buildClen

fn buildClen(
    dyn_bits: []const u4,
    out_values: []u8,
    out_extra: []u8,
    out_freqs: *[19]u16,
) struct { u16, u16 } {
    assert(dyn_bits.len <= out_values.len);
    assert(out_values.len == out_extra.len);

    var len: u16 = 0;
    var extra_bitsize: u16 = 0;

    var remaining_bits = dyn_bits;
    var prev: u4 = 0;
    while (true) {
        const b = remaining_bits[0];
        const n_max = @min(@as(u8, if (b != 0)
            if (b != prev) 1 else 6
        else
            138), remaining_bits.len);
        prev = b;

        var n: u8 = 0;
        while (true) {
            remaining_bits = remaining_bits[1..];
            n += 1;
            if (n == n_max or remaining_bits[0] != b) break;
        }
        const code, const extra, const xsize = switch (n) {
            0 => unreachable,
            1...2 => .{ b, 0, 0 },
            3...10 => .{
                @as(u8, 16) + @intFromBool(b == 0),
                n - 3,
                @as(u8, 2) + @intFromBool(b == 0),
            },
            11...138 => .{ 18, n - 11, 7 },
            else => unreachable,
        };
        while (true) {
            out_values[len] = code;
            out_extra[len] = extra;
            out_freqs[code] += 1;
            extra_bitsize += xsize;
            len += 1;
            if (n != 2) {
                @branchHint(.likely);
                break;
            }
            // Code needs outputted once more
            n = 1;
        }
        if (remaining_bits.len == 0) break;
    }

    return .{ len, extra_bitsize };
}