package lo

import (
	"math/rand"
)

// Filter iterates over elements of collection, returning an array of all elements predicate returns truthy for.
func Filter[V any](collection []V, predicate func(V, int) bool) []V {
	result := []V{}

	for i, item := range collection {
		if predicate(item, i) {
			result = append(result, item)
		}
	}

	return result
}

// Map manipulates a slice and transforms it to a slice of another type.
func Map[T any, R any](collection []T, iteratee func(T, int) R) []R {
	result := make([]R, len(collection))

	for i, item := range collection {
		result[i] = iteratee(item, i)
	}

	return result
}

// FlatMap manipulates a slice and transforms and flattens it to a slice of another type.
func FlatMap[T any, R any](collection []T, iteratee func(T, int) []R) []R {
	result := []R{}

	for i, item := range collection {
		result = append(result, iteratee(item, i)...)
	}

	return result
}

// Reduce reduces collection to a value which is the accumulated result of running each element in collection
// through accumulator, where each successive invocation is supplied the return value of the previous.
func Reduce[T any, R any](collection []T, accumulator func(R, T, int) R, initial R) R {
	for i, item := range collection {
		initial = accumulator(initial, item, i)
	}

	return initial
}

// ForEach iterates over elements of collection and invokes iteratee for each element.
func ForEach[T any](collection []T, iteratee func(T, int)) {
	for i, item := range collection {
		iteratee(item, i)
	}
}

// Times invokes the iteratee n times, returning an array of the results of each invocation.
// The iteratee is invoked with index as argument.
func Times[T any](count int, iteratee func(int) T) []T {
	result := make([]T, count)

	for i := 0; i < count; i++ {
		result[i] = iteratee(i)
	}

	return result
}

// Uniq returns a duplicate-free version of an array, in which only the first occurrence of each element is kept.
// The order of result values is determined by the order they occur in the array.
func Uniq[T comparable](collection []T) []T {
	result := make([]T, 0, len(collection))
	seen := make(map[T]struct{}, len(collection))

	for _, item := range collection {
		if _, ok := seen[item]; ok {
			continue
		}

		seen[item] = struct{}{}
		result = append(result, item)
	}

	return result
}

// Uniq returns a duplicate-free version of an array, in which only the first occurrence of each element is kept.
// The order of result values is determined by the order they occur in the array. It accepts `iteratee` which is
// invoked for each element in array to generate the criterion by which uniqueness is computed.
func UniqBy[T any, U comparable](collection []T, iteratee func(T) U) []T {
	result := make([]T, 0, len(collection))
	seen := make(map[U]struct{}, len(collection))

	for _, item := range collection {
		key := iteratee(item)

		if _, ok := seen[key]; ok {
			continue
		}

		seen[key] = struct{}{}
		result = append(result, item)
	}

	return result
}

// GroupBy returns an object composed of keys generated from the results of running each element of collection through iteratee.
func GroupBy[T any, U comparable](collection []T, iteratee func(T) U) map[U][]T {
	result := map[U][]T{}

	for _, item := range collection {
		key := iteratee(item)

		if _, ok := result[key]; !ok {
			result[key] = []T{}
		}

		result[key] = append(result[key], item)
	}

	return result
}

// Chunk returns an array of elements split into groups the length of size. If array can't be split evenly,
// the final chunk will be the remaining elements.
func Chunk[T any](collection []T, size int) [][]T {
	if size <= 0 {
		panic("Second parameter must be greater than 0")
	}

	result := make([][]T, 0, len(collection)/2+1)
	length := len(collection)

	for i := 0; i < length; i++ {
		chunk := i / size

		if i%size == 0 {
			result = append(result, make([]T, 0, size))
		}

		result[chunk] = append(result[chunk], collection[i])
	}

	return result
}

// PartitionBy returns an array of elements split into groups. The order of grouped values is
// determined by the order they occur in collection. The grouping is generated from the results
// of running each element of collection through iteratee.
func PartitionBy[T any, K comparable](collection []T, iteratee func(x T) K) [][]T {
	result := [][]T{}
	seen := map[K]int{}

	for _, item := range collection {
		key := iteratee(item)

		resultIndex, ok := seen[key]
		if !ok {
			resultIndex = len(result)
			seen[key] = resultIndex
			result = append(result, []T{})
		}

		result[resultIndex] = append(result[resultIndex], item)
	}

	return result

	// unordered:
	// groups := GroupBy[T, K](collection, iteratee)
	// return Values[K, []T](groups)
}

// Flattens returns an array a single level deep.
func Flatten[T any](collection [][]T) []T {
	result := []T{}

	for _, item := range collection {
		result = append(result, item...)
	}

	return result
}

// Shuffle returns an array of shuffled values. Uses the Fisher-Yates shuffle algorithm.
func Shuffle[T any](collection []T) []T {
	rand.Shuffle(len(collection), func(i, j int) {
		collection[i], collection[j] = collection[j], collection[i]
	})

	return collection
}

// Reverse reverses array so that the first element becomes the last, the second element becomes the second to last, and so on.
func Reverse[T any](collection []T) []T {
	length := len(collection)
	half := length / 2

	for i := 0; i < half; i = i + 1 {
		j := length - 1 - i
		collection[i], collection[j] = collection[j], collection[i]
	}

	return collection
}

// Fill fills elements of array with `initial` value.
func Fill[T Clonable[T]](collection []T, initial T) []T {
	result := make([]T, 0, len(collection))

	for _ = range collection {
		result = append(result, initial.Clone())
	}

	return result
}

// Repeat builds a slice with N copies of initial value.
func Repeat[T Clonable[T]](count int, initial T) []T {
	result := make([]T, 0, count)

	for i := 0; i < count; i++ {
		result = append(result, initial.Clone())
	}

	return result
}

// KeyBy transforms a slice or an array of structs to a map based on a pivot callback.
func KeyBy[K comparable, V any](collection []V, iteratee func(V) K) map[K]V {
	result := make(map[K]V, len(collection))

	for _, v := range collection {
		k := iteratee(v)
		result[k] = v
	}

	return result
}