Compression device and control method

A control method of a compression device, comprising:

• determining a plurality of code lengths that are respectively associated with a plurality of symbols, based on a frequency of occurrence of each of the plurality of symbols;
• when the plurality of symbols include one or more first symbols that are respectively associated with one or more first code lengths exceeding an upper limit, changing the one or more first code lengths to the upper limit, selecting, from one or more second symbols of the plurality of symbols that are respectively associated with one or more second code lengths shorter than the upper limit, at least one third symbol in descending associated code length order, changing at least one third code length associated with the at least one third symbol to the upper limit, and generating coding information that indicates the plurality of symbols and a plurality of variable-length codes that are respectively associated with the plurality of symbols, using at least the one or more first code lengths and the at least one third code length that are changed;
• and
• converting a symbol of the plurality of symbols into a variable-length code of the plurality of variable-length codes using the coding information,
• wherein a code length of each of the variable-length codes is a bit length from one to the upper limit.

A memory system comprising:

• a nonvolatile memory;
• and
• a controller connected to the nonvolatile memory,
• wherein the controller is configured to:
• determine a plurality of code lengths that are respectively associated with a plurality of symbols, based on a frequency of occurrence of each of the plurality of symbols;
• when the plurality of symbols include one or more first symbols that are respectively associated with one or more first code lengths exceeding an upper limit, change the one or more first code lengths to the upper limit, select, from one or more second symbols of the plurality of symbols that are respectively associated with one or more second code lengths shorter than the upper limit, at least one third symbol in descending associated code length order, change at least one third code length associated with the at least one third symbol to the upper limit, and generate coding information that indicates the plurality of symbols and a plurality of variable-length codes that are respectively associated with the plurality of symbols, using at least the one or more first code lengths and the at least one third code length that are changed;
• convert a symbol of the plurality of symbols into a variable-length code of the plurality of variable-length codes using the coding information;
• and
• write data based on the variable-length code, which is obtained by the conversion, into the nonvolatile memory,
• wherein a code length of each of the plurality of variable-length codes is a bit length from one to the upper limit.