RangeDecoder

pub const RangeDecoder = struct

File

Code

pub const RangeDecoder = struct {
    range: u32,
    code: u32,

    pub fn init(reader: *Reader) !RangeDecoder {
        var counter: u64 = 0;
        return initCounting(reader, &counter);
    }

    pub fn initCounting(reader: *Reader, n_read: *u64) !RangeDecoder {
        const reserved = try reader.takeByte();
        n_read.* += 1;
        if (reserved != 0) return error.InvalidRangeCode;
        const code = try reader.takeInt(u32, .big);
        n_read.* += 4;
        return .{
            .range = 0xFFFF_FFFF,
            .code = code,
        };
    }

    pub fn isFinished(self: RangeDecoder) bool {
        return self.code == 0;
    }

    fn normalize(self: *RangeDecoder, reader: *Reader, n_read: *u64) !void {
        if (self.range < 0x0100_0000) {
            self.range <<= 8;
            self.code = (self.code << 8) ^ @as(u32, try reader.takeByte());
            n_read.* += 1;
        }
    }

    fn getBit(self: *RangeDecoder, reader: *Reader, n_read: *u64) !bool {
        self.range >>= 1;

        const bit = self.code >= self.range;
        if (bit) self.code -= self.range;

        try self.normalize(reader, n_read);
        return bit;
    }

    pub fn get(self: *RangeDecoder, reader: *Reader, count: usize, n_read: *u64) !u32 {
        var result: u32 = 0;
        for (0..count) |_| {
            result = (result << 1) ^ @intFromBool(try self.getBit(reader, n_read));
        }
        return result;
    }

    pub fn decodeBit(self: *RangeDecoder, reader: *Reader, prob: *u16, n_read: *u64) !bool {
        const bound = (self.range >> 11) * prob.*;

        if (self.code < bound) {
            prob.* += (0x800 - prob.*) >> 5;
            self.range = bound;

            try self.normalize(reader, n_read);
            return false;
        } else {
            prob.* -= prob.* >> 5;
            self.code -= bound;
            self.range -= bound;

            try self.normalize(reader, n_read);
            return true;
        }
    }

    fn parseBitTree(
        self: *RangeDecoder,
        reader: *Reader,
        num_bits: u5,
        probs: []u16,
        n_read: *u64,
    ) !u32 {
        var tmp: u32 = 1;
        var i: @TypeOf(num_bits) = 0;
        while (i < num_bits) : (i += 1) {
            const bit = try self.decodeBit(reader, &probs[tmp], n_read);
            tmp = (tmp << 1) ^ @intFromBool(bit);
        }
        return tmp - (@as(u32, 1) << num_bits);
    }

    pub fn parseReverseBitTree(
        self: *RangeDecoder,
        reader: *Reader,
        num_bits: u5,
        probs: []u16,
        offset: usize,
        n_read: *u64,
    ) !u32 {
        var result: u32 = 0;
        var tmp: usize = 1;
        var i: @TypeOf(num_bits) = 0;
        while (i < num_bits) : (i += 1) {
            const bit = @intFromBool(try self.decodeBit(reader, &probs[offset + tmp], n_read));
            tmp = (tmp << 1) ^ bit;
            result ^= @as(u32, bit) << i;
        }
        return result;
    }
}