std.mem: add indexOfMin/Max

[ghost]

I think the usefulness of the index is obvious.

[nektro]

can you clarify what the goal of this issue is?

[ghost]

std.mem has min which gives you the min val. Getting the min val is a common task but sometimes we also need the index and I didn't find an fn that does that.

[andrewrk]

This issue is missing components that would make it viable:

• A better description of the motivation. Don't assume it is obvious. You should be able to link to some code in a zig project that had to do something worse, due to not having this feature.
• The proposed function prototype and doc comments

[ghost]

The presumed purpose of this function is to find the index of the smallest/largest value in a slice. This functionality is good to have, because otherwise it is necessary to first call mem.min to get the value and then mem.indexOfScalar to get the location, which traverses the array twice. I have used these functions several times in a numerical computing context (in Julia).

In case someone wants to make a PR: #9915

const testing = @import("std").testing;

/// Returns the index of the smallest number in a slice. O(n).
/// `slice` must not be empty.
pub fn minIndex(comptime T: type, slice: []const T) usize {
    var best = slice[0];
    var index: usize = 0;
    for (slice[1..]) |item, i| {
        if (item < best) {
            best = item;
            index = i + 1;
        }
    }
    return index;
}

test "mem.minIndex" {
    try testing.expect(minIndex(u8, "abcdefg") == 0);
}

/// Returns the index of the largest number in a slice. O(n).
/// `slice` must not be empty.
pub fn maxIndex(comptime T: type, slice: []const T) usize {
    var best = slice[0];
    var index: usize = 0;
    for (slice[1..]) |item, i| {
        if (item > best) {
            best = item;
            index = i + 1;
        }
    }
    return index;
}

test "mem.maxIndex" {
    try testing.expect(maxIndex(u8, "abcdefg") == 6);
}