Reverse specific key when sorting with multiple keys

Here's a similar approach to the problem: create a function for chaining multiple orderings:

/// chain two orderings: the first one gets more priority
fn chain_ordering(o1: Ordering, o2: Ordering) -> Ordering {
    match o1 {
        Ordering::Equal => o2,
        _ => o1,
    }
}

Then use sort_by, possibly with pattern matching, to produce the ordering of each key:

#[derive(Debug, PartialEq)]
struct HeroSkill(&'static str, &'static str);

fn main() {
    // a vector of hero names and super powers
    let mut v = vec![
        HeroSkill("Bob", "X"),
        HeroSkill("Bob", "Y"),
        HeroSkill("Alice", "X")
    ];

    // sort by name, then by super power, where Y is more powerful than X
    v.sort_by(|&HeroSkill(name1, power1), &HeroSkill(name2, power2)| {
        chain_ordering(name1.cmp(name2), power1.cmp(power2).reverse())
    });

    assert_eq!(v, vec![
        HeroSkill("Alice", "X"),
        HeroSkill("Bob", "Y"),
        HeroSkill("Bob", "X")
    ]);
}

Playground

Since Rust 1.19, the std::cmp::Reverse struct wraps a value and implements PartialOrd and Ord by calling partial_cmp and cmp with swapped arguments in order to return the reversed order. Just wrap the key to sort in descending order:

vec.sort_by_key(|k| (Reverse(foo(k)), bar(k)));

Before Rust 1.19, you can use the revord crate (documentation) which provides the struct RevOrd which provides the same benefit::

vec.sort_by_key(|k| (RevOrd(foo(k)), bar(k)));

You can use sort_by paired with Ordering::reverse instead of sort_by_key.

use std::cmp::Ordering;

#[derive(Debug)]
struct Foo(&'static str, u8);

impl Foo {
    fn name(&self) -> &str { self.0 }
    fn len(&self) -> u8 { self.1 }
}

fn main() {
    let mut vec = vec![Foo("alpha", 1), Foo("beta", 2), Foo("beta", 1)];

    vec.sort_by(|a, b| {
        match a.name().cmp(b.name()).reverse() {
            Ordering::Equal => a.len().cmp(&b.len()),
            other => other,
        }
    });

    println!("{:?}", vec);
}

This sorts in reverse alphabetical order, then ties are sorted in ascending numerical order:

[Foo("beta", 1), Foo("beta", 2), Foo("alpha", 1)]

Since Rust 1.17 (via RFC 1677), you can write it like this:

vec.sort_by(|a, b| {
    a.name().cmp(b.name()).reverse()
        .then(a.len().cmp(&b.len()))
});

If you have something that can naturally be negated / inverted, you can simply negate the key.