Zig 0.17.0-dev (Split by item)

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ShortCode

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.

token.ShortCode

fn ShortCode(Value: type, HighBits: type, HighLog2: type, len_special: bool) type

File

lib/std/compress/flate/token.zig:73

Code

fn 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);
        }
    };
}