const std = @import("std");

/// Convert a string to lowercase using a pre-allocated buffer.
/// Returns null if the buffer is too small.
pub fn toLower(s: []const u8, buf: []u8) ?[]const u8 {
    if (s.len > buf.len) return null;
    for (s, 0..) |c, i| {
        buf[i] = std.ascii.toLower(c);
    }
    return buf[0..s.len];
}

test "toLower" {
    const testing = std.testing;
    var buf: [64]u8 = undefined;

    try testing.expectEqualStrings("hello", toLower("HELLO", &buf).?);
    try testing.expectEqualStrings("hello", toLower("hello", &buf).?);
    try testing.expectEqualStrings("hello123", toLower("HeLLo123", &buf).?);

    // Buffer too small
    var small_buf: [2]u8 = undefined;
    try testing.expect(toLower("hello", &small_buf) == null);
}