fn ShortCode(Value: type, HighBits: type, HighLog2: type, len_special: bool) typehigh_bits: HighBitsBits preceding high bit or start if none
high_log2: HighLog2High bit, 0 means none, otherwise it is at bit x + high_log2 - 1
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112fn ShortCode(Value: type, HighBits: type, HighLog2: type, len_special: bool) type {
return packed struct(u5) {
/// Bits preceding high bit or start if none
high_bits: HighBits,
/// High bit, 0 means none, otherwise it is at bit `x + high_log2 - 1`
high_log2: HighLog2,
pub fn fromVal(v: Value) @This() {
if (len_special and v == 255) return .fromInt(28);
const high_bits = @bitSizeOf(HighBits) + 1;
const bits = @bitSizeOf(Value) - @clz(v);
if (bits <= high_bits) return @bitCast(@as(u5, @intCast(v)));
const high = v >> @intCast(bits - high_bits);
return .{ .high_bits = @truncate(high), .high_log2 = @intCast(bits - high_bits + 1) };
}
/// `@ctz(return) >= extraBits()`
pub fn base(c: @This()) Value {
if (len_special and c.toInt() == 28) return 255;
if (c.high_log2 <= 1) return @as(u5, @bitCast(c));
const high_value = (@as(Value, @intFromBool(c.high_log2 != 0)) << @bitSizeOf(HighBits)) | c.high_bits;
const high_start = @as(std.math.Log2Int(Value), c.high_log2 - 1);
return @shlExact(high_value, high_start);
}
const max_extra = @bitSizeOf(Value) - (1 + @bitSizeOf(HighLog2));
pub fn extraBits(c: @This()) std.math.IntFittingRange(0, max_extra) {
if (len_special and c.toInt() == 28) return 0;
return @intCast(c.high_log2 -| 1);
}
pub fn toInt(c: @This()) u5 {
return @bitCast(c);
}
pub fn fromInt(x: u5) @This() {
return @bitCast(x);
}
};
}
For length and distance codes, they having this format.
For example, length code 0b1101 (13 or literal 270) has high_bits=0b01 and high_log2=3 and is 1_01_xx (2 extra bits). It is then offsetted by the min length of 3. ^ bit 4 = 2 + high_log2 - 1
An exception is Length codes, where value 255 is assigned the special zero-bit code 28 or literal 285.