// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.

//! Persist schema evolution.

use std::sync::Arc;

use arrow::array::{new_null_array, Array, AsArray, ListArray, StructArray};
use arrow::datatypes::{DataType, Field, FieldRef, Fields, SchemaBuilder};
use itertools::Itertools;

/// Returns a function to migrate arrow data encoded by `old` to be the same
/// DataType as arrow data encoded by `new`, if `new` is backward compatible
/// with `old`. Exposed for testing.
pub fn backward_compatible(old: &DataType, new: &DataType) -> Option<Migration> {
    backward_compatible_typ(old, new).map(Migration)
}

/// See [backward_compatible].
#[derive(Debug, PartialEq)]
pub struct Migration(ArrayMigration);

impl Migration {
    /// Returns true if the migration requires dropping data, including nested
    /// structs.
    pub fn contains_drop(&self) -> bool {
        self.0.contains_drop()
    }

    /// Returns true if the output array will preserve the input's sortedness.
    pub fn preserves_order(&self) -> bool {
        // If we add support for reordering fields, then this will also have to
        // account for that.
        !self.contains_drop()
    }

    /// For the `old` and `new` schemas used at construction time, migrates data
    /// encoded by `old` to be the same arrow DataType as data encoded by `new`.
    pub fn migrate(&self, array: Arc<dyn Array>) -> Arc<dyn Array> {
        self.0.migrate(array)
    }
}

#[derive(Debug, PartialEq)]
pub(crate) enum ArrayMigration {
    NoOp,
    Struct(Vec<StructArrayMigration>),
    List(FieldRef, Box<ArrayMigration>),
}

#[derive(Debug, PartialEq)]
pub(crate) enum StructArrayMigration {
    // TODO: Support adding fields not at the end, if/when that becomes
    // necessary.
    AddFieldNullableAtEnd {
        name: String,
        typ: DataType,
    },
    DropField {
        name: String,
    },
    AlterFieldNullable {
        name: String,
    },
    Recurse {
        name: String,
        migration: ArrayMigration,
    },
}

impl ArrayMigration {
    fn contains_drop(&self) -> bool {
        use ArrayMigration::*;
        match self {
            NoOp => false,
            Struct(xs) => xs.iter().any(|x| x.contains_drop()),
            List(_f, x) => x.contains_drop(),
        }
    }

    fn migrate(&self, array: Arc<dyn Array>) -> Arc<dyn Array> {
        use ArrayMigration::*;
        match self {
            NoOp => array,
            Struct(migrations) => {
                let len = array.len();

                let (mut fields, mut arrays, nulls) = match array.data_type() {
                    DataType::Null => {
                        let all_add_nullable = migrations.iter().all(|action| {
                            matches!(action, StructArrayMigration::AddFieldNullableAtEnd { .. })
                        });
                        assert!(all_add_nullable, "invalid migrations, {migrations:?}");
                        (Fields::empty(), Vec::new(), None)
                    }
                    DataType::Struct(_) => {
                        let array = array
                            .as_any()
                            .downcast_ref::<StructArray>()
                            .expect("known to be StructArray")
                            .clone();
                        array.into_parts()
                    }
                    other => panic!("expected Struct or Null got {other:?}"),
                };

                for migration in migrations {
                    migration.migrate(len, &mut fields, &mut arrays);
                }
                Arc::new(StructArray::new(fields, arrays, nulls))
            }
            List(field, entry_migration) => {
                let list_array: ListArray = if let Some(list_array) = array.as_list_opt() {
                    list_array.clone()
                } else if let Some(map_array) = array.as_map_opt() {
                    map_array.clone().into()
                } else {
                    panic!("expected list-like array; got {:?}", array.data_type())
                };

                let (_field, offsets, entries, nulls) = list_array.into_parts();
                let entries = entry_migration.migrate(entries);
                Arc::new(ListArray::new(Arc::clone(field), offsets, entries, nulls))
            }
        }
    }
}

impl StructArrayMigration {
    fn contains_drop(&self) -> bool {
        use StructArrayMigration::*;
        match self {
            AddFieldNullableAtEnd { .. } => false,
            DropField { .. } => true,
            AlterFieldNullable { .. } => false,
            Recurse { migration, .. } => migration.contains_drop(),
        }
    }

    fn migrate(&self, len: usize, fields: &mut Fields, arrays: &mut Vec<Arc<dyn Array>>) {
        use StructArrayMigration::*;
        match self {
            AddFieldNullableAtEnd { name, typ } => {
                arrays.push(new_null_array(typ, len));
                let mut f = SchemaBuilder::from(&*fields);
                f.push(Arc::new(Field::new(name, typ.clone(), true)));
                *fields = f.finish().fields;
            }
            DropField { name } => {
                let (idx, _) = fields
                    .find(name)
                    .unwrap_or_else(|| panic!("expected to find field {} in {:?}", name, fields));
                arrays.remove(idx);
                let mut f = SchemaBuilder::from(&*fields);
                f.remove(idx);
                *fields = f.finish().fields;
            }
            AlterFieldNullable { name } => {
                let (idx, _) = fields
                    .find(name)
                    .unwrap_or_else(|| panic!("expected to find field {} in {:?}", name, fields));
                let mut f = SchemaBuilder::from(&*fields);
                let field = f.field_mut(idx);
                // Defensively assert field is not nullable.
                assert_eq!(field.is_nullable(), false);
                *field = Arc::new(Field::new(field.name(), field.data_type().clone(), true));
                *fields = f.finish().fields;
            }
            Recurse { name, migration } => {
                let (idx, _) = fields
                    .find(name)
                    .unwrap_or_else(|| panic!("expected to find field {} in {:?}", name, fields));
                arrays[idx] = migration.migrate(Arc::clone(&arrays[idx]));
                let mut f = SchemaBuilder::from(&*fields);
                *f.field_mut(idx) = Arc::new(Field::new(
                    name,
                    arrays[idx].data_type().clone(),
                    f.field(idx).is_nullable(),
                ));
                *fields = f.finish().fields;
            }
        }
    }
}

fn backward_compatible_typ(old: &DataType, new: &DataType) -> Option<ArrayMigration> {
    use ArrayMigration::NoOp;
    use DataType::*;
    match (old, new) {
        (Null, Struct(fields)) if fields.iter().all(|field| field.is_nullable()) => {
            let migrations = fields
                .iter()
                .map(|field| StructArrayMigration::AddFieldNullableAtEnd {
                    name: field.name().clone(),
                    typ: field.data_type().clone(),
                })
                .collect();
            Some(ArrayMigration::Struct(migrations))
        }
        (
            Null | Boolean | Int8 | Int16 | Int32 | Int64 | UInt8 | UInt16 | UInt32 | UInt64
            | Float16 | Float32 | Float64 | Binary | Utf8 | Date32 | Date64 | LargeBinary
            | BinaryView | LargeUtf8 | Utf8View,
            _,
        ) => (old == new).then_some(NoOp),
        (FixedSizeBinary(o), FixedSizeBinary(n)) => (o == n).then_some(NoOp),
        (FixedSizeBinary(_), _) => None,
        (Struct(o), Struct(n)) => backward_compatible_struct(o, n),
        (Struct(_), _) => None,
        (List(o), List(n)) | (Map(o, _), List(n)) => {
            // The list migration can proceed if the entry types are compatible and the
            // field metadata is compatible. (In practice, this requires making sure that
            // nullable fields don't become non-nullable.)
            if o.is_nullable() && !n.is_nullable() {
                None
            } else {
                let nested = backward_compatible_typ(o.data_type(), n.data_type())?;
                let migration =
                    if matches!(old, DataType::List(_)) && o == n && nested == ArrayMigration::NoOp
                    {
                        ArrayMigration::NoOp
                    } else {
                        ArrayMigration::List(Arc::clone(n), nested.into())
                    };
                Some(migration)
            }
        }
        (List(_), _) => None,
        (Map(o, _), Map(n, _)) => (o == n).then_some(NoOp),
        (Map(_, _), _) => None,
        (
            Timestamp(_, _)
            | Time32(_)
            | Time64(_)
            | Duration(_)
            | Interval(_)
            | ListView(_)
            | FixedSizeList(_, _)
            | LargeList(_)
            | LargeListView(_)
            | Union(_, _)
            | Dictionary(_, _)
            | Decimal128(_, _)
            | Decimal256(_, _)
            | RunEndEncoded(_, _),
            _,
        ) => unimplemented!("not used in mz: old={:?} new={:?}", old, new),
    }
}

fn backward_compatible_struct(old: &Fields, new: &Fields) -> Option<ArrayMigration> {
    use ArrayMigration::*;
    use StructArrayMigration::*;

    let mut added_field = false;
    let mut field_migrations = Vec::new();
    for n in new.iter() {
        // This find (and the below) make the overall runtime of this O(n^2). We
        // could get it down to O(n log n) by indexing fields in old and new by
        // name, but the number of fields is expected to be small, so for now
        // avoid the allocation.
        let o = old.find(n.name());
        let o = match o {
            // If we've already added a field and then encounter another that
            // exists in old, the field we added wasn't at the end. We only
            // support adding fields at the end for now, so return incompatible.
            Some(_) if added_field => return None,
            Some((_, o)) => o,
            // Allowed to add a new field but it must be nullable.
            None if !n.is_nullable() => return None,
            None => {
                added_field = true;
                field_migrations.push(AddFieldNullableAtEnd {
                    name: n.name().to_owned(),
                    typ: n.data_type().clone(),
                });
                continue;
            }
        };

        // Not allowed to make a nullable field into non-nullable.
        if o.is_nullable() && !n.is_nullable() {
            return None;
        }
        // However, allowed to make a non-nullable field nullable.
        let make_nullable = !o.is_nullable() && n.is_nullable();

        match backward_compatible_typ(o.data_type(), n.data_type()) {
            None => return None,
            Some(NoOp) if make_nullable => {
                field_migrations.push(AlterFieldNullable {
                    name: n.name().clone(),
                });
            }
            Some(NoOp) => continue,
            // For now, don't support both making a field nullable and also
            // modifying it in some other way. It doesn't seem that we need this for
            // mz usage.
            Some(_) if make_nullable => return None,
            Some(migration) => field_migrations.push(Recurse {
                name: n.name().clone(),
                migration,
            }),
        }
    }

    for o in old.iter() {
        let n = new.find(o.name());
        if n.is_none() {
            // Allowed to drop a field regardless of nullability.
            field_migrations.push(DropField {
                name: o.name().to_owned(),
            });
        }
    }

    // Now detect if any re-ordering happened.
    let same_order = new
        .iter()
        .flat_map(|n| old.find(n.name()))
        .tuple_windows()
        .all(|((i, _), (j, _))| i <= j);
    if !same_order {
        return None;
    }

    if field_migrations.is_empty() {
        Some(NoOp)
    } else {
        Some(Struct(field_migrations))
    }
}

#[cfg(test)]
mod tests {
    use arrow::array::new_empty_array;
    use arrow::datatypes::Field;

    use super::*;

    // NB: We also have proptest coverage of all this, but it works on
    // RelationDesc+SourceData and so lives in src/storage-types.
    #[mz_ore::test]
    fn backward_compatible() {
        use DataType::*;

        #[track_caller]
        fn testcase(old: DataType, new: DataType, expected: Option<bool>) {
            let migration = super::backward_compatible_typ(&old, &new);
            let actual = migration.as_ref().map(|x| x.contains_drop());
            assert_eq!(actual, expected);
            // If it's backward compatible, make sure that the migration
            // logic works.
            if let Some(migration) = migration {
                let (old, new) = (new_empty_array(&old), new_empty_array(&new));
                let migrated = migration.migrate(old);
                assert_eq!(new.data_type(), migrated.data_type());
            }
        }

        fn struct_(fields: impl IntoIterator<Item = (&'static str, DataType, bool)>) -> DataType {
            let fields = fields
                .into_iter()
                .map(|(name, typ, nullable)| Field::new(name, typ, nullable))
                .collect();
            DataType::Struct(fields)
        }

        // Matching primitive types
        testcase(Boolean, Boolean, Some(false));
        testcase(Utf8, Utf8, Some(false));

        // Non-matching primitive types
        testcase(Boolean, Utf8, None);
        testcase(Utf8, Boolean, None);

        // Matching structs.
        testcase(
            struct_([("a", Boolean, true)]),
            struct_([("a", Boolean, true)]),
            Some(false),
        );
        testcase(
            struct_([("a", Boolean, false)]),
            struct_([("a", Boolean, false)]),
            Some(false),
        );

        // Changing nullability in a struct.
        testcase(
            struct_([("a", Boolean, true)]),
            struct_([("a", Boolean, false)]),
            None,
        );
        testcase(
            struct_([("a", Boolean, false)]),
            struct_([("a", Boolean, true)]),
            Some(false),
        );

        // Add/remove field in a struct.
        testcase(struct_([]), struct_([("a", Boolean, true)]), Some(false));
        testcase(struct_([]), struct_([("a", Boolean, false)]), None);
        testcase(struct_([("a", Boolean, true)]), struct_([]), Some(true));
        testcase(struct_([("a", Boolean, false)]), struct_([]), Some(true));

        // Add AND remove field in a struct.
        testcase(
            struct_([("a", Boolean, true)]),
            struct_([("b", Boolean, true)]),
            Some(true),
        );

        // Add two fields.
        testcase(
            struct_([]),
            struct_([("a", Boolean, true), ("b", Boolean, true)]),
            Some(false),
        );

        // Nested struct.
        testcase(
            struct_([("a", struct_([("b", Boolean, false)]), false)]),
            struct_([("a", struct_([("b", Boolean, false)]), false)]),
            Some(false),
        );
        testcase(
            struct_([("a", struct_([]), false)]),
            struct_([("a", struct_([("b", Boolean, true)]), false)]),
            Some(false),
        );
        testcase(
            struct_([("a", struct_([]), false)]),
            struct_([("a", struct_([("b", Boolean, false)]), false)]),
            None,
        );
        testcase(
            struct_([("a", struct_([("b", Boolean, true)]), false)]),
            struct_([("a", struct_([]), false)]),
            Some(true),
        );
        testcase(
            struct_([("a", struct_([("b", Boolean, false)]), false)]),
            struct_([("a", struct_([]), false)]),
            Some(true),
        );

        // For now, don't support both making a field nullable and also
        // modifying it in some other way. It doesn't seem that we need this for
        // mz usage.
        testcase(
            struct_([("a", struct_([]), false)]),
            struct_([("a", struct_([("b", Boolean, false)]), true)]),
            None,
        );

        // Similarly, don't support reordering fields. This matters to persist
        // because it affects sortedness, which is used in the consolidating
        // iter.
        testcase(
            struct_([("a", Boolean, false), ("b", Utf8, false)]),
            struct_([("b", Utf8, false), ("a", Boolean, false)]),
            None,
        );

        // Regression test for a bug in the (fundamentally flawed) original
        // impl.
        testcase(
            struct_([("2", Boolean, false), ("10", Utf8, false)]),
            struct_([("10", Utf8, false)]),
            Some(true),
        );

        // Regression test for another bug caught during code review where a
        // field was added not at the end.
        testcase(
            struct_([("a", Boolean, true), ("c", Boolean, true)]),
            struct_([
                ("a", Boolean, true),
                ("b", Boolean, true),
                ("c", Boolean, true),
            ]),
            None,
        );

        // Regression test for migrating a RelationDesc with no columns
        // (which gets encoded as a NullArray) to a RelationDesc with one
        // nullable column.
        testcase(Null, struct_([("a", Boolean, true)]), Some(false));

        // Test that we can migrate the old maparray columns to listarrays if the contents match.
        testcase(
            Map(
                Field::new_struct(
                    "map_entries",
                    vec![
                        Field::new("keys", Utf8, false),
                        Field::new("values", Boolean, true),
                    ],
                    false,
                )
                .into(),
                true,
            ),
            List(
                Field::new_struct(
                    "map_entries",
                    vec![
                        Field::new("keys", Utf8, false),
                        Field::new("values", Boolean, true),
                    ],
                    false,
                )
                .into(),
            ),
            Some(false),
        );

        // Recursively migrate list contents
        testcase(
            List(Field::new_struct("entries", vec![Field::new("keys", Utf8, false)], true).into()),
            List(
                Field::new_struct(
                    "entries",
                    vec![
                        Field::new("keys", Utf8, false),
                        Field::new("values", Boolean, true),
                    ],
                    true,
                )
                .into(),
            ),
            Some(false),
        );
    }
}