// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/12/Memory.jack

/**
 * This library provides two services: direct access to the computer's main
 * memory (RAM), and allocation and recycling of memory blocks. The Hack RAM
 * consists of 32,768 words, each holding a 16-bit binary number.
 */ 
class Memory {
    static int freeList;
    
    /** Initializes the class. */
    function void init() {
        let freeList = 2048;
        
        //freeList番地にfreeListLengthを入れる
        do Memory.poke(freeList, 16383 - freeList + 1);
        
        //(freeList + 1)番地にfreeListNext(-1=行き先ナシ)を入れる
        do Memory.poke(freeList + 1, -1);
        
        return;
    }

    /** Returns the RAM value at the given address. */
    function int peek(int j) {
        var Array memory;
        var int x;
        
        let memory = 0;
        let x = memory[j];
        
        return x;
    }

    /** Sets the RAM value at the given address to the given value. */
    function void poke(int j, int y) {
        var Array memory;
        let memory = 0;
        let memory[j] = y;
        
        return;
    }

    /** Finds an available RAM block of the given size and returns
     *  a reference to its base address. */
    function int alloc(int size) {
        var int pointer, segmentLength, segment, segmentRest;
        
        if(size < 1) {
            do Sys.error(44);
        }
        
        let segment = freeList;
        let segmentLength = Memory.peek(segment);
        let segmentRest = segmentLength - size;
        while(segmentRest < 2) {
            let segment = Memory.peek(segment + 1);
            if(segment = -1) {
                //割り当てできる領域がない(メモリ不足)
                do Sys.error(41);
            }
            let segmentLength = Memory.peek(segment);
        }
        
        //確保したメモリの頭
        let pointer = segment + segmentLength - size;
        //確保したメモリの前のアドレスにサイズを入れる
        do Memory.poke(pointer - 1, size + 1);
        //現在のsegmentのサイズを更新する
        do Memory.poke(segment, segmentLength - (size + 1));
        
        return pointer;
    }

    /** De-allocates the given object (cast as an array) by making
     *  it available for future allocations. */
    function void deAlloc(Array o) {
        var int segment;
        
        //freeListにsegmentを追加する
        //次のアドレスがない(=-1)まで移動
        let segment = freeList;
        while(~(Memory.peek(segment + 1) = -1)) {
            let segment = Memory.peek(segment + 1);
        }
        
        //freeListの最後にsegmentを追加する
        do Memory.poke(segment + 1, o - 1);
        //次のアドレスに-1を入れておく(-1: 次はなし)
        do Memory.poke(o, -1);
        
        return;
    }
}