如何使用 Itertools 创建一系列 Chrono 日期的所有组合？

Question

我正在尝试使用 Rust itertools 生成日期范围的所有组合，但它说不满足特征界限。

extern crate chrono;
extern crate itertools;

use itertools::Itertools;
use chrono::prelude::*;

fn main() {
    let min = NaiveDate::from_ymd(2018, 10, 1);
    let max = NaiveDate::from_ymd(2018, 10, 14);
    let combinations = (min..=max).combinations(5);
}

错误信息：

error[E0599]: no method named `combinations` found for type `std::ops::RangeInclusive<chrono::NaiveDate>` in the current scope
  --> src/main.rs:46:36
   |
46 |     let combinations = (min..=max).combinations(5);
   |                                    ^^^^^^^^^^^^
   |
   = note: the method `combinations` exists but the following trait bounds were not satisfied:
           `std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`
           `&std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`
           `&mut std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`

我希望 Itertools 是为通用 RangeInclusive 实现的。我正在学习 Rust，所以我可能会遗漏一些明显的东西。

Answer 1

Itertools，顾名思义，适用于 Iterators。 not possible 在稳定的 Rust（版本 1.29）中从标准库之外创建一个类型的可迭代范围。

相反，我们可以为日期范围创建自定义迭代器，基于looping over dates

extern crate chrono; // 0.4.6
extern crate itertools; // 0.7.8

use chrono::{Duration, NaiveDate};
use itertools::Itertools;
use std::mem;

struct DateRange(NaiveDate, NaiveDate);

impl Iterator for DateRange {
    type Item = NaiveDate;
    fn next(&mut self) -> Option<Self::Item> {
        if self.0 < self.1 {
            let next = self.0 + Duration::days(1);
            Some(mem::replace(&mut self.0, next))
        } else {
            None
        }
    }
}

fn main() {
    let min = NaiveDate::from_ymd(2018, 10, 1);
    let max = NaiveDate::from_ymd(2018, 10, 14);
    let combinations: Vec<_> = DateRange(min, max).combinations(5).collect();
    println!("{:?}", combinations)
}

• Iterating over a range of generic type
• Loop over date range