package merge

import "github.com/databricks/cli/libs/dyn"

type elementsByKey struct {
	key     string
	keyFunc func(dyn.Value) string
}

func (e elementsByKey) Map(v dyn.Value) (dyn.Value, error) {
	// We know the type of this value is a sequence.
	// For additional defence, return self if it is not.
	elements, ok := v.AsSequence()
	if !ok {
		return v, nil
	}

	seen := make(map[string]dyn.Value, len(elements))
	keys := make([]string, 0, len(elements))

	// Iterate in natural order. For a given key, we first see the
	// base definition and merge instances that come after it.
	for i := range elements {
		kv := elements[i].Get(e.key)
		key := e.keyFunc(kv)

		// Register element with key if not yet seen before.
		ref, ok := seen[key]
		if !ok {
			keys = append(keys, key)
			seen[key] = elements[i]
			continue
		}

		// Merge this instance into the reference.
		nv, err := Merge(ref, elements[i])
		if err != nil {
			return v, err
		}

		// Overwrite reference.
		seen[key] = nv
	}

	// Gather resulting elements in natural order.
	out := make([]dyn.Value, 0, len(keys))
	for _, key := range keys {
		nv, err := dyn.Set(seen[key], e.key, dyn.V(key))
		if err != nil {
			return dyn.InvalidValue, err
		}
		out = append(out, nv)
	}

	return dyn.NewValue(out, v.Location()), nil
}

// ElementsByKey returns a [dyn.MapFunc] that operates on a sequence
// where each element is a map. It groups elements by a key and merges
// elements with the same key.
//
// The function that extracts the key from an element is provided as
// a parameter. The resulting elements get their key field overwritten
// with the value as returned by the key function.
func ElementsByKey(key string, keyFunc func(dyn.Value) string) dyn.MapFunc {
	return elementsByKey{key, keyFunc}.Map
}