utf8ToUtf16LeImpl

pub fn utf8ToUtf16LeImpl(utf16le: []u16, utf8: []const u8, comptime surrogates: Surrogates) !usize {
    var dest_index: usize = 0;

    var remaining = utf8;
    vectorized: {
        const chunk_len = std.simd.suggestVectorLength(u16) orelse break :vectorized;
        const Chunk = @Vector(chunk_len, u8);

        // Fast path. Check for and encode ASCII characters at the start of the input.
        while (remaining.len >= chunk_len) {
            const chunk: Chunk = remaining[0..chunk_len].*;
            const mask: Chunk = @splat(0x80);
            if (@reduce(.Or, chunk & mask == mask)) {
                // found a non ASCII code unit
                break;
            }
            const utf16_chunk = mem.nativeToLittle(@Vector(chunk_len, u16), chunk);
            utf16le[dest_index..][0..chunk_len].* = utf16_chunk;
            dest_index += chunk_len;
            remaining = remaining[chunk_len..];
        }
    }

    const view = switch (surrogates) {
        .cannot_encode_surrogate_half => try Utf8View.init(remaining),
        .can_encode_surrogate_half => try Wtf8View.init(remaining),
    };
    var it = view.iterator();
    while (it.nextCodepoint()) |codepoint| {
        if (codepoint < 0x10000) {
            utf16le[dest_index] = mem.nativeToLittle(u16, @intCast(codepoint));
            dest_index += 1;
        } else {
            const high = @as(u16, @intCast((codepoint - 0x10000) >> 10)) + 0xD800;
            const low = @as(u16, @intCast(codepoint & 0x3FF)) + 0xDC00;
            utf16le[dest_index..][0..2].* = .{ mem.nativeToLittle(u16, high), mem.nativeToLittle(u16, low) };
            dest_index += 2;
        }
    }
    return dest_index;
}