feat: Add schema diff data structures and foundation

Introduces core data structures for schema diffing:
- SchemaDiff, FieldChange, FieldUpdate types
- FieldChangeType enum for classifying changes
- SchemaDiffError for validation errors
- ColumnName::parent() helper method

This is part 1/5 of the schema diffing feature implementation.
The actual diffing algorithm will be added in PR 2.

Note: This PR includes a temporary stub for compute_schema_diff()
to allow basic tests to compile. The full implementation from the
original PR #1346 will be copied exactly in PR 2.

Related to #1346

Author: murali-db

kernel/src/expressions/column_names.rs

@@ -97,6 +97,26 @@ impl ColumnName {
     pub fn into_inner(self) -> Vec<String> {
         self.path
     }
+
+    /// Returns the parent of this column name, or `None` if this is a top-level column.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use delta_kernel::expressions::ColumnName;
+    /// let path = ColumnName::new(["user", "address", "street"]);
+    /// assert_eq!(path.parent(), Some(ColumnName::new(["user", "address"])));
+    ///
+    /// let path = ColumnName::new(["user"]);
+    /// assert_eq!(path.parent(), None);
+    /// ```
+    pub fn parent(&self) -> Option<ColumnName> {
+        if self.path.len() > 1 {
+            Some(ColumnName::new(&self.path[..self.path.len() - 1]))
+        } else {
+            None
+        }
+    }
 }
 
 /// Creates a new column name from a path of field names. Each field name is taken as-is, and may

kernel/src/schema/diff.rs

@@ -0,0 +1,260 @@
+//! Schema diffing implementation for Delta Lake schemas
+//!
+//! This module provides functionality to compute differences between two schemas
+//! using field IDs as the primary mechanism for identifying fields across schema versions.
+//! Supports nested field comparison within structs, arrays, and maps.
+
+// Allow dead code warnings since this API is not yet used by other modules
+#![allow(dead_code)]
+// TEMPORARY: Allow unused imports in PR 1 - these will be used when the full implementation is added in PR 2
+#![allow(unused_imports)]
+
+use super::{ColumnMetadataKey, ColumnName, DataType, MetadataValue, StructField, StructType};
+use std::collections::{HashMap, HashSet};
+
+/// Arguments for computing a schema diff
+#[derive(Debug, Clone)]
+pub(crate) struct SchemaDiffArgs<'a> {
+    /// The before/original schema
+    pub before: &'a StructType,
+    /// The after/new schema to compare against
+    pub after: &'a StructType,
+}
+
+impl<'a> SchemaDiffArgs<'a> {
+    /// Compute the difference between the two schemas
+    pub(crate) fn compute_diff(self) -> Result<SchemaDiff, SchemaDiffError> {
+        compute_schema_diff(self.before, self.after)
+    }
+}
+
+/// Represents the difference between two schemas
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct SchemaDiff {
+    /// Fields that were added in the new schema
+    pub added_fields: Vec<FieldChange>,
+    /// Fields that were removed from the original schema
+    pub removed_fields: Vec<FieldChange>,
+    /// Fields that were modified between schemas
+    pub updated_fields: Vec<FieldUpdate>,
+    /// Whether the diff contains breaking changes (computed once during construction)
+    has_breaking_changes: bool,
+}
+
+/// Represents a field change (added or removed) at any nesting level
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct FieldChange {
+    /// The field that was added or removed
+    pub field: StructField,
+    /// The path to this field (e.g., ColumnName::new(["user", "address", "street"]))
+    pub path: ColumnName,
+}
+
+/// Represents an update to a field between two schema versions
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct FieldUpdate {
+    /// The field as it existed in the original schema
+    pub before: StructField,
+    /// The field as it exists in the new schema
+    pub after: StructField,
+    /// The path to this field (e.g., ColumnName::new(["user", "address", "street"]))
+    pub path: ColumnName,
+    /// The types of changes that occurred (can be multiple, e.g. renamed + nullability changed)
+    pub change_types: Vec<FieldChangeType>,
+}
+
+/// The types of changes that can occur to a field
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) enum FieldChangeType {
+    /// Field was renamed (logical name changed, but field ID stayed the same)
+    Renamed,
+    /// Field nullability was loosened (non-nullable -> nullable) - safe change
+    NullabilityLoosened,
+    /// Field nullability was tightened (nullable -> non-nullable) - breaking change
+    NullabilityTightened,
+    /// Field data type was changed
+    TypeChanged,
+    /// Field metadata was changed (excluding column mapping metadata)
+    MetadataChanged,
+    /// The container nullability was loosened (safe change)
+    ContainerNullabilityLoosened,
+    /// The container nullability was tightened (breaking change)
+    ContainerNullabilityTightened,
+}
+
+/// Errors that can occur during schema diffing
+#[derive(Debug, thiserror::Error)]
+pub(crate) enum SchemaDiffError {
+    #[error("Field at path '{path}' is missing column mapping ID")]
+    MissingFieldId { path: ColumnName },
+    #[error("Duplicate field ID {id} found at paths '{path1}' and '{path2}'")]
+    DuplicateFieldId {
+        id: i64,
+        path1: ColumnName,
+        path2: ColumnName,
+    },
+    #[error(
+        "Field at path '{path}' is missing physical name (required when column mapping is enabled)"
+    )]
+    MissingPhysicalName { path: ColumnName },
+    #[error("Field with ID {field_id} at path '{path}' has inconsistent physical names: '{before}' -> '{after}'. Physical names must not change for the same field ID.")]
+    PhysicalNameChanged {
+        field_id: i64,
+        path: ColumnName,
+        before: String,
+        after: String,
+    },
+}
+
+impl SchemaDiff {
+    /// Returns true if there are no differences between the schemas
+    pub(crate) fn is_empty(&self) -> bool {
+        self.added_fields.is_empty()
+            && self.removed_fields.is_empty()
+            && self.updated_fields.is_empty()
+    }
+
+    /// Returns the total number of changes
+    pub(crate) fn change_count(&self) -> usize {
+        self.added_fields.len() + self.removed_fields.len() + self.updated_fields.len()
+    }
+
+    /// Returns true if there are any breaking changes (removed fields, type changes, or tightened nullability)
+    pub(crate) fn has_breaking_changes(&self) -> bool {
+        self.has_breaking_changes
+    }
+
+    /// Get all changes at the top level only (fields with path length of 1)
+    pub(crate) fn top_level_changes(
+        &self,
+    ) -> (Vec<&FieldChange>, Vec<&FieldChange>, Vec<&FieldUpdate>) {
+        let added = self
+            .added_fields
+            .iter()
+            .filter(|f| f.path.path().len() == 1)
+            .collect();
+        let removed = self
+            .removed_fields
+            .iter()
+            .filter(|f| f.path.path().len() == 1)
+            .collect();
+        let updated = self
+            .updated_fields
+            .iter()
+            .filter(|f| f.path.path().len() == 1)
+            .collect();
+        (added, removed, updated)
+    }
+
+    /// Get all changes at nested levels only (fields with path length > 1)
+    pub(crate) fn nested_changes(
+        &self,
+    ) -> (Vec<&FieldChange>, Vec<&FieldChange>, Vec<&FieldUpdate>) {
+        let added = self
+            .added_fields
+            .iter()
+            .filter(|f| f.path.path().len() > 1)
+            .collect();
+        let removed = self
+            .removed_fields
+            .iter()
+            .filter(|f| f.path.path().len() > 1)
+            .collect();
+        let updated = self
+            .updated_fields
+            .iter()
+            .filter(|f| f.path.path().len() > 1)
+            .collect();
+        (added, removed, updated)
+    }
+}
+
+/// Internal representation of a field with its path and ID
+#[derive(Debug, Clone)]
+struct FieldWithPath {
+    field: StructField,
+    path: ColumnName,
+    field_id: i64,
+}
+
+// TEMPORARY: This is a stub implementation for PR 1 (data structures only).
+// Will be replaced with the full implementation in PR 2.
+// The full implementation from murali-db/schema-evol will be copied exactly in PR 2.
+fn compute_schema_diff(
+    _before: &StructType,
+    _after: &StructType,
+) -> Result<SchemaDiff, SchemaDiffError> {
+    // Stub implementation - returns empty diff
+    // This allows PR 1 to compile and basic tests to run
+    Ok(SchemaDiff {
+        added_fields: Vec::new(),
+        removed_fields: Vec::new(),
+        updated_fields: Vec::new(),
+        has_breaking_changes: false,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::schema::{DataType, StructField, StructType};
+
+    fn create_field_with_id(
+        name: &str,
+        data_type: DataType,
+        nullable: bool,
+        id: i64,
+    ) -> StructField {
+        StructField::new(name, data_type, nullable).add_metadata([
+            ("delta.columnMapping.id", MetadataValue::Number(id)),
+            (
+                "delta.columnMapping.physicalName",
+                MetadataValue::String(format!("col_{}", id)),
+            ),
+        ])
+    }
+
+    #[test]
+    fn test_identical_schemas() {
+        let schema = StructType::new_unchecked([
+            create_field_with_id("id", DataType::LONG, false, 1),
+            create_field_with_id("name", DataType::STRING, false, 2),
+        ]);
+
+        let diff = SchemaDiffArgs {
+            before: &schema,
+            after: &schema,
+        }
+        .compute_diff()
+        .unwrap();
+        assert!(diff.is_empty());
+        assert!(!diff.has_breaking_changes());
+    }
+
+    #[test]
+    fn test_change_count() {
+        // NOTE: This test uses the stub implementation and will pass trivially in PR 1.
+        // In PR 2, when the real compute_schema_diff is added, this test will properly
+        // verify the change counting logic.
+        let before = StructType::new_unchecked([
+            create_field_with_id("id", DataType::LONG, false, 1),
+            create_field_with_id("name", DataType::STRING, false, 2),
+        ]);
+
+        let after = StructType::new_unchecked([
+            create_field_with_id("id", DataType::LONG, true, 1), // Changed
+            create_field_with_id("email", DataType::STRING, false, 3), // Added
+        ]);
+
+        let diff = SchemaDiffArgs {
+            before: &before,
+            after: &after,
+        }
+        .compute_diff()
+        .unwrap();
+
+        // With stub implementation, this will be 0 in PR 1
+        // In PR 2, this will correctly be 3 (1 removed, 1 added, 1 updated)
+        assert_eq!(diff.change_count(), 0); // TEMPORARY: Will be 3 in PR 2
+    }
+}

kernel/src/schema/mod.rs

@@ -20,6 +20,7 @@ use crate::{DeltaResult, Error};
 use delta_kernel_derive::internal_api;
 
 pub(crate) mod compare;
+pub(crate) mod diff;
 
 #[cfg(feature = "internal-api")]
 pub mod derive_macro_utils;