icydb_schema/

fragment.rs

//! Store-free reusable schema fragments.

use std::collections::BTreeSet;

use serde::{Deserialize, Serialize};

use crate::{
    ConstraintSourceKey, Decimal, DeclaredEntityVersion, EntitySourceKey, FieldSourceKey,
    IndexSourceKey, MAX_FRAGMENT_CONSTRAINTS, MAX_FRAGMENT_ENTITIES, MAX_FRAGMENT_FIELDS,
    MAX_FRAGMENT_INDEXES, MAX_FRAGMENT_RELATIONS, MAX_FRAGMENT_TYPES, MAX_RELATION_PATH_STEPS,
    MAX_SCHEMA_FIELD_TYPE_DEPTH, RelationSourceKey, RuleSourceKey, ScalarKind, ScalarLiteral,
    SchemaContractError, SchemaName, SourceCheckExpr, SourceRuleOperation, TypeSourceKey,
};

/// Logical type reference in a proposal fragment.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum FieldType {
    /// Exact built-in scalar contract.
    Scalar(ScalarType),
    /// Ordered repeated values with one exact element contract.
    List(Box<Self>),
    /// Named record, enum, newtype, or collection definition.
    Named(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        TypeSourceKey,
    ),
}

/// Exact scalar field contract required by accepted-schema lowering.
#[derive(Clone, Copy, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum ScalarType {
    /// ICRC account.
    Account,
    /// Binary value with an optional maximum byte length.
    Blob {
        /// Maximum bytes, or no field-specific maximum.
        max_len: Option<u32>,
    },
    /// Boolean.
    Bool,
    /// Day-precision date.
    Date,
    /// Fixed-point decimal with exact accepted scale.
    Decimal {
        /// Accepted decimal scale.
        scale: u32,
    },
    /// Millisecond duration.
    Duration,
    /// Finite 32-bit float.
    Float32,
    /// Finite 64-bit float.
    Float64,
    /// Signed 8-bit integer.
    Int8,
    /// Signed 16-bit integer.
    Int16,
    /// Signed 32-bit integer.
    Int32,
    /// Signed 64-bit integer.
    Int64,
    /// Signed 128-bit integer.
    Int128,
    /// Principal.
    Principal,
    /// Fixed-width subaccount.
    Subaccount,
    /// Text with an optional maximum Unicode-scalar length.
    Text {
        /// Maximum Unicode scalar count, or no field-specific maximum.
        max_len: Option<u32>,
    },
    /// Millisecond timestamp.
    Timestamp,
    /// Unsigned 8-bit integer.
    Nat8,
    /// Unsigned 16-bit integer.
    Nat16,
    /// Unsigned 32-bit integer.
    Nat32,
    /// Unsigned 64-bit integer.
    Nat64,
    /// Unsigned 128-bit integer.
    Nat128,
    /// Fixed-width unsigned 256-bit integer.
    U256,
    /// ULID.
    Ulid,
    /// Unit.
    Unit,
}

impl ScalarType {
    /// Return the intrinsic scalar capability kind.
    #[must_use]
    pub const fn kind(self) -> ScalarKind {
        match self {
            Self::Account => ScalarKind::Account,
            Self::Blob { .. } => ScalarKind::Blob,
            Self::Bool => ScalarKind::Bool,
            Self::Date => ScalarKind::Date,
            Self::Decimal { .. } => ScalarKind::Decimal,
            Self::Duration => ScalarKind::Duration,
            Self::Float32 => ScalarKind::Float32,
            Self::Float64 => ScalarKind::Float64,
            Self::Int8 | Self::Int16 | Self::Int32 | Self::Int64 => ScalarKind::Int,
            Self::Int128 => ScalarKind::Int128,
            Self::Principal => ScalarKind::Principal,
            Self::Subaccount => ScalarKind::Subaccount,
            Self::Text { .. } => ScalarKind::Text,
            Self::Timestamp => ScalarKind::Timestamp,
            Self::Nat8 | Self::Nat16 | Self::Nat32 | Self::Nat64 => ScalarKind::Nat,
            Self::Nat128 => ScalarKind::Nat128,
            Self::U256 => ScalarKind::U256,
            Self::Ulid => ScalarKind::Ulid,
            Self::Unit => ScalarKind::Unit,
        }
    }

    pub(crate) const fn validate(self) -> Result<(), SchemaContractError> {
        match self {
            Self::Decimal { scale } if scale > Decimal::max_supported_scale() => {
                Err(SchemaContractError::InvalidFieldType)
            }
            _ => Ok(()),
        }
    }

    pub(crate) fn accepts_literal(self, literal: &ScalarLiteral) -> bool {
        match (self, literal) {
            (Self::Account, ScalarLiteral::Account(_))
            | (Self::Bool, ScalarLiteral::Bool(_))
            | (Self::Date, ScalarLiteral::Date(_))
            | (Self::Duration, ScalarLiteral::Duration(_))
            | (Self::Float32, ScalarLiteral::Float32(_))
            | (Self::Float64, ScalarLiteral::Float64(_))
            | (Self::Int128, ScalarLiteral::Int(_))
            | (Self::Principal, ScalarLiteral::Principal(_))
            | (Self::Subaccount, ScalarLiteral::Subaccount(_))
            | (Self::Timestamp, ScalarLiteral::Timestamp(_))
            | (Self::Nat128, ScalarLiteral::Nat(_))
            | (Self::U256, ScalarLiteral::U256(_))
            | (Self::Ulid, ScalarLiteral::Ulid(_))
            | (Self::Unit, ScalarLiteral::Unit(_)) => true,
            (Self::Blob { max_len }, ScalarLiteral::Blob(value)) => {
                max_len.is_none_or(|max| value.len() <= max as usize)
            }
            (Self::Text { max_len }, ScalarLiteral::Text(value)) => {
                max_len.is_none_or(|max| value.chars().count() <= max as usize)
            }
            (Self::Int8, ScalarLiteral::Int(value)) => i8::try_from(*value).is_ok(),
            (Self::Int16, ScalarLiteral::Int(value)) => i16::try_from(*value).is_ok(),
            (Self::Int32, ScalarLiteral::Int(value)) => i32::try_from(*value).is_ok(),
            (Self::Int64, ScalarLiteral::Int(value)) => i64::try_from(*value).is_ok(),
            (Self::Nat8, ScalarLiteral::Nat(value)) => u8::try_from(*value).is_ok(),
            (Self::Nat16, ScalarLiteral::Nat(value)) => u16::try_from(*value).is_ok(),
            (Self::Nat32, ScalarLiteral::Nat(value)) => u32::try_from(*value).is_ok(),
            (Self::Nat64, ScalarLiteral::Nat(value)) => u64::try_from(*value).is_ok(),
            (Self::Decimal { scale }, ScalarLiteral::Decimal(value)) => {
                decimal_fits_scale(*value, scale)
            }
            _ => false,
        }
    }
}

impl FieldType {
    /// Append named-type references through inline list layers.
    ///
    /// This preserves the caller's existing entries and does not resolve the
    /// referenced definition or compute its transitive dependency closure.
    pub fn append_named_type_dependencies(&self, pending: &mut Vec<TypeSourceKey>) {
        let mut field_type = self;
        while let Self::List(item) = field_type {
            field_type = item;
        }
        if let Self::Named(source) = field_type {
            pending.push(source.clone());
        }
    }

    pub(crate) const fn validate(&self) -> Result<(), SchemaContractError> {
        self.validate_at_depth(0)
    }

    const fn validate_at_depth(&self, depth: usize) -> Result<(), SchemaContractError> {
        let Some(depth) = depth.checked_add(1) else {
            return Err(SchemaContractError::FieldTypeDepthExceeded);
        };
        if depth > MAX_SCHEMA_FIELD_TYPE_DEPTH {
            return Err(SchemaContractError::FieldTypeDepthExceeded);
        }
        match self {
            Self::Scalar(scalar) => scalar.validate(),
            Self::List(item) => item.validate_at_depth(depth),
            Self::Named(_) => Ok(()),
        }
    }
}

/// Insert policy authored for one field.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum FieldInsertPolicy {
    /// Omission rejects.
    Required,
    /// Omission resolves to explicit null.
    Nullable,
    /// Omission or explicit database `DEFAULT` resolves to this constant.
    Default(ScalarLiteral),
    /// IcyDB generates the value.
    Generated,
}

/// Accepted database-owned lifecycle policy.
#[derive(Clone, Copy, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum FieldManagementPolicy {
    /// Set once on insert and preserve thereafter.
    CreatedAt,
    /// Set on insert and on each logical row change.
    UpdatedAt,
}

/// One field definition keyed by its current source name.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct FieldFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: FieldSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    field_type: FieldType,
    nullable: bool,
    insert_policy: FieldInsertPolicy,
    management: Option<FieldManagementPolicy>,
}

impl FieldFragment {
    /// Construct one field definition.
    #[must_use]
    pub fn new(
        name: SchemaName,
        field_type: FieldType,
        nullable: bool,
        insert_policy: FieldInsertPolicy,
        management: Option<FieldManagementPolicy>,
    ) -> Self {
        Self {
            source_key: FieldSourceKey::from_name(&name),
            name,
            field_type,
            nullable,
            insert_policy,
            management,
        }
    }

    /// Borrow the typed proposal key derived from the current field name.
    #[must_use]
    pub const fn source_key(&self) -> &FieldSourceKey {
        &self.source_key
    }

    /// Borrow the current field name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow the logical field type.
    #[must_use]
    pub const fn field_type(&self) -> &FieldType {
        &self.field_type
    }

    /// Return whether the field admits authored null.
    #[must_use]
    pub const fn nullable(&self) -> bool {
        self.nullable
    }

    /// Borrow the future-write insert policy.
    #[must_use]
    pub const fn insert_policy(&self) -> &FieldInsertPolicy {
        &self.insert_policy
    }

    /// Return the optional accepted management policy.
    #[must_use]
    pub const fn management(&self) -> Option<FieldManagementPolicy> {
        self.management
    }

    pub(crate) fn validate(&self) -> Result<(), SchemaContractError> {
        ensure_current_name_key(self.source_key.as_str(), &self.name)?;
        self.field_type.validate()?;
        if let FieldInsertPolicy::Default(literal) = &self.insert_policy {
            literal.validate()?;
            match &self.field_type {
                FieldType::Scalar(scalar) if scalar.accepts_literal(literal) => {}
                FieldType::Named(_) if matches!(literal, ScalarLiteral::EnumUnit { .. }) => {}
                FieldType::Scalar(_) | FieldType::List(_) | FieldType::Named(_) => {
                    return Err(SchemaContractError::LiteralTypeMismatch);
                }
            }
        }
        if matches!(self.insert_policy, FieldInsertPolicy::Nullable) && !self.nullable {
            return Err(SchemaContractError::InvalidFieldPolicy);
        }
        if self.management.is_some()
            && (!matches!(self.field_type, FieldType::Scalar(ScalarType::Timestamp))
                || self.nullable
                || !matches!(self.insert_policy, FieldInsertPolicy::Required))
        {
            return Err(SchemaContractError::InvalidFieldPolicy);
        }
        Ok(())
    }
}

/// One ordered index key component.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum IndexKeyFragment {
    /// Direct field key.
    Field(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Lower-cased text expression.
    Lower(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Upper-cased text expression.
    Upper(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Trimmed text expression.
    Trim(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Lower-cased and trimmed text expression.
    LowerTrim(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Date extraction expression.
    Date(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Year extraction expression.
    Year(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Month extraction expression.
    Month(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
    /// Day extraction expression.
    Day(
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        FieldSourceKey,
    ),
}

impl IndexKeyFragment {
    /// Borrow the field source key consumed by this component.
    #[must_use]
    pub const fn field(&self) -> &FieldSourceKey {
        match self {
            Self::Field(field)
            | Self::Lower(field)
            | Self::Upper(field)
            | Self::Trim(field)
            | Self::LowerTrim(field)
            | Self::Date(field)
            | Self::Year(field)
            | Self::Month(field)
            | Self::Day(field) => field,
        }
    }
}

/// One secondary-index proposal definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct IndexFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: IndexSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_INDEX_FIELDS }>"
    )]
    key: Vec<IndexKeyFragment>,
    unique: bool,
    predicate: Option<SourceCheckExpr>,
}

impl IndexFragment {
    /// Construct one bounded index definition.
    ///
    /// # Errors
    ///
    /// Returns a typed contract error for an engine-reserved name or an empty key.
    pub fn try_new(
        name: SchemaName,
        key: Vec<IndexKeyFragment>,
        unique: bool,
        predicate: Option<SourceCheckExpr>,
    ) -> Result<Self, SchemaContractError> {
        if crate::is_engine_reserved_schema_name(name.as_str()) {
            return Err(SchemaContractError::InvalidSourceKey);
        }
        if key.is_empty() {
            return Err(SchemaContractError::InvalidReferenceList);
        }
        if let Some(predicate) = &predicate {
            predicate.validate()?;
        }
        Ok(Self {
            source_key: IndexSourceKey::from_name(&name),
            name,
            key,
            unique,
            predicate,
        })
    }

    /// Borrow the typed proposal key derived from the current index name.
    #[must_use]
    pub const fn source_key(&self) -> &IndexSourceKey {
        &self.source_key
    }

    /// Borrow the current index name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow ordered index key components.
    #[must_use]
    pub fn key(&self) -> &[IndexKeyFragment] {
        &self.key
    }

    /// Return whether the index enforces uniqueness.
    #[must_use]
    pub const fn unique(&self) -> bool {
        self.unique
    }

    /// Borrow the optional source predicate.
    #[must_use]
    pub const fn predicate(&self) -> Option<&SourceCheckExpr> {
        self.predicate.as_ref()
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        let rebuilt = Self::try_new(
            self.name.clone(),
            self.key.clone(),
            self.unique,
            self.predicate.clone(),
        )?;
        ensure_canonical_rebuild(self, &rebuilt)
    }
}

/// Maintained referential action in the current proposal contract.
#[derive(Clone, Copy, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum RelationDeleteAction {
    /// Reject deletion while a source row refers to the target.
    Restrict,
}

/// One deterministic step below an entity-root relation field.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum RelationPathStepFragment {
    /// Enter one accepted named record, enum, or transparent newtype.
    EnterNamed { r#type: TypeSourceKey },
    /// Continue through a present optional value; `NULL` means no target.
    OptionalSome,
    /// Select one accepted member of an accepted record.
    RecordMember {
        /// Record declaration containing the member.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        record: TypeSourceKey,
        /// Stable proposal key for the member.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        field: FieldSourceKey,
    },
    /// Select one payload-bearing accepted enum variant.
    EnumVariantPayload {
        /// Enum declaration containing the variant.
        r#enum: TypeSourceKey,
        /// Stable proposal key for the variant.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        variant: TypeSourceKey,
    },
    /// Traverse every item of an accepted ordered collection.
    ListItems,
    /// Traverse every item of an accepted unique collection.
    SetItems,
    /// Traverse every value of an accepted map in canonical key order.
    MapValues,
}

/// Current source program for one relation edge.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum RelationSourceFragment {
    /// Ordered entity-root fields for the direct relation fast path.
    Direct {
        #[serde(
            deserialize_with = "crate::codec::deserialize_keys::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
        )]
        fields: Vec<FieldSourceKey>,
    },
    /// One deterministic path from an entity-root field to scalar targets.
    Nested {
        /// Entity-root field that owns the finite nested value.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        root: FieldSourceKey,
        /// Complete ordered traversal program below `root`.
        #[serde(
            deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_RELATION_PATH_STEPS }>"
        )]
        steps: Vec<RelationPathStepFragment>,
    },
}

impl RelationSourceFragment {
    /// Construct a direct relation source from ordered root fields.
    #[must_use]
    pub const fn direct(fields: Vec<FieldSourceKey>) -> Self {
        Self::Direct { fields }
    }

    /// Borrow every entity-root field owned by this relation source.
    pub fn root_fields(&self) -> impl Iterator<Item = &FieldSourceKey> {
        let direct = match self {
            Self::Direct { fields } => fields.as_slice(),
            Self::Nested { .. } => &[],
        };
        let nested = match self {
            Self::Nested { root, .. } => Some(root),
            Self::Direct { .. } => None,
        };
        direct.iter().chain(nested)
    }

    fn validate(&self, target_field_count: usize) -> Result<(), SchemaContractError> {
        match self {
            Self::Direct { fields } => {
                if fields.is_empty() || fields.len() != target_field_count {
                    return Err(SchemaContractError::InvalidReferenceList);
                }
                ensure_unique(fields)
            }
            Self::Nested { steps, .. } => {
                if steps.is_empty()
                    || steps.len() > MAX_RELATION_PATH_STEPS
                    || target_field_count != 1
                {
                    return Err(SchemaContractError::InvalidReferenceList);
                }
                Ok(())
            }
        }
    }
}

/// One source-owned relation definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct RelationFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: RelationSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    source: RelationSourceFragment,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    target_entity: EntitySourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_keys::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
    )]
    target_fields: Vec<FieldSourceKey>,
    on_delete: RelationDeleteAction,
}

impl RelationFragment {
    /// Construct one relation with one bounded source program and ordered target components.
    ///
    /// # Errors
    ///
    /// Returns a typed reference-list error for empty or arity-mismatched
    /// components.
    pub fn try_new(
        name: SchemaName,
        source: RelationSourceFragment,
        target_entity: EntitySourceKey,
        target_fields: Vec<FieldSourceKey>,
        on_delete: RelationDeleteAction,
    ) -> Result<Self, SchemaContractError> {
        if target_fields.is_empty() {
            return Err(SchemaContractError::InvalidReferenceList);
        }
        source.validate(target_fields.len())?;
        ensure_unique(&target_fields)?;
        Ok(Self {
            source_key: RelationSourceKey::from_name(&name),
            name,
            source,
            target_entity,
            target_fields,
            on_delete,
        })
    }

    /// Borrow the typed proposal key derived from the current relation name.
    #[must_use]
    pub const fn source_key(&self) -> &RelationSourceKey {
        &self.source_key
    }

    /// Borrow the current relation name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow the complete current relation source program.
    #[must_use]
    pub const fn source(&self) -> &RelationSourceFragment {
        &self.source
    }

    /// Borrow the target entity source key.
    #[must_use]
    pub const fn target_entity(&self) -> &EntitySourceKey {
        &self.target_entity
    }

    /// Borrow ordered target fields.
    #[must_use]
    pub fn target_fields(&self) -> &[FieldSourceKey] {
        &self.target_fields
    }

    /// Return the maintained delete action.
    #[must_use]
    pub const fn on_delete(&self) -> RelationDeleteAction {
        self.on_delete
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        let rebuilt = Self::try_new(
            self.name.clone(),
            self.source.clone(),
            self.target_entity.clone(),
            self.target_fields.clone(),
            self.on_delete,
        )?;
        ensure_canonical_rebuild(self, &rebuilt)
    }
}

/// One source constraint kind.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum ConstraintFragmentKind {
    /// General row check over top-level source fields.
    Check(SourceCheckExpr),
    /// Closed durable operation over one nominal target below a persisted root.
    TargetedRule(TargetedRuleFragment),
}

impl ConstraintFragmentKind {
    fn validate(&self) -> Result<(), SchemaContractError> {
        match self {
            Self::Check(expression) => expression.validate(),
            Self::TargetedRule(rule) => rule.validate(),
        }
    }
}

/// One source-bound nominal durable-rule target.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct TargetedRuleFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    root: FieldSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    target_type: TypeSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    rule: RuleSourceKey,
    operation: SourceRuleOperation,
}

impl TargetedRuleFragment {
    /// Construct one targeted rule below a persisted root field.
    #[must_use]
    pub fn new(
        root: FieldSourceKey,
        target_type: TypeSourceKey,
        rule: SchemaName,
        operation: SourceRuleOperation,
    ) -> Self {
        Self {
            root,
            target_type,
            rule: RuleSourceKey::from_name(&rule),
            operation,
        }
    }

    /// Borrow the persisted root-field identity.
    #[must_use]
    pub const fn root(&self) -> &FieldSourceKey {
        &self.root
    }

    /// Borrow the ruled nominal type identity.
    #[must_use]
    pub const fn target_type(&self) -> &TypeSourceKey {
        &self.target_type
    }

    /// Borrow the current durable-rule name as a typed proposal key.
    #[must_use]
    pub const fn rule(&self) -> &RuleSourceKey {
        &self.rule
    }

    /// Borrow the closed durable operation.
    #[must_use]
    pub const fn operation(&self) -> &SourceRuleOperation {
        &self.operation
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        self.operation.validate()
    }
}

/// One source constraint declaration.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct ConstraintFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: ConstraintSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    kind: ConstraintFragmentKind,
}

impl ConstraintFragment {
    /// Construct one general source check.
    #[must_use]
    pub fn check(name: SchemaName, expression: SourceCheckExpr) -> Self {
        Self {
            source_key: ConstraintSourceKey::from_name(&name),
            name,
            kind: ConstraintFragmentKind::Check(expression),
        }
    }

    /// Construct one source-bound targeted durable rule.
    #[must_use]
    pub fn targeted_rule(rule: TargetedRuleFragment) -> Self {
        let source_key = ConstraintSourceKey::for_targeted_field_rule(
            rule.root(),
            rule.target_type(),
            rule.rule(),
        );
        let name = SchemaName::for_targeted_rule(&source_key);
        Self {
            source_key,
            name,
            kind: ConstraintFragmentKind::TargetedRule(rule),
        }
    }

    /// Borrow the typed proposal key derived from the current declaration.
    #[must_use]
    pub const fn source_key(&self) -> &ConstraintSourceKey {
        &self.source_key
    }

    /// Borrow the current constraint name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow the exact source constraint kind.
    #[must_use]
    pub const fn kind(&self) -> &ConstraintFragmentKind {
        &self.kind
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        self.kind.validate()?;
        let rebuilt = match &self.kind {
            ConstraintFragmentKind::Check(expression) => {
                if crate::is_engine_reserved_schema_name(self.name.as_str()) {
                    return Err(SchemaContractError::InvalidSourceKey);
                }
                Self::check(self.name.clone(), expression.clone())
            }
            ConstraintFragmentKind::TargetedRule(rule) => Self::targeted_rule(rule.clone()),
        };
        ensure_canonical_rebuild(self, &rebuilt)
    }
}

/// Store-free logical entity definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct EntityFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: EntitySourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    version: DeclaredEntityVersion,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
    )]
    fields: Vec<FieldFragment>,
    #[serde(
        deserialize_with = "crate::codec::deserialize_keys::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
    )]
    primary_key: Vec<FieldSourceKey>,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_INDEXES }>"
    )]
    indexes: Vec<IndexFragment>,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_RELATIONS }>"
    )]
    relations: Vec<RelationFragment>,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_CONSTRAINTS }>"
    )]
    constraints: Vec<ConstraintFragment>,
}

impl EntityFragment {
    /// Construct and canonicalize one entity definition.
    ///
    /// # Errors
    ///
    /// Returns a typed contract error for collection overflow, duplicate
    /// source keys, malformed policy, expression, or primary-key references.
    /// Primary keys require non-null scalar key fields; Unit is valid only as
    /// the sole primary-key field.
    pub fn try_new(
        name: SchemaName,
        version: DeclaredEntityVersion,
        mut fields: Vec<FieldFragment>,
        primary_key: Vec<FieldSourceKey>,
        mut indexes: Vec<IndexFragment>,
        mut relations: Vec<RelationFragment>,
        mut constraints: Vec<ConstraintFragment>,
    ) -> Result<Self, SchemaContractError> {
        let source_key = EntitySourceKey::from_name(&name);
        check_len("entity fields", fields.len(), MAX_FRAGMENT_FIELDS)?;
        check_len("entity indexes", indexes.len(), MAX_FRAGMENT_INDEXES)?;
        check_len("entity relations", relations.len(), MAX_FRAGMENT_RELATIONS)?;
        check_len(
            "entity constraints",
            constraints.len(),
            MAX_FRAGMENT_CONSTRAINTS,
        )?;
        if primary_key.is_empty() {
            return Err(SchemaContractError::InvalidReferenceList);
        }
        ensure_unique(&primary_key)?;
        // Equal source keys are rejected below, so stable tie ordering cannot
        // be observed and need not retain stable-sort machinery in Wasm.
        crate::compact_sort_unstable_by(&mut fields, |a, b| a.source_key.cmp(&b.source_key));
        crate::compact_sort_unstable_by(&mut indexes, |a, b| a.source_key.cmp(&b.source_key));
        crate::compact_sort_unstable_by(&mut relations, |a, b| a.source_key.cmp(&b.source_key));
        crate::compact_sort_unstable_by(&mut constraints, |a, b| a.source_key.cmp(&b.source_key));
        ensure_unique_sorted_by(&fields, FieldFragment::source_key)?;
        ensure_unique_sorted_by(&indexes, IndexFragment::source_key)?;
        ensure_unique_sorted_by(&relations, RelationFragment::source_key)?;
        ensure_unique_sorted_by(&constraints, ConstraintFragment::source_key)?;
        ensure_unique_names(fields.iter().map(FieldFragment::name))?;
        ensure_unique_names(indexes.iter().map(IndexFragment::name))?;
        ensure_unique_names(relations.iter().map(RelationFragment::name))?;
        ensure_unique_names(constraints.iter().map(ConstraintFragment::name))?;
        for field in &fields {
            field.validate()?;
        }
        validate_management_cardinality(&fields)?;
        for index in &indexes {
            index.validate()?;
        }
        for relation in &relations {
            relation.validate()?;
        }
        for constraint in &constraints {
            constraint.validate()?;
        }
        let field_keys = fields
            .iter()
            .map(|field| field.source_key.clone())
            .collect::<BTreeSet<_>>();
        if primary_key.iter().any(|field| !field_keys.contains(field)) {
            return Err(SchemaContractError::InvalidLocalReference);
        }
        validate_insert_generation(&fields, &primary_key)?;
        validate_primary_key_field_shapes(&fields, &primary_key)?;
        for index in &indexes {
            if index
                .key()
                .iter()
                .any(|component| !field_keys.contains(component.field()))
                || index.predicate().is_some_and(|predicate| {
                    predicate
                        .dependencies()
                        .iter()
                        .any(|field| !field_keys.contains(field))
                })
            {
                return Err(SchemaContractError::InvalidLocalReference);
            }
        }
        for relation in &relations {
            if relation
                .source()
                .root_fields()
                .any(|field| !field_keys.contains(field))
                || (relation.target_entity() == &source_key
                    && relation
                        .target_fields()
                        .iter()
                        .any(|field| !field_keys.contains(field)))
            {
                return Err(SchemaContractError::InvalidLocalReference);
            }
        }
        for constraint in &constraints {
            let invalid = match constraint.kind() {
                ConstraintFragmentKind::Check(expression) => expression
                    .dependencies()
                    .iter()
                    .any(|field| !field_keys.contains(field)),
                ConstraintFragmentKind::TargetedRule(rule) => !field_keys.contains(rule.root()),
            };
            if invalid {
                return Err(SchemaContractError::InvalidLocalReference);
            }
        }
        Ok(Self {
            source_key,
            name,
            version,
            fields,
            primary_key,
            indexes,
            relations,
            constraints,
        })
    }

    /// Borrow the typed proposal key derived from the current entity name.
    #[must_use]
    pub const fn source_key(&self) -> &EntitySourceKey {
        &self.source_key
    }

    /// Borrow the current entity name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Return the application-declared current entity version.
    #[must_use]
    pub const fn version(&self) -> DeclaredEntityVersion {
        self.version
    }

    /// Borrow canonical field definitions.
    #[must_use]
    pub fn fields(&self) -> &[FieldFragment] {
        &self.fields
    }

    /// Borrow ordered primary-key fields.
    #[must_use]
    pub fn primary_key(&self) -> &[FieldSourceKey] {
        &self.primary_key
    }

    /// Borrow canonical secondary-index definitions.
    #[must_use]
    pub fn indexes(&self) -> &[IndexFragment] {
        &self.indexes
    }

    /// Borrow canonical relation definitions.
    #[must_use]
    pub fn relations(&self) -> &[RelationFragment] {
        &self.relations
    }

    /// Borrow canonical source constraint definitions.
    #[must_use]
    pub fn constraints(&self) -> &[ConstraintFragment] {
        &self.constraints
    }

    pub(crate) fn validate(&self) -> Result<(), SchemaContractError> {
        let rebuilt = Self::try_new(
            self.name.clone(),
            self.version,
            self.fields.clone(),
            self.primary_key.clone(),
            self.indexes.clone(),
            self.relations.clone(),
            self.constraints.clone(),
        )?;
        ensure_canonical_rebuild(self, &rebuilt)
    }
}

// Share intrinsic key capabilities with generated model admission. A named
// type or collection cannot substitute for an exact scalar key.
fn validate_primary_key_field_shapes(
    fields: &[FieldFragment],
    primary_key: &[FieldSourceKey],
) -> Result<(), SchemaContractError> {
    if fields.iter().any(|field| {
        primary_key.contains(&field.source_key)
            && (field.nullable
                || !matches!(field.field_type, FieldType::Scalar(scalar)
                    if scalar.kind().is_primary_key_component_encodable()
                        && (primary_key.len() == 1 || scalar != ScalarType::Unit)))
    }) {
        return Err(SchemaContractError::InvalidFieldPolicy);
    }
    Ok(())
}

// Keep the public proposal contract exact even when fragments are constructed
// without the source macro. Numeric generation is unsigned identity
// generation and therefore belongs only to one non-null scalar primary key.
fn validate_insert_generation(
    fields: &[FieldFragment],
    primary_key: &[FieldSourceKey],
) -> Result<(), SchemaContractError> {
    for field in fields {
        if !matches!(field.insert_policy(), FieldInsertPolicy::Generated) {
            continue;
        }
        if field.nullable() || field.management().is_some() {
            return Err(SchemaContractError::InvalidFieldPolicy);
        }
        match field.field_type() {
            FieldType::Scalar(ScalarType::Ulid | ScalarType::Timestamp) => {}
            FieldType::Scalar(
                ScalarType::Nat8
                | ScalarType::Nat16
                | ScalarType::Nat32
                | ScalarType::Nat64
                | ScalarType::Nat128,
            ) if primary_key.len() == 1 && primary_key.first() == Some(field.source_key()) => {}
            FieldType::Scalar(_) | FieldType::List(_) | FieldType::Named(_) => {
                return Err(SchemaContractError::InvalidFieldPolicy);
            }
        }
    }
    Ok(())
}

/// One structural field in a named record type.
///
/// Composite fields carry exact type and nullability facts only. Insert,
/// generation, and management policies belong to persisted entity fields.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct RecordFieldFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: FieldSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    field_type: FieldType,
    nullable: bool,
}

impl RecordFieldFragment {
    /// Construct one exact structural record field.
    #[must_use]
    pub fn new(name: SchemaName, field_type: FieldType, nullable: bool) -> Self {
        Self {
            source_key: FieldSourceKey::from_name(&name),
            name,
            field_type,
            nullable,
        }
    }

    /// Borrow the typed proposal key derived from the current field name.
    #[must_use]
    pub const fn source_key(&self) -> &FieldSourceKey {
        &self.source_key
    }

    /// Borrow the current record-field name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow the exact logical field type.
    #[must_use]
    pub const fn field_type(&self) -> &FieldType {
        &self.field_type
    }

    /// Return whether the structural field admits null.
    #[must_use]
    pub const fn nullable(&self) -> bool {
        self.nullable
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        if !current_name_key_matches(self.source_key.as_str(), &self.name) {
            return Err(SchemaContractError::NonCanonical);
        }
        self.field_type.validate()
    }
}

/// One positional tuple member and its explicit-null policy.

#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct TupleElementFragment {
    field_type: FieldType,
    nullable: bool,
}

impl TupleElementFragment {
    /// Construct one exact tuple-member contract.
    #[must_use]
    pub const fn new(field_type: FieldType, nullable: bool) -> Self {
        Self {
            field_type,
            nullable,
        }
    }

    /// Borrow the exact logical member type.
    #[must_use]
    pub const fn field_type(&self) -> &FieldType {
        &self.field_type
    }

    /// Return whether this tuple member admits explicit null.
    #[must_use]
    pub const fn nullable(&self) -> bool {
        self.nullable
    }

    const fn validate(&self) -> Result<(), SchemaContractError> {
        self.field_type.validate()
    }
}

/// Named record-type definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct RecordTypeFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: TypeSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
    )]
    fields: Vec<RecordFieldFragment>,
}

impl RecordTypeFragment {
    /// Construct and canonicalize a record definition.
    ///
    /// # Errors
    ///
    /// Returns a typed contract error for overflow, duplicates, or malformed
    /// fields.
    pub fn try_new(
        name: SchemaName,
        mut fields: Vec<RecordFieldFragment>,
    ) -> Result<Self, SchemaContractError> {
        check_len("record fields", fields.len(), MAX_FRAGMENT_FIELDS)?;
        crate::compact_sort_unstable_by(&mut fields, |left, right| {
            left.source_key.cmp(&right.source_key)
        });
        ensure_unique_sorted_by(&fields, RecordFieldFragment::source_key)?;
        ensure_unique_names(fields.iter().map(RecordFieldFragment::name))?;
        for field in &fields {
            field.validate()?;
        }
        Ok(Self {
            source_key: TypeSourceKey::from_name(&name),
            name,
            fields,
        })
    }

    /// Borrow the typed proposal key derived from the current record name.
    #[must_use]
    pub const fn source_key(&self) -> &TypeSourceKey {
        &self.source_key
    }

    /// Borrow the current record name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow canonical record fields.
    #[must_use]
    pub fn fields(&self) -> &[RecordFieldFragment] {
        &self.fields
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        let rebuilt = Self::try_new(self.name.clone(), self.fields.clone())?;
        if rebuilt != *self {
            return Err(SchemaContractError::NonCanonical);
        }
        Ok(())
    }
}

/// One named enum variant.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct EnumVariantFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: TypeSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    payload: Option<FieldType>,
}

impl EnumVariantFragment {
    /// Construct one unit variant.
    #[must_use]
    pub fn new(name: SchemaName) -> Self {
        Self {
            source_key: TypeSourceKey::from_name(&name),
            name,
            payload: None,
        }
    }

    /// Construct one payload-bearing variant.
    #[must_use]
    pub fn with_payload(name: SchemaName, payload: FieldType) -> Self {
        Self {
            source_key: TypeSourceKey::from_name(&name),
            name,
            payload: Some(payload),
        }
    }

    /// Borrow the typed proposal key derived from the current variant name.
    #[must_use]
    pub const fn source_key(&self) -> &TypeSourceKey {
        &self.source_key
    }

    /// Borrow the current variant name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow the optional exact payload contract.
    #[must_use]
    pub const fn payload(&self) -> Option<&FieldType> {
        self.payload.as_ref()
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        if !current_name_key_matches(self.source_key.as_str(), &self.name) {
            return Err(SchemaContractError::NonCanonical);
        }
        match &self.payload {
            Some(payload) => payload.validate(),
            None => Ok(()),
        }
    }
}

/// Named enum-type definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct EnumTypeFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
    )]
    source_key: TypeSourceKey,
    #[serde(
        deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
    )]
    name: SchemaName,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
    )]
    variants: Vec<EnumVariantFragment>,
}

impl EnumTypeFragment {
    /// Construct and canonicalize an enum definition.
    ///
    /// # Errors
    ///
    /// Returns a typed contract error for empty, oversized, or duplicate
    /// variants.
    pub fn try_new(
        name: SchemaName,
        mut variants: Vec<EnumVariantFragment>,
    ) -> Result<Self, SchemaContractError> {
        if variants.is_empty() {
            return Err(SchemaContractError::InvalidReferenceList);
        }
        check_len("enum variants", variants.len(), MAX_FRAGMENT_FIELDS)?;
        crate::compact_sort_unstable_by(&mut variants, |left, right| {
            left.source_key.cmp(&right.source_key)
        });
        ensure_unique_sorted_by(&variants, |variant| &variant.source_key)?;
        ensure_unique_names(variants.iter().map(EnumVariantFragment::name))?;
        for variant in &variants {
            variant.validate()?;
        }
        Ok(Self {
            source_key: TypeSourceKey::from_name(&name),
            name,
            variants,
        })
    }

    /// Borrow the typed proposal key derived from the current enum name.
    #[must_use]
    pub const fn source_key(&self) -> &TypeSourceKey {
        &self.source_key
    }

    /// Borrow the current enum name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        &self.name
    }

    /// Borrow canonical enum variants.
    #[must_use]
    pub fn variants(&self) -> &[EnumVariantFragment] {
        &self.variants
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        let rebuilt = Self::try_new(self.name.clone(), self.variants.clone())?;
        if rebuilt != *self {
            return Err(SchemaContractError::NonCanonical);
        }
        Ok(())
    }
}

/// Named reusable type definition.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum NamedTypeFragment {
    /// Record type.
    Record(RecordTypeFragment),
    /// Enum type.
    Enum(EnumTypeFragment),
    /// Transparent named wrapper.
    Newtype {
        /// Typed proposal key derived from the current name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        source_key: TypeSourceKey,
        /// Current schema name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
        )]
        name: SchemaName,
        /// Wrapped logical type.
        inner: FieldType,
    },
    /// Homogeneous ordered collection.
    List {
        /// Typed proposal key derived from the current name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        source_key: TypeSourceKey,
        /// Current schema name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
        )]
        name: SchemaName,
        /// Element type.
        item: FieldType,
    },
    /// Homogeneous unique collection.
    Set {
        /// Typed proposal key derived from the current name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        source_key: TypeSourceKey,
        /// Current schema name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
        )]
        name: SchemaName,
        /// Element type.
        item: FieldType,
    },
    /// Homogeneous key/value collection.
    Map {
        /// Typed proposal key derived from the current name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        source_key: TypeSourceKey,
        /// Current schema name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
        )]
        name: SchemaName,
        /// Key type.
        key: FieldType,
        /// Value type.
        value: FieldType,
    },
    /// Ordered heterogeneous product.
    Tuple {
        /// Typed proposal key derived from the current name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SOURCE_KEY_BYTES }>"
        )]
        source_key: TypeSourceKey,
        /// Current schema name.
        #[serde(
            deserialize_with = "crate::codec::deserialize_key::<_, _, { crate::MAX_SCHEMA_NAME_BYTES }>"
        )]
        name: SchemaName,
        /// Ordered member types.
        #[serde(
            deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_FIELDS }>"
        )]
        members: Vec<TupleElementFragment>,
    },
}

impl NamedTypeFragment {
    /// Append direct named-type dependencies in fragment order.
    ///
    /// Inline list layers are traversed, but referenced definitions are not.
    /// Existing entries and duplicate references are preserved so callers own
    /// reachability, canonicalization and accepted-catalog validation.
    pub fn append_named_type_dependencies(&self, pending: &mut Vec<TypeSourceKey>) {
        match self {
            Self::Record(record) => {
                for field in record.fields() {
                    field.field_type().append_named_type_dependencies(pending);
                }
            }
            Self::Enum(r#enum) => {
                for variant in r#enum.variants() {
                    if let Some(payload) = variant.payload() {
                        payload.append_named_type_dependencies(pending);
                    }
                }
            }
            Self::Newtype { inner, .. }
            | Self::List { item: inner, .. }
            | Self::Set { item: inner, .. } => {
                inner.append_named_type_dependencies(pending);
            }
            Self::Map { key, value, .. } => {
                key.append_named_type_dependencies(pending);
                value.append_named_type_dependencies(pending);
            }
            Self::Tuple { members, .. } => {
                for member in members {
                    member.field_type().append_named_type_dependencies(pending);
                }
            }
        }
    }

    /// Construct one transparent named wrapper from its current name.
    #[must_use]
    pub fn newtype(name: SchemaName, inner: FieldType) -> Self {
        Self::Newtype {
            source_key: TypeSourceKey::from_name(&name),
            name,
            inner,
        }
    }

    /// Construct one named ordered collection from its current name.
    #[must_use]
    pub fn list(name: SchemaName, item: FieldType) -> Self {
        Self::List {
            source_key: TypeSourceKey::from_name(&name),
            name,
            item,
        }
    }

    /// Construct one named unique collection from its current name.
    #[must_use]
    pub fn set(name: SchemaName, item: FieldType) -> Self {
        Self::Set {
            source_key: TypeSourceKey::from_name(&name),
            name,
            item,
        }
    }

    /// Construct one named key/value collection from its current name.
    #[must_use]
    pub fn map(name: SchemaName, key: FieldType, value: FieldType) -> Self {
        Self::Map {
            source_key: TypeSourceKey::from_name(&name),
            name,
            key,
            value,
        }
    }

    /// Construct one named heterogeneous product from its current name.
    #[must_use]
    pub fn tuple(name: SchemaName, members: Vec<TupleElementFragment>) -> Self {
        Self::Tuple {
            source_key: TypeSourceKey::from_name(&name),
            name,
            members,
        }
    }

    /// Borrow the typed proposal key derived from the current type name.
    #[must_use]
    pub const fn source_key(&self) -> &TypeSourceKey {
        match self {
            Self::Record(record) => record.source_key(),
            Self::Enum(r#enum) => r#enum.source_key(),
            Self::Newtype { source_key, .. }
            | Self::List { source_key, .. }
            | Self::Set { source_key, .. }
            | Self::Map { source_key, .. }
            | Self::Tuple { source_key, .. } => source_key,
        }
    }

    /// Borrow the current type name.
    #[must_use]
    pub const fn name(&self) -> &SchemaName {
        match self {
            Self::Record(record) => record.name(),
            Self::Enum(r#enum) => r#enum.name(),
            Self::Newtype { name, .. }
            | Self::List { name, .. }
            | Self::Set { name, .. }
            | Self::Map { name, .. }
            | Self::Tuple { name, .. } => name,
        }
    }

    fn validate(&self) -> Result<(), SchemaContractError> {
        ensure_current_name_key(self.source_key().as_str(), self.name())?;
        match self {
            Self::Record(record) => record.validate(),
            Self::Enum(r#enum) => r#enum.validate(),
            Self::Newtype { inner, .. }
            | Self::List { item: inner, .. }
            | Self::Set { item: inner, .. } => inner.validate(),
            Self::Map { key, value, .. } => {
                key.validate()?;
                value.validate()
            }
            Self::Tuple { members, .. } => {
                if members.is_empty() {
                    return Err(SchemaContractError::InvalidReferenceList);
                }
                check_len("tuple members", members.len(), MAX_FRAGMENT_FIELDS)?;
                members.iter().try_for_each(TupleElementFragment::validate)
            }
        }
    }
}

/// Reusable store-free collection of entity and type definitions.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct SchemaFragment {
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_ENTITIES }>"
    )]
    entities: Vec<EntityFragment>,
    #[serde(
        deserialize_with = "crate::codec::deserialize_list::<_, _, { crate::MAX_FRAGMENT_TYPES }>"
    )]
    types: Vec<NamedTypeFragment>,
}

impl SchemaFragment {
    /// Construct and canonicalize one reusable fragment.
    ///
    /// # Errors
    ///
    /// Returns a typed contract error for overflow, duplicate definitions, or
    /// malformed nested definitions.
    pub fn try_new(
        mut entities: Vec<EntityFragment>,
        mut types: Vec<NamedTypeFragment>,
    ) -> Result<Self, SchemaContractError> {
        check_len("fragment entities", entities.len(), MAX_FRAGMENT_ENTITIES)?;
        check_len("fragment types", types.len(), MAX_FRAGMENT_TYPES)?;
        crate::compact_sort_unstable_by(&mut entities, |left, right| {
            left.source_key.cmp(&right.source_key)
        });
        crate::compact_sort_unstable_by(&mut types, |left, right| {
            left.source_key().cmp(right.source_key())
        });
        ensure_unique_sorted_by(&entities, EntityFragment::source_key)?;
        ensure_unique_sorted_by(&types, NamedTypeFragment::source_key)?;
        ensure_unique_names(entities.iter().map(EntityFragment::name))?;
        ensure_unique_names(types.iter().map(NamedTypeFragment::name))?;
        for entity in &entities {
            entity.validate()?;
        }
        for r#type in &types {
            r#type.validate()?;
        }
        Ok(Self { entities, types })
    }

    /// Borrow entity definitions.
    #[must_use]
    pub fn entities(&self) -> &[EntityFragment] {
        &self.entities
    }

    /// Borrow named type definitions.
    #[must_use]
    pub fn types(&self) -> &[NamedTypeFragment] {
        &self.types
    }

    pub(crate) fn validate(&self) -> Result<(), SchemaContractError> {
        for r#type in &self.types {
            r#type.validate()?;
        }
        let rebuilt = Self::try_new(self.entities.clone(), self.types.clone())?;
        if rebuilt != *self {
            return Err(SchemaContractError::NonCanonical);
        }
        Ok(())
    }
}

pub(crate) const fn check_len(
    kind: &'static str,
    len: usize,
    max: usize,
) -> Result<(), SchemaContractError> {
    if len > max {
        return Err(SchemaContractError::TooManyItems { kind, len, max });
    }
    Ok(())
}

fn current_name_key_matches(source_key: &str, name: &SchemaName) -> bool {
    source_key == name.as_str()
}

fn ensure_current_name_key(source_key: &str, name: &SchemaName) -> Result<(), SchemaContractError> {
    if !current_name_key_matches(source_key, name) {
        return Err(SchemaContractError::NonCanonical);
    }
    Ok(())
}

fn ensure_canonical_rebuild<T: PartialEq>(
    current: &T,
    rebuilt: &T,
) -> Result<(), SchemaContractError> {
    if current != rebuilt {
        return Err(SchemaContractError::NonCanonical);
    }
    Ok(())
}

fn ensure_unique<T>(values: &[T]) -> Result<(), SchemaContractError>
where
    T: Ord,
{
    let mut seen = BTreeSet::new();
    if values.iter().any(|value| !seen.insert(value)) {
        return Err(SchemaContractError::InvalidReferenceList);
    }
    Ok(())
}

fn ensure_unique_sorted_by<T, K>(
    values: &[T],
    key: impl Fn(&T) -> &K,
) -> Result<(), SchemaContractError>
where
    K: Eq,
{
    if values.windows(2).any(|pair| key(&pair[0]) == key(&pair[1])) {
        return Err(SchemaContractError::DuplicateSourceKey);
    }
    Ok(())
}

fn ensure_unique_names<'a>(
    names: impl IntoIterator<Item = &'a SchemaName>,
) -> Result<(), SchemaContractError> {
    let mut seen = BTreeSet::new();
    if names.into_iter().any(|name| !seen.insert(name)) {
        return Err(SchemaContractError::DuplicateName);
    }
    Ok(())
}

fn validate_management_cardinality(fields: &[FieldFragment]) -> Result<(), SchemaContractError> {
    for policy in [
        FieldManagementPolicy::CreatedAt,
        FieldManagementPolicy::UpdatedAt,
    ] {
        if fields
            .iter()
            .filter(|field| field.management() == Some(policy))
            .count()
            > 1
        {
            return Err(SchemaContractError::InvalidFieldPolicy);
        }
    }
    Ok(())
}

fn decimal_fits_scale(value: Decimal, scale: u32) -> bool {
    match value.scale().cmp(&scale) {
        std::cmp::Ordering::Equal | std::cmp::Ordering::Greater => true,
        std::cmp::Ordering::Less => value.scale_to_integer(scale).is_some(),
    }
}

#[cfg(test)]
mod tests {
    use super::{
        FieldFragment, FieldInsertPolicy, FieldSourceKey, FieldType, ScalarType,
        SchemaContractError, SchemaName,
    };

    #[test]
    fn independently_decoded_field_key_and_name_must_match() {
        let field = FieldFragment {
            source_key: FieldSourceKey::try_new("legacy_name").expect("fixture key should admit"),
            name: SchemaName::try_new("current_name").expect("fixture name should admit"),
            field_type: FieldType::Scalar(ScalarType::Nat64),
            nullable: false,
            insert_policy: FieldInsertPolicy::Required,
            management: None,
        };

        assert_eq!(field.validate(), Err(SchemaContractError::NonCanonical));
    }
}