Method for operating decoder to reduce computational complexity and method for operating data storage device having the same

In the variable nodes and the method of operation of a decoder including a check node, the number of messages from the check nodes in the first check node has a certain size, from among the steps of: receiving the V2C message from the variable nodule, the V2C messages to a count based on the step of generating a count value, the V2C message size of the count value and the variable claim among nodes first variable node, the step of determining the size of the C2V messages to be transmitted to the first variable node, It includes.