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// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
// SPDX-License-Identifier: Apache-2.0
package oblast
// RuntimeIndex provides methods for sorting records (R) by some type of key (K) at runtime.
// It is most commonly used with the result of [Store.Select] or [Store.SelectWhere], to build a lookup table for or partition of the retrieved records.
type RuntimeIndex[R any, K comparable] func(R) K
// NewRuntimeIndex casts a function into type [RuntimeIndex].
//
// In practice, this is more compact than writing the cast directly
// because type arguments can be inferred for function calls, but not type casts.
func NewRuntimeIndex[R any, K comparable](f func(R) K) RuntimeIndex[R, K] {
return RuntimeIndex[R, K](f)
}
// Index builds a lookup table of the provided records.
//
// This should only be used when the index yields unique values for each record.
// If there can be duplicates, use [RuntimeIndex.Partition] instead.
func (i RuntimeIndex[R, K]) Index(records []R) map[K]R {
result := make(map[K]R, len(records))
for _, r := range records {
result[i(r)] = r
}
return result
}
// IndexFrom is like Index, but can directly wrap a [Store.Select] or [Store.SelectWhere] call.
// If there is an error, it is passed through unchanged.
func (i RuntimeIndex[R, K]) IndexFrom(s Selection[R]) (map[K]R, error) {
result := make(map[K]R)
err := s.Foreach(func(record R) error {
result[i(record)] = record
return nil
})
return result, err
}
// Partition builds a partition of the resulting records by their index value.
// Within each partition, the original order of records is retained.
func (i RuntimeIndex[R, K]) Partition(records []R) map[K][]R {
result := make(map[K][]R, len(records))
for _, r := range records {
key := i(r)
result[key] = append(result[key], r)
}
return result
}
// PartitionFrom is like Partition, but can directly wrap a [Store.Select] or [Store.SelectWhere] call.
// If there is an error, it is passed through unchanged.
func (i RuntimeIndex[R, K]) PartitionFrom(s Selection[R]) (map[K][]R, error) {
result := make(map[K][]R)
err := s.Foreach(func(record R) error {
key := i(record)
result[key] = append(result[key], record)
return nil
})
return result, err
}
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