// Copyright (c) 2025 Alexey Mayshev and contributors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package xiter

import "iter"

// Concat returns an iterator over the concatenation of the sequences.
func Concat[V any](seqs ...iter.Seq[V]) iter.Seq[V] {
	return func(yield func(V) bool) {
		for _, seq := range seqs {
			for e := range seq {
				if !yield(e) {
					return
				}
			}
		}
	}
}

// MergeFunc merges two sequences of values ordered by the function f.
// Values appear in the output once for each time they appear in x
// and once for each time they appear in y.
// When equal values appear in both sequences,
// the output contains the values from x before the values from y.
// If the two input sequences are not ordered by f,
// the output sequence will not be ordered by f,
// but it will still contain every value from x and y exactly once.
func MergeFunc[V any](x, y iter.Seq[V], f func(V, V) int) iter.Seq[V] {
	return func(yield func(V) bool) {
		next, stop := iter.Pull(y)
		defer stop()
		v2, ok2 := next()
		for v1 := range x {
			for ok2 && f(v1, v2) > 0 {
				if !yield(v2) {
					return
				}
				v2, ok2 = next()
			}
			if !yield(v1) {
				return
			}
		}
		for ok2 {
			if !yield(v2) {
				return
			}
			v2, ok2 = next()
		}
	}
}