERVATION";

/// One observer-supplied observation row, as it arrives on the wire
/// (`memstead verify-anchors --observations`): permissive and string-typed
/// so a malformed row refuses with a typed [`ObservationValidationError`]
/// before any state changes. The engine never fetches; for a grain it
/// cannot observe itself (`url`) this is how an observation enters the one
/// resolution funnel.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct SuppliedObservationInput {
    /// The anchor's artifact reference, exactly as stored (a URL for a
    /// `url` row).
    #[serde(default)]
    pub artifact: Option<String>,
    /// The prepared-content hash the observer computed. Exactly one of
    /// `hash`, `content`, `absent: true`.
    #[serde(default)]
    pub hash: Option<String>,
    /// The observed artifact CONTENT (UTF-8 text); the engine hashes it
    /// under the same canonicalization the write path applies to a `url`
    /// anchor's `content`.
    #[serde(default)]
    pub content: Option<String>,
    /// The observer could not retrieve the artifact.
    #[serde(default)]
    pub absent: Option<bool>,
    /// When the observation was made (ISO-8601 UTC, `YYYY-MM-DDTHH:MM:SSZ`
    /// or a bare `YYYY-MM-DD`). Defaults to the engine's clock at the run.
    #[serde(default)]
    pub observed_at: Option<String>,
}

/// What an observer saw for one artifact.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SuppliedOutcome {
    /// The artifact was retrieved; `hash` is its prepared-content hash.
    Present { hash: String },
    /// The observer could not retrieve the artifact.
    Absent,
}

/// A validated supplied observation.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SuppliedObservation {
    pub artifact: String,
    /// ISO-8601 timestamp of the observation.
    pub at: String,
    pub outcome: SuppliedOutcome,
}

/// Why a supplied observation row was refused.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum ObservationValidationError {
    #[error("observation row {row}: `artifact` is required and must be non-empty")]
    MissingArtifact { row: usize },
    #[error(
        "observation row {row} (`{artifact}`): give exactly one of `hash`, `content`, or \
         `absent: true`"
    )]
    OutcomeAmbiguous { row: usize, artifact: String },
    #[error(
        "observation row {row} (`{artifact}`): `observed_at` '{got}' is not an ISO-8601 \
         timestamp (`YYYY-MM-DDTHH:MM:SSZ`) or date (`YYYY-MM-DD`)"
    )]
    BadTimestamp {
        row: usize,
        artifact: String,
        got: String,
    },
    #[error("observation rows name `{artifact}` more than once (rows {first} and {second})")]
    DuplicateArtifact {
        artifact: String,
        first: usize,
        second: usize,
    },
}

impl ObservationValidationError {
    /// The stable typed code — always [`INVALID_OBSERVATION_CODE`].
    pub fn code(&self) -> &'static str {
        INVALID_OBSERVATION_CODE
    }

    /// Structured recovery detail for the typed envelope.
    pub fn detail(&self) -> BTreeMap<String, serde_json::Value> {
        let mut d = BTreeMap::new();
        match self {
            ObservationValidationError::MissingArtifact { row } => {
                d.insert("row".into(), serde_json::json!(row));
                d.insert("field".into(), "artifact".into());
            }
            ObservationValidationError::OutcomeAmbiguous { row, artifact } => {
                d.insert("row".into(), serde_json::json!(row));
                d.insert("artifact".into(), serde_json::json!(artifact));
                d.insert(
                    "expected".into(),
                    serde_json::json!("exactly one of `hash`, `content`, `absent: true`"),
                );
            }
            ObservationValidationError::BadTimestamp { row, artifact, got } => {
                d.insert("row".into(), serde_json::json!(row));
                d.insert("artifact".into(), serde_json::json!(artifact));
                d.insert("field".into(), "observed_at".into());
                d.insert("got".into(), serde_json::json!(got));
            }
            ObservationValidationError::DuplicateArtifact {
                artifact,
                first,
                second,
            } => {
                d.insert("artifact".into(), serde_json::json!(artifact));
                d.insert("rows".into(), serde_json::json!([first, second]));
            }
        }
        d
    }
}

/// Accept `YYYY-MM-DD` or `YYYY-MM-DDTHH:MM:SSZ` (any `T…Z` time part of the
/// second-granularity form).
fn timestamp_is_wellformed(ts: &str) -> bool {
    let b = ts.as_bytes();
    let date_ok = b.len() >= 10
        && b[..10].iter().enumerate().all(|(i, c)| {
            if i == 4 || i == 7 {
                *c == b'-'
            } else {
                c.is_ascii_digit()
            }
        });
    if !date_ok {
        return false;
    }
    if b.len() == 10 {
        return true;
    }
    b.len() == 20
        && b[10] == b'T'
        && b[19] == b'Z'
        && b[11..19].iter().enumerate().all(|(i, c)| {
            if i == 2 || i == 5 {
                *c == b':'
            } else {
                c.is_ascii_digit()
            }
        })
}

/// Validate a batch of supplied observation rows; all-or-nothing, so a
/// malformed row refuses before any state changes. `now` is the timestamp
/// stamped on rows that carry no `observed_at`. Returns the observations
/// keyed by artifact.
pub fn validate_supplied_observations(
    rows: &[SuppliedObservationInput],
    now: &str,
) -> Result<BTreeMap<String, SuppliedObservation>, ObservationValidationError> {
    let mut out: BTreeMap<String, SuppliedObservation> = BTreeMap::new();
    let mut first_row: BTreeMap<String, usize> = BTreeMap::new();
    for (i, row) in rows.iter().enumerate() {
        let n = i + 1;
        let artifact = row
            .artifact
            .as_deref()
            .map(str::trim)
            .filter(|s| !s.is_empty())
            .ok_or(ObservationValidationError::MissingArtifact { row: n })?
            .to_string();
        let absent = row.absent.unwrap_or(false);
        let given = usize::from(row.hash.is_some())
            + usize::from(row.content.is_some())
            + usize::from(absent);
        if given != 1 {
            return Err(ObservationValidationError::OutcomeAmbiguous { row: n, artifact });
        }
        let at = match row.observed_at.as_deref().map(str::trim) {
            None | Some("") => now.to_string(),
            Some(ts) if timestamp_is_wellformed(ts) => ts.to_string(),
            Some(ts) => {
                return Err(ObservationValidationError::BadTimestamp {
                    row: n,
                    artifact,
                    got: ts.to_string(),
                });
            }
        };
        if let Some(first) = first_row.get(&artifact) {
            return Err(ObservationValidationError::DuplicateArtifact {
                artifact,
                first: *first,
                second: n,
            });
        }
        let outcome = if absent {
            SuppliedOutcome::Absent
        } else if let Some(hash) = &row.hash {
            SuppliedOutcome::Present {
                hash: hash.trim().to_string(),
            }
        } else {
            SuppliedOutcome::Present {
                hash: prepared_content_hash(row.content.as_deref().unwrap_or_default().as_bytes()),
            }
        };
        first_row.insert(artifact.clone(), n);
        out.insert(
            artifact.clone(),
            SuppliedObservation {
                artifact,
                at,
                outcome,
            },
        );
    }
    Ok(out)
}

/// Days since the Unix epoch of an ISO timestamp's date part, for aging a
/// recorded observation (`unobserved for N days`). `None` when the string
/// does not start with a well-formed `YYYY-MM-DD`.
pub fn iso_days_since_epoch(ts: &str) -> Option<i64> {
    if !timestamp_is_wellformed(ts) {
        return None;
    }
    let y: i64 = ts[..4].parse().ok()?;
    let m: u32 = ts[5..7].parse().ok()?;
    let d: u32 = ts[8..10].parse().ok()?;
    if !(1..=12).contains(&m) || !(1..=31).contains(&d) {
        return None;
    }
    let y = if m <= 2 { y - 1 } else { y };
    let era = if y >= 0 { y } else { y - 399 } / 400;
    let yoe = y - era * 400;
    let mp = ((m + 9) % 12) as i64;
    let doy = (153 * mp + 2) / 5 + d as i64 - 1;
    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
    Some(era * 146097 + doe - 719468)
}

/// Whole days between a recorded observation and `now` (both ISO), floored
/// at zero; `None` when either fails to parse.
pub fn days_between(observed_at: &str, now: &str) -> Option<u64> {
    let a = iso_days_since_epoch(observed_at)?;
    let b = iso_days_since_epoch(now)?;
    Some((b - a).max(0) as u64)
}

// ---------------------------------------------------------------------------
// Resolution
// ---------------------------------------------------------------------------

/// The resolved state of one anchor against the current medium.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum AnchorState {
    /// The artifact is present and matches (hash equal, or a non-hash
    /// class whose artifact still exists).
    Resolves,
    /// The artifact is present but its prepared-content hash differs and
    /// the medium is `stable` — a real content drift.
    Drifted,
    /// The artifact is present but drift cannot be asserted — the medium
    /// is `unstable`, or the hash is unavailable on one side. Flagged for
    /// re-examination, never reported as drift.
    Recheck,
    /// The artifact the anchor references is no longer present in the
    /// medium.
    Orphaned,
}

impl AnchorState {
    /// Stable wire form.
    pub fn as_wire(&self) -> &'static str {
        match self {
            AnchorState::Resolves => "resolves",
            AnchorState::Drifted => "drifted",
            AnchorState::Recheck => "recheck",
            AnchorState::Orphaned => "orphaned",
        }
    }
}

/// What the engine observed about an anchor's artifact when resolving.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ArtifactObservation {
    /// The artifact could not be found in the medium.
    Absent,
    /// The artifact is present; `current_hash` is its prepared-content
    /// hash when the medium could compute one (`None` when the medium has
    /// no hash for it this pass — e.g. enumeration without preparation).
    Present { current_hash: Option<String> },
}

/// Resolve one anchor against a current observation, honouring the class's
/// hash semantics and the medium's declared stability.
///
/// - `authored` / `informed-by` are excluded from hash-drift adjudication:
///   they [`Resolves`](AnchorState::Resolves) as long as the artifact
///   exists, [`Orphaned`](AnchorState::Orphaned) when it does not — a
///   content change never produces a drift state for them.
/// - `anchored` / `derived` compare the recorded prepared-content hash to
///   the current one: equal ⇒ resolves; different ⇒ `drifted` on a stable
///   medium, `recheck` on an unstable one; unavailable on either side ⇒
///   `recheck` (cannot adjudicate).
pub fn resolve_anchor(anchor: &Anchor, observation: &ArtifactObservation) -> AnchorState {
    let current_hash = match observation {
        ArtifactObservation::Absent => return AnchorState::Orphaned,
        ArtifactObservation::Present { current_hash } => current_hash,
    };
    if !anchor.class.is_hash_bearing() {
        return AnchorState::Resolves;
    }
    match (&anchor.hash, current_hash) {
        (Some(recorded), Some(current)) if recorded == current => AnchorState::Resolves,
        (Some(_), Some(_)) => match anchor.hash_stability {
            AnchorHashStability::Stable => AnchorState::Drifted,
            AnchorHashStability::Unstable => AnchorState::Recheck,
        },
        // Missing hash on either side — cannot adjudicate drift.
        _ => AnchorState::Recheck,
    }
}

/// Per-entity provenance-class + grain composition, computed from an
/// entity's anchor list. Tree-grain fan-out is surfaced distinctly so a
/// single entity anchored to a large tree is never laundered into
/// full per-file credit — the count of tree anchors is visible on its own
/// axis, and downstream (E3b) reads the fan-out counts from resolution.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct EntityAnchorComposition {
    /// Anchor count keyed by provenance-class wire string.
    pub by_class: BTreeMap<String, usize>,
    /// Anchor count keyed by grain wire string.
    pub by_grain: BTreeMap<String, usize>,
    /// The `derived_from` input lists of every `derived` anchor, in
    /// anchor order — E3b's derived-input provenance.
    pub derived_inputs: Vec<Vec<String>>,
    /// Artifact refs of every `tree`-grain anchor — the fan-out axis. A
    /// tree anchor is one row here regardless of how many files the tree
    /// contains; the file count is an observation resolution supplies, not
    /// a credit this composition grants.
    pub tree_grain_artifacts: Vec<String>,
}

/// Compose an entity's anchors into class/grain counts, derived inputs,
/// and the tree-grain fan-out axis.
pub fn compose_entity_anchors(anchors: &[Anchor]) -> EntityAnchorComposition {
    let mut comp = EntityAnchorComposition::default();
    for a in anchors {
        *comp
            .by_class
            .entry(a.class.as_wire().to_string())
            .or_insert(0) += 1;
        *comp
            .by_grain
            .entry(a.grain.as_wire().to_string())
            .or_insert(0) += 1;
        if a.class == AnchorProvenanceClass::Derived {
            comp.derived_inputs.push(a.derived_from.clone());
        }
        if a.grain == AnchorGrain::Tree {
            comp.tree_grain_artifacts.push(a.artifact.clone());
        }
    }
    comp
}

// ---------------------------------------------------------------------------
// Sidecar document
// ---------------------------------------------------------------------------

/// The engine-owned anchors sidecar document persisted at
/// [`ANCHOR_SIDECAR_PATH`] on the mem branch: entity id → its anchors.
///
/// Written only through engine commits (the [`crate::backend::MemBackend`]
/// sidecar seam). Rename rewrites the key atomically in the same commit as
/// the entity move; delete drops the key in the same commit as the entity
/// delete.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct AnchorSidecar {
    /// Document schema version.
    pub version: u32,
    /// Entity id (`mem--slug`) → its anchors. An entity with no anchors
    /// carries no key (an empty vec is pruned on write).
    #[serde(default)]
    pub entities: BTreeMap<String, Vec<Anchor>>,
}

impl Default for AnchorSidecar {
    fn default() -> Self {
        Self {
            version: ANCHOR_SIDECAR_VERSION,
            entities: BTreeMap::new(),
        }
    }
}

impl AnchorSidecar {
    /// Parse sidecar bytes; an absent/empty payload yields an empty
    /// document so callers need not special-case a fresh mem.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, serde_json::Error> {
        if bytes.iter().all(u8::is_ascii_whitespace) {
            return Ok(Self::default());
        }
        let sidecar: Self = serde_json::from_slice(bytes)?;
        // The version field is a contract, not decoration. Every sibling
        // store refuses an unknown one — the binding record with
        // `UNKNOWN_BINDING_VERSION`, the workspace stores with
        // `WORKSPACE_STORE_FORMAT_MISMATCH` — and this one silently accepted
        // it, so a sidecar written by a future engine parsed as whatever
        // today's field names happened to match and verified CLEAN. Reading
        // an unknown format optimistically is how a measurement ends up
        // confidently describing something it does not understand.
        if !ANCHOR_SIDECAR_VERSIONS_READ.contains(&sidecar.version) {
            return Err(serde::de::Error::custom(format!(
                "unsupported anchors sidecar version {} (this engine reads versions {}) — \
                 the file was written by a different engine; upgrade, or remove the sidecar \
                 to re-record anchors",
                sidecar.version,
                ANCHOR_SIDECAR_VERSIONS_READ
                    .iter()
                    .map(u32::to_string)
                    .collect::<Vec<_>>()
                    .join(", ")
            )));
        }
        // An older readable version is upgraded in memory: the rows are
        // unchanged (a version-2 field is simply absent on them) and the
        // next write persists the current version.
        let mut sidecar = sidecar;
        sidecar.version = ANCHOR_SIDECAR_VERSION;
        Ok(sidecar)
    }

    /// Serialise to canonical pretty JSON with a trailing newline —
    /// diff-friendly on the mem branch.
    pub fn to_bytes(&self) -> Vec<u8> {
        let mut s = serde_json::to_string_pretty(self).expect("anchor sidecar serialises");
        s.push('\n');
        s.into_bytes()
    }

    /// The anchors recorded for `entity_id`, or an empty slice.
    pub fn get(&self, entity_id: &str) -> &[Anchor] {
        self.entities
            .get(entity_id)
            .map(Vec::as_slice)
            .unwrap_or(&[])
    }

    /// Replace `entity_id`'s anchors. An empty list prunes the key so the
    /// sidecar never accumulates empty rows.
    pub fn set(&mut self, entity_id: &str, anchors: Vec<Anchor>) {
        if anchors.is_empty() {
            self.entities.remove(entity_id);
        } else {
            self.entities.insert(entity_id.to_string(), anchors);
        }
    }

    /// Merge `incoming` into `entity_id`'s anchor row after applying
    /// `unsets` — the write-path set arithmetic.
    ///
    /// Unset applies **first**: each selector removes its matching anchors
    /// (a selector matching nothing is a no-op). Then each incoming anchor
    /// **replaces** the surviving anchor with the same
    /// `(artifact, grain, class)` triple in place, and **appends**
    /// otherwise — untouched anchors keep their bytes and their position.
    /// Writing anchors never removes an anchor the call did not name in
    /// `unsets`; an empty `incoming` merges nothing. A row emptied by
    /// unsets prunes its key so the sidecar never accumulates empty rows.
    pub fn merge(&mut self, entity_id: &str, unsets: &[AnchorUnset], incoming: Vec<Anchor>) {
        let mut row = self.entities.remove(entity_id).unwrap_or_default();
        row.retain(|a| !unsets.iter().any(|u| u.matches(a)));
        for mut anchor in incoming {
            match row.iter_mut().find(|e| {
                e.artifact == anchor.artifact && e.grain == anchor.grain && e.class == anchor.class
            }) {
                Some(existing) => {
                    // Carry the stored baseline forward when the incoming row
                    // does not mention one (consistency-sweep 03/03,
                    // criterion 5). A re-pin usually exists to update the
                    // artifact reference, and dropping the hash made the next
                    // verify re-baseline silently, so drift became
                    // unfalsifiable with nothing recording that it had. A
                    // caller who supplies a hash still replaces it, and one
                    // who means to CLEAR a baseline unsets the row and writes
                    // it fresh — unsets are applied above, before this merge.
                    if anchor.hash.is_none()
                        && let Some(kept) = existing.hash.clone()
                    {
                        anchor.hash = Some(kept);
                        anchor.hash_source = existing.hash_source;
                    }
                    *existing = anchor;
                }
                None => row.push(anchor),
            }
        }
        if !row.is_empty() {
            self.entities.insert(entity_id.to_string(), row);
        }
    }

    /// Blank every artifact reference — `artifact` and each `derived_from`
    /// entry — to [`REDACTED_ARTIFACT_SENTINEL`], keeping everything else:
    /// class, grain, `at_version`, hash, hash-stability, binding, source,
    /// and the per-entity anchor counts. Redact, not strip: a consumer
    /// still reads *how strongly* each entity claims fidelity to a source
    /// without learning *which* source. Publish-time only by design — no
    /// engine path calls this against workspace state.
    pub fn redact_artifact_references(&mut self) {
        for anchors in self.entities.values_mut() {
            for anchor in anchors {
                anchor.artifact = REDACTED_ARTIFACT_SENTINEL.to_string();
                for input in &mut anchor.derived_from {
                    *input = REDACTED_ARTIFACT_SENTINEL.to_string();
                }
            }
        }
    }

    /// Structural check on artifact references: every `artifact` and every
    /// `derived_from` entry must be non-empty. The mutation surface never
    /// admits an empty reference (`INVALID_ANCHOR`), so a sidecar carrying
    /// one is corruption — including a botched redaction that blanked to
    /// nothing instead of the pinned sentinel. Returns the first offence.
    pub fn validate_artifact_references(&self) -> Result<(), String> {
        for (entity_id, anchors) in &self.entities {
            for anchor in anchors {
                if anchor.artifact.trim().is_empty() {
                    return Err(format!(
                        "entity `{entity_id}` carries an anchor with an empty artifact \
                         reference"
                    ));
                }
                if anchor.derived_from.iter().any(|d| d.trim().is_empty()) {
                    return Err(format!(
                        "entity `{entity_id}` carries an anchor with an empty \
                         `derived_from` entry"
                    ));
                }
            }
        }
        Ok(())
    }

    /// Drop `entity_id`'s anchors entirely (delete leg). Idempotent.
    pub fn remove(&mut self, entity_id: &str) {
        self.entities.remove(entity_id);
    }

    /// Move `from`'s anchors to `to` (rename leg), leaving zero rows under
    /// the old id. No-op when `from` has no anchors. When `to` already has
    /// anchors they are overwritten — a rename onto a live id is refused
    /// upstream, so this is the residual-stub case only.
    pub fn rename(&mut self, from: &str, to: &str) {
        if let Some(anchors) = self.entities.remove(from) {
            self.entities.insert(to.to_string(), anchors);
        }
    }

    /// Whether the document holds no anchors for any entity.
    pub fn is_empty(&self) -> bool {
        self.entities.is_empty()
    }
}

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

    /// Redaction blanks exactly the two artifact-reference fields — to the
    /// pinned sentinel, never removal — and keeps everything else: class,
    /// grain, `at_version`, hash, hash-stability, binding, source, and the
    /// per-entity anchor counts.
    #[test]
    fn redaction_blanks_references_and_keeps_trust_metadata() {
        let mut sidecar = AnchorSidecar::default();
        sidecar.set(
            "m--alpha",
            vec![
                Anchor {
                    artifact: "src/lib.rs".into(),
                    grain: AnchorGrain::File,
                    class: AnchorProvenanceClass::Anchored,
                    at_version: Some(AnchorVersion::Commit("abc123".into())),
                    hash: Some("h1".into()),
                    hash_stability: AnchorHashStability::Stable,
                    derived_from: vec![],
                    binding: Some("bhash".into()),
                    source: Some("source-tree".into()),
                    span_unvalidated: false,
                    hash_source: None,
                    last_observed: None,
                },
                Anchor {
                    artifact: "docs/summary.md".into(),
                    grain: AnchorGrain::File,
                    class: AnchorProvenanceClass::Derived,
                    at_version: None,
                    hash: Some("h2".into()),
                    hash_stability: AnchorHashStability::Unstable,
                    derived_from: vec!["notes/a.md".into(), "notes/b.md".into()],
                    binding: None,
                    source: None,
                    span_unvalidated: false,
                    hash_source: None,
                    last_observed: None,
                },
            ],
        );

        sidecar.redact_artifact_references();

        let anchors = sidecar.get("m--alpha");
        assert_eq!(anchors.len(), 2, "no anchor entry is dropped");
        for a in anchors {
            assert_eq!(a.artifact, REDACTED_ARTIFACT_SENTINEL);
            for d in &a.derived_from {
                assert_eq!(d, REDACTED_ARTIFACT_SENTINEL);
            }
        }
        assert_eq!(
            anchors[0].at_version,
            Some(AnchorVersion::Commit("abc123".into()))
        );
        assert_eq!(anchors[0].hash.as_deref(), Some("h1"));
        assert_eq!(anchors[0].binding.as_deref(), Some("bhash"));
        assert_eq!(anchors[0].source.as_deref(), Some("source-tree"));
        assert_eq!(anchors[1].class, AnchorProvenanceClass::Derived);
        assert_eq!(anchors[1].derived_from.len(), 2, "derivation arity kept");
        // A redacted sidecar is structurally valid — the sentinel is not
        // an empty reference.
        sidecar.validate_artifact_references().unwrap();
    }

    /// The structural reference check refuses empty `artifact` and empty
    /// `derived_from` entries — including a botched redaction that blanked
    /// to nothing instead of the sentinel.
    #[test]
    fn empty_artifact_references_are_refused() {
        let mut sidecar = AnchorSidecar::default();
        sidecar.set(
            "m--alpha",
            vec![Anchor {
                artifact: "".into(),
                grain: AnchorGrain::File,
                class: AnchorProvenanceClass::Anchored,
                at_version: None,
                hash: None,
                hash_stability: AnchorHashStability::Stable,
                derived_from: vec![],
                binding: None,
                source: None,
                span_unvalidated: false,
                hash_source: None,
                last_observed: None,
            }],
        );
        assert!(sidecar.validate_artifact_references().is_err());

        let mut sidecar = AnchorSidecar::default();
        sidecar.set(
            "m--beta",
            vec![Anchor {
                artifact: "docs/x.md".into(),
                grain: AnchorGrain::File,
                class: AnchorProvenanceClass::Derived,
                at_version: None,
                hash: None,
                hash_stability: AnchorHashStability::Stable,
                derived_from: vec!["  ".into()],
                binding: None,
                source: None,
                span_unvalidated: false,
                hash_source: None,
                last_observed: None,
            }],
        );
        assert!(sidecar.validate_artifact_references().is_err());
    }

    // -- wire vocabulary is the contract -----------------------------------

    #[test]
    fn class_wire_strings_are_stable() {
        assert_eq!(AnchorProvenanceClass::Anchored.as_wire(), "anchored");
        assert_eq!(AnchorProvenanceClass::Derived.as_wire(), "derived");
        assert_eq!(AnchorProvenanceClass::Authored.as_wire(), "authored");
        assert_eq!(AnchorProvenanceClass::InformedBy.as_wire(), "informed-by");
        for w in AnchorProvenanceClass::WIRE_VALUES {
            assert_eq!(AnchorProvenanceClass::from_wire(w).unwrap().as_wire(), *w);
        }
        assert!(AnchorProvenanceClass::from_wire("bogus").is_none());
    }

    #[test]
    fn grain_wire_strings_are_stable() {
        for w in AnchorGrain::WIRE_VALUES {
            assert_eq!(AnchorGrain::from_wire(w).unwrap().as_wire(), *w);
        }
        assert_eq!(
            AnchorGrain::WIRE_VALUES,
            &["span", "file", "tree", "url", "entity"]
        );
        assert!(AnchorGrain::from_wire("chunk").is_none());
    }

    #[test]
    fn stability_and_state_wire_strings_are_stable() {
        assert_eq!(AnchorHashStability::Stable.as_wire(), "stable");
        assert_eq!(AnchorHashStability::Unstable.as_wire(), "unstable");
        assert_eq!(AnchorState::Resolves.as_wire(), "resolves");
        assert_eq!(AnchorState::Drifted.as_wire(), "drifted");
        assert_eq!(AnchorState::Recheck.as_wire(), "recheck");
        assert_eq!(AnchorState::Orphaned.as_wire(), "orphaned");
    }

    #[test]
    fn only_anchored_and_derived_are_hash_bearing() {
        assert!(AnchorProvenanceClass::Anchored.is_hash_bearing());
        assert!(AnchorProvenanceClass::Derived.is_hash_bearing());
        assert!(!AnchorProvenanceClass::Authored.is_hash_bearing());
        assert!(!AnchorProvenanceClass::InformedBy.is_hash_bearing());
    }

    // -- grain / namespace matrix ------------------------------------------

    #[test]
    fn grain_namespace_support_matches_capability_matrix() {
        // path-shaped grains need path / path+commit.
        for g in [AnchorGrain::Span, AnchorGrain::File, AnchorGrain::Tree] {
            assert!(g.supported_by_namespace("path"));
            assert!(g.supported_by_namespace("path+commit"));
            assert!(!g.supported_by_namespace("url"));
            assert!(!g.supported_by_namespace("entity"));
        }
        assert!(AnchorGrain::Url.supported_by_namespace("url"));
        // A URL is an absolute reference: admitted beside every medium.
        assert!(AnchorGrain::Url.supported_by_namespace("path"));
        assert!(AnchorGrain::Url.supported_by_namespace("entity"));
        assert!(AnchorGrain::Entity.supported_by_namespace("entity"));
        assert!(!AnchorGrain::Entity.supported_by_namespace("path"));
    }

    // -- validation refusals -----------------------------------------------

    fn valid_input() -> AnchorInput {
        AnchorInput {
            artifact: Some("src/lib.rs".into()),
            grain: Some("file".into()),
            class: Some("anchored".into()),
            hash_stability: Some("stable".into()),
            hash: Some("abc123".into()),
            ..Default::default()
        }
    }

    fn span_input(artifact: &str) -> AnchorInput {
        AnchorInput {
            artifact: Some(artifact.into()),
            grain: Some("span".into()),
            class: Some("anchored".into()),
            ..Default::default()
        }
    }

    /// Criterion 1 (consistency-sweep 03/03): a locator that can never
    /// address anything is refused at the moment of writing. Each of these
    /// used to write successfully and could then never be adjudicated.
    #[test]
    fn a_span_locator_that_addresses_nothing_is_refused() {
        for artifact in [
            "src/lib.rs#",      // announces a span, names none
            "src/lib.rs#   ",   // the same, in whitespace
            "src/lib.rs#L0",    // lines are 1-based
            "src/lib.rs#L0-L4", // and so is a range's start
            "src/lib.rs#L9-L2", // ends before it starts
            "src/lib.rs#L4-L",  // half a range
            "src/lib.rs#L4-x",  // a range that stops being one
        ] {
            let err = span_input(artifact)
                .validate(Some(("codebase", "path")))
                .expect_err(artifact);
            assert!(
                matches!(err, AnchorValidationError::SpanLocatorUnusable { .. }),
                "{artifact} refused as {err:?}"
            );
            assert_eq!(err.code(), INVALID_ANCHOR_CODE);
            assert!(err.detail().contains_key("expected"), "carries the repair");
        }
    }

    /// Criterion 4's first half: a span that names something real still
    /// writes. `Lx` forms within the artifact, a preparation's unit key, and
    /// a bare path (which addresses the whole file, and is what a span's hash
    /// covers anyway) are all legal.
    #[test]
    fn a_usable_span_locator_still_writes() {
        for artifact in [
            "src/lib.rs",
            "src/lib.rs#L1",
            "src/lib.rs#L4-L7",
            "logs/ops.md#2026-08-25T00:00:00",
        ] {
            span_input(artifact)
                .validate(Some(("codebase", "path")))
                .unwrap_or_else(|e| panic!("{artifact} refused: {e}"));
        }
    }

    /// Criterion 2: where the content is already in hand, a range beyond the
    /// artifact's end is refused rather than stored as an anchor pointing at
    /// lines the file does not have.
    #[test]
    fn a_span_beyond_supplied_content_is_refused() {
        let mut i = span_input("src/lib.rs#L2-L9");
        i.content = Some(
            "one
two
three
"
            .into(),
        );
        let err = i.validate(Some(("codebase", "path"))).unwrap_err();
        match err {
            AnchorValidationError::SpanOutsideContent { lines, .. } => assert_eq!(lines, 3),
            other => panic!("wrong refusal: {other:?}"),
        }

        let mut ok = span_input("src/lib.rs#L2-L3");
        ok.content = Some(
            "one
two
three
"
            .into(),
        );
        let a = ok.validate(Some(("codebase", "path"))).unwrap();
        assert!(
            !a.span_unvalidated,
            "a span checked against content is not unvalidated"
        );
    }

    /// Criterion 3: where the write path holds no content, the span cannot be
    /// checked without a read it deliberately does not perform. The anchor is
    /// accepted and the row says the span is unverified, so no later surface
    /// reports it as adjudicated.
    #[test]
    fn an_uncheckable_span_is_accepted_and_recorded_as_unchecked() {
        let a = span_input("src/lib.rs#L4-L7")
            .validate(Some(("codebase", "path")))
            .unwrap();
        assert!(a.span_unvalidated);

        let whole_file = span_input("src/lib.rs")
            .validate(Some(("codebase", "path")))
            .unwrap();
        assert!(
            !whole_file.span_unvalidated,
            "no locator addresses the whole artifact, which the existence gate checks"
        );

        let file_grain = valid_input().validate(Some(("codebase", "path"))).unwrap();
        assert!(!file_grain.span_unvalidated, "never set off the span grain");
    }

    /// Criterion 8: a hash the writer supplied is recorded as theirs, so a
    /// reader can later tell it from one the backfill inferred.
    #[test]
    fn an_authored_hash_records_that_the_author_pinned_it() {
        let a = valid_input().validate(Some(("codebase", "path"))).unwrap();
        assert_eq!(a.hash_source, Some(AnchorHashSource::Author));

        let mut hashless = valid_input();
        hashless.hash = None;
        let b = hashless.validate(Some(("codebase", "path"))).unwrap();
        assert_eq!(b.hash_source, None, "no baseline, no origin to record");
    }

    /// Criteria 5 and 6: a re-pin that says nothing about the hash keeps the
    /// baseline it did not mention, one that supplies a hash replaces it, and
    /// unsetting the row first is the explicit way to clear it.
    #[test]
    fn a_re_pin_keeps_the_baseline_it_did_not_mention() {
        let mut sc = AnchorSidecar::default();
        let mut pinned = file_anchor("src/a.rs", "h-original");
        pinned.hash_source = Some(AnchorHashSource::Author);
        sc.set("m--e", vec![pinned]);

        let mut repin = file_anchor("src/a.rs", "");
        repin.hash = None;
        repin.hash_source = None;
        sc.merge("m--e", &[], vec![repin]);
        let row = &sc.entities["m--e"][0];
        assert_eq!(
            row.hash.as_deref(),
            Some("h-original"),
            "the baseline the caller did not mention survives"
        );
        assert_eq!(row.hash_source, Some(AnchorHashSource::Author));

        sc.merge("m--e", &[], vec![file_anchor("src/a.rs", "h-new")]);
        assert_eq!(
            sc.entities["m--e"][0].hash.as_deref(),
            Some("h-new"),
            "a supplied hash still replaces"
        );

        // The explicit clear: unset the row, then write it fresh. Unsets are
        // applied before the merge, so the old row is gone first.
        let unset = AnchorUnset {
            artifact: "src/a.rs".into(),
            grain: None,
            class: None,
        };
        let mut fresh = file_anchor("src/a.rs", "");
        fresh.hash = None;
        fresh.hash_source = None;
        sc.merge("m--e", &[unset], vec![fresh]);
        assert_eq!(
            sc.entities["m--e"][0].hash, None,
            "unset-then-write is how a caller clears a baseline"
        );
    }

    #[test]
    fn validate_accepts_a_well_formed_anchor() {
        let a = valid_input().validate(Some(("codebase", "path"))).unwrap();
        assert_eq!(a.artifact, "src/lib.rs");
        assert_eq!(a.grain, AnchorGrain::File);
        assert_eq!(a.class, AnchorProvenanceClass::Anchored);
        assert_eq!(a.hash.as_deref(), Some("abc123"));
        assert_eq!(a.hash_stability, AnchorHashStability::Stable);
    }

    /// Path grains keep their `stable` default — pinned, because the
    /// per-grain default that gives `url` its `unstable` must not leak.
    #[test]
    fn validate_defaults_hash_stability_to_stable() {
        for grain in ["span", "file", "tree"] {
            let mut i = valid_input();
            i.grain = Some(grain.into());
            i.hash_stability = None;
            let a = i.validate(None).unwrap();
            assert_eq!(a.hash_stability, AnchorHashStability::Stable, "{grain}");
        }
        let mut e = valid_input();
        e.grain = Some("entity".into());
        e.artifact = Some("m--e".into());
        e.hash_stability = None;
        assert_eq!(
            e.validate(None).unwrap().hash_stability,
            AnchorHashStability::Stable
        );
    }

    /// A `url` anchor defaults to `unstable` (a served page is a moving
    /// target — a hash break resolves `recheck`, never `drifted`) unless the
    /// author asserts `stable`.
    #[test]
    fn validate_defaults_url_grain_to_unstable_unless_declared() {
        let mut i = valid_input();
        i.grain = Some("url".into());
        i.artifact = Some("https://example.invalid/doc".into());
        i.hash_stability = None;
        assert_eq!(
            i.validate(None).unwrap().hash_stability,
            AnchorHashStability::Unstable
        );
        i.hash_stability = Some("stable".into());
        assert_eq!(
            i.validate(None).unwrap().hash_stability,
            AnchorHashStability::Stable
        );
    }

    /// Supplied `content` becomes the registry's prepared hash: for a `url`
    /// anchor the same canonicalization the path grains use over what the
    /// observer read; for `file`/`span` the hash the engine would compute
    /// from the file itself. `hash` beside it is refused, as is content on
    /// a grain the registry never prepares from bytes, or on a non-hash
    /// class.
    #[test]
    fn content_yields_the_prepared_hash_through_the_registry() {
        let mut u = valid_input();
        u.grain = Some("url".into());
        u.artifact = Some("https://example.invalid/doc".into());
        u.hash = None;
        u.hash_stability = None;
        u.content = Some("<p>hello</p>\r\n".into());
        let a = u.validate(None).unwrap();
        assert_eq!(
            a.hash.as_deref(),
            Some(crate::preparation::url_prepared_hash(b"<p>hello</p>\n").as_str())
        );
        assert_eq!(a.hash_stability, AnchorHashStability::Unstable);

        let mut f = valid_input();
        f.hash = None;
        f.content = Some("fn a() {}\n".into());
        assert_eq!(
            f.validate(None).unwrap().hash.as_deref(),
            Some(prepared_content_hash(b"fn a() {}").as_str())
        );

        let mut both = valid_input();
        both.content = Some("x".into());
        assert_eq!(
            both.validate(None).unwrap_err(),
            AnchorValidationError::ContentAndHash
        );

        let mut ent = valid_input();
        ent.grain = Some("entity".into());
        ent.artifact = Some("m--e".into());
        ent.hash = None;
        ent.content = Some("x".into());
        let err = ent.validate(None).unwrap_err();
        assert_eq!(
            err,
            AnchorValidationError::ContentNotAcceptedForGrain { grain: "entity" }
        );
        assert_eq!(err.detail()["field"], "content");

        let mut tree = valid_input();
        tree.grain = Some("tree".into());
        tree.hash = None;
        tree.content = Some("x".into());
        assert!(matches!(
            tree.validate(None).unwrap_err(),
            AnchorValidationError::ContentNotAcceptedForGrain { grain: "tree" }
        ));

        let mut informed = valid_input();
        informed.class = Some("informed-by".into());
        informed.hash = None;
        informed.content = Some("x".into());
        assert!(matches!(
            informed.validate(None).unwrap_err(),
            AnchorValidationError::HashOnNonHashClass { .. }
        ));
    }

    #[test]
    fn validate_refuses_unknown_class() {
        let mut i = valid_input();
        i.class = Some("guessed".into());
        let err = i.validate(None).unwrap_err();
        assert_eq!(err.code(), INVALID_ANCHOR_CODE);
        assert!(matches!(err, AnchorValidationError::UnknownClass { .. }));
        assert_eq!(err.detail()["field"], serde_json::json!("class"));
    }

    #[test]
    fn validate_refuses_unknown_grain() {
        let mut i = valid_input();
        i.grain = Some("paragraph".into());
        let err = i.validate(None).unwrap_err();
        assert!(matches!(err, AnchorValidationError::UnknownGrain { .. }));
    }

    #[test]
    fn validate_refuses_missing_artifact() {
        let mut i = valid_input();
        i.artifact = Some("   ".into());
        let err = i.validate(None).unwrap_err();
        assert!(matches!(err, AnchorValidationError::MissingArtifact));
        i.artifact = None;
        assert!(matches!(
            valid_input_with_artifact(None).validate(None).unwrap_err(),
            AnchorValidationError::MissingArtifact
        ));
        let _ = i;
    }

    fn valid_input_with_artifact(a: Option<String>) -> AnchorInput {
        AnchorInput {
            artifact: a,
            ..valid_input()
        }
    }

    #[test]
    fn validate_refuses_hash_on_non_hash_class() {
        let mut i = valid_input();
        i.class = Some("authored".into());
        // hash still supplied → refuse
        let err = i.validate(None).unwrap_err();
        assert!(matches!(
            err,
            AnchorValidationError::HashOnNonHashClass { class: "authored" }
        ));
    }

    #[test]
    fn validate_accepts_non_hash_class_without_hash() {
        let mut i = valid_input();
        i.class = Some("informed-by".into());
        i.hash = None;
        let a = i.validate(None).unwrap();
        assert_eq!(a.class, AnchorProvenanceClass::InformedBy);
        assert!(a.hash.is_none());
    }

    #[test]
    fn validate_refuses_grain_unsupported_by_medium_namespace() {
        // span grain on a web (url namespace) medium.
        let mut i = valid_input();
        i.grain = Some("span".into());
        i.class = Some("authored".into());
        i.hash = None;
        let err = i.validate(Some(("web", "url"))).unwrap_err();
        match err {
            AnchorValidationError::GrainNamespaceUnsupported {
                grain,
                anchor_namespace,
                ..
            } => {
                assert_eq!(grain, "span");
                assert_eq!(anchor_namespace, "url");
            }
            other => panic!("expected GrainNamespaceUnsupported, got {other:?}"),
        }
    }

    #[test]
    fn validate_skips_namespace_check_without_medium_context() {
        // span grain, no medium → namespace rule not applied.
        let mut i = valid_input();
        i.grain = Some("span".into());
        assert!(i.validate(None).is_ok());
    }

    // -- prepared-content hash ----------------------------------------------

    /// The prepared form is stable across meaningless byte noise: BOM,
    /// line-ending convention, and final-newline presence never move the
    /// hash — a real content change always does.
    #[test]
    fn prepared_hash_is_stable_across_byte_noise() {
        let base = prepared_content_hash(b"fn a() {}\nfn b() {}\n");
        // CRLF and lone-CR line endings normalize away.
        assert_eq!(prepared_content_hash(b"fn a() {}\r\nfn b() {}\r\n"), base);
        assert_eq!(prepared_content_hash(b"fn a() {}\rfn b() {}\r"), base);
        // Final-newline presence (missing, single, several) is noise.
        assert_eq!(prepared_content_hash(b"fn a() {}\nfn b() {}"), base);
        assert_eq!(prepared_content_hash(b"fn a() {}\nfn b() {}\n\n\n"), base);
        // A leading UTF-8 BOM is stripped.
        assert_eq!(
            prepared_content_hash("\u{feff}fn a() {}\nfn b() {}\n".as_bytes()),
            base
        );
        // A real content change moves the hash.
        assert_ne!(prepared_content_hash(b"fn a() {}\nfn c() {}\n"), base);
        // House hash shape: 16 lowercase hex chars.
        assert_eq!(base.len(), 16);
        assert!(
            base.chars()
                .all(|c| c.is_ascii_hexdigit() && !c.is_ascii_uppercase())
        );
    }

    /// Interior whitespace is content, not noise: a trailing space inside a
    /// line (markdown hard break) changes the hash.
    #[test]
    fn prepared_hash_preserves_interior_whitespace() {
        assert_ne!(
            prepared_content_hash(b"line one  \nline two\n"),
            prepared_content_hash(b"line one\nline two\n")
        );
    }

    /// Non-UTF-8 bytes hash raw — no text canonicalization is applied, and
    /// any byte change moves the hash.
    #[test]
    fn prepared_hash_hashes_binary_bytes_raw() {
        let bin_a = [0xff_u8, 0xfe, 0x00, 0x0d, 0x0a];
        let bin_b = [0xff_u8, 0xfe, 0x00, 0x0a];
        assert_ne!(prepared_content_hash(&bin_a), prepared_content_hash(&bin_b));
        // Deterministic.
        assert_eq!(prepared_content_hash(&bin_a), prepared_content_hash(&bin_a));
    }

    // -- resolution --------------------------------------------------------

    fn anchor(
        class: AnchorProvenanceClass,
        hash: Option<&str>,
        stab: AnchorHashStability,
    ) -> Anchor {
        Anchor {
            artifact: "src/lib.rs".into(),
            grain: AnchorGrain::File,
            class,
            at_version: None,
            hash: hash.map(str::to_string),
            hash_stability: stab,
            derived_from: Vec::new(),
            binding: None,
            source: None,
            span_unvalidated: false,
            hash_source: None,
            last_observed: None,
        }
    }

    #[test]
    fn resolves_when_hash_matches() {
        let a = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        let obs = ArtifactObservation::Present {
            current_hash: Some("h1".into()),
        };
        assert_eq!(resolve_anchor(&a, &obs), AnchorState::Resolves);
    }

    #[test]
    fn stable_hash_break_drifts_unstable_rechecks() {
        let stable = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        let unstable = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Unstable,
        );
        let obs = ArtifactObservation::Present {
            current_hash: Some("h2".into()),
        };
        assert_eq!(resolve_anchor(&stable, &obs), AnchorState::Drifted);
        assert_eq!(resolve_anchor(&unstable, &obs), AnchorState::Recheck);
    }

    #[test]
    fn absent_artifact_is_orphaned() {
        let a = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        assert_eq!(
            resolve_anchor(&a, &ArtifactObservation::Absent),
            AnchorState::Orphaned
        );
    }

    #[test]
    fn non_hash_classes_never_drift() {
        for class in [
            AnchorProvenanceClass::Authored,
            AnchorProvenanceClass::InformedBy,
        ] {
            let a = anchor(class, None, AnchorHashStability::Stable);
            // Content moved underneath — still resolves (excluded from
            // hash-drift adjudication).
            let obs = ArtifactObservation::Present {
                current_hash: Some("whatever".into()),
            };
            assert_eq!(resolve_anchor(&a, &obs), AnchorState::Resolves);
            // But an absent artifact is still orphaned.
            assert_eq!(
                resolve_anchor(&a, &ArtifactObservation::Absent),
                AnchorState::Orphaned
            );
        }
    }

    #[test]
    fn unavailable_hash_rechecks_not_drifts() {
        let a = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        let obs = ArtifactObservation::Present { current_hash: None };
        assert_eq!(resolve_anchor(&a, &obs), AnchorState::Recheck);
    }

    // -- composition -------------------------------------------------------

    #[test]
    fn composition_counts_classes_grains_and_tree_fanout() {
        let anchors = vec![
            Anchor {
                artifact: "a.rs".into(),
                grain: AnchorGrain::File,
                class: AnchorProvenanceClass::Anchored,
                at_version: None,
                hash: Some("h".into()),
                hash_stability: AnchorHashStability::Stable,
                derived_from: Vec::new(),
                binding: None,
                source: None,
                span_unvalidated: false,
                hash_source: None,
                last_observed: None,
            },
            Anchor {
                artifact: "src/".into(),
                grain: AnchorGrain::Tree,
                class: AnchorProvenanceClass::Derived,
                at_version: None,
                hash: Some("t".into()),
                hash_stability: AnchorHashStability::Stable,
                derived_from: vec!["a.rs".into(), "b.rs".into()],
                binding: None,
                source: None,
                span_unvalidated: false,
                hash_source: None,
                last_observed: None,
            },
        ];
        let comp = compose_entity_anchors(&anchors);
        assert_eq!(comp.by_class["anchored"], 1);
        assert_eq!(comp.by_class["derived"], 1);
        assert_eq!(comp.by_grain["file"], 1);
        assert_eq!(comp.by_grain["tree"], 1);
        // Tree fan-out is a distinct axis — one row, never per-file credit.
        assert_eq!(comp.tree_grain_artifacts, vec!["src/".to_string()]);
        assert_eq!(
            comp.derived_inputs,
            vec![vec!["a.rs".to_string(), "b.rs".to_string()]]
        );
    }

    // -- sidecar round-trip -------------------------------------------------

    #[test]
    fn sidecar_round_trips_and_prunes_empty() {
        let mut sc = AnchorSidecar::default();
        assert!(sc.is_empty());
        let a = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        sc.set("specs--x", vec![a.clone()]);
        assert_eq!(sc.get("specs--x").len(), 1);

        let bytes = sc.to_bytes();
        let round = AnchorSidecar::from_bytes(&bytes).unwrap();
        assert_eq!(round, sc);

        // Setting empty prunes the key.
        sc.set("specs--x", vec![]);
        assert!(sc.is_empty());
        assert!(sc.get("specs--x").is_empty());
    }

    // -- merge / unset arithmetic ------------------------------------------

    fn file_anchor(artifact: &str, hash: &str) -> Anchor {
        Anchor {
            artifact: artifact.into(),
            grain: AnchorGrain::File,
            class: AnchorProvenanceClass::Anchored,
            at_version: None,
            hash: Some(hash.into()),
            hash_stability: AnchorHashStability::Stable,
            derived_from: Vec::new(),
            binding: None,
            source: None,
            span_unvalidated: false,
            hash_source: None,
            last_observed: None,
        }
    }

    /// Merge appends a new triple and leaves the existing set untouched —
    /// the incremental-anchoring contract (N existing + 1 new ⇒ N+1).
    #[test]
    fn merge_appends_new_triple_without_touching_others() {
        let mut sc = AnchorSidecar::default();
        sc.set(
            "m--e",
            vec![file_anchor("a.rs", "h-a"), file_anchor("b.rs", "h-b")],
        );
        sc.merge("m--e", &[], vec![file_anchor("c.rs", "h-c")]);
        let row = sc.get("m--e");
        assert_eq!(row.len(), 3);
        assert_eq!(row[0], file_anchor("a.rs", "h-a"));
        assert_eq!(row[1], file_anchor("b.rs", "h-b"));
        assert_eq!(row[2], file_anchor("c.rs", "h-c"));
    }

    /// An incoming anchor with an existing `(artifact, grain, class)`
    /// triple replaces exactly that one, in place; others stay
    /// byte-identical.
    #[test]
    fn merge_replaces_same_triple_in_place() {
        let mut sc = AnchorSidecar::default();
        sc.set(
            "m--e",
            vec![file_anchor("a.rs", "h-old"), file_anchor("b.rs", "h-b")],
        );
        sc.merge("m--e", &[], vec![file_anchor("a.rs", "h-new")]);
        let row = sc.get("m--e");
        assert_eq!(row.len(), 2);
        assert_eq!(row[0], file_anchor("a.rs", "h-new"));
        assert_eq!(row[1], file_anchor("b.rs", "h-b"));
    }

    /// Same artifact under a different grain or class is a different
    /// identity — it appends rather than replaces (the triple is the merge
    /// key, not the artifact alone).
    #[test]
    fn merge_treats_grain_and_class_as_identity() {
        let mut sc = AnchorSidecar::default();
        sc.set("m--e", vec![file_anchor("a.rs", "h-a")]);
        let mut span = file_anchor("a.rs", "h-span");
        span.grain = AnchorGrain::Span;
        let mut informed = file_anchor("a.rs", "h-a");
        informed.class = AnchorProvenanceClass::InformedBy;
        informed.hash = None;
        sc.merge("m--e", &[], vec![span, informed]);
        assert_eq!(sc.get("m--e").len(), 3);
    }

    /// Re-sending an entity's full current set is a no-op on the stored
    /// bytes, and merging an empty list changes nothing.
    #[test]
    fn merge_full_resend_and_empty_are_noops() {
        let mut sc = AnchorSidecar::default();
        sc.set(
            "m--e",
            vec![file_anchor("a.rs", "h-a"), file_anchor("b.rs", "h-b")],
        );
        let before = sc.to_bytes();
        sc.merge(
            "m--e",
            &[],
            vec![file_anchor("a.rs", "h-a"), file_anchor("b.rs", "h-b")],
        );
        assert_eq!(sc.to_bytes(), before, "full re-send is byte-stable");
        sc.merge("m--e", &[], Vec::new());
        assert_eq!(sc.to_bytes(), before, "empty merge is a no-op");
    }

    /// A bare-artifact unset removes all of that artifact's anchors and
    /// nothing else; a grain/class-narrowed unset removes only the match;
    /// a selector matching nothing is a no-op.
    #[test]
    fn unset_selects_by_artifact_with_optional_narrowing() {
        let mut span = file_anchor("a.rs", "h-span");
        span.grain = AnchorGrain::Span;
        let mut sc = AnchorSidecar::default();
        sc.set(
            "m--e",
            vec![
                file_anchor("a.rs", "h-a"),
                span.clone(),
                file_anchor("b.rs", "h-b"),
            ],
        );

        // Narrowed: only the span-grain anchor on a.rs goes.
        let narrowed = AnchorUnset {
            artifact: "a.rs".into(),
            grain: Some(AnchorGrain::Span),
            class: None,
        };
        sc.merge("m--e", &[narrowed], Vec::new());
        assert_eq!(
            sc.get("m--e"),
            &[file_anchor("a.rs", "h-a"), file_anchor("b.rs", "h-b")]
        );

        // Nonexistent target: idempotent no-op.
        let missing = AnchorUnset {
            artifact: "never-there.rs".into(),
            grain: None,
            class: None,
        };
        sc.merge("m--e", &[missing], Vec::new());
        assert_eq!(sc.get("m--e").len(), 2);

        // Bare artifact: everything on a.rs goes, b.rs untouched.
        let bare = AnchorUnset {
            artifact: "a.rs".into(),
            grain: None,
            class: None,
        };
        sc.merge("m--e", &[bare], Vec::new());
        assert_eq!(sc.get("m--e"), &[file_anchor("b.rs", "h-b")]);
    }

    /// Unset applies before merge in the same call: unsetting an artifact
    /// and writing a new anchor on it lands the new anchor (full-replace
    /// stays expressible in one call).
    #[test]
    fn unset_applies_before_merge() {
        let mut span = file_anchor("a.rs", "h-span");
        span.grain = AnchorGrain::Span;
        let mut sc = AnchorSidecar::default();
        sc.set("m--e", vec![file_anchor("a.rs", "h-old"), span]);
        let bare = AnchorUnset {
            artifact: "a.rs".into(),
            grain: None,
            class: None,
        };
        sc.merge("m--e", &[bare], vec![file_anchor("a.rs", "h-new")]);
        assert_eq!(sc.get("m--e"), &[file_anchor("a.rs", "h-new")]);
    }

    /// A row emptied by unsets prunes its key — the sidecar never keeps
    /// empty rows.
    #[test]
    fn merge_prunes_row_emptied_by_unset() {
        let mut sc = AnchorSidecar::default();
        sc.set("m--e", vec![file_anchor("a.rs", "h-a")]);
        let bare = AnchorUnset {
            artifact: "a.rs".into(),
            grain: None,
            class: None,
        };
        sc.merge("m--e", &[bare], Vec::new());
        assert!(sc.is_empty());
        assert!(!sc.to_bytes().windows(5).any(|w| w == b"m--e\""));
    }

    /// The unset validator: artifact required; grain/class, when supplied,
    /// must be known wire strings; absent narrowing means "any".
    #[test]
    fn unset_input_validates_typed() {
        let ok = AnchorUnsetInput {
            artifact: Some("  a.rs  ".into()),
            grain: Some("span".into()),
            class: None,
        }
        .validate()
        .unwrap();
        assert_eq!(ok.artifact, "a.rs");
        assert_eq!(ok.grain, Some(AnchorGrain::Span));
        assert_eq!(ok.class, None);

        let missing = AnchorUnsetInput::default().validate().unwrap_err();
        assert!(matches!(missing, AnchorValidationError::MissingArtifact));
        assert_eq!(missing.code(), INVALID_ANCHOR_CODE);

        let bad_grain = AnchorUnsetInput {
            artifact: Some("a.rs".into()),
            grain: Some("paragraph".into()),
            class: None,
        }
        .validate()
        .unwrap_err();
        assert!(matches!(
            bad_grain,
            AnchorValidationError::UnknownGrain { .. }
        ));

        let bad_class = AnchorUnsetInput {
            artifact: Some("a.rs".into()),
            grain: None,
            class: Some("guessed".into()),
        }
        .validate()
        .unwrap_err();
        assert!(matches!(
            bad_class,
            AnchorValidationError::UnknownClass { .. }
        ));
    }

    #[test]
    fn sidecar_rename_leaves_zero_rows_under_old_id() {
        let mut sc = AnchorSidecar::default();
        sc.set(
            "specs--old",
            vec![anchor(
                AnchorProvenanceClass::Anchored,
                Some("h"),
                AnchorHashStability::Stable,
            )],
        );
        sc.rename("specs--old", "specs--new");
        assert!(sc.get("specs--old").is_empty());
        assert_eq!(sc.get("specs--new").len(), 1);
    }

    #[test]
    fn sidecar_remove_drops_entity_anchors() {
        let mut sc = AnchorSidecar::default();
        sc.set(
            "specs--gone",
            vec![anchor(
                AnchorProvenanceClass::Anchored,
                Some("h"),
                AnchorHashStability::Stable,
            )],
        );
        sc.remove("specs--gone");
        assert!(sc.get("specs--gone").is_empty());
        // Idempotent.
        sc.remove("specs--gone");
    }

    #[test]
    fn empty_bytes_parse_as_empty_sidecar() {
        assert!(AnchorSidecar::from_bytes(b"").unwrap().is_empty());
        assert!(AnchorSidecar::from_bytes(b"  \n ").unwrap().is_empty());
    }

    #[test]
    fn anchor_json_shape_omits_empty_optionals() {
        let a = anchor(
            AnchorProvenanceClass::Anchored,
            Some("h1"),
            AnchorHashStability::Stable,
        );
        let v = serde_json::to_value(&a).unwrap();
        assert_eq!(v["artifact"], "src/lib.rs");
        assert_eq!(v["grain"], "file");
        assert_eq!(v["class"], "anchored");
        assert_eq!(v["hash"], "h1");
        assert_eq!(v["hash_stability"], "stable");
        // Absent optionals are skipped, not null.
        assert!(v.get("at_version").is_none());
        assert!(v.get("derived_from").is_none());
        assert!(v.get("binding").is_none());
    }

    #[test]
    fn anchor_version_serialises_tagged() {
        let a = Anchor {
            at_version: Some(AnchorVersion::Commit("deadbeef".into())),
            ..anchor(
                AnchorProvenanceClass::Anchored,
                Some("h"),
                AnchorHashStability::Stable,
            )
        };
        let v = serde_json::to_value(&a).unwrap();
        assert_eq!(v["at_version"]["kind"], "commit");
        assert_eq!(v["at_version"]["value"], "deadbeef");
    }

    /// `source` rides validation: a non-empty name is carried, absent
    /// stays absent, and present-but-empty refuses `INVALID_ANCHOR`
    /// with `field: source` in the recovery detail.
    #[test]
    fn validate_source_carried_absent_or_refused_when_empty() {
        let mut input = AnchorInput {
            artifact: Some("src/lib.rs".into()),
            grain: Some("file".into()),
            class: Some("anchored".into()),
            ..Default::default()
        };
        assert_eq!(
            input.validate(None).unwrap().source,
            None,
            "absent stays absent"
        );

        input.source = Some("  api-docs  ".into());
        assert_eq!(
            input.validate(None).unwrap().source.as_deref(),
            Some("api-docs"),
            "non-empty name is carried (trimmed)"
        );

        input.source = Some("   ".into());
        let err = input.validate(None).unwrap_err();
        assert_eq!(err.code(), INVALID_ANCHOR_CODE);
        assert!(matches!(err, AnchorValidationError::EmptySource));
        assert_eq!(
            err.detail().get("field"),
            Some(&serde_json::json!("source"))
        );
    }

    /// A sidecar written before the `source` field existed loads
    /// unchanged (additive, optional — no migration, no version bump),
    /// and a sourced anchor round-trips through serde.
    #[test]
    fn source_is_additive_on_the_persisted_shape() {
        let pre_plan = r#"{
            "artifact": "src/lib.rs",
            "grain": "file",
            "class": "anchored",
            "hash_stability": "stable"
        }"#;
        let a: Anchor = serde_json::from_str(pre_plan).expect("pre-plan anchor loads");
        assert_eq!(a.source, None, "no backfill, no default");

        let sourced = Anchor {
            source: Some("api-docs".into()),
            ..a
        };
        let json = serde_json::to_string(&sourced).unwrap();
        let back: Anchor = serde_json::from_str(&json).unwrap();
        assert_eq!(back.source.as_deref(), Some("api-docs"));
    }

    // --- sidecar version 2, supplied observations, the url namespace rule ---

    #[test]
    fn sidecar_v1_loads_and_upgrades_in_memory_v3_refuses() {
        let v1 = br#"{"version":1,"entities":{"m--e":[{"artifact":"https://x.test/a","grain":"url","class":"informed-by","hash_stability":"unstable"}]}}"#;
        let sc = AnchorSidecar::from_bytes(v1).expect("version 1 loads");
        assert_eq!(sc.version, ANCHOR_SIDECAR_VERSION, "upgraded in memory");
        assert_eq!(sc.get("m--e").len(), 1);
        assert!(sc.get("m--e")[0].last_observed.is_none(), "rows unchanged");
        let rewritten = String::from_utf8(sc.to_bytes()).unwrap();
        assert!(rewritten.contains("\"version\": 2"), "{rewritten}");

        let v3 = br#"{"version":3,"entities":{}}"#;
        let err = AnchorSidecar::from_bytes(v3).expect_err("unknown higher version refuses");
        assert!(
            err.to_string()
                .contains("unsupported anchors sidecar version 3"),
            "{err}"
        );
    }

    #[test]
    fn last_observed_round_trips_and_is_absent_when_none() {
        let mut a = valid_input().validate(None).unwrap();
        let json = serde_json::to_value(&a).unwrap();
        assert!(json.get("last_observed").is_none());
        a.last_observed = Some(AnchorObservation {
            at: "2026-09-01T10:00:00Z".into(),
            hash: Some("abc".into()),
            state: AnchorState::Resolves,
        });
        let json = serde_json::to_value(&a).unwrap();
        assert_eq!(json["last_observed"]["state"], "resolves");
        let back: Anchor = serde_json::from_value(json).unwrap();
        assert_eq!(back, a);
    }

    #[test]
    fn url_grain_is_admitted_beside_a_path_medium_and_path_grains_refuse_a_url_artifact() {
        let mut i = valid_input();
        i.grain = Some("url".into());
        i.artifact = Some("https://example.org/doc.pdf".into());
        i.class = Some("anchored".into());
        i.hash = None;
        i.content = Some("the document text".into());
        i.hash_stability = None;
        let a = i
            .validate(Some(("filesystem", "path")))
            .expect("url beside a path medium is legal");
        assert_eq!(a.grain, AnchorGrain::Url);
        assert_eq!(a.hash_source, Some(AnchorHashSource::Author));
        assert_eq!(
            a.hash_stability,
            AnchorHashStability::Unstable,
            "url default"
        );

        for grain in ["span", "file", "tree"] {
            let mut i = valid_input();
            i.grain = Some(grain.into());
            i.artifact = Some("https://example.org/doc.pdf#L1-L3".into());
            i.class = Some("informed-by".into());
            i.hash = None;
            let err = i.validate(Some(("filesystem", "path"))).unwrap_err();
            assert!(
                matches!(&err, AnchorValidationError::PathGrainOnUrlArtifact { grain: g, .. } if *g == grain),
                "{grain}: {err:?}"
            );
            assert_eq!(err.code(), INVALID_ANCHOR_CODE);
            assert!(err.to_string().contains("never enters a path namespace"));
        }
        assert!(looks_like_url("https://a.b/c"));
        assert!(looks_like_url("file://x"));
        assert!(!looks_like_url("src/main.rs"));
        assert!(!looks_like_url("://nope"));
        assert!(!looks_like_url("http://"));
    }

    #[test]
    fn supplied_observations_validate_all_or_nothing() {
        let now = "2026-09-02T12:00:00Z";
        let rows = vec![
            SuppliedObservationInput {
                artifact: Some("https://a.test/1".into()),
                hash: Some("h1".into()),
                ..Default::default()
            },
            SuppliedObservationInput {
                artifact: Some("https://a.test/2".into()),
                content: Some("body\r\n".into()),
                observed_at: Some("2026-08-01".into()),
                ..Default::default()
            },
            SuppliedObservationInput {
                artifact: Some("https://a.test/3".into()),
                absent: Some(true),
                ..Default::default()
            },
        ];
        let ok = validate_supplied_observations(&rows, now).unwrap();
        assert_eq!(ok.len(), 3);
        assert_eq!(ok["https://a.test/1"].at, now);
        assert_eq!(
            ok["https://a.test/2"].outcome,
            SuppliedOutcome::Present {
                hash: prepared_content_hash(b"body\r\n")
            },
            "content hashes under the write path's canonicalization"
        );
        assert_eq!(ok["https://a.test/2"].at, "2026-08-01");
        assert_eq!(ok["https://a.test/3"].outcome, SuppliedOutcome::Absent);

        // hash + content on one row: ambiguous, refused by row number.
        let bad = vec![SuppliedObservationInput {
            artifact: Some("https://a.test/1".into()),
            hash: Some("h".into()),
            content: Some("c".into()),
            ..Default::default()
        }];
        let err = validate_supplied_observations(&bad, now).unwrap_err();
        assert!(matches!(
            err,
            ObservationValidationError::OutcomeAmbiguous { row: 1, .. }
        ));
        assert_eq!(err.code(), INVALID_OBSERVATION_CODE);
        // nothing at all
        let bad = vec![SuppliedObservationInput {
            artifact: Some("https://a.test/1".into()),
            ..Default::default()
        }];
        assert!(matches!(
            validate_supplied_observations(&bad, now).unwrap_err(),
            ObservationValidationError::OutcomeAmbiguous { .. }
        ));
        let bad = vec![SuppliedObservationInput {
            artifact: Some("https://a.test/1".into()),
            hash: Some("h".into()),
            observed_at: Some("yesterday".into()),
            ..Default::default()
        }];
        assert!(matches!(
            validate_supplied_observations(&bad, now).unwrap_err(),
            ObservationValidationError::BadTimestamp { .. }
        ));
        let dup = vec![rows[0].clone(), rows[0].clone()];
        assert!(matches!(
            validate_supplied_observations(&dup, now).unwrap_err(),
            ObservationValidationError::DuplicateArtifact {
                first: 1,
                second: 2,
                ..
            }
        ));
        assert!(matches!(
            validate_supplied_observations(&[SuppliedObservationInput::default()], now)
                .unwrap_err(),
            ObservationValidationError::MissingArtifact { row: 1 }
        ));
    }

    #[test]
    fn days_between_ages_by_civil_date() {
        assert_eq!(days_between("2026-08-01", "2026-09-02T00:00:00Z"), Some(32));
        assert_eq!(
            days_between("2026-09-02T23:59:59Z", "2026-09-02T00:00:00Z"),
            Some(0)
        );
        assert_eq!(days_between("2026-09-03", "2026-09-02"), Some(0), "floored");
        assert_eq!(days_between("garbage", "2026-09-02"), None);
        assert_eq!(iso_days_since_epoch("1970-01-01"), Some(0));
        assert_eq!(iso_days_since_epoch("2000-03-01"), Some(11017));
    }
}